Diagnosis of respiratory failure

A number of modern research methods are used to diagnose respiratory failure, allowing one to form an idea of the specific causes, mechanisms and severity of the course of respiratory failure, concomitant functional and organic changes in the internal organs, the state of hemodynamics, acid-base balance, etc. For this purpose, the function of external respiration, blood gas composition, respiratory and minute ventilation volumes, hemoglobin and hematocrit levels, blood oxygen saturation, arterial and central venous pressure, heart rate, ECG, if necessary - pulmonary artery wedge pressure (PAWP) are determined, echocardiography, etc. are performed (A.P. Zilber).

Evaluation of external respiratory function

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

1. Diagnosis of respiratory function disorders and objective assessment of the severity of respiratory failure.
2. Differential diagnostics of obstructive and restrictive disorders of pulmonary ventilation.
3. Justification for pathogenetic therapy of respiratory failure.
4. Evaluation of the effectiveness of the treatment.

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

The most common methods of studying the function of external respiration are spirometry and spirography. Spirometry provides not only measurement, but also graphic recording of the main ventilation indicators during calm and formed breathing, physical activity, and pharmacological tests. In recent years, the use of computer spirographic systems has significantly simplified and accelerated the examination and, most importantly, has made it possible to measure the volumetric speed of inspiratory and expiratory air flows as a function of lung volume, i.e. to analyze the flow-volume loop. Such computer systems include, for example, spirographs from Fukuda (Japan) and Erich Eger (Germany), etc.

Research method. The simplest spirograph consists of an air-filled sliding cylinder immersed in a container of water and connected to the recording device (for example, a calibrated drum rotating at a certain speed, on which the spirograph readings are recorded). The patient in a sitting position breathes through a tube connected to the cylinder with air. Changes in lung volume during breathing are recorded by changes in the volume of the cylinder connected to the rotating drum. The study is usually conducted in two modes:

• In conditions of basal metabolism - in the early morning hours, on an empty stomach, after 1-hour rest in a lying position; medications should be discontinued 12-24 hours before the study.
• 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 examination, a 15-minute rest in a sitting position is required.

The study is conducted in a separate, dimly lit room with an air temperature of 18-24 C, after the patient has been familiarized with the procedure. When conducting the study, it is important to achieve full contact with the patient, since his negative attitude to the procedure and the lack of necessary skills can significantly change the results and lead to an inadequate assessment of the data obtained.

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

Main indicators of pulmonary ventilation

Classical spirography allows to determine:

1. the size of most lung volumes and capacities,
2. main indicators of pulmonary ventilation,
3. oxygen consumption by the body and ventilation efficiency.

There are 4 primary lung volumes and 4 capacities. The latter include two or more primary volumes.

Lung volumes

1. Tidal volume (TV) is the volume of gas inhaled and exhaled during quiet breathing.
2. Inspiratory reserve volume ( _IRV ) is the maximum volume of gas that can be additionally inhaled after a calm inhalation.
3. _Expiratory reserve volume ( ERV) is the maximum volume of gas that can be additionally exhaled after a calm exhalation.
4. Residual volume of the lungs (RV) is the volume of air remaining in the lungs after maximum exhalation.

Lung capacity

1. Vital capacity (VC) is the sum of VL, RO _in and RO _exp, i.e. the maximum volume of gas that can be exhaled after a maximal deep inhalation.
2. Inspiratory capacity (IC) is the sum of DI and PO _, i.e. the maximum volume of gas that can be inhaled after a calm exhalation. This capacity characterizes the ability of the lung tissue to stretch.
3. Functional residual capacity (FRC) is the sum of the FRC and PO _exp, i.e. the volume of gas remaining in the lungs after a calm exhalation.
4. Total lung capacity (TLC) is the total amount of gas contained in the lungs after a maximal inspiration.

Conventional spirographs, widely used in clinical practice, allow determining only 5 lung volumes and capacities: RV, RO _in, RO exp, _VC, EVP (or, respectively, VT, IRV, ERV, VC and VC). To find the most important indicator of lung ventilation - the functional residual capacity (FRC) and calculate the residual volume of the lungs (RV) and total lung capacity (TLC), it is necessary to use special techniques, in particular, the methods of helium dilution, nitrogen washout or whole-body plethysmography (see below).

The main indicator in the traditional spirography method is the vital capacity of the lungs (VC). To measure VC, the patient, after a period of calm breathing (CB), first takes a maximum breath and then, possibly, a full exhalation. In this case, it is advisable to evaluate not only the integral value of VC) and the inspiratory and expiratory vital capacity (respectively, VCin, VCex), i.e. the maximum volume of air that can be inhaled or exhaled.

The second mandatory technique used in traditional spirography is a test to determine the forced (expiratory) vital capacity of the lungs (FVC, or forced vital capacity expiratory), which allows one to determine the most (formative speed indicators of pulmonary ventilation during forced exhalation, characterizing, in particular, the degree of obstruction of the intrapulmonary airways. As in the test to determine the VC, the patient takes the deepest possible breath, and then, unlike determining the VC, exhales air at the maximum possible speed (forced exhalation). In this case, a gradually flattening spontaneous curve is recorded. When evaluating the spirogram of this expiratory maneuver, several indicators are calculated:

