Diagnosis of respiratory failure

To diagnose respiratory failure, a number of modern research methods are used that allow one to comprehend the specific causes, mechanisms and severity of the course of respiratory failure, the accompanying functional and organic changes in internal organs, the state of hemodynamics, the acid-base state, and so on. To this end, determine the function of external respiration, the gas composition of the blood, respiratory and minute volumes of ventilation, hemoglobin and hematocrit levels, blood oxygen saturation, arterial and central venous pressure, heart rate, ECG, if necessary - pulmonary artery wedge pressure and others (AP Zilber).

Evaluation of the function of external respiration

The most important method of diagnosing respiratory failure is the evaluation of the external respiration function of the HPF), the main tasks of which can be formulated as follows:

1. Diagnosis of violations of the function of external respiration and an objective assessment of the severity of respiratory failure.
2. Differential diagnosis of obstructive and restrictive pulmonary ventilation disorders.
3. Justification of pathogenetic therapy of respiratory failure.
4. Evaluation of the effectiveness of the treatment.

These tasks are solved with the help of a number of instrumental and laboratory methods: pyrometry, spirography, pneumotachometry, tests for the diffusion capacity of the lungs, violation of ventilation-perfusion relations, etc. The scope of examinations is determined by many factors, including the severity of the patient's condition and the possibility (and appropriateness!) full-fledged and comprehensive investigation of HPF.

The most common methods of studying the function of external respiration are spirometry and spirography. Spirography provides not only a measurement, but a graphic recording of the main ventilation parameters with calm and formed breathing, physical activity, and carrying out pharmacological tests. In recent years, the use of computer spirographic systems has greatly simplified and accelerated the conduct of the survey and, most importantly, allowed to measure the volumetric rate of inspiratory and expiratory airflow as a function of lung volume, i.e. Analyze the flow-volume loop. Such computer systems include, for example, spirographs of the firms "Fukuda" (Japan) and "Erich Eger" (Germany), etc.

Methods of research. The simplest spirograph consists of an air-filled double cylinder immersed in a container with water and connected to a recordable device (for example, with a calibrated and rotating drum with a certain speed, on which the spirograph readings are recorded). The patient in sitting position breathes through the tube connected to the cylinder with air. Changes in the volume of the lungs during respiration are recorded from the change in the volume of the cylinder connected to the rotating drum. The study is usually conducted in two modes:

• In the conditions of the main exchange - in the early morning hours, on an empty stomach, after a 1-hour rest in the supine position; for 12-24 hours before the study should be canceled taking medication.
• In conditions of relative rest - in the morning or afternoon, on an empty stomach or not earlier than 2 hours after a light breakfast; Before the study, rest for 15 minutes in a sitting position is necessary.

The study is carried out in a separate, poorly lit room with an air temperature of 18-24 ° C, having previously acquainted the patient with the procedure. In the study, it is important to achieve full contact with the patient, since his negative attitude towards the procedure and lack of necessary skills can greatly change the results and lead to an inadequate evaluation of the data.

[1], [2], [3], [4], [5]

Basic indicators of pulmonary ventilation

Classical spirography allows to determine:

1. the value of most pulmonary volumes and capacities,
2. basic indicators of pulmonary ventilation,
3. oxygen consumption by the body and ventilation efficiency.

There are 4 primary pulmonary volumes and 4 vessels. The latter include two or more primary volumes.

Pulmonary volumes

1. The respiratory volume (DO, or VT - tidal volume) is the volume of gas inhaled and exhaled with quiet breathing.
2. Inspiratory reserve volume (PO _tm or IRV - inspiratory reserve volume) - the maximum amount of gas that can be further inhale after inhaling relaxing.
3. Reserve expiratory volume (PO _vyd, or ERV - expiratory reserve volume) is the maximum volume of gas that can be exhaled after a quiet exhalation.
4. Residual lung volume (OOJI, or RV - residual volume) is the volume of the reptile that remains in the lungs after maximum expiration.

Pulmonary capacity

1. Vital capacity (VC or VC - vital capacity) is the amount to, PO _tm and PO _vyd, i.e. The maximum volume of gas that can be exhaled after the maximum deep inspiration.
2. The inspiratory capacity (Eud, or 1C - inspiratory capacity) is the sum of DO and RO _vs, i.e. The maximum volume of gas that can be inhaled after a quiet exhalation. This capacity characterizes the ability of the lung tissue to stretch.
3. Functional residual capacity (FOE, or FRC - functional residual capacity) is the sum of the OOL and PO _output. The volume of gas remaining in the lungs after a calm exhalation.
4. Total lung capacity (OEL, or TLC - total lung capacity) is the total amount of gas contained in the lungs after a maximum inspiration.

Conventional spirographs, widespread in clinical practice, only 5 allow us to determine lung volumes and capacities: TO, RO _hp, PO _vyd. YEL, Evd (or, respectively, VT, IRV, ERV, VC and 1C). It is necessary to use special techniques, in particular, helium dilution methods, flushing techniques, for the determination of the most important index of ventilation - the functional residual capacity (FOE or FRC) and calculation of the residual volume of the lungs (OOL or RV) and total lung capacity (OEL, or TLC) nitrogen or plethysmography of the whole body (see below).

The main indicator in the traditional technique of spirography is the vital capacity of the lungs (ZHEL, or VC). To measure LEL, the patient after a period of calm breathing (DO) produces at first a maximum breath, and then, perhaps, a full exhalation. It is advisable to estimate not only the integral value of the ZHEL) and the inspiratory and expiratory life capacity (VCin, VCex, respectively), i.e. The maximum volume of air that can be inhaled or exhaled.

The second mandatory method used in traditional spirography is a test with the definition of the forced vital capacity of the lungs, or FVC - forced vital capacity expiratory, which allows to determine the most (formative speed parameters of pulmonary ventilation in forced expiration, characterizing, in particular, the degree obstruction of the intrapulmonary airways.As with a test with the definition of VC, the patient produces the deepest inhalation, and then, in contrast to the definition of the LEL, the air is exhaled to a maximum but possible speed (forced expiration) When this is registered preceding the exponential curve flattens progressively Evaluating spirogram expiratory this maneuver is calculated several indicators..:

1. The volume of the forced exhalation in one second (FEV1, or FEV1 - forced expiratory volume after 1 second) is the amount of air withdrawn from the lungs during the first second of expiration. This indicator decreases both in the obstruction of the airways (due to the increase in bronchial resistance) and in restrictive disorders (due to the reduction of all pulmonary volumes).
2. Tiffon index (FEV1 / FVC,%) is the ratio of the forced expiratory volume in the first second (FEV1 or FEV1) to the forced vital capacity of the lung (FVC, or FVC). This is the main indicator of expiratory maneuver with forced expiration. It significantly decreases with bronchial obstructive syndrome, because the slowing of exhalation due to bronchial obstruction is accompanied by a decrease in the volume of forced expiration in 1 s (FEV1 or FEV1) with or without a slight decrease in the overall FVC. With restrictive disorders, the Tiffno index is practically unchanged, as FEV1 (FEV1) and FVC (FVC) decrease almost equally.
3. The maximum volumetric exhalation rate at 25%, 50% and 75% of the forced vital capacity of the lungs (MOC25%, MOS50%, MOS75%, or MEF25, MEF50, MEF75 - maximum expiratory flow at 25%, 50%, 75% of FVC) . These values are calculated by dividing the corresponding volumes (in liters) of forced expiration (at the level of 25%, 50% and 75% of the total FVC) for the time of reaching these volumes with forced expiration (in seconds).
4. The average volumetric expiratory flow rate is 25 ~ 75% of FVC (COS25-75%. Or FEF25-75). This indicator is less dependent on the patient's arbitrary effort and more objectively reflects the patency of the bronchi.
5. Peak volume rate of forced expiration (PIC _vyd, or PEF - peak expiratory flow) - the maximum volume rate of forced expiration.

