Cardiovascular disorders

II./3.2.: Echocardiography in the diagnostics of heart defects

II./3.2.1.: Techniques of cardiologic ultrasound

Echocardiography is essential in the diagnostics of heart valve diseases and in the therapeutic management of patients, as it is cost-effective, easily available, and can be repeated as often as needed. During its development that lasted for more than a half century we have come from the initial one-dimension imaging (M-mode echocardiography) to the real-time three-dimensional imaging. In addition to analyzing the fine details of structural abnormities of the valves, the use of Doppler technique provides a detailed information also on the functions of the valve and, with the help of biophysical formulas, non-invasive calculation of intracardial pressures and by this way an assessment of the hemodynamic consequences of heart valve diseases is also feasible, what is a unique feature of this imaging modality.

The conventional two-dimensional echocardiography provides cross-sectional images of the heart in certain planes, while three-dimensional imaging maps a pyramid-shaped field of space. B-mode (brightness) allows us to assess the shape, size and shift of the tissue structures (valves, myocardium) (Image 1), while Doppler technique enables us to analyze the speed and direction of the movement of any tissue structures (tissue Doppler echocardiography) or the flow velocity and direction of blood (pulsatile wave Doppler echocardiography, continuous wave Doppler echocardiography, color mapping) in two or three dimensions (Images 2 and 3). Of the newer techniques speckle tracking echocardiography can be emphasized, which allows the analysis of the changes in the myocardium’s shape (elongation, shortening, thickening, torsion) by exploiting the ”speckled” or “granulated” structure of the myocardium observed during its ultrasound examination.

Image 1: Viewing the aortic valve with imaging techniques. Image of a tricuspid aortic valve with echocardiography. Transthoracic, parasternal, short-axis recording.

Image 2: Aortic insufficiency with echocardiography. Transthoracic, parasternal, short-axis recording with color Doppler. Central regurgitation jet.

Image 3: Aortic insufficiency with echocardiography. Transthoracic, apical, long-axis recording with color Doppler.

The use of ultrasound contrast media allows a better assessment of blood flow within the cardiac chambers, what makes the quantification of valvular stenoses and regurgitations considerably more accurate, and helps us examining myocardial perfusion, what is important in the diagnostics of coronary heart disease. The tissue density of the myocardium can be measured and its structure can be examined with the special method of integrated back scatter technique. Echocardiographic examinations may be performed at rest, under pharmacologic or even physical stress; the latter is also a unique feature of this technique. Examining heads which contain piezo-electric crystals can be used also in a transthoracic or transesophageal way.

Miniaturized transducers can be directed through the vessels into the heart in order to perform intracardial echocardiography. The various echo techniques usually provide images with excellent resolution in space and time; however at present the accuracy and reproducibility of measurements yet lag behind those of the CT and MR examinations. Ultrasound exerts no harmful effect on the tissues or the whole body; a small warming of the tissues is clinically irrelevant. Contrast media may evoke hypersensitivity reactions in a small percentage of patients. Echocardiographic examinations should be performed with Class 1 indication in patients with valvular diseases. In addition to the diagnostics of heart valve diseases, several interventions e.g. implantation or plastic surgery of the valves are also performed with ultrasound control.

II./3.2.2.: Ultrasound diagnostics of aortic stenosis

Echocardiography plays a truly manifold role in the assessment of aortic stenosis, beginning with the examination of the valve’s anatomy and morphology (Images 4 and 5), through the quantification of the grade of stenosis, and until the appraisal of the harmful effects exerted on other structures as well as the hemodynamic consequences. Echo techniques to be used and parameters to be measured are laid down in regularly updated international guidelines. Based on parameters obtained by echo, the severity of heart defects is divided into three groups. Mild, moderate and severe or significant aortic stenoses are distinguished. A therapeutic strategy can be determined by integrating the echo findings with the results of other examinations into a clinical picture.

Image 4: Bicuspid aorta with echocardiography. Transthoracic, parasternal, short-axis recording.

Image 5: Quadricuspid aorta with echocardiography. Transthoracic, parasternal, short-axis recording.

With the two- or three-dimensional imaging we assess the morphologic abnormities, determine the number of cusps, their fibrotic or calcified degeneration, the abnormities of commissures (e.g. commissural fusion), the movement of the cusps and the anatomical characteristics of the entire region of aortic root. Sizes of outflow track and anulus, diameter of the ascending aorta, and a description of a subvalvular hypertrophic, so called sigmoidal septum may be important additional information for establishing a plan for the surgery. The severity of AS can be judged by determining the valvular area as well as the pressure gradient developing on the stenotic valve. The opening area of a normal valve, through which the blood can flow freely, is 3 to 4 cm². If this area is under 0.7 cm², the stenosis is considered severely significant.

Measurement of the pressure gradient developing on a narrowed valve is made possible by the fact that the acceleration of blood flow is related to the gradient; this relation is expressed by the simplified Bernoulli’s equation that is often used in echocardiography: gradient (mmHg) = 4V², where V is flow velocity measured with Doppler (m/sec). The American and European guidelines somewhat differ in their gradients; usually mean gradients calculated to the whole systole and exceeding 40 to 50 mmHg are considered as significant. In these cases the velocity of blood flow accelerates from the normal 1 to 1.8 m/sec to over 4 m/sec. Anyone who performs an echo examination should keep in mind that the pressure gradient always depends on the extent of flow, so it is not authentic in patients with weak left ventricular pump function, concomitant significant mitral stenosis or e.g. significant VSD.

