Alternative Approaches to the Assessment of the Systemic Circulation and Left Ventricular Performance: A...

The purpose of this article is to examine the systemic circulation and left ventricular (LV) performance by alternative, nonconventional approaches: systemic vascular conductance (G ) and the head-capacity relation (ie, the relation between LV pressure and cardiac output), respectively; in so doing, we aspired to present a novel and improved interpretation of integrated cardiovascular function.

In 16 open-chest, anaesthetized pigs, we measured LV pressure (P ), central aortic pressure (P ), and central venous pressure (P ) and aortic flow (Q ). We calculated heart rate (HR), stroke volume, cardiac index (CI = cardiac output/body weight), mean P_LV ( $$ , and the average arteriovenous pressure difference ( $$ ); G  = CI/( $$ ). We studied the effects of changing loading conditions with the administration of phenylephrine (Δ $$ ≥ +25 mm Hg), isoproterenol (ΔHR ∼+25%), sodium nitroprusside (Δ $$ ≥ -25 mm Hg), and proximal aortic constriction (to maximize developed P and minimize Q ).

Sodium nitroprusside and isoproterenol increased G compared with phenylephrine and constriction. A maximum head-capacity curve was derived from pooled data using nonlinear regression on the maximum $$ values in Q bins 12.5 mL/min/kg wide. The head-capacity relation and the plots of conductance were combined using CI as a common axis, which illustrated that CI is the output of the heart and the input of the circulation.

Thus, at a given CI, G determines the driving pressure and, thereby, P . We also demonstrated how decreases in G compensate for arterial hypotension by restoring the arteriovenous pressure difference and arterial pressure.