Based on the results of the spirographic study, the following is also calculated:

1. number of respiratory movements with quiet breathing (BH, or BF - breathing freguency) and
2. minute breathing volume (MOU, or MV - minute volume) - the value of total ventilation of the lungs per minute with quiet breathing.

[6], [7]

Investigation of the "flow-volume" relation

Computer spirography

Modern computer spirographic systems allow you to automatically analyze not only the above spirographic indicators, but also the flow-volume ratio, i.e. The dependence of the volumetric flow velocity of the air during inspiration and expiration on the value of the pulmonary volume. Automatic computer analysis of the inspiratory and expiratory parts of the flow-volume loop is the most promising method for quantifying pulmonary ventilation disorders. Although the flow-volume loop itself contains essentially the same information as a simple spirogram, the visibility of the relationship between the volumetric flow rate of airflow and the volume of the lung allows for a more detailed study of the functional characteristics of both the upper and lower airways.

The basic element of all modern spirographic computer systems is a pneumotachograph sensor that records the volumetric airflow velocity. The sensor is a wide tube through which the patient breathes freely. In this case, as a result of the small, known, aerodynamic resistance of the tube between its beginning and end, a certain pressure difference is directly proportional to the volumetric flow velocity of the air. In this way, it is possible to register changes in the volumetric air flow rate during the doha and expiration - a piracy chart.

The automatic integration of this signal also makes it possible to obtain traditional spirographic indices - the volume of lungs in liters. Thus, at each moment of time, information about the volumetric air flow rate and the volume of the lungs at a given time is simultaneously fed into the computer's memory. This allows you to build a flow-volume curve on the monitor screen. An essential advantage of this method is that the device operates in an open system, i.e. The subject breathes through the tube through the open contour, without experiencing additional resistance to breathing, as in ordinary spirography.

The procedure for performing respiratory maneuvers when registering the flow-volume curve and resembling the recording of an ordinary coroutine. After a period of difficult breathing, the patient takes a maximum breath, as a result of which the inspiratory part of the flow-volume curve is recorded. The volume of the lung at point "3" corresponds to the total lung capacity (OEL, or TLC). Following this, the patient makes a forced exhalation, and the expiratory part of the flow-volume curve (curve "3-4-5-1") is recorded on the monitor screen. At the beginning of the forced exhalation ("3-4"), the volumetric air velocity rapidly increases, reaching a peak (peak space velocity - PIC _output, or PEF), and then decreases linearly until the end of the forced exhalation, when the forced expiratory curve returns to its original position.

In a healthy person, the shape of the inspiratory and expiratory parts of the flow-volume curve is significantly different: the maximum volumetric space velocity during inspiration is reached at about 50% YEL (MO50% inspiration> or MIF50), while during forced exhalation the peak expiratory flow POSSvid or PEF) occurs very early. The maximum inspiratory flow (MOC50% inspiration, or MIF50) is approximately 1.5 times greater than the maximum expiratory flow in the middle of the vital capacity (Vmax50%).

The described sample of the flow-volume curve is carried out several times until the coincidence results coincide. In most modern instruments, the procedure for collecting the best curve for further material processing is automatic. The flow-volume curve is printed along with numerous indicators of pulmonary ventilation.

With the help of a pneumotogeographic sensor, a curve of the volumetric flow velocity of air is recorded. The automatic integration of this curve makes it possible to obtain a curve of respiratory volumes.

[8], [9], [10]

Evaluation of research results

The majority of pulmonary volumes and capacities, both in healthy patients and in patients with lung diseases, depend on a number of factors, including age, sex, chest size, body position, level of fitness, etc. For example, the vital capacity of the lungs (ZHEL, or VC) in healthy people decreases with age, while the residual volume of the lungs (OOL, or RV) increases, and the total lung capacity (OEL, or TLS) remains practically unchanged. ZHEL is proportional to the size of the chest and, accordingly, the growth of the patient. Women were on average 25% lower than men.

Therefore, from a practical point of view, it is not practical to compare the quantities of pulmonary volumes and capacities obtained during spirographic research: by single "standards", the fluctuations in their values due to the influence of the above and other factors are very significant (for example, GEL can normally vary from 3 to 6 liters) .

The most acceptable way of evaluating the spirographic indices obtained in the study is to compare them with the so-called proper values that were obtained by examining large groups of healthy people, taking into account their age, gender and growth.

The proper values of ventilation indicators are determined by special formulas or tables. In modern computer spirographs they are calculated automatically. For each indicator, the boundaries of the normal values in percent relative to the calculated proper value are given. For example, LEL (VC) or FVC (FVC) is considered to be reduced if its actual value is less than 85% of the calculated proper value. Decrease in FEV1 (FEV1) is ascertained if the actual value of this indicator is less than 75% of the proper value, and the decrease in FEV1 / FVC (FEV1 / FVC) - at an actual value less than 65% of the proper value.

Limits of normal values of the basic spirographic indices (in percent relative to the calculated proper value).

Indicators	Norm	Conditional Norm	Deviations
			Moderate	Significant	Sharp
ZHEL	> 90	85-89	70-84	50-69	<50
FEV1	> 85	75-84	55-74	35-54	<35
FEV1 / FVC	> 70	65-69	55-64	40-54	<40
OOL	90-125	126-140	141-175	176-225	> 225
		85-89	70-84	50-69	<50
OEL	90-110	110-115	116-125	126-140	> 140
		85-89	75-84	60-74	<60
OOL / OEL	<105	105-108	109-115	116-125	> 125

In addition, when evaluating the results of spirography, it is necessary to take into account some additional conditions under which the study was conducted: the atmospheric pressure, temperature and humidity levels of the ambient air. Indeed, the amount of air exhaled by the patient is usually somewhat less than that which the same air held in the lungs, since its temperature and humidity are generally higher than the ambient air. To exclude differences in the measured values associated with the study conditions, all pulmonary volumes, whether due (calculated) or actual (measured in this patient), are given for conditions corresponding to their values at a body temperature of 37 ° C and full saturation with water in pairs (BTPS - Body Temperature, Pressure, Saturated). In modern computer spirographs, such correction and recalculation of pulmonary volumes in the BTPS system is automatic.