In patients with weak left ventricular pump function a dobutamine stress test may be required for determining the severity of their stenosis. The valvular area can be measured in a smaller part of cases directly by planimetry on the cross-sectional imaging of the valvular orifice, but much more frequently it is calculated from Doppler parameters. Inaccuracy of these measurements may be caused when the angle of incidence of the Doppler ultrasound beam is not quite identical with the direction of the flow to be measured; this may lead to consequent underestimation of the stenosis. In addition to the visualization of the valve, it is also important to measure the thickness of left ventricular myocardium which becomes hypertrophic under the stress of increased pressure, and to calculate the muscular mass. Increase of the whole myocardial mass is a cardiovascular risk factor even alone and it also significantly increases the risk of a surgery.

Systolic and diastolic left ventricular function is essential for choosing a therapy, as its decrease may mean an indication for surgery even in an asymptomatic patient. A dilation of the ascending aorta is often associated with aortic stenosis; its accurate measurement may require a transesophageal examination in patients with a weak echo window. Based on the size of the aortic anulus the surgeon can choose the artificial valve of appropriate size, and can prepare himself for the difficulties of surgery expected in case of a narrow anulus. Although CT angiography and angiography are required as well, echocardiography is also very important in the selection of patients eligible for transcatheter valve implantation. In the modern era of heart surgery the intraoperative echocardiography, performed from the esophagus, is a part of the everyday routine for examining left ventricular function and the functioning of the newly implanted artificial valve.

II./3.2.3.: Ultrasound diagnostics of aortic regurgitation

Ultrasound examination of heart valve regurgitations is based on the same basic principles as that of the stenoses, however the determination of the pathogenic mechanism and in particular the quantification of the regurgitation’s extent often poses a greater challenge, and it is less accurate. The examinations of anatomical characteristics at the aortic root’s region and directly the material of the aortic valve are performed, as described in the previous chapter, mainly from parasternal and apical echo windows. When aortic insufficiency is detected in young patients, yet before the appearance of senile, degenerative lesions; underlying malformations of the cusps, mostly a bicuspid anomaly should always be looked for. An asymmetric line of closure and an abnormal opening are always confirming signs.

Infective endocarditis and aortic dissection, which are responsible for the majority of acute regurgitations, give characteristic ultrasound images. Vegetations of the endocarditis develop almost always on the ventricular surface of the cusps, and appear as mobile, irregularly shaped masses which oscillate synchronously with the blood flow. The thin intimal flap of aortic dissection, if the rupture extends down to the root, can also be detected by a transthoracic basic ultrasound examination. The real and spurious lumina, separated by the flap, can be differentiated based on their pattern of flow. With color Doppler technique one can obtain a quick surveying image on conditions of flow in a chosen area within the heart. When examining from the apical direction, the jet of aortic insufficiency appears in the outflow track as a turbulent flow advancing toward the transducer, with red color coding.

The narrowest neck of the regurgitating jet (vena contracta) is seen immediately at the regurgitating orifice; apically from here the jet spreads out in the outflow track and in the left ventricle. The extension of the vena contracta and the jet provide semiquantitative information on the severity of insufficiency. Quantification of the regurgitation is based on the determination of anatomic or effective orifice of regurgitation and the calculation of regurgitating volume and fraction. An insufficiency is regarded significant when the orifice of regurgitation is larger than 0.3 cm², and the volume of blood flowing back in every heart cycle exceeds 60 ml. The volume of blood flowing through a given area is calculated from the size of the area and the flow’s Doppler spectrum. The volume of regurgitation is conventionally given by calculating the difference between the left ventricular stroke volume and the volume flowing from the direction of the mitral valve.

For quantifying the grade of insufficiency, proximal isovelocity surface area (PISA) is a relatively novel technique; it is based on the fact that the velocity of corpuscular elements of the blood which reflect ultrasound becomes homogeneous at the orifice and the elements with identical speed form an approximate spherical shell pattern. Several other echo parameters are also helpful in assessing the extent of insufficiency, such as deceleration of the regurgitation jet during systole or the reverse flow detected in the descending aorta.

In a significant part of patients, aortic stenosis and insufficiency are combined, what makes the evaluation of echo parameters complicated. As in patients with aortic stenosis, for determining the significance of this defect, also the hemodynamic consequences should be assessed, including primarily the myocardial hypertrophy that developed due to the chronic volume and pressure load, the increase in the size of the left ventricular chamber, the impairments of systolic and diastolic functions, the decrease of myocardial contractility, and the appearance of pulmonary hypertension. Surgical correction of the defect should be performed yet before the development of an irreversible impairment of the myocardium.

For the evaluation of both stenoses and regurgitations; in their guidelines the international echocardiographic societies recommend an integration of various echocardiographic signs and data, what requires a great professional experience.