1. Forced expiratory volume after 1 second (FEV1) is the amount of air expelled from the lungs in the first second of exhalation. This indicator decreases both with airway obstruction (due to increased bronchial resistance) and with restrictive disorders (due to a decrease in all lung volumes).
2. The Tiffno index (FEV1/FVC, %) is the ratio of the forced expiratory volume in the first second (FEV1) to the forced vital capacity of the lungs (FVC). This is the main indicator of the expiratory maneuver with forced expiration. It decreases significantly in broncho-obstructive syndrome, since the slowing of exhalation caused by bronchial obstruction is accompanied by a decrease in the forced expiratory volume in 1 second (FEV1) in the absence or insignificant decrease in the overall value of FVC. In restrictive disorders, the Tiffno index remains virtually unchanged, since FEV1 and FVC decrease almost equally.
3. Maximum expiratory flow at 25%, 50%, and 75% of forced vital capacity (MEF25, MEF50, MEF75, or MEF25, MEF50, MEF75). These values are calculated by dividing the corresponding volumes (in liters) of forced expiration (at 25%, 50%, and 75% of the total FVC) by the time it takes to achieve these volumes during forced expiration (in seconds).
4. Average expiratory flow rate at the level of 25~75% of FVC (AEF25-75). This indicator is less dependent on the patient's voluntary effort and more objectively reflects the patency of the bronchi.
5. Peak expiratory flow ( _PEF ) is the maximum volumetric flow rate of forced expiration.

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

1. the number of respiratory movements during quiet breathing (RR, or BF - breathing frequency) and
2. minute volume of respiration (MV) is the amount of total ventilation of the lungs per minute during calm breathing.

[ 6 ], [ 7 ]

Investigation of the flow-volume relationship

Computerized spirography

Modern computer spirographic systems allow automatic analysis of not only the above spirographic indices, but also the flow-volume ratio, i.e. the dependence of the volumetric air flow rate during inhalation and exhalation on the value of the lung volume. Automatic computer analysis of the inspiratory and expiratory parts of the flow-volume loop is the most promising method for quantitative assessment of pulmonary ventilation disorders. Although the flow-volume loop itself contains basically the same information as a simple spirogram, the clarity of the relationship between the volumetric air flow rate and the lung volume allows a more detailed study of the functional characteristics of both the upper and lower airways.

The main element of all modern spirographic computer systems is a pneumotachographic sensor, which records the volumetric speed of air flow. The sensor is a wide tube through which the patient breathes freely. At the same time, as a result of a small, previously known, aerodynamic resistance of the tube between its beginning and end, a certain pressure difference is created, directly proportional to the volumetric speed of air flow. In this way, it is possible to record changes in the volumetric speed of air flow during inhalation and exhalation - a pneumotachogram.

Automatic integration of this signal also allows obtaining traditional spirographic indices - lung volume values in liters. Thus, at each moment in time, information about the volumetric air flow rate and lung volume at a given moment is simultaneously received by the computer's memory device. This allows plotting a flow-volume curve on the monitor screen. A significant advantage of this method is that the device operates in an open system, i.e. the subject breathes through a tube along an open circuit, without experiencing additional breathing resistance, as with conventional spirography.

The procedure for performing respiratory maneuvers when recording the flow-volume curve resembles recording a regular co-routine. After a period of complex breathing, the patient inhales maximally, as a result of which the inspiratory part of the flow-volume curve is recorded. The lung volume at point "3" corresponds to the total lung capacity (TLC). Following this, the patient exhales forcefully, 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 flow rate rapidly increases, reaching a peak (peak expiratory flow rate - _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 individual, the shapes of the inspiratory and expiratory parts of the flow-volume curve differ significantly from each other: the maximum volume flow during inspiration is achieved at approximately 50% of vital capacity (MIF50), whereas during forced expiration, the peak expiratory flow (PEF) occurs very early. The maximum inspiratory flow (MIF50) is approximately 1.5 times greater than the maximum expiratory flow at mid-vital capacity (Vmax50%).

The described flow-volume curve registration test is performed several times until the results coincide. In most modern devices, the procedure for collecting the best curve for further processing of the material is carried out automatically. The flow-volume curve is printed out together with numerous pulmonary ventilation indices.

The pneumotochographic sensor records the curve of the volumetric air flow rate. Automatic integration of this curve makes it possible to obtain a curve of respiratory volumes.

[ 8 ], [ 9 ], [ 10 ]

Evaluation of research results

Most lung volumes and capacities, both in healthy patients and in patients with lung diseases, depend on a number of factors, including age, gender, chest size, body position, level of training, etc. For example, vital capacity (VC) in healthy people decreases with age, while residual volume (RV) increases, and total lung capacity (TLC) remains virtually unchanged. VC is proportional to chest size and, accordingly, to the patient's height. In women, VC is on average 25% lower than in men.

Therefore, from a practical point of view, it is impractical to compare the values of lung volumes and capacities obtained during a spirographic study with uniform “standards”, the fluctuations in the values of which, due to the influence of the above and other factors, are quite significant (for example, the vital capacity can normally fluctuate from 3 to 6 liters).

The most acceptable way to evaluate the spirographic indicators obtained during the study is to compare them with the so-called normal values, which were obtained during the examination of large groups of healthy people, taking into account their age, gender and height.

The required values of ventilation parameters are determined by special formulas or tables. In modern computer spirographs, they are calculated automatically. For each parameter, the normal value limits are given as a percentage in relation to the calculated required value. For example, VC or FVC are considered reduced if their actual value is less than 85% of the calculated required value. A decrease in FEV1 is noted if the actual value of this parameter is less than 75% of the required value, and a decrease in FEV1/FVC is noted if the actual value is less than 65% of the required value.

Limits of normal values of the main spirographic indicators (as a percentage of the calculated expected value).