Interpretation of results

The practical physician should well represent the true possibilities of the spirographic research method, limited, as a rule, by the lack of information about the values of the residual volume of the lungs, functional residual capacity (FOE) and total lung capacity (OEL), which does not allow for a full analysis of the OEL structure. At the same time, spirography makes it possible to compose a general idea of the state of external respiration, in particular:

1. to detect a decrease in the vital capacity of the lungs (ZHEL);
2. to reveal violations of tracheobronchial patency, and using modern computer analysis of flow-volume loops - at the earliest stages of development of obstructive syndrome;
3. to reveal the presence of restrictive pulmonary ventilation disorders in cases when they are not combined with violations of bronchial patency.

Modern computer spirography allows obtaining reliable and complete information about the presence of bronchial obstructive syndrome. More or less reliable detection of restrictive ventilation disorders with the help of the spirographic method (without the use of gas analytical methods for evaluating the structure of the OEL) is possible only in relatively simple, classical cases of pulmonary dilatability disorder when they do not combine with impaired bronchial patency.

[11], [12], [13], [14], [15]

Diagnosis of obstructive syndrome

The main spirographic sign of obstructive syndrome is the slowing of forced exhalation due to increased airway resistance. When registering a classical spirogram, the forced expiratory curve becomes stretched, such indicators as FEV1 and Tiffno index (FEV1 / FVC, or FEV, / FVC) decrease. The VC (VC) either does not change, or decreases slightly.

A more reliable sign of bronchial obstructive syndrome is a decrease in the Tiffno index (FEV1 / FVC, or FEV1 / FVC), since the absolute value of FEV1 may decrease not only in bronchial obstruction but also in restrictive disorders due to a proportional decrease in all pulmonary volumes and capacities, including FEV1 (FEV1) and FVC (FVC).

Already in the early stages of the development of obstructive syndrome, the calculated value of the mean space velocity at the level of 25-75% of FVC (COC25-75%) - O "is the most sensitive spirographic indicator, which earlier than others indicates an increase in airway resistance. Accurate manual measurements of the descending knee of the FVC curve, which is not always possible according to the classical spirogram.

More accurate and reliable data can be obtained by analyzing the flow-volume loop using modern computer spirographic systems. Obstructive disorders are accompanied by changes in the predominantly expiratory part of the flow-volume loop. If in most healthy people this part of the loop resembles a triangle with an almost linear decrease in the volumetric air flow rate during expiration, in patients with impaired bronchial patency there is a kind of "sagging" of the expiratory part of the loop and a decrease in the volume flow rate of air for all lung volume values. Often, due to the increase in lung volume, the expiratory part of the loop is shifted to the left.

Reduced such spirographic indicators as FEV1 (FEV1), FEV1 / FVC (FEV1 / FVS), the peak volumetric exhalation rate (PIC _vyd or REF) MOS25% (MEF25) MOS50% (MEF50) MOS75% (MEF75) and СОС25-75% (FЕF25-75).

The vital capacity of the lungs (JEL) may remain unchanged or decrease, even in the absence of concomitant restrictive disorders. It is also important to estimate the size of the reserve volume of expiration (PO _vyd ), which naturally decreases with obstructive syndrome, especially when an early expiratory closure (collapse) of the bronchi occurs.

According to some researchers, a quantitative analysis of the expiratory part of the flow-volume loop also makes it possible to form an idea of the predominant severity of large or small bronchi. It is believed that the obstruction of large bronchi is characterized by a decrease in the volume rate of forced exhalation, mainly in the initial part of the loop, and therefore sharply reduced such indicators as peak space velocity (PIC) and maximum space velocity at 25% of FVC (25% MEF25). At the same time, the volumetric flow rate of air in the middle and the end of the exhalation (MOC50% and MOS75%) also decreases, but to a lesser extent than PIC _vyd and MOC25%. Conversely, with obstruction of the small bronchi, a decrease in the MOC50% is detected predominantly. MOS75% whereas PIC _vyd normal or slightly reduced and MOS25% reduced moderately.

However, it should be emphasized that these provisions are currently quite controversial and can not be recommended for use in broad clinical practice. In any case, there is more reason to believe that the unevenness of the decrease in the volume velocity of the air flow during forced exhalation rather reflects the degree of bronchial obstruction than its localization. The early stages of bronchial constriction are accompanied by a slowing down of the expiratory flow of air at the end and middle of the exhalation (a decrease in MOC50%, MOC75%, COC25-75% with low values of MOC25%, FEV1 / FVC and PIC), whereas with pronounced bronchial obstruction, speed indicators, including the Tiffno index (FEV1 / FVC), PIC and MOC25%.

It is of interest to diagnose obstruction of the upper airways (larynx, trachea) using computer spirographs. There are three types of such obstruction:

1. fixed obstruction;
2. variable non-obstructive obstruction;
3. variable intrathoracic obstruction.

An example of a fixed obstruction of the upper airways is stenosis of the fallow deer, due to the presence of a tracheostomy. In these cases, breathing is carried out through a rigid relatively narrow tube, the lumen of which does not change during inhalation and exhalation. This fixed obstruction limits the flow of air both on inhalation and on exhalation. Therefore, the expiratory part of the curve resembles an inspiratory shape; the volumetric rates of inspiration and expiration are significantly reduced and almost equal to each other.

In the clinic, however, it is more often necessary to encounter two variants of variable obstruction of the upper airways, when the larynx or trachea lumen changes the time of inspiration or expiration, leading to selective restriction of respiratory or expiratory airflow, respectively.

Variable hilar obstruction is observed in various types of stenosis of the larynx (swelling of the vocal cords, swelling, etc.). As is known, during respiratory movements, the lumen of the extrathoracic airways, especially the narrowed ones, depends on the ratio of intra-tracheal and atmospheric pressures. During inspiration, the pressure in the trachea (as well as the vitrualveolar and intrapleural) becomes negative, i.e. Below atmospheric. This contributes to the narrowing of the lumen of the extrathoracic airways and to a significant limitation of the ipspirator air flow and to a decrease (flattening) of the inspiratory part of the flow-volume loop. During forced exhalation, the intra-tracheal pressure becomes significantly higher than the atmospheric pressure, so that the diameter of the airways approaches normal, and the expiratory part of the flow-volume loop changes little. Variable intrathoracic obstruction of the upper airways is observed and tumors of the trachea and dyskinesia of the membrane part of the trachea. The diameter of the airway in the airway is largely determined by the ratio of intra-tracheal and intrapleural pressures. With forced expiration, when the intrapleural pressure increases significantly, exceeding the pressure in the trachea, the intrathoracic airways narrow, and their obstruction develops. During inspiration, the pressure in the trachea slightly exceeds the negative intrapleural pressure, and the degree of narrowing of the trachea decreases.

Thus, with variable intra-thoracic obstruction of the upper airways, a selective restriction of the air flow on the exhalation and flattening of the inspiratory part of the loop takes place. Its inspiratory part almost does not change.

With variable extra-thoracic obstruction of the upper airways, selective restriction of the volumetric air flow velocity is observed predominantly on inspiration, with intrathoracic obstruction - on exhalation.

It should also be noted that in clinical practice, cases where the narrowing of the lumen of the upper airways are accompanied by the flattening of only the inspiratory or only the expiratory part of the loop is quite rare. Usually, airflow restriction is detected in both phases of breathing, although during one of them the process is much more pronounced.

