X-ray examination of cardiac function

In a healthy person, an excitation wave spreads through the myocardium approximately once per second - the heart contracts and then relaxes. The simplest and most accessible method of recording them is fluoroscopy. It allows visually assessing the contractions and relaxation of the heart, the pulsation of the aorta and pulmonary artery. At the same time, by changing the patient's position behind the screen, it is possible to bring out on the contour, i.e. make all sections of the heart and blood vessels edge-forming. However, recently, due to the development of ultrasound diagnostics and its widespread introduction into clinical practice, the role of fluoroscopy in studying the functional activity of the heart has noticeably decreased due to the fairly high radiation load that exists with it.

The main method for studying the contractile function of the heart muscle is ultrasound examination (ultrasound).

In cardiology, several ultrasound techniques are used: one-dimensional echocardiography - M-method; two-dimensional echocardiography (sonography) - B-method; one-dimensional Doppler echocardiography; two-dimensional color Doppler mapping. An effective method for studying the heart is also a duplex study - a combination of sonography and Dopplerography.

A one-dimensional echocardiogram has the appearance of a group of curves, each of which corresponds to a specific structure of the heart: the wall of the ventricle and atrium, the interatrial and interventricular septum, valves, pericardium, etc. The amplitude of the curve on the echocardiogram indicates the range of systolic movements of the recorded anatomical structure.

Sonography allows one to observe the movements of the heart walls and valves on the display screen in real time. To study a number of parameters characterizing the function of the heart, the contour of the heart is outlined on the monitor screen on freeze frames recorded at the apex of the R wave of the electrocardiogram and the descending knee of the T wave. A special computer program available in the ultrasound device allows one to compare and analyze these two images and obtain the parameters of the end-systolic and end-diastolic volumes of the left ventricle and atria, the size of the right ventricle surface, the value of the ventricular ejection fraction, the atrial emptying fraction, systolic and minute volumes, and the thickness of the myocardial walls. It is very valuable that this can also provide parameters of regional contractility of the left ventricular wall, which is extremely important in the diagnosis of coronary heart disease and other lesions of the heart muscle.

Dopplerography of the heart is performed mainly in pulse mode. It allows not only to study the movement of the valves and walls of the heart in any phase of the cardiac cycle, but also to measure the speed of blood flow, direction and nature of its flow in the selected control volume. New methods of Dopplerography have acquired special significance in the study of functional parameters of the heart: color mapping, energy and tissue Doppler. Currently, the specified options of ultrasound examination are the leading instrumental methods for examining cardiac patients, especially in outpatient practice.

Along with ultrasound diagnostics, radionuclide methods of examining the heart and blood vessels have recently been rapidly developing. Among these methods, three should be highlighted: equilibrium ventriculography (dynamic radiocardiography), radionuclide angiocardiography, and perfusion syntigraphy. They provide important, sometimes unique information about heart function, do not require vascular catheterization, and can be performed both at rest and after functional loads. The latter circumstance is most important when assessing the reserve capacity of the heart muscle.

Equilibrium ventriculography is one of the most common methods of examining the heart. It is used to determine the pumping function of the heart and the nature of the movement of its walls. The object of the study is usually the left ventricle, but special techniques have been developed for studying the right ventricle of the heart. The principle of the method is to record a series of images in the memory of a gamma camera computer. These images are obtained from the gamma radiation of radiopharmaceuticals introduced into the blood and remaining in the bloodstream for a long time, i.e. not diffusing through the vascular wall. The concentration of such radiopharmaceuticals in the bloodstream remains constant for a long time, so it is customary to say that the blood pool is being studied (from the English pool - a puddle, a pool).

The simplest way to create a blood pool is to introduce albumin into the blood. However, the protein is still broken down in the body, and the radionuclide released in this process leaves the bloodstream, and the radioactivity of the blood gradually decreases, which reduces the accuracy of the study. A more adequate way to create a stable radioactive pool was to label the patient's erythrocytes. For this purpose, a small amount of pyrophosphate is first injected intravenously - about 0.5 mg. It is actively absorbed by the erythrocytes. After 30 minutes, 600 MBq of 99mTc-pertechnetate is injected intravenously, which instantly combines with the pyrophosphate absorbed by the erythrocytes. This results in a strong connection. Note that this is the first time we have encountered a radionuclide study technique in which the RFP is "prepared" in the patient's body.

The passage of radioactive blood through the chambers of the heart is recorded in the computer memory using an electronic device called a trigger. It "links" the collection of information from the gamma camera detector to the electrical signals of the electrocardiograph. Having collected information on 300-500 cardiac cycles (after complete dilution of the radiopharmaceutical in the blood, i.e. stabilization of the blood pool), the computer groups them into a series of images, the main ones of which are those reflecting the end-systolic and end-diastolic phases. Several intermediate images of the heart are created simultaneously throughout the cardiac cycle, for example, every 0.1 s.