Indicators	Norm	Conditional norm	Deviations
			Moderate	Significant	Sharp
YELLOW	>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 spirography results, it is necessary to take into account some additional conditions under which the study was conducted: atmospheric pressure, temperature and humidity of the surrounding air. Indeed, the volume of air exhaled by the patient is usually somewhat less than that which the same air occupied in the lungs, since its temperature and humidity are usually higher than those of the surrounding air. In order to exclude differences in the measured values associated with the conditions of the study, all lung volumes, both expected (calculated) and actual (measured in a given patient), are given for conditions corresponding to their values at a body temperature of 37°C and full saturation with water vapor (BTPS system - Body Temperature, Pressure, Saturated). In modern computer spirographs, such a correction and recalculation of lung volumes in the BTPS system are made automatically.

Interpretation of results

A practicing physician should have a good understanding of the true capabilities of the spirographic method of research, limited, as a rule, by the lack of information on the values of residual lung volume (RLV), functional residual capacity (FRC) and total lung capacity (TLC), which does not allow for a full analysis of the TLC structure. At the same time, spirography makes it possible to form a general idea of the state of external respiration, in particular:

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

Modern computer spirography allows obtaining reliable and complete information about the presence of broncho-obstructive syndrome. More or less reliable detection of restrictive ventilation disorders using the spirographic method (without using gas-analytical methods for assessing the structure of the OEL) is possible only in relatively simple, classic cases of impaired lung compliance, when they are not combined with impaired bronchial patency.

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

Diagnosis of obstructive syndrome

The main spirographic sign of obstructive syndrome is a slowdown in forced exhalation due to an increase in airway resistance. When recording a classic spirogram, the forced exhalation curve becomes stretched, and such indicators as FEV1 and the Tiffno index (FEV1/FVC) decrease. VC either does not change or decreases slightly.

A more reliable sign of broncho-obstructive syndrome is a decrease in the Tiffeneau index (FEV1/FVC), since the absolute value of FEV1 can decrease not only with bronchial obstruction, but also with restrictive disorders due to a proportional decrease in all lung volumes and capacities, including FEV1 and FVC.

Already at the early stages of development of obstructive syndrome, the calculated indicator of the average volumetric speed decreases to the level of 25-75% of FVC (SOC25-75%) - O" is the most sensitive spirographic indicator, indicating an increase in airway resistance before others. However, its calculation requires fairly accurate manual measurements of the descending knee of the FVC curve, which is not always possible using a classic 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 exhalation, then in patients with bronchial patency disorders a peculiar "sagging" of the expiratory part of the loop and a decrease in the volumetric air flow rate at all values of lung volume are observed. Often, due to an increase in lung volume, the expiratory part of the loop is shifted to the left.

The following spirographic parameters decrease: FEV1, FEV1/FVC, peak expiratory flow rate (PEF ₎, MEF25% (MEF25), MEF50% (MEF50), MEF75% (MEF75), and FEF25-75%.

Vital capacity of the lungs (VC) may remain unchanged or decrease even in the absence of concomitant restrictive disorders. It is also important to evaluate the value of the expiratory reserve volume (ERV ₎, which naturally decreases in obstructive syndrome, especially in the event of early expiratory closure (collapse) of the bronchi.

According to some researchers, quantitative analysis of the expiratory part of the flow-volume loop also allows us to get an idea of the predominant narrowing of large or small bronchi. It is believed that obstruction of large bronchi is characterized by a decrease in the volumetric flow rate of forced expiration mainly in the initial part of the loop, due to which such indicators as the peak volumetric flow rate (PVF) and the maximum volumetric flow rate at 25% of the FVC (MEF25) sharply decrease. At the same time, the volumetric flow rate of air in the middle and end of expiration (MEF50% and MEF75%) also decreases, but to a lesser extent than MEF _exp and MEF25%. Conversely, with obstruction of small bronchi, a predominantly decrease in MEF50% and MEF75% is detected, while MEF _exp is normal or slightly reduced, and MEF25% is moderately reduced.

However, it should be emphasized that these provisions currently seem quite controversial and cannot be recommended for use in widespread clinical practice. In any case, there are more grounds to believe that the unevenness of the decrease in the volumetric air flow rate during forced expiration rather reflects the degree of bronchial obstruction than its localization. Early stages of bronchial narrowing are accompanied by a slowdown in the expiratory air flow at the end and middle of expiration (a decrease in the MEF50%, MEF75%, SEF25-75% with slightly changed values of MEF25%, FEV1/FVC and PEF), whereas with severe bronchial obstruction, a relatively proportional decrease in all velocity indices is observed, including the Tiffeneau index (FEV1/FVC), PEF and MEF25%.

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

1. fixed obstruction;
2. variable extrathoracic obstruction;
3. variable intrathoracic obstruction.

An example of a fixed obstruction of the upper airways is tracheostomy stenosis. In these cases, breathing is done through a rigid, relatively narrow tube, the lumen of which does not change during inhalation and exhalation. Such a fixed obstruction limits the airflow both during inhalation and exhalation. Therefore, the expiratory part of the curve resembles the inspiratory one in shape; the volumetric speeds of inhalation and exhalation are significantly reduced and almost equal to each other.

In the clinic, however, one often encounters two variants of variable obstruction of the upper airways, when the lumen of the larynx or trachea changes during inhalation or exhalation, which leads to selective limitation of the inspiratory or expiratory air flow, respectively.