[16], [17], [18], [19], [20], [21]

Diagnosis of restrictive disorders

Restrictive pulmonary ventilation disorders are accompanied by a restriction of lung filling with air due to a decrease in the respiratory surface of the lung, switching off the lung part from breathing, reducing the elastic properties of the lung and chest, and the ability of the lung tissue to stretch (inflammatory or hemodynamic pulmonary edema, massive pneumonia, pneumoconiosis, pneumosclerosis and so-called). In this case, if the restrictive disorders are not combined with the above-described violations of bronchial patency, the resistance of the airways usually does not increase.

The main consequence of restrictive (limiting) ventilation disorders detected by classical spirography - is almost proportional decrease in the majority of lung volumes and capacities: BEFORE, VC, RC _hp, PO _vyd, FEV, FEV 1, etc. It is important that, unlike obstructive syndrome, the decrease in FEV1 is not accompanied by a decrease in the FEV1 / FVC ratio. This indicator remains within the limits of the norm or even slightly increases due to a more significant decrease in the LEL.

With computer spirography, the flow-volume curve is a reduced copy of the normal curve, due to the overall decrease in lung volume shifted to the right. The peak space velocity (PIC) of the expiratory flow of FEV1 is decreased, although the FEV1 / FVC ratio is normal or increased. Due to the limitation of lung expansion and, consequently, the reduction of its elastic thrust, flow rates (for example, СОС25-75% "MOS50%, MOS75%) in some cases can also be reduced even in the absence of airway obstruction.

The most important diagnostic criteria for restrictive ventilation disorders, which make it possible to reliably distinguish them from obstructive disorders, are:

1. an almost proportional decrease in pulmonary volumes and capacities measured in spirography, as well as in flow rates, and, accordingly, a normal or slightly altered shape of the flow-volume loop curve shifted to the right;
2. normal or even increased Tiffon index (FEV1 / FVC);
3. the decrease in the reserve volume of inspiration (PO _d ) is almost proportional to the reserve expiratory volume (PO _vyd ).

It should be emphasized once again that for the diagnosis of even "pure" restrictive ventilation disorders, one can not focus only on the reduction of GEL, since the sweat rate with a pronounced obstructive syndrome can also be significantly reduced. More reliable differential-diagnostic features are no changes form part expiratory flow-volume curve (in particular, normal or increased values OFB1 / FVC), and the proportional reduction PO _tm and PO _vyd.

[22], [23], [24]

Determination of the structure of total lung capacity (OEL, or TLC)

As mentioned above, the methods of classical spirography, as well as computer processing of the flow-volume curve, make it possible to comprehend the changes in only five of the eight pulmonary volumes and capacities (DO, ROVD, ROVID, YEL, Evd, or VT, IRV, ERV , VC and 1C), which makes it possible to evaluate mainly the degree of obstructive pulmonary ventilation disorders. Restrictive disorders can be sufficiently reliably diagnosed only if they are not combined with a violation of bronchial patency, i.e. In the absence of mixed pulmonary ventilation disorders. Nevertheless, in the practice of a doctor, it is often such mixed disorders (for example, in chronic obstructive bronchitis or bronchial asthma, complicated by emphysema and pneumosclerosis, etc.). In these cases, the mechanisms of pulmonary ventilation disorders can be detected only by analysis of the structure of the OEL.

To solve this problem, it is necessary to use additional methods to determine the functional residual capacity (FOE, or FRC) and calculate the residual lung volume (RV) and total lung capacity (OEL, or TLC). Because FOE is the amount of air remaining in the lungs after maximum expiration, it is measured only by indirect methods (gas analysis or whole-body plethysmography).

The principle of gas analytical methods is that to the lungs either by introducing an inert gas helium (dilution method) or by washing out the nitrogen contained in the alveolar air, causing the patient to breathe pure oxygen. In both cases, the FOE is calculated from the final gas concentration (RF Schmidt, G. Thews).

Method of helium dilution. Helium, as is known, is an inert and harmless to the body gas, which practically does not pass through the alveolar-capillary membrane and does not participate in gas exchange.

The dilution method is based on measuring the helium concentration in the closed capacity of the spirometer before and after mixing the gas with the lung volume. A spirometer of a closed type with a known volume (V _cn ) is filled with a gas mixture consisting of oxygen and helium. The volume occupied by helium (V _cn ) and its initial concentration (FHe1) are also known. After a quiet exhalation, the patient begins to breathe from the spirometer, and helium is evenly distributed between lung volume (FOE, or FRC) and spirometry volume (V _cn ). After a few minutes, the concentration of helium in the general system ("spirometer-lungs") decreases (FHe ₂ ).

Method of nitrogen washout. When using this method, the spirometer is filled with oxygen. The patient breathes for a few minutes into the closed loop of the spirometer, while measuring the volume of exhaled air (gas), the initial content of nitrogen in the lungs and its final content in the spirometer. FRU (FRC) is calculated using an equation similar to that for the helium dilution method.

The accuracy of both of the above methods for determining the OPE (RNS) depends on the completeness of the mixing of gases in the lungs, which in healthy people occurs within a few minutes. However, in some diseases accompanied by severe uneven ventilation (for example, in obstructive pulmonary pathology), equilibration of the gas concentration takes a long time. In these cases, the measurement of FOE (FRC) by the methods described can be inaccurate. These defects are devoid of a more technically sophisticated method of plethysmography of the whole body.

Whole body plethysmography. The method of plethysmography of the whole body is one of the most informative and complex research methods used in pulmonology for the determination of pulmonary volumes, tracheobronchial resistance, elastic properties of lung tissue and thorax, and also for evaluating some other parameters of pulmonary ventilation.

The integral plethysmograph is a sealed chamber with a volume of 800 liters, in which the patient is freely placed. The patient breathes through a pneumotachograph tube connected to a hose open to the atmosphere. The hose has a damper that allows you to automatically shut off the air flow at the right time. Special pressure barometric sensors measure the pressure in the chamber (Rkam) and in the mouth (mouth). The last one with a closed hose flap is equal to the inside of the alveolar pressure. The Pythagotometer allows you to determine the air flow (V).

The principle of the integral plethysmograph is based on the Boyle Moriosta law, according to which, at a constant temperature, the relationship between the pressure (P) and the gas volume (V) remains constant:

P1xV1 = P2xV2, where P1 is the initial gas pressure, V1 is the initial gas volume, P2 is the pressure after the gas volume change, and V2 is the volume after the gas pressure changes.

The patient inside the plethysmograph chamber inhales and exhales slowly, after which (at the level of the FOE, or FRC), the hose flap is closed, and the subject attempts to "breath" and "expiration" (breathing maneuver) With this "breathing" maneuver the intra-alveolar pressure changes, and the pressure in the closed chamber of the plethysmograph changes inversely with it. When attempting to "inhale" with a closed flap, the volume of the chest increases, which leads, on the one hand, to a decrease in intra-alveolar pressure, and on the other hand to a corresponding increase in pressure in the plethysmograph chamber (P _kam ). On the contrary, when you try to "exhale" the alveolar pressure increases, and the volume of the chest and the pressure in the chamber decrease.

