Area of the aortic pressure-flow loop measures energy storage by the proximal arteries.

We show that power measured by the area of the pressure-flow loop (APQ) is directly proportional to reactive power [Im(W)], which measures pulsatile energy storage in the proximal arteries. Analytically, Im(W) differs from APQ by K.2 pi, where K is a postulated constant of proportionality. To determine the value of K, 338 heartbeats obtained from 8 dogs under a wide range of hemodynamic conditions were analyzed. The relationship between APQ and Im(W) remained highly linear (r = 0.98), and the pooled value of K was -1.19 +/- 0.01. For the individual dogs, values of K differed at most by +/- 9% from its average value. There was also a small offset in the relationship between APQ and Im(W), probably caused by truncating the calculation of Im(W) at the 10th harmonic. Ignoring the small differences in the values of K and the offset, we found that APQ still proved to be an excellent predictor of Im(W). We also found that APQ was sensitive to changes in heart rate, left ventricular stroke output, and peripheral resistance. This reflects the dependence of pulsatile energy storage on these variables and indicates that APQ cannot serve as an index for changes in just the reactive properties of the proximal arteries.