It is axiomatic that, except for transient imbalances, right ventricular (RV) and LV output must be equal. Since SDf and/or DDf rarely, if ever, impair the ventricles symmetrically, their earliest effect is to unbalance the circulation, that is, to lower the output of one ventricle more than the other.10–11
An example is shown in Figure 1
, where SDf and/or DDf transforms a normal LV output curve (Starling curve), curve I, into curve II. With no immediate change in LV end-diastolic pressure (LVEDP), LV output drops,11–
from 5 to 2 L/min, point x. Meanwhile the RV continues pumping 5 L/min (right-heart curves are omitted from Fig 1
for clarity). The LV is thus unable to empty the pulmonary circuit as fast as the RV fills it, and pulmonary blood volume expands. This shifts the normal pulmonary venous return12
(Guyton) curve, A, upward to B, re-setting the Starling-Guyton intersection to a higher LVEDP and a partially compensating LV output, 4.5 L/min (intersection II-B). Since total blood volume is fixed, the volume expanding the pulmonary circuit is equally and simultaneously removed from the systemic circuit. This shifts the systemic Guyton curve and RV end-diastolic pressure (RVEDP) downward. Thus, by Starling’s law of the heart and within a few heartbeats, the altered Guyton curves simultaneously raise the lowered LV output while lowering the normal RV output until biventricular output re-equilibrates at an intermediate CO (intersection II-B).10–11
Low CO also evokes a reflex sympathetic response proportional to its hypotensive effect, the cardiostimulatory component tending to shift the Starling curve back toward normal.