Part II: Radionuclide evaluation of aortic regurgitation

Abstract

The timing for valve replacement in patients with aortic regurgitation remains a complex clinical problem. Rest radionuclide angiography measurement of ejection fraction is a simple informative study to help evaluate the appropriate timing for valve replacement in the asymptomatic patients or those with mild symptoms. In patients with normal ejection fractions the disease probably has not yet evolved to the phase in which valve replacement is essential. If the ejection fraction is mildly depressed (0.40 to 0.49) the time is right for intervention. By the time the ejection fraction falls to < 0.40 the left ventricle is likely damaged and unlikely to regain normal function. If the patient has severe symptoms with maximal medical therapy, surgery is indicated no matter what the ejection fraction. The latter situation can arise especially when aortic regurgitation evolves over a short period, as might be the case in patients with bacterial endocarditis. A single ejection fraction measurement is not as reliable as serial studies. If, for example, the ejection fraction (under similar circumstances) falls from the ≥0.50 range to the 0.40 to 0.49 range, the physician should be alerted to the possibility that the left ventricle is deteriorating, and surgery should be considered. It should be understood that multiple hemodynamic factors in aortic regurgitation can alter the ejection fraction and could limit its use as the sole measure of left ventricular performance. Other systolic or diastolic parameters cannot be relied on in isolation as an indication or contraindication for aortic valve replacement. The exercise ejection fraction response reflects the total stroke volume and does not distinguish between regurgitant flow and forward flow. It is therefore possible to observe a decrease in ejection fraction in association with an increase in forward stroke volume during exercise as a result of an increase in heart rate and a decrease in peripheral resistance. Accordingly, it is not appropriate to compare the ejection fraction during exercise in aortic insufficiency with the expected response of the normal ventricle. Exercise position (sitting vs. supine) affects loading conditions and ejection fraction response. 44Because of the complexity of the exercise ejection fraction response, it is not clear that there is a role for exercise ejection fraction measurements in determining the appropriate time for aortic valve replacement. Criteria based on supine exercise may not be applicable to studies in the upright position. Other markers of left ventricular performance, including end-systolic volumes and pressure-volume relationships, also have been examined. These parameters are preload independent and incorporate the effects of afterload. End-ejection pressure-volume ratios appear to be reasonable predictors of left ventricular contractile performance in chronic aortic regurgitation. 45 The continued problem of the timing of aortic valve replacement inspires the hope that serial measurement left ventricular function with exercise may have some predictive power. Parameters other than ejection fraction, such as regurgitant fraction and end-systolic and end-diastolic volume response, also may be useful. Careful consideration of aortic valve replacement should be given to patients with deterioration of these other radionuclide angiographic parameters, particularly when associated with a decrease in functional performance and worsening of symptoms. Nevertheless, only the resting LVEF has been shown to correlate with survival after valve replacement. Radionuclide studies provide a means to assess the change in left ventricular function after valve replacement. It has been observed that soon after valve replacement LVEF decreases, likely due to acute reduction in preload and increase in heart rate. This reduction should not be misconstrued as reduction in left ventricular performance, and under these circumstances end-systolic volume-endsystolic pressure may be a more useful test of left ventricular function. However, over the ensuing weeks to months ejection fraction tends to normalize in those patients in whom surgery was appropriately timed. The frequent occurrence of late postoperative deaths due to chronic congestive heart failure (78% in the National Institutes of Health study 46) underscores the importance of continued serial radionuclide evaluation of left ventricular performance in the improved methods to better time when aortic valve replacement should be performed. Operated and unoperated patients receiving chronic vasodilator therapy also benefit from serial evaluation of ventricular function to assess the response to therapy and define prognosis.