Variable extrathoracic obstruction is observed in various types of laryngeal stenosis (vocal cord edema, tumor, etc.). As is known, during respiratory movements, the lumen of the extrathoracic airways, especially narrowed ones, depends on the ratio of intratracheal and atmospheric pressure. During inhalation, the pressure in the trachea (as well as the intraalveolar and intrapleural pressure) becomes negative, i.e. lower than atmospheric. This contributes to the narrowing of the lumen of the extrathoracic airways and a significant limitation of the inspiratory air flow and a decrease (flattening) of the inspiratory part of the flow-volume loop. During forced exhalation, the intratracheal pressure becomes significantly higher than atmospheric, due to which 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 in tracheal tumors and dyskinesia of the membranous part of the trachea. The diameter of the atrium of the thoracic airways is largely determined by the ratio of intratracheal and intrapleural pressures. During forced exhalation, when the intrapleural pressure increases significantly, exceeding the pressure in the trachea, the intrathoracic airways narrow and their obstruction develops. During inhalation, the pressure in the trachea slightly exceeds the negative intrapleural pressure, and the degree of tracheal narrowing decreases.

Thus, with variable intrathoracic obstruction of the upper airways, there is a selective restriction of the air flow during exhalation and a flattening of the inspiratory part of the loop. Its inspiratory part remains almost unchanged.

With variable extrathoracic obstruction of the upper airways, selective limitation of the volumetric air flow rate is observed mainly during inhalation, and with intrathoracic obstruction - during exhalation.

It should also be noted that in clinical practice, cases are quite rare when narrowing of the upper airway lumen is accompanied by flattening of only the inspiratory or only the expiratory part of the loop. Usually, air flow limitation is revealed in both phases of breathing, although during one of them this process is much more pronounced.

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

Diagnosis of restrictive disorders

Restrictive disorders of pulmonary ventilation are accompanied by a limitation of filling the lungs with air due to a decrease in the respiratory surface of the lung, exclusion of part of the lung from breathing, a decrease in the elastic properties of the lung and chest, as well as the ability of the lung tissue to stretch (inflammatory or hemodynamic pulmonary edema, massive pneumonia, pneumoconiosis, pneumosclerosis, etc.). At the same time, if restrictive disorders are not combined with the above-described disorders of bronchial patency, the resistance of the airways usually does not increase.

The main consequence of restrictive ventilation disorders revealed by classical spirography is an almost proportional decrease in most lung volumes and capacities: RV, VC, RO _in, RO _exp, FEV, FEV1, etc. It is important that, unlike obstructive syndrome, a decrease in FEV1 is not accompanied by a decrease in the FEV1/FVC ratio. This indicator remains within the normal range or even increases slightly due to a more significant decrease in VC.

In computer spirography, the flow-volume curve is a reduced copy of the normal curve, shifted to the right due to the overall decrease in lung volume. The peak volume rate (PVR) of the expiratory flow FEV1 is reduced, although the FEV1/FVC ratio is normal or increased. Due to the limited expansion of the lung and, accordingly, a decrease in its elastic traction, the flow indicators (e.g., PVR25-75%, MVR50%, MVR75%) in some cases can also be reduced even in the absence of airway obstruction.

The most important diagnostic criteria for restrictive ventilation disorders, which allow them to be distinguished reliably from obstructive disorders, are:

1. an almost proportional decrease in lung volumes and capacities measured by spirography, as well as flow indicators and, accordingly, a normal or slightly changed shape of the flow-volume loop curve, shifted to the right;
2. normal or even increased value of the Tiffeneau index (FEV1/FVC);
3. the decrease in the inspiratory reserve volume (IRV ₎ is almost proportional to the expiratory reserve volume (ERV ₎.

It should be emphasized once again that for the diagnosis of even "pure" restrictive ventilation disorders, one cannot rely only on the decrease in VCF, since this indicator in severe obstructive syndrome can also decrease significantly. More reliable differential diagnostic signs are the absence of changes in the shape of the expiratory part of the flow-volume curve (in particular, normal or increased values of FEV1/FVC), as well as a proportional decrease in PO _in and PO _out.

[ 22 ], [ 23 ], [ 24 ]

Determination of the structure of total lung capacity (TLC)

As was stated above, the methods of classical spirography, as well as computer processing of the flow-volume curve, allow us to form an idea of the changes in only five of the eight pulmonary volumes and capacities (VO, ROin, ROout, VC, Evd, or, respectively, VT, IRV, ERV, VC and 1C), which makes it possible to assess mainly the degree of obstructive disorders of pulmonary ventilation. Restrictive disorders can be reliably diagnosed only if they are not combined with impaired bronchial patency, i.e. in the absence of mixed disorders of pulmonary ventilation. Nevertheless, in medical practice, such mixed disorders are most often encountered (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 identified only by analyzing the structure of the OEL.

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

The principle of gas analytical methods is that either the inert gas helium is introduced into the lungs (dilution method), or the nitrogen contained in the alveolar air is washed out, forcing the patient to breathe pure oxygen. In both cases, the FRC is calculated based on the final concentration of the gas (RF Schmidt, G. Thews).

Helium dilution method. Helium is known to be an inert and harmless gas for the body, 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 a closed spirometer container before and after mixing the gas with the lung volume. A closed spirometer with a known volume (Vsp ₎ is filled with a gas mixture consisting of oxygen and helium. The volume occupied by helium (Vsp ₎ and its initial concentration (FHe1) are also known. After a calm exhalation, the patient begins to breathe from the spirometer, and the helium is evenly distributed between the lung volume (FRC) and the spirometer volume (Vsp ₎. After a few minutes, the helium concentration in the general system ("spirometer-lungs") decreases (FHe2 ₎.

Nitrogen washout method. In this method, the spirometer is filled with oxygen. The patient breathes into the closed circuit of the spirometer for several minutes, and the volume of exhaled air (gas), the initial nitrogen content in the lungs, and its final content in the spirometer are measured. FRC is calculated using an equation similar to that for the helium dilution method.