Thus, the whole-body plethysmography method allows calculating the intrathoracic gas volume (VGO) with high accuracy, which in healthy individuals corresponds quite accurately to the value of the functional residual capacity of the lungs (VON, or CS); the difference between VGO and FOB usually does not exceed 200 ml. However, it should be remembered that with violation of bronchial patency and some other pathological conditions, VGO can significantly exceed the value of true FOB due to an increase in the number of unventilated and poorly ventilated alveoli. In these cases, a combined study with the help of gas analytical methods of the whole-body plethysmography method is advisable. By the way, the difference between VOG and FOB is one of the important indicators of uneven ventilation of the lungs.

Interpretation of results

The main criterion for the presence of restrictive pulmonary ventilation disorders is a significant decrease in OEL. With "pure" restriction (without the combination of bronchial obstruction), the structure of the OEL does not change significantly, or there was a slight decrease in the ratio of OOL / OEL. If restrictive disorders occur against a background of violations of bronchial patency (mixed type of ventilation disorders), along with a distinct decrease in OEL, there is a significant change in its structure, characteristic of bronchial obstructive syndrome: an increase in OOL / OEL (> 35%) and FOE / OEL (> 50% ). In both variants of restrictive disorders, ZHEL significantly decreases.

Thus, the analysis of the structure of the OEL makes it possible to differentiate all three variants of ventilation disorders (obstructive, restrictive and mixed), whereas an evaluation of only spirographic indices does not enable one to reliably distinguish a mixed variant from an obstructive one, accompanied by a decrease in ZHEL.

The main criterion of obstructive syndrome is a change in the structure of the OEL, in particular an increase in OOL / OEL (greater than 35%) and FOE / OEL (more than 50%). For "pure" restrictive disorders (without combination with obstruction), the most common reduction in OEL without a change in its structure. The mixed type of ventilation disorders is characterized by a significant decrease in OEL and an increase in the ratio of OOL / OEL and FOE / OEL.

[25], [26], [27], [28], [29], [30],

Determination of uneven ventilation

In a healthy person, there is a certain physiological uneven ventilation of different parts of the lungs, due to differences in the mechanical properties of the airways and lung tissue, as well as by the so-called vertical gradient of pleural pressure. If the patient is in a vertical position, at the end of exhalation, the pleural pressure in the upper parts of the lung is more negative than in the lower (basal) regions. The difference can reach 8 cm of water column. Therefore, before the start of the next breath, the alveoli of the apex of the lungs are stretched more than the alveoli of the lower-bilobial divisions. In this connection, during inhalation, a greater volume of air enters the alveoli of the basal regions.

The alveoli of the lower basal parts of the lung are normally ventilated better than the apex regions, which is due to the presence of a vertical intrapleural pressure gradient. However, normally this uneven ventilation is not accompanied by a marked disturbance of gas exchange, since the blood flow in the lungs is also uneven: the basal parts are perfused better than the apical ones.

With some diseases of the respiratory system, the degree of uneven ventilation can significantly increase. The most common causes of such pathological uneven ventilation are:

• Diseases, accompanied by an uneven increase in airway resistance (chronic bronchitis, bronchial asthma).
• Diseases with unequal regional extensibility of pulmonary tissue (emphysema, pneumosclerosis).
• Inflammation of the lung tissue (focal pneumonia).
• Diseases and syndromes, combined with local restriction of alveolar distension (restrictive), - exudative pleurisy, hydrothorax, pneumosclerosis, etc.

Often different reasons are combined. For example, with chronic obstructive bronchitis complicated by emphysema and pneumosclerosis, regional violations of bronchial patency and extensibility of lung tissue develop.

With uneven ventilation, the physiological dead space increases substantially, gas exchange in which does not occur or is weakened. This is one of the reasons for the development of respiratory failure.

To assess the unevenness of pulmonary ventilation, gas analytical and barometric methods are more often used. Thus, a general idea of the unevenness of lung ventilation can be obtained, for example, by analyzing the mixing curves (dilutions) of helium or the washing out of nitrogen, which are used to measure the FOE.

In healthy people, the mixing of helium with alveolar air or the washing out of nitrogen occurs within three minutes. In case of violations of bronchial patency, the amount (volume) of poorly ventilated alveoli increases dramatically, and therefore the time of mixing (or washing out) increases significantly (up to 10-15 minutes), which is an indicator of uneven pulmonary ventilation.

More accurate data can be obtained by using a sample to wash out nitrogen with a single inhalation of oxygen. The patient exits the maximum exhalation, and then inhales as much as possible deeply pure oxygen. Then he exerts a slow exhalation into the closed system of the spirograph equipped with a device for determining the nitrogen concentration (azotograph). Throughout the exhalation the volume of the exhaled gas mixture is continuously measured, and the changing nitrogen concentration in the exhaled gas mixture containing alveolar air nitrogen is determined.

The nitrogen washout curve consists of 4 phases. At the very beginning of exhalation, the air enters the spirograph from the upper airways, 100% consisting of oxygen, which filled them during the preceding inspiration. The nitrogen content in this portion of the exhaled gas is zero.

The second phase is characterized by a sharp increase in the nitrogen concentration, which is due to the leaching of this gas from the anatomical dead space.

During a prolonged third phase, the nitrogen concentration of the alveolar air is recorded. In healthy people this phase of the curve is flat - in the form of a plateau (alveolar plateau). In the presence of uneven ventilation during this phase, the nitrogen concentration increases due to gas flushed out of poorly ventilated alveoli, which are emptied in the last turn. Thus, the greater the rise in the nitrogen washout curve at the end of the third phase, the more pronounced is the unevenness of pulmonary ventilation.

The fourth phase of the nitrogen washout curve is associated with the expiratory closure of the small airways of the basal parts of the lungs and the intake of air predominantly from the apical parts of the lungs, the alveolar air in which contains nitrogen of higher concentration.

[31], [32], [33], [34], [35], [36]

Assessment of ventilation-perfusion ratio

Gas exchange in the lungs depends not only on the level of general ventilation and the degree of its unevenness in various parts of the organ, but also on the ratio of ventilation and perfusion to the level of the alveoli. Therefore, the value of the ventilation-perfusion ratio VPO) is one of the most important functional characteristics of the respiratory organs, which ultimately determines the level of gas exchange.

In normal HPV for the lung as a whole is 0.8-1.0. With a decrease in HPI below 1.0 perfusion of poorly ventilated areas of the lung leads to hypoxemia (reduction in oxygenation of the arterial blood). An increase in HPV greater than 1.0 is observed with preserved or excessive ventilation of the zones, perfusion of which is significantly reduced, which can lead to a violation of the elimination of CO2 - hypercapnia.

Causes of violation of HPE:

1. All diseases and syndromes that cause uneven ventilation of the lungs.
2. The presence of anatomical and physiological shunts.
3. Thromboembolism of small branches of the pulmonary artery.
4. Disturbance of microcirculation and thrombus formation in small vessels.

Capnography. Several methods have been proposed to identify violations of HPE, one of the most simple and accessible is capnography. It is based on the continuous recording of the CO2 content in the exhaled gas mixture using special gas analyzers. These devices measure the absorption of carbon dioxide by infrared rays transmitted through a cuvette with exhaled gas.

