Final Feigenbaum’s Echocardiography DIGITAL

Feigenbaum’s Echocardiography

120

Feigenbaum’s Echocardiography

myocardial contractility but rather as a consequence of contractile dyssynchrony and overall pump ine ciency. Left Ventricular Wall Stress Most clinically used parameters of ventricular function including stroke volume and ejection fraction are a erload dependent, that is, they are dependent on the pressure developed and impedance against which the le ventricle must contract. Several methods have been proposed for correcting a erload or creating a erload-independent indices of le ventricular performance, including calculating ven- tricular wall stress and creation of pressure volume loops. ese calculations have been used as a measure of myocardial contractility in the investigation of cardiomyopathy and valvular heart disease. Because it accounts for wall thickness and pressure generation, wall stress is more a erload-independent than parame- ters such as fractional shortening or ejection fraction. Le ventric- ular stress can be calculated either globally or regionally. ere are three dierent regional stress calculations: radial, circumferential, and meridional, each of which is mutually orthogonal. In its sim- plest form, meridional stress is dened by the formula: stress = (pressure × radius) ÷ h (where h = wall thickness) (Fig. 5.42). is formula assumes spherical geometry, which obviously is not the case in the le ventricle. As such, while correlating with other measures of le ventricular stress, it may not truly represent the actual value. Regional stress can be calculated along any of the ventricular seg- ments using a similar equation for which the radius is independently determined for that segment rather than for the le ventricular cav- ity as a whole. Because of le ventricular–right ventricular interac- tion and changes in the radius of curvature of the ventricle, regional stress varies from apex to base and around the circumference of the le ventricle. Calculation of stress indexed to ventricular volume has been used as an index of ventricular performance in valvular heart disease and cardiomyopathy. In this instance, it is an additional renement of the determination of le ventricular reserve and ven- tricular compensation in either pressure or volume overload states. A nal highly detailed assessment of le ventricular contractility involves creation of a pressure volume loop, which provides load- independent information regarding ventricular contractility. is can be accomplished by exporting instantaneous volume data from automatically determined borders and combing the continuous vol- ume data with simultaneously determined pressure recordings. Clinicians have used Doppler spectral proles to evaluate global le ventricular function since the early 1970s. e earliest, concep- tually simplest, and still probably one of the more clinically use- ful methods for following le ventricular function with Doppler is to evaluate the time velocity integral (TVI) of the le ventricular out–ow tract or ascending aorta. Basically, the principle is that if the cross-sectional area of –ow is known, then the product of that cross-sectional area and the mean velocity of –ow equals the volu- metric –ow (Fig. 5.43). Typically, the area evaluated for determina- tion of systolic –ow, and hence global le ventricular performance, has been the le ventricular out–ow tract. e calculated stroke volume can be multiplied by the heart rate to obtain cardiac output. e principles and limitations of this measurement are further dis- cussed in Chapter 8. Although measurement of the actual out–ow tract area may be subject to signicant error, there are no commonly encountered dis- ease states in which the area of the out–ow tract would be expected to change over a short period. With this in mind, the out–ow tract area can be considered a constant over time in most patients. In this instance, the TVI is the only variable to change over time, and therefore calculation of this value alone can be used to track serial DOPPLER EVALUATION OF GLOBAL LEFT VENTRICULAR FUNCTION

Au: Please provide better quality art if available.

A

B

C

FIGURE 5.41. Continuous-wave Doppler imaging–derived left ventricular d P/ d t in three patients with varying degrees of left ventricular systolic dysfunction. A: Recorded in a patient with relatively mild systolic dysfunction and a d P/ d t of 967 mm Hg/s. B: Recorded in a patient with intermediate dysfunction. C: Recorded in a patient with severe systolic dysfunc- tion and a d P/ d t of 425 mm Hg/s. Either a reduced positive or negative d P /d t carries signicant prog- nostic implications. ere are contributors to le ventricular d P /d t in addition to intrinsic myocardial contractility. In the presence of marked mechanical dyssynchrony (as typied by le bundle branch block), d P /d t may be reduced, not due to intrinsically decreased

26

LWBK1670_C05_100-127.indd 120

5/15/18 11:27 AM