The accuracy of both of the above methods for determining FRC (Fluorescence Resonance Index) depends on the completeness of gas mixing in the lungs, which in healthy people occurs within a few minutes. However, in some diseases accompanied by pronounced unevenness of ventilation (for example, in obstructive pulmonary pathology), equilibration of gas concentration takes a long time. In these cases, measuring FRC (Fluorescence Resonance Index) using the described methods may be inaccurate. The more technically complex method of whole-body plethysmography is free of these shortcomings.

Whole-body plethysmography. Whole-body plethysmography is one of the most informative and complex research methods used in pulmonology to determine lung volumes, tracheobronchial resistance, elastic properties of lung tissue and chest, and to assess some other parameters of pulmonary ventilation.

The integral plethysmograph is a hermetically sealed chamber with a volume of 800 l, in which the patient is freely placed. The patient breathes through a pneumotachographic tube connected to a hose open to the atmosphere. The hose has a valve that allows the air flow to be automatically closed at the right moment. Special barometric sensors measure the pressure in the chamber (Pcam) and in the oral cavity (Pmouth). The latter, with the hose valve closed, is equal to the intra-alveolar pressure. The pneumotachograph allows the air flow (V) to be determined.

The operating principle of the integral plethysmograph is based on Boyle-Moriost's law, according to which, at a constant temperature, the ratio between the pressure (P) and the volume of gas (V) remains constant:

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

The patient, located inside the plethysmograph chamber, inhales and exhales calmly, after which (at the FRC level) the hose valve is closed, and the subject attempts to "inhale" and "exhale" (the "breathing" maneuver). During this "breathing" maneuver, the intra-alveolar pressure changes, and the pressure in the closed chamber of the plethysmograph changes inversely proportionally. During an attempt to "inhale" with the valve closed, the volume of the chest increases, which leads, on the one hand, to a decrease in the intra-alveolar pressure, and on the other hand, to a corresponding increase in the pressure in the plethysmograph chamber (Pcam ₎. Conversely, during an attempt to "exhale", the alveolar pressure increases, and the volume of the chest and the pressure in the chamber decrease.

Thus, the method of whole-body plethysmography allows to calculate with high accuracy the intrathoracic gas volume (ITG), which in healthy individuals quite accurately corresponds to the value of the functional residual capacity of the lungs (FRC, or CS); the difference between ITG and FRC usually does not exceed 200 ml. However, it should be remembered that in case of impaired bronchial patency and some other pathological conditions, ITG can significantly exceed the value of the true FRC due to an increase in the number of non-ventilated and poorly ventilated alveoli. In these cases, a combined study using gas analytical methods of the method of whole-body plethysmography is advisable. By the way, the difference between ITG and FRC 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 the OLC. With "pure" restriction (without combination with bronchial obstruction), the OLC structure does not change significantly, or some decrease in the OLC/OLC ratio was observed. If restrictive disorders occur against the background of bronchial patency disorders (mixed type of ventilation disorders), along with a distinct decrease in the OLC, a significant change in its structure is observed, characteristic of broncho-obstructive syndrome: an increase in the OLC/OLC (more than 35%) and FRC/OLC (more than 50%). With both types of restrictive disorders, the VC is significantly reduced.

Thus, the analysis of the structure of the VC allows differentiating all three variants of ventilation disorders (obstructive, restrictive and mixed), whereas the assessment of only spirographic indicators does not make it possible to reliably distinguish the mixed variant from the obstructive one, accompanied by a decrease in VC).

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

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

Determination of uneven ventilation of the lungs

In a healthy person, there is a certain physiological unevenness in the ventilation of different parts of the lungs, caused by differences in the mechanical properties of the airways and lung tissue, as well as the presence of the so-called vertical pleural pressure gradient. 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) parts. The difference can reach 8 cm of water column. Therefore, before the beginning of the next inhalation, the alveoli of the apex of the lungs are stretched more than the alveoli of the lower basal parts. In this regard, during inhalation, a greater volume of air enters the alveoli of the basal parts.

The alveoli of the lower basal parts of the lungs are normally ventilated better than the apical areas, which is associated with the presence of a vertical gradient of intrapleural pressure. However, normally such uneven ventilation is not accompanied by a noticeable disruption of gas exchange, since the blood flow in the lungs is also uneven: the basal parts are perfused better than the apical ones.

In some respiratory diseases, the degree of ventilation unevenness may increase significantly. The most common causes of such pathological unevenness of ventilation are:

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

Often, various causes are combined. For example, in chronic obstructive bronchitis complicated by emphysema and pneumosclerosis, regional disorders of bronchial patency and lung tissue elasticity develop.

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

Gas analytical and barometric methods are most often used to assess the unevenness of pulmonary ventilation. Thus, a general idea of the unevenness of pulmonary ventilation can be obtained, for example, by analyzing the helium mixing (dilution) curves or nitrogen washout, which are used to measure FRC.

In healthy people, helium mixes with alveolar air or washes out nitrogen from it within three minutes. In case of bronchial obstruction, the number (volume) of poorly ventilated alveoli increases sharply, due to which the mixing (or washing) time increases significantly (up to 10-15 minutes), which is an indicator of uneven pulmonary ventilation.

More accurate data can be obtained by using a single-breath nitrogen washout test. The patient exhales as much as possible and then inhales pure oxygen as deeply as possible. He then exhales slowly into the closed system of a spirograph equipped with a device for determining the concentration of nitrogen (an azotograph). Throughout the exhalation, the volume of the exhaled gas mixture is continuously measured, and the changing concentration of nitrogen in the exhaled gas mixture containing alveolar nitrogen is determined.