Such a procedure of forming medical images from a large series is necessary to obtain sufficient "counting statistics" so that the resulting images will have a high enough quality for analysis. This applies to any analysis - both visual and computer.

In radionuclide diagnostics, as in all radiation diagnostics, the main rule of “quality of reliability” applies: collecting the largest possible amount of information (quanta, electrical signals, cycles, images, etc.).

Using a computer, the ejection fraction, the rate of filling and emptying of the ventricle, the duration of systole and diastole are calculated from the integral curve constructed based on the results of the analysis of cardiac images. The ejection fraction (EF) is determined by the formula:

Where DO and CO are the values of the count rate (radioactivity levels) in the end-diastolic and end-systolic phases of the cardiac cycle.

The ejection fraction is one of the most sensitive indicators of ventricular function. Normally, it fluctuates around 50% for the right ventricle and 60% for the left ventricle. In patients with myocardial infarction, EF is always reduced proportionally to the volume of the lesion, which has a known prognostic value. This indicator is also reduced in a number of cardiac muscle lesions: cardiosclerosis, myocardiopathy, myocarditis, etc.

Equilibrium ventriculography can be used to detect limited disorders of the left ventricle contractility: local dyskinesia, hypokinesia, akinesia. For this purpose, the ventricle image is divided into several segments - from 8 to 40. For each segment, the movement of the ventricle wall during heart contractions is studied. Equilibrium ventriculography is of considerable value for detecting patients with reduced functional reserves of the heart muscle. Such people form a high-risk group for developing acute heart failure or myocardial infarction. They undergo this study under conditions of a dosed bicycle ergometric load to detect areas of the ventricle wall that cannot cope with the load, although no deviations are observed in the patient's calm state. This condition is called stress-induced myocardial ischemia.

Equilibrium ventriculography makes it possible to calculate the regurgitation fraction, i.e. the amount of backflow of blood in heart defects accompanied by valvular insufficiency. Another advantage of the method is that the study can be conducted over a long period of time, for several hours, studying, for example, the effect of drugs on heart activity.

Radionuclide angiocardiography is a method of alternating the first passage of radiopharmaceuticals through the chambers of the heart after its rapid intravenous administration in a small volume (bolus).

Usually 99mTc-pertechnetate with an activity of 4-6 MBq per 1 kg of body weight in a volume of 0.5-1.0 ml is used. The study is conducted on a gamma camera equipped with a high-performance computer. A series of images of the heart during the passage of the radiopharmaceutical through it (15-20 frames for no more than 30 s) is recorded in the computer memory. Then, having selected the "zone of interest" (usually this is the area of the root of the lung or the right ventricle), the radiation intensity of the radiopharmaceutical is analyzed. Normally, the curves of the passage of the radiopharmaceutical through the right chambers of the heart and through the lungs have the appearance of one high steep peak. In pathological conditions, the curve flattens (when the radiopharmaceutical is diluted in the heart chambers) or lengthens (when the radiopharmaceutical is retained in the chamber).

In some congenital heart defects, arterial blood is shunted from the left chambers of the heart to the right. Such shunts (called left-right shunts) occur with defects in the cardiac septum. On radionuclide angiocardiograms, a left-right shunt is revealed as a repeated rise in the curve in the "zone of interest" of the lungs. In other congenital heart defects, venous blood, not yet enriched with oxygen, again enters, bypassing the lungs, into the systemic circulation (right-left shunts). A sign of such shunting on a radionuclide angiocardiogram is the appearance of a peak of radioactivity in the left ventricle and aorta before the maximum radioactivity is registered in the lungs. In acquired heart defects, angiocardiograms allow the degree of regurgitation through the mitral and aortic openings to be determined.

Myocardial perfusion scintigraphy is used mainly to study myocardial blood flow and, to a certain extent, to assess the level of metabolism in the heart muscle. It is performed with the drugs ^99m T1-chloride and ^99m Tc-sesamibi. Both radiopharmaceuticals, passing through the vessels that feed the heart muscle, quickly diffuse into the surrounding muscle tissue and are included in metabolic processes, simulating potassium ions. Thus, the intensity of accumulation of these radiopharmaceuticals in the heart muscle reflects the volume of blood flow and the level of metabolic processes in the heart muscle.

The accumulation of radiopharmaceuticals in the myocardium occurs quite quickly and reaches its maximum in 5-10 minutes. This allows the study to be conducted in various projections. A normal perfusion image of the left ventricle on scintigrams looks like a homogeneous horseshoe-shaped shadow with a central defect that corresponds to the ventricular cavity. The ischemic zones that arise during an infarction will be displayed as areas with reduced radiopharmaceutical fixation. More visual and, most importantly, reliable data in the study of myocardial perfusion can be obtained using single-photon emission tomography. In recent years, interesting and important physiological data on the functioning of the heart muscle have been obtained using ultra-short-lived positron-emitting nuclides as radiopharmaceuticals, such as F-DG, i.e. using two-photon emission tomography. However, so far this is only possible in certain large research centers.

