ASSESSMENT OF VENTRICULAR FUNCTION
Plotting cardiac output or stroke volume against preload is useful in evaluating pathological states and understanding drug therapy. Normal right and left ventricular function curves are shown in Figure 20–6.
Ventricular pressure–volume diagrams are use-ful because they dissociate contractility from both preload and afterload. Two points are identified on such diagrams: the end-systolic point (ESP) and the end-diastolic point (EDP) (Figure 20–7). ESP is reflective of systolic function, whereas EDP is more reflective of diastolic function. For any given con-tractile state, all ESPs are on the same line (ie, the relationship between end-systolic volume and end-systolic pressure is fixed).
The change in ventricular pressure over time dur-ing systole (dP/dt) is defined by the first derivative of the ventricular pressure curve and is often used as a measure of contractility. Contractility is directly proportional to dP/dt, but accurate measurement of this value requires a high-fidelity (“Millar”) ven-tricular catheter; however, it can be estimated with echocardiography. Although arterial pressure trac-ings are distorted due to properties of the vascular tree, the initial rate of rise in pressure (the slope) can serve as a rough approximation; the more proxi-mally the arterial line catheter is located in the arte-rial tree, the more accurate the extrapolation will be. The usefulness of dP/dt is also limited in that it may be affected by preload, afterload, and heart rate.
The ventricular ejection fraction (EF), the fraction of the end-diastolic ventricular volume ejected, is the most commonly used clinical measurement of systolic function. EF can be calcu-lated by the following equation:
where EDV is left ventricular diastolic volume and ESV is end-systolic volume. Normal EF is
approximately 0.67 ± 0.08. Measurements can be made preoperatively from cardiac catheterization, radionucleotide studies, or transthoracic (TTE) or transesophageal echocardiography (TEE).Pulmonary artery catheters with fast-response thermistors allow measurement of the right ventric-ular EF. Unfortunately, when pulmonary vascular resistance increases, decreases in right ventricular
EF may reflect afterload rather than contractility. Left ventricular EF is not an accurate measure of ventricular contractility in the presence of mitral insufficiency.
Left ventricular diastolic function can be assessed clinically by Doppler echocardiography on a transthoracic or transesophageal examination. Flow velocities are measured across the mitral valve during diastole. Three patterns of diastolic dysfunction are generally recognized based on isovolumetric relaxation time, the ratio of peak early diastolic flow (E) to peak atrial sys-tolic flow (A), and the deceleration time (DT) of E (DTE) (Figure 20–8). Tissue Doppler is frequently used to distinguish “pseudonormal” from normal diastolic function. Tissue Doppler is also an excel-lent way to detect “conventional” diastolic dysfunc-tion. An e’ wave peak velocity of less than 8 cm/sec is associated with impaired diastolic function. An E/e’ wave ratio that is greater than 15 is consistent with elevated left ventricular end-diastolic pressure (Figure 20–9).