Alternative Approaches to the Assessment of the Systemic Circulation and Left Ventricular Performance: A Proof-of-Concept Study

BACKGROUND: The purpose of this article is to examine the systemic circulation and left ventricular (LV) performance by alternative, nonconventional approaches: systemic vascular conductance (G(SV)) 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. METHODS: In 16 open-chest, anaesthetized pigs, we measured LV pressure (P(LV)), central aortic pressure (P(Ao)), and central venous pressure (P(CV)) and aortic flow (Q(Ao)). We calculated heart rate (HR), stroke volume, cardiac index (CI = cardiac output/body weight), mean P(LV) ([Formula: see text] , and the average arteriovenous pressure difference ([Formula: see text]); G(SV) = CI/([Formula: see text]). We studied the effects of changing loading conditions with the administration of phenylephrine (Δ [Formula: see text] ≥ +25 mm Hg), isoproterenol (ΔHR ∼+25%), sodium nitroprusside (Δ [Formula: see text] ≥ −25 mm Hg), and proximal aortic constriction (to maximize developed P(LV) and minimize Q(Ao)). RESULTS: Sodium nitroprusside and isoproterenol increased G(SV) compared with phenylephrine and constriction. A maximum head-capacity curve was derived from pooled data using nonlinear regression on the maximum [Formula: see text] values in Q(Ao) 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. CONCLUSIONS: Thus, at a given CI, G(SV) determines the driving pressure and, thereby, P(Ao). We also demonstrated how decreases in G(SV) compensate for arterial hypotension by restoring the arteriovenous pressure difference and arterial pressure.