When analyzing the capnogram, three indicators are usually calculated:

1. slope of the alveolar phase of the curve (segment BC),
2. the value of the concentration of CO2 at the end of exhalation (at point C),
3. ratio of functional dead space (MP) to the tidal volume (DO) - MP / DO.

[37], [38], [39], [40], [41], [42]

Determination of the diffusion of gases

Diffusion of gases through the alveolar-capillary membrane obeys Fick's law, according to which the diffusion rate is directly proportional:

1. gradient of the partial pressure of gases (O2 and CO2) on both sides of the membrane (P1 - P2) and
2. diffusion ability of alveolar-cainillary membrane (Dm):

VG = Dm x (P1 - P2), where VG is the gas transfer velocity (C) through the alveolar-capillary membrane, Dm is the diffusion capacity of the membrane, P1-P2 is the gradient of the partial pressure of gases on both sides of the membrane.

To calculate the diffusivity of light oxygen for oxygen, it is necessary to measure the absorbance 62 (VO ₂ ) and the average gradient of the partial pressure O ₂. Values of VO ₂ are measured using a spirograph of an open or closed type. To determine the oxygen partial pressure gradient (P ₁ - P ₂ ), more sophisticated gas analytical methods are used, since it is difficult to measure the partial pressure of O ₂ in pulmonary capillaries under clinical conditions .

The determination of the diffusivity of light ne ne for O ₂, and for carbon monoxide (CO) is more often used . Since CO is 200 times more actively associated with hemoglobin than oxygen, its concentration in the blood of pulmonary capillaries can be neglected. To determine DlCO, it is therefore sufficient to measure the rate of CO transmission through the alveolar-capillary membrane and the gas pressure in the alveolar air.

The most widely used method of solitary inhalation is in the clinic. The subject inhales a gas mixture with a small content of CO and helium, and at the height of a deep breath for 10 seconds holds his breath. After this, the composition of the exhaled gas is determined by measuring the concentration of CO and helium, and the diffusion capacity of the lungs for CO is calculated.

In norm DlCO, reduced to the area of the body, is 18 ml / min / mm Hg. Item / m2. The diffusion capacity of the lungs for oxygen (DlO2) is calculated by multiplying DlCO by a factor of 1.23.

The most common decrease in the diffusivity of the lungs is caused by the following diseases.

• Emphysema of the lungs (due to a decrease in the surface area of the alveolar-capillary contact and the volume of capillary blood).
• Diseases and syndromes accompanied by diffuse lesions of the lung parenchyma and thickening of the alveolar-capillary membrane (massive pneumonia, inflammatory or hemodynamic pulmonary edema, diffuse pneumosclerosis, alveolitis, pneumoconiosis, cystic fibrosis, etc.).
• Diseases, accompanied by the defeat of the capillary bed of the lungs (vasculitis, embolism of small branches of the pulmonary artery, etc.).

To correctly interpret the changes in the diffusivity of the lungs, it is necessary to take into account the hematocrit index. The increase in hematocrit with polycythemia and secondary erythrocytosis is accompanied by an increase, and its decrease in anemia - a decrease in the diffusivity of the lungs.

[43], [44]

Measurement of airway resistance

Measurement of airway resistance is a diagnostic parameter of pulmonary ventilation. Aspiration air moves along the airways under the influence of a pressure gradient between the oral cavity and the alveoli. During the inhalation, chest expansion leads to a decrease in the vWU and, consequently, intra-alveolar pressure, which becomes lower than the pressure in the oral cavity (atmospheric). As a result, the air flow is directed into the lungs. During exhalation, the effect of the elastic thrust of the lungs and the chest is aimed at increasing the intra-alveolar pressure, which becomes higher than the pressure in the oral cavity, resulting in a backflow of air. Thus, the pressure gradient (ΔP) is the main force that ensures air transport through airway paths.

The second factor determining the amount of gas flow through the airways is the aerodynamic resistance (Raw), which in turn depends on the lumen and the length of the airways, as well as on the viscosity of the gas.

The value of the volumetric air flow velocity obeys Poiseuille's law: V = ΔP / Raw, where

• V is the volumetric velocity of the laminar air flow;
• ΔP - pressure gradient in the oral cavity and alveoli;
• Raw - aerodynamic resistance of airways.

It follows that in order to calculate the aerodynamic resistance of airways, it is necessary to simultaneously measure the difference between the pressure in the oral cavity in the alveoli (ΔP), as well as the flow velocity of the air.

There are several methods for determining Raw based on this principle:

• method of plethysmography of the whole body;
• method of overlapping the air flow.

Determination of blood gases and acid-base state

The main method for diagnosing acute respiratory failure is the examination of arterial blood gases, which involves the measurement of PaO2, PaCO2, and pH. It is also possible to measure the saturation of hemoglobin with oxygen (oxygen saturation) and some other parameters, in particular, the content of buffer bases (BB), standard bicarbonate (SB) and base excess (deficiency).

The parameters of PaO2 and PaCO2 most accurately characterize the ability of the lungs to saturate the blood with oxygen (oxygenation) and to remove carbon dioxide (ventilation). The latter function is also determined by pH and BE.

To determine the gas composition of blood in patients with acute respiratory failure, residing in the intensive care unit, use a complex invasive procedure for obtaining arterial blood by puncture a large artery. More often, the puncture of the radial artery is carried out, since the risk of complication development is lower here. On the hand there is a good collateral blood flow, which is carried out by the ulnar artery. Therefore, even with damage to the radial artery during the puncture or operation of the arterial catheter, the blood supply of the hand remains.

Indications for puncture of the radial artery and the installation of an arterial catheter are:

• the need for frequent measurement of the arterial blood gas composition;
• marked hemodynamic instability on the background of acute respiratory failure and the need for constant monitoring of hemodynamic parameters.

Contraindication to the placement of the catheter is a negative test Allen. To conduct the test, the ulnar and radial arteries are squeezed with fingers so as to turn the arterial blood flow; After a while the hand pales. After that, the ulnar artery is released, continuing to pinch the radial. Usually, brushing the brush quickly (within 5 seconds) is restored. If this does not happen then the brush remains pale, the ulnar artery occlusion is diagnosed, the result of the test is considered negative, and the puncture of the radial artery does not produce.

In case of a positive test result, the palm and forearm of the patient are fixed. After the preparation of the operating field in the distal sections, the radial guests palpate the pulse on the radial artery, perform anesthesia at this site, and puncture the artery at an angle of 45 °. The catheter is pushed upward until the blood appears in the needle. The needle is removed, leaving a catheter in the artery. To prevent excessive bleeding, the proximal portion of the radial artery is pressed with a finger for 5 minutes. The catheter is fixed to the skin with silk sutures and covered with a sterile bandage.

Complications (bleeding, clot artery occlusion and infection) during the establishment of the catheter are relatively rare.

The blood for research is preferable to dial into a glass, and not into a plastic syringe. It is important that the blood sample does not come into contact with the surrounding air, i.e. The collection and transportation of blood should be carried out under anaerobic conditions. Otherwise, the penetration of ambient air into the sample leads to a determination of the level of PaO2.