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

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

During the long third phase, the concentration of nitrogen in 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 washed out of poorly ventilated alveoli, which are emptied last. Thus, the greater the rise in the nitrogen washout curve at the end of the third phase, the more pronounced the unevenness of pulmonary ventilation is.

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 flow of air predominantly from the apical parts of the lungs, the alveolar air in which contains nitrogen of a higher concentration.

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

Ventilation-perfusion ratio assessment

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 at the level of the alveoli. Therefore, the value of the ventilation-perfusion ratio (VPR) is one of the most important functional characteristics of the respiratory organs, ultimately determining the level of gas exchange.

Normally, the VPO for the lung as a whole is 0.8-1.0. When the VPO decreases below 1.0, perfusion of poorly ventilated areas of the lungs leads to hypoxemia (reduced oxygenation of arterial blood). An increase in VPO greater than 1.0 is observed with preserved or excessive ventilation of areas whose perfusion is significantly reduced, which can lead to impaired CO2 removal - hypercapnia.

Reasons for violation of the VPO:

1. All diseases and syndromes that cause uneven ventilation of the lungs.
2. Presence of anatomical and physiological shunts.
3. Thromboembolism of small branches of the pulmonary artery.
4. Microcirculation disorders and thrombus formation in the vessels of the pulmonary circulation.

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

When analyzing a capnogram, three indicators are usually calculated:

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

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

Determination of gas diffusion

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

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

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

To calculate the diffusion capacity of the lungs for oxygen, it is necessary to measure the absorption of 62 (VO ₂ ) and the average gradient of partial pressure of O ₂. VO ₂ values are measured using an open or closed type spirograph. More complex gas analytical methods are used to determine the gradient of partial pressure of oxygen (P ₁ - P _{2 ), since it is difficult to measure the partial pressure of O2} in the pulmonary capillaries in clinical conditions.

The definition of the diffusion capacity of the lungs is more often used for O ₂, but for carbon monoxide (CO). Since CO binds to hemoglobin 200 times more actively than oxygen, its concentration in the blood of the pulmonary capillaries can be neglected. Then, to determine DlCO, it is sufficient to measure the rate of passage of CO through the alveolar-capillary membrane and the gas pressure in the alveolar air.

The single-breath method is most widely used 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 holds his breath for 10 seconds. 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.

Normally, DlСО, normalized to body area, is 18 ml/min/mm Hg/m2. The diffusion capacity of the lungs for oxygen (DlО2) is calculated by multiplying DlСО by a coefficient of 1.23.

The most common diseases that cause a decrease in the diffusion capacity of the lungs are the following.

• Pulmonary emphysema (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 damage to 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 damage to the capillary bed of the lungs (vasculitis, embolism of small branches of the pulmonary artery, etc.).

For correct interpretation of changes in the diffusion capacity of the lungs, it is necessary to take into account the hematocrit index. An increase in hematocrit in polycythemia and secondary erythrocytosis is accompanied by an increase, and its decrease in anemia - a decrease in the diffusion capacity of the lungs.

[ 43 ], [ 44 ]

Measuring airway resistance

Measuring the airway resistance is a diagnostically important parameter of pulmonary ventilation. During inhalation, air moves through the airways under the action of the pressure gradient between the oral cavity and the alveoli. During inhalation, expansion of the chest leads to a decrease in the vitripleural and, accordingly, 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 action of the elastic traction of the lungs and chest is aimed at increasing the intra-alveolar pressure, which becomes higher than the pressure in the oral cavity, resulting in a reverse air flow. Thus, the pressure gradient (∆P) is the main force ensuring air transfer through the airways.

The second factor that determines the magnitude of gas flow through the airways is aerodynamic resistance (Raw), which, in turn, depends on the clearance and length of the airways, as well as on the viscosity of the gas.

The magnitude of the volumetric air flow velocity obeys Poiseuille's law: V = ∆P / Raw, where

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

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

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

• whole body plethysmography method;
• air flow blocking method.

Determination of blood gases and acid-base balance

The main method for diagnosing acute respiratory failure is the study of arterial blood gases, which includes 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 the value of excess (deficit) of bases (BE).

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

To determine the gas composition of the blood in patients with acute respiratory failure in intensive care units, a complex invasive technique is used to obtain arterial blood by puncturing a large artery. The radial artery is punctured more often, since the risk of complications is lower. The hand has good collateral blood flow, which is carried out by the ulnar artery. Therefore, even if the radial artery is damaged during puncture or use of an arterial catheter, the blood supply to the hand is maintained.

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

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

A negative Allen test is a contraindication to catheter placement. To perform the test, the ulnar and radial arteries are compressed with the fingers so as to turn off the arterial blood flow; the hand turns pale after a while. After this, the ulnar artery is released, while continuing to compress the radial. Usually, the color of the hand is quickly restored (within 5 seconds). If this does not happen, then the hand remains pale, ulnar artery occlusion is diagnosed, the test result is considered negative, and the radial artery puncture is not performed.

If the test result is positive, the patient's palm and forearm are immobilized. After preparing the surgical field in the distal sections of the radial artery, the pulse on the radial artery is palpated, anesthesia is administered at this site, and the artery is punctured at an angle of 45°. The catheter is advanced upward until blood appears in the needle. The needle is removed, leaving the catheter in the artery. To prevent excessive bleeding, the proximal section 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, arterial occlusion by a thrombus, and infection) during catheter placement are relatively rare.

It is preferable to collect blood for testing in a glass syringe rather than a plastic one. 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 entry of ambient air into the blood sample leads to the determination of the PaO2 level.

