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.