Effect of acute endotoxemia on analog estimates of mean systemic pressure

Dynamic estimates of mean systemic pressure based on a Guytonian analog model (Pmsa) appear accurate under baseline conditions but may not remain so during septic shock because blood volume distribution and resistances between arterial and venous beds may change. Thus, we examined the effect of acute endotoxemia on the ability of Pmsa, estimated from steady-state cardiac output, right atrial pressure, and mean arterial pressure, to reflect our previously validated instantaneous venous return measure of mean systemic pressure (Pmsi), derived from beat-to-beat measures of right ventricular stroke volume and right atrial pressure during positive pressure ventilation. We studied 6 splenectomized pentobarbital-anesthetized close chested dogs. Right ventricular stroke volume was measured by a pulmonary arterial electromagnetic flow probe. Instantaneous venous return measure of mean systemic pressure and Pmsa were calculated during volume loading and removal (±100-mL bolus increments×5) both before (control) and 30 minutes after endotoxin infusion (endo). Cardiac output increased (2628±905 vs 3560±539 mL/min; P<.05) and mean arterial pressure decreased (107±16 vs 56±12 mm Hg; P<.01) during endo. Changes in Pmsi and Pmsa correlated during both control and endo (r2=0.7) with minimal bias by Bland-Altman analysis (mean difference±95% confidence interval, 0.47±5.04 mm Hg). We conclude that changes in Pmsa accurately tracts Pmsi under both control and endo.