Blood gas determination should be performed no later than 10 minutes after drawing arterial blood. Otherwise, the ongoing metabolic processes in the blood sample (initiated mainly by the activity of leukocytes) significantly change the results of blood gas determination, reducing the level of PaO2 and pH, and increasing PaCO2. Particularly pronounced changes are observed in leukemia and in pronounced leukocytosis.

[ 45 ], [ 46 ], [ 47 ]

Methods for assessing acid-base balance

Measuring 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, thereby changing color.
• The pH-metry method allows for more accurate and rapid determination of the concentration of hydrogen ions using special polarographic electrodes, on the surface of which, when immersed in a solution, a potential difference is created, depending on the pH of the medium being studied.

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

The electrodes are immersed in the solution being studied (e.g. blood) and polarized by the current source. As a result, a current is generated in the closed electric circuit. Since the platinum (active) electrode is additionally 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 balance is assessed using the Astrup method on the microAstrup device. The BB, BE, and PaCO2 indices are determined. Two portions of the arterial blood being examined are brought into equilibrium with two gas mixtures of known composition, differing in the partial pressure of CO2. The pH is measured in each portion of blood. The pH and PaCO2 values in each portion of blood are plotted as two points on the nomogram. A straight line is drawn through the two points marked on the nomogram until it intersects with the standard BB and BE graphs, and the actual values of these indices are determined. Then the pH of the blood being examined is measured, and a point corresponding to this measured pH value is found on the resulting straight line. The actual pressure of CO2 in the blood (PaCO2) is determined by the projection of this point on the ordinate axis.

Direct measurement of CO2 pressure (PaCO2)

In recent years, a modification of polarographic electrodes intended for pH measurement has been used for direct measurement of PaCO2 in a small volume. Both electrodes (active and reference) are immersed in an electrolyte solution, which is separated from the blood by another membrane permeable only for gases, but not for hydrogen ions. CO2 molecules, diffusing through this membrane from the blood, change the pH of the solution. As was said above, the active electrode is additionally separated from the NaHCO3 solution by a glass membrane permeable only for 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 blood. Thus, the pressure in the circuit is proportional to PaCO2 in the blood.

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

[ 48 ], [ 49 ], [ 50 ]

Determination of BE based on direct measurement of pH and PaCO2

Direct determination of pH and PaCO2 of blood allows to significantly simplify the method of determining the third indicator of acid-base balance - excess bases (BE). The last indicator can be determined using special nomograms. After direct measurement of pH and PaCO2, the actual values of these indicators are plotted on the corresponding scales of the nomogram. The points are connected by a straight line and continue until they intersect with the BE scale.

This method of determining the main indicators of acid-base balance does not require equilibrating the blood with a gas mixture, as when using 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 an oxygen concentration of 21% (FiO2 ₌ 0.21) and normal atmospheric pressure (760 mm Hg), PaO2 is 90-95 mm Hg. With a change in barometric pressure, ambient temperature and some other conditions, PaO2 in a healthy person can reach 80 mm Hg.

Lower values of PaO2 (less than 80 mm Hg) can be considered an initial manifestation of hypoxemia, especially against the background of acute or chronic damage to the lungs, chest, respiratory muscles or central regulation of respiration. A decrease in PaO2 to 70 mm Hg in most cases indicates compensated respiratory failure and is usually accompanied by clinical signs of decreased functional capacity of the external respiratory system:

• slight tachycardia;
• shortness of breath, respiratory discomfort, appearing mainly during physical exertion, although at rest the respiratory rate does not exceed 20-22 per minute;
• a noticeable decrease in exercise tolerance;
• participation in breathing of accessory respiratory muscles, etc.

At first glance, these criteria of arterial hypoxemia contradict the definition of respiratory failure by E. Campbell: "respiratory failure is characterized by a decrease in PaO2 below 60 mm Hg...". However, as already noted, this definition refers to decompensated respiratory failure, which is manifested by a large number of clinical and instrumental signs. Indeed, a decrease in PaO2 below 60 mm Hg, as a rule, indicates severe 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 with PaO2 below 40-45 mm Hg.

PaO2 from 80 to 61 mm Hg, especially against the background of acute or chronic damage to the lungs and external respiratory system, should be regarded as the initial manifestation of arterial hypoxemia. In most cases, it indicates the formation of mild compensated respiratory failure. A decrease in PaO2 _below 60 mm Hg indicates moderate or severe pre-compensated respiratory failure, the clinical manifestations of which are clearly expressed.

Normally, the pressure of CO2 in arterial blood (PaCO2 ₎ is 35-45 mm Hg. Hypercapia is diagnosed when PaCO2 increases above 45 mm Hg. PaCO2 values above 50 mm Hg usually correspond to the clinical picture of severe ventilation (or mixed) respiratory failure, and above 60 mm Hg are an indication for mechanical ventilation aimed at restoring the minute respiratory volume.

Diagnosis of various forms of respiratory failure (ventilatory, 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 an assessment of the gas composition of the blood.

_{Some features of the change in PaO 2} and PaCO ₂ in ventilatory and parenchymatous respiratory failure have already been noted above. Let us recall that ventilatory respiratory failure, in which the process of CO ₂ release from the body is primarily disrupted in the lungs, is characterized by hypercapnia (PaCO ₂ greater than 45-50 mm Hg), often accompanied by compensated or decompensated respiratory acidosis. At the same time, progressive hypoventilation of the alveoli naturally leads to a decrease in the oxygenation of the alveolar air and the pressure of O ₂ in the arterial blood (PaO ₂ ), resulting in hypoxemia. Thus, the detailed picture of ventilatory respiratory failure is accompanied by both hypercapnia and increasing hypoxemia.