Determination of blood gases should be carried out no later than 10 minutes after the instruction of arterial blood. Otherwise, the metabolic processes that continue in the blood sample (initiated primarily by the activity of leukocytes) significantly change the results of the determination of blood gases, reducing the level of PaO2 and pH, and increasing PaCO2. Especially pronounced changes are observed in leukemia and in severe leukocytosis.

[45], [46], [47]

Methods for estimating the acid-base state

Measurement of blood pH

The pH value of blood plasma can be determined by two methods:

• The indicator method is based on the property of some weak acids or bases used as indicators to dissociate at certain pH values while changing the color.
• The pH-metry method makes it possible to more accurately and quickly determine the concentration of hydrogen ions by means of special polarographic electrodes, on the surface of which, when immersed in a solution, a potential difference is created that depends on the pH of the medium under study.

One of the electrodes - active, or measuring, is made of a noble metal (platinum or gold). The other (reference) serves as a reference electrode. The platinum electrode is separated from the rest of the system by a glass membrane that is permeable only to hydrogen ions (H ⁺ ). Inside the electrode is filled with a buffer solution.

The electrodes are immersed in the test solution (eg, blood) and polarized from the current source. As a result, a current appears in the closed electrical circuit. Since the platinum (active) electrode is further separated from the electrolyte solution by a glass membrane permeable only to H ⁺ ions , the pressure on both surfaces of this membrane is proportional to the pH of the blood.

Most often, the acid-base state is estimated by the Astrup method on the micro-Astrup apparatus. Determine the values of BB, BE and PaCO2. Two portions of the investigated arterial blood are equilibrated with two gas mixtures of known composition, differing in the partial pressure of CO2. In each portion of blood, pH is measured. Values of pH and PaCO2 in each portion of blood are applied as two points in a nomogram. After 2 the points marked on the nomogram are drawn straight to the intersection with the standard graphs BB and BE and determine the actual values of these indicators. The pH of the blood is then measured and a point is obtained on the resulting straight line corresponding to this measured pH value. From the projection of this point, the actual pressure of CO2 in the blood (PaCO2) is determined on the ordinate.

Direct measurement of the pressure of CO2 (PaCO2)

In recent years, for a direct measurement of PaCO2 in a small volume, a modification of polarographic electrodes intended for measuring pH is used. Both electrodes (active and reference) are immersed in a solution of electrolytes, which is separated from the blood by another membrane, permeable only to gases, but not to hydrogen ions. Molecules of CO2, diffusing through this membrane from the blood, change the pH of the solution. As mentioned above, the active electrode is further separated from the NaHCO3 solution by a glass membrane permeable only to H ⁺ ions . After immersing the electrodes in the test solution (for example, blood), the pressure on both surfaces of this membrane is proportional to the pH of the electrolyte (NaHCO3). In turn, the pH of the NaHCO3 solution depends on the concentration of CO2 in the sprinkling. Thus, the value of the pressure in the chain is proportional to PaCO2 of the blood.

The polarographic method is also used to determine the PaO2 in the arterial blood.

[48], [49], [50]

The determination of BE by the results of direct measurement of pH and PaCO2

Direct determination of pH and PaCO2 of blood makes it possible to substantially simplify the procedure for determining the third index of the acid-base state-excess bases (BE). The last indicator can be determined by special nomograms. After a direct measurement of pH and PaCO2, the actual values of these indicators are plotted on the corresponding nomogram scales. The points are connected by a straight line and continue it to the intersection with the scale BE.

Such a method of determining the basic parameters of the acid-base state does not require balancing the blood with the gas mixture, as with the classical Astrup method.

Interpretation of results

Partial pressure of O2 and CO2 in arterial blood

The values of PaO2 and PaCO2 serve as the main objective indicators of respiratory failure. In a healthy adult, breathing room air with 21% oxygen concentration (FiO ₂ = 0.21) and normal atmospheric pressure (760 mm Hg. V.), PaO2 90-95 mm Hg. Art. When the barometric pressure, ambient temperature and some other conditions of RaO2 change in a healthy person, it can reach 80 mm Hg. Art.

Lower values of PaO2 (less than 80 mmHg) can be considered the initial manifestation of hypoxemia, especially against the background of acute or chronic lesions of the lungs, chest, respiratory muscles, or central regulation of respiration. Reduction of PaO2 to 70 mm Hg. Art. In most cases, indicates a compensated respiratory failure and, as a rule, is accompanied by clinical signs of a decrease in the functionality of the external respiration system:

• small tachycardia;
• shortness of breath, respiratory discomfort, appearing mainly with physical exertion, although at rest, the respiration rate does not exceed 20-22 per minute;
• a marked decrease in tolerance to loads;
• participation in respiration of the respiratory musculature and the like.

At first glance, these criteria for arterial hypoxemia contradict the definition of respiratory failure E. Campbell: "respiratory failure is characterized by a decrease in PaO2 below 60 mm Hg. St ... ". However, as already noted, this definition refers to decompensated respiratory failure, manifested by a large number of clinical and instrumental signs. Indeed, the decrease in PaO2 is below 60 mm Hg. As a rule, it indicates a pronounced decompensated respiratory failure, and is accompanied by dyspnea at rest, an increase in the number of respiratory movements to 24-30 per minute, cyanosis, tachycardia, significant pressure of the respiratory muscles, etc. Neurological disorders and signs of hypoxia of other organs usually develop at PaO2 below 40-45 mm Hg. Art.

PaO2 from 80 to 61 mm Hg. Especially against a background of acute or chronic lung injury and external respiration apparatus, should be regarded as the initial manifestation of arterial hypoxemia. In most cases, it indicates the formation of light compensated respiratory failure. Reduction of PaO ₂ below 60 mm Hg. Art. Indicates a moderate or severe precompensated respiratory failure, the clinical manifestations of which are pronounced.

Normally, the pressure of CO2 in the arterial blood (PaCO ₂ ) is 35-45 mm Hg. Hypercupy is diagnosed with an increase in PaCO2 greater than 45 mm Hg. Art. The values of PaCO2 are greater than 50 mmHg. Art. Usually correspond to the clinical picture of severe ventilation (or mixed) respiratory failure, and above 60 mm Hg. Art. - serve as an indication for ventilation, aimed at restoring the minute volume of breathing.

Diagnosis of various forms of respiratory failure (ventilation, parenchymatous, etc.) is based on the results of a comprehensive examination of patients - the clinical picture of the disease, the results of determining the function of external respiration, chest X-ray, laboratory tests, including blood gas composition.

Some features of the changes in PaO ₂ and PaCO ₂ in ventilating and parenchymal respiratory failure have already been noted above . Recall that for ventilation respiratory failure, in which the process of CO ₂ release from the body is violated primarily in the lungs, hypercapnia (PaCO ₂ more than 45-50 mm Hg), often accompanied by compensated or decompensated respiratory acidosis , is characteristic . At the same time, progressive hypoventilation of alveoli naturally leads to a decrease in oxygenation of alveolar air and pressure of O ₂ in arterial blood (PaO ₂ ), as a result of which hypoxemia develops. Thus, a detailed picture of ventilation respiratory failure is accompanied by both hypercapnia and growing hypoxemia.