New opportunities in assessing cardiac function have emerged with the improvement of computer tomography, when it became possible to perform a series of tomograms with short exposures against the background of a bolus injection of a radiopaque substance. 50-100 ml of a non-ionic contrast agent - omnipaque or ultravist - is injected into the vein of the elbow using an automatic syringe. Comparative analysis of heart sections using computer densitometry allows one to determine the movement of blood in the cavities of the heart throughout the cardiac cycle.

Computer tomography has made particularly significant progress in cardiac research with the development of electron beam computer tomographs. Such devices not only allow for a large number of images to be taken with very short exposure times, but also for the creation of a real-time simulation of cardiac contraction dynamics and even for the performance of a three-dimensional reconstruction of a moving heart.

Another no less dynamically developing method of studying cardiac function is magnetic resonance imaging. Due to the high intensity of the magnetic field and the creation of a new generation of high-performance computers, it became possible to collect the information necessary for image reconstruction in very short periods of time, in particular, to analyze the end-systolic and end-diastolic phases of the cardiac cycle in real time.

The physician has at his disposal many radiological methods for assessing the contractile function of the heart muscle and myocardial blood flow. However, no matter how much the physician tries to limit himself to noninvasive methods, in a number of patients it is necessary to use more complex procedures associated with vascular catheterization and artificial contrast of the heart cavities and coronary vessels - X-ray ventriculographitis and coronary angiography.

Ventriculography is necessary because it has higher sensitivity and accuracy in assessing left ventricular function than other methods. This is especially true for identifying disorders of local contractility of the left ventricle. Information on regional myocardial disorders is necessary to determine the severity of coronary heart disease, assess indications for surgical interventions, transluminal angioplasty of the coronary arteries, thrombolysis in myocardial infarction. In addition, ventriculography allows for an objective assessment of the results of stress and diagnostic tests for coronary heart disease (atrial stimulation test, bicycle ergometric test, etc.).

The radiopaque substance is injected in a volume of 50 ml at a rate of 10-15 ml/s and filming is performed. The film frames clearly show changes in the shadow of the contrast substance in the cavity of the left ventricle. Upon careful examination of the film frames, it is possible to notice pronounced disturbances in myocardial contractility: lack of wall movement in any area or paradoxical movements, i.e. bulging at the moment of systole.

To identify less pronounced and local contractility disorders, it is common to conduct a separate analysis of 5-8 standard segments of the left ventricle silhouette (for a picture in the right anterior oblique projection at an angle of 30). Fig. 111.66 shows the division of the ventricle into 8 segments. Different methods have been proposed to assess contractility by segments. One of them is that 60 radii are drawn from the middle of the long axis of the ventricle to the contours of the ventricle shadow. Each radius is measured in the end-diastolic phase and, accordingly, the degree of its shortening during ventricular contraction. Based on these measurements, computer processing and diagnostics of regional contractility disorders are performed.

An indispensable direct method for studying coronary blood flow is selective coronary angiography. Through a catheter inserted sequentially into the left and then into the right coronary artery, a radiopaque substance is injected with an automatic injector and filming is performed. The resulting images reflect both the morphology of the entire coronary artery system and the nature of blood circulation in all parts of the heart.

Indications for coronary angiography are quite broad. Firstly, coronary angiography is indicated in all insufficiently clear cases for verification of ischemic heart disease, choice of treatment method for acute myocardial infarction, differential diagnostics of myocardial infarction and cardiomyopathy. As well as in combination with repeated heart biopsy - if there is a suspicion of rejection reaction during its transplantation. Secondly, coronary angiography is used in cases of strict professional selection if there is a suspicion of possible damage to the coronary arteries in pilots, air traffic controllers, drivers of intercity buses and trains, since the development of acute myocardial infarction in such workers poses a threat to passengers and people around them.

An absolute contraindication to coronary angiography is intolerance to the contrast agent. Relative contraindications include severe damage to internal organs: liver, kidneys, etc. Coronary angiography can only be performed in specially equipped X-ray operating units, which are provided with all the means to restore cardiac activity. In some cases, the introduction of a contrast agent (and it has to be introduced several times into each coronary artery if functional tests are used) can be accompanied by brachycardia, extrasystole, and sometimes temporary transverse heart block and even fibrillation. In addition to visual analysis of coronary angiograms, they are computer processed. To analyze the contours of the shadow of the arteries, only the outlines of the artery are highlighted on the display. In case of stenosis, a stenosis graph is plotted.