The early stages of parenchymatous respiratory failure are characterized by a decrease in PaO ₂ (hypoxemia), in most cases combined with pronounced hyperventilation of the alveoli (tachypnea) and the resulting hypocapnia and respiratory alkalosis. If this condition cannot be relieved, signs of progressive total reduction in ventilation, minute respiratory volume, and hypercapnia (PaCO ₂ greater than 45-50 mm Hg) gradually appear. This indicates the addition of ventilatory respiratory failure caused by fatigue of the respiratory muscles, severe obstruction of the airways, or a critical drop in the volume of functioning alveoli. Thus, the later stages of parenchymatous respiratory failure are characterized by a progressive decrease in PaO ₂ (hypoxemia) combined with hypercapnia.

Depending on the individual characteristics of the development of the disease and the predominance of certain pathophysiological mechanisms of respiratory failure, other combinations of hypoxemia and hypercapnia are possible, which are discussed in the following chapters.

Acid-base imbalances

In most cases, for an accurate diagnosis of respiratory and non-respiratory acidosis and alkalosis, as well as for assessing the degree of compensation of these disorders, it is sufficient to determine the blood pH, pCO2, BE and SB.

During the period of decompensation, a decrease in blood pH is observed, and in alkalosis, the acid-base balance is determined quite simply: in acidity, it is increased. It is also easy to determine the respiratory and non-respiratory types of these disorders by laboratory indicators: changes in pCO ₂ and BE in each of these two types are in different directions.

The situation is more complicated with the assessment of the parameters of the acid-base balance during the period of compensation of its disturbances, when the blood pH 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 general clinical situation helps, allowing us to understand whether the corresponding changes in pCO ₂ or BE are primary or secondary (compensatory).

Compensated respiratory alkalosis is characterized by a primary increase in PaCO2, which is essentially the cause of this disturbance of the acid-base balance; in these cases, the corresponding changes in BE are secondary, i.e., they reflect 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, and the shifts in pCO2 reflect compensatory hyperventilation of the lungs (if possible).

Thus, comparison of the parameters of acid-base imbalance with the clinical picture of the disease in most cases allows for a fairly reliable diagnosis of the nature of these imbalances even during the period of their compensation. Evaluation of changes in the electrolyte composition of the blood can also help establish the correct diagnosis in these cases. Hypernatremia (or normal Na ⁺ concentration ) and hyperkalemia are often observed in respiratory and metabolic acidosis, while hypo- (or normo) natremia and hypokalemia are observed in respiratory alkalosis.

Pulse oximetry

Oxygen supply to peripheral organs and tissues depends not only on the absolute values of D2 pressure _in arterial blood, but also on the ability of hemoglobin to bind oxygen in the lungs and release 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 O2 pressure values corresponds to the horizontal section of this curve. Therefore, even with fluctuations in arterial blood oxygen pressure from 95 to 60-70 mm Hg, the saturation of hemoglobin with oxygen (SaO2 ₎ remains at a sufficiently high level. Thus, in a healthy young person with PaO2 ₌ 95 mm Hg, the saturation of hemoglobin with oxygen is 97%, and with PaO2 ₌ 60 mm Hg - 90%. The steep slope of the middle section of the oxyhemoglobin dissociation curve indicates very favorable conditions for the release of oxygen in the tissues.

Under the influence of certain factors (increased temperature, 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 the tissues. The figure shows that in these cases, more PaO2 is required to maintain the oxygen saturation of hemoglobin at the same _level.

A leftward shift in the oxyhemoglobin dissociation curve indicates increased affinity of hemoglobin for O ₂ and its lower release into tissues. Such a shift occurs under the influence of hypocapnia, alkalosis, and lower temperatures. In these cases, high hemoglobin oxygen saturation is maintained even at lower values of PaO ₂

Thus, the value of hemoglobin oxygen saturation in respiratory failure acquires an independent value 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 light-sensitive sensor located opposite the light-emitting diode). Two wavelengths of radiation are usually used: 660 nm (red light) and 940 nm (infrared). Oxygen saturation is determined by the absorption of red and infrared light, respectively, by reduced hemoglobin (Hb) and oxyhemoglobin (HbJ ₂ ). The result is displayed as SaO2 (saturation obtained by pulse oximetry).

Normally, oxygen saturation exceeds 90%. This indicator decreases with hypoxemia and a decrease in PaO2 _below 60 mm Hg.

When evaluating the results of pulse oximetry, one should keep in mind the rather large error of the method, reaching ±4-5%. It should also be remembered that the results of indirect determination of oxygen saturation depend on many other factors. For example, on the presence of nail polish on the subject's nails. The polish absorbs part of the anode radiation with a wavelength of 660 nm, thereby underestimating the values of the SaO ₂ indicator.

The pulse oximeter readings are affected by the shift in the hemoglobin dissociation curve, which occurs 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 pH fluctuations lead to significant changes in the SaO2 indicator; in alkalosis (for example, respiratory, developed against the background of hyperventilation), SaO2 is overestimated, and in acidosis, it is underestimated.

In addition, this technique does not allow for the appearance in the peripheral blood of pathological types of hemoglobin - carboxyhemoglobin and methemoglobin, which absorb light of the same wavelength as oxyhemoglobin, which leads to an overestimation of SaO2 values.

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

Evaluation of hemodynamic parameters

For a complete analysis of the clinical situation in acute respiratory failure, it is necessary to dynamically determine a number of hemodynamic parameters:

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

Many of these parameters (BP, HR, SaO2, ECG, etc.) can be determined using modern monitoring equipment in intensive care and resuscitation departments. In seriously ill patients, it is advisable to catheterize the right heart with the installation of a temporary floating intracardiac catheter to determine CVP and PAOP.

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