ObjectivesTo investigate the effects of Nomega-mono-methyl-L-arginine (L-NMMA), an inhibitor of nitric oxide synthesis, on hemodynamics, oxygen transport, and regional blood flow in an ovine model of hyperdynamic sepsis and to compare these effects with the responses to norepinephrine.DesignProspective, nonrandomized, controlled experimental study with repeated measures.SettingInvestigational intensive care unit at a university medical center.SubjectsTwenty-five female, healthy, adult sheep of the Merino breed, divided into three groups: nine control sheep; eight sheep treated with L-NMMA; and eight sheep treated with norepinephrine.InterventionsAll sheep were chronically instrumented. After a 5-day recovery period, a continuous infusion of live Pseudomonas aeruginosa (2.5 times 106colony-forming units/min) was started and maintained for the remainder of the experiment. After 24 hrs of sepsis, eight sheep received L-NMMA (7 mg/kg/hr), eight sheep received norepinephrine, and nine sheep received the vehicle alone (0.9% saline). The norepinephrine dosage was continuously and individually adjusted to achieve the same increase in blood pressure as was observed in a matched sheep of the L-NMMA group.Measurements and Main ResultsAfter 24 hrs of sepsis, all sheep developed a hyperdynamic circulatory state with increased cardiac indices and reduced arterial pressures, and systemic vascular resistances. L-NMMA reversed the hyperdynamic circulation, causing an increase in arterial pressure by peripheral vasoconstriction. Norepinephrine led to an increase in blood pressure by