LEFT-VENTRICULAR END-SYSTOLIC ELASTANCE IS INCORRECTLY ESTIMATED BY THE USE OF STEPWISE AFTERLOAD VARIATIONS IN CONSCIOUS, UNSEDATED, AUTONOMICALLY INTACT DOGS

Background End-systolic elastance (E(es)), the slope parameter of the end-systolic pressure (ESP)-volume (ESV) relation (ESPVR), is usually estimated in patients by producing stepwise, steady-state pharmacological afterload variations and collecting one ESP-ESV point from each step. The ESPVR is then constructed by fitting a linear equation to these points. In sedated, autonomically blocked dogs, it has been shown that when one point from control, one point from a state of increased afterload, and one point from a state of decreased afterload are used, the resulting E(es) incorrectly estimates true E(es), defined as the slope of the ESPVR obtained by transient vena caval occlusion. We investigated if this was also the case in unsedated, autonomically intact dogs when the points used belonged to steady states of progressively decreasing or progressively increasing afterload pressure. Methods and Results In 10 conscious dogs instrumented with left ventricular (LV) endocardial sonomicrometers to measure LV volume, a LV pressure transducer, and an inferior vena caval (IVC) occluder, two protocols were carried out on separate days. In each protocol, an ESPVR was generated by IVC occlusion in the control state and in two steady-state levels of afterload change produced by stepwise infusion of nitroprusside (protocol 1, afterload decrease) and angiotensin II (protocol 2, afterload increase). In each protocol, steady-state ESP-ESV data points were averaged from the control state and from each level of afterload variation. Linear equations were fitted to the three steady-state points from each protocol, and the estimated E(es) values obtained (E(es)EST) were compared with the E(es) values of the control ESPVRs obtained by IVC occlusion (E(es)TRUE). In protocol 1, E(es)EST underestimated E(es)TRUE by about 16% (E(es)EST, 6.49+/-1.55 mm Hg/mL; E(es)TRUE, 7.48+/-1.29 mm Hg/mL; P<.02). In protocol 2, E(es)EST overestimated E(es)TRUE by about 37% (E(es)EST, 9.99+/-3.97 mm Hg/mL; E(es)TRUE, 6.43+/-3.88 mm Hg/mL; P<.007). Conclusions In conscious, autonomically intact dogs, the use of stepwise, steady-state afterload variations to obtain ESP-ESV data points to construct the ESPVR incorrectly estimates E(es). In the case of afterload reduction, E(es)TRUE is underestimated an average of 16.3%, and in the case of afterload increase, E(es)TRUE is overestimated an average of 37.1%. These errors should be taken into account when interpreting clinical studies using this methodology.