Early stages of parenchymal respiratory failure are characterized by a decrease in PaO ₂ (hypoxemia), in most cases combined with severe hyperventilation of alveoli (tachypnea) and developing in connection with this hypocapnia and respiratory alkalosis. If this condition is not managed to stop, gradually there are signs of progressive total decrease in ventilation, minute volume of respiration and hypercapnia (PaCO _{2 is} more than 45-50 mm Hg). This indicates the attachment of ventilation respiratory failure due to fatigue of the respiratory muscles, severe obstruction of the airways or a critical drop in the volume of functioning alveoli. Thus, for the later stages of parenchymal respiratory failure, a progressive decrease in PaO ₂ (hypoxemia) in combination with hypercapnia is characteristic .

Depending on the specific features of the development of the disease and the prevalence of certain pathophysiological mechanisms of respiratory failure, other combinations of hypoxemia and hypercapnia are possible, which are discussed in later chapters.

Violations of the acid-base state

In most cases, it is quite sufficient to determine the pH of the blood, pCO2, BE and SB, in order to accurately diagnose respiratory and non-respiratory acidosis and alkalosis, and also to estimate the degree of compensation for these disorders.

In the period of decompensation, a decrease in the pH of the blood is observed, and for alkalozenes of the acid-base state it is quite simple to determine: with acidide an increase. It is also easy for laboratory parameters opredelit respiratory and non-respiratory type of these disorders: changes rS0 ₂ and BE in each of these two types of multidirectional.

The situation is more complicated with the assessment of the parameters of the acid-base state in the period of compensation of its disturbances, when the pH of the blood is not changed. Thus, a decrease in pCO ₂ and BE can be observed both in non-respiratory (metabolic) acidosis and in respiratory alkalosis. In these cases, an assessment of the overall clinical situation helps to understand whether the corresponding changes in pCO ₂ or BE are primary or secondary (compensatory).

For compensated respiratory alkalosis, a primary increase in PaCO2 is characteristic, which is in fact the cause of this disturbance of the acid-base state; in these cases, the corresponding changes in BE are secondary, that is, the inclusion of various compensatory mechanisms aimed at reducing the concentration of bases. On the contrary, for compensated metabolic acidosis, the changes in BE are primary, o pCO2 shifts reflect compensatory hyperventilation of the lungs (if possible).

Thus, the comparison of the parameters of the disturbances of the acid-base state with the clinical picture of the disease in most cases makes it possible to reliably diagnose the nature of these disorders even in the period of their compensation. The establishment of a correct diagnosis in these cases can also help assess the changes in electrolyte blood composition. With respiratory and metabolic acidosis, hypernatremia (or normal concentration of Na ⁺ ) and hyperkalemia are often observed , and with respiratory alkalosis - hypo- (or normo) sodiumemia and hypokalemia

Pulse Oximetry

Provision of oxygen to peripheral organs and tissues depends not only on the absolute values of pressure D ₂ in the arterial blood, and on the ability of hemoglobin to bind oxygen in the lungs and secrete it in tissues. This ability is described by the S-shaped form of the oxyhemoglobin dissociation curve. The biological meaning of this form of the dissociation curve is that the region of high pressure O2 corresponds to the horizontal portion of this curve. Therefore, even with fluctuations in oxygen pressure in the arterial blood from 95 to 60-70 mm Hg. Art. Saturation (saturation) of hemoglobin with oxygen (SaO ₂ ) is maintained at a sufficiently high level. Thus, in a healthy young man with PaO ₂ = 95 mm Hg. Art. The saturation of hemoglobin with oxygen is 97%, and at PaO ₂ = 60 mm Hg. Art. - 90%. The steep slope of the middle portion of the oxyhemoglobin dissociation curve indicates very favorable conditions for the release of oxygen in the tissues.

Under the influence of some factors (temperature increase, hypercapnia, acidosis), the dissociation curve shifts to the right, which indicates a decrease in the affinity of hemoglobin for oxygen and the possibility of its easier release in tissues. It is seen in these figures that in order to maintain saturation of hemoglobin with oxygen The former level requires a larger PAO ₂.

The shift of the dissociation curve of oxyhemoglobin to the left indicates an increased affinity of hemoglobin for O ₂ and a smaller release of it in the tissues. Such a shift occurs by the action of hypocapnia, alkalosis and lower temperatures. In these cases, high saturation of hemoglobin with oxygen persists even at lower values of PaO ₂

Thus, the value of saturation of hemoglobin with oxygen during respiratory failure acquires independent significance for characterizing the provision of peripheral tissues with oxygen. The most common non-invasive method for determining this indicator is pulse oximetry.

Modern pulse oximeters contain a microprocessor connected to a sensor containing a light-emitting diode and a photosensitive sensor located opposite the light-emitting diode). Usually 2 wavelengths of radiation are used: 660 nm (red light) and 940 nm (infrared). Saturation with oxygen is determined by the absorption of red and infrared light, respectively, by reduced hemoglobin (Hb) and oxyhemoglobin (HbJ ₂ ). The result is displayed as Sa2 (saturation, obtained by pulse oximetry).

Normally, oxygen saturation exceeds 90%. This index decreases with hypoxemia and a decrease in PaO _{2 of} less than 60 mm Hg. Art.

When evaluating the results of pulse oximetry, one should keep in mind the sufficiently large error of the method, which is ± 4-5%. It should also be remembered that the results of the indirect determination of oxygen saturation depend on many other factors. For example, on the presence of nails on the nail polish. The lacquer absorbs some of the anode radiation with a wavelength of 660 nm, thereby underestimating the values of the Sau ₂ index .

The readings of the pulse oximeter are affected by the shift in the hemoglobin dissociation curve that occur under the influence of various factors (temperature, blood pH, PaCO2 level), skin pigmentation, anemia with a hemoglobin level below 50-60 g / l, etc. For example, small fluctuations in pH lead to significant changes the indicator of SаО2, with alkalosis (for example, respiratory, developed against a background of hyperventilation), SаО2 is overestimated, with acidosis - understated.

In addition, this technique does not allow to take into account the appearance of pathological varieties of hemoglobin - carboxyhemoglobin and methemoglobin, which absorb light of the same wavelength as oxyhemoglobin, which leads to an overestimation of the values of SaO2.

Nevertheless, pulse oximetry is now widely used in clinical practice, in particular in intensive care and intensive care units for simple, indicative, dynamic monitoring of the saturation state of hemoglobin with oxygen.

Assessment of hemodynamic parameters

For a full-fledged analysis of the clinical situation with acute respiratory failure, a dynamic determination of a number of hemodynamic parameters is necessary:

• blood pressure;
• heart rate (heart rate);
• central venous pressure (CVP);
• pulmonary artery wedge pressure (DZLA);
• cardiac output;
• ECG monitoring (including for the timely detection of arrhythmias).

Many of these parameters (blood pressure, heart rate, SаО2, ECG, etc.) allow to determine the modern monitor equipment of intensive care and resuscitation departments. Severe patients are advisable to catheterize the right heart with the installation of a temporary floating intracardiac catheter for determining CVP and ZDLA.

[51], [52], [53], [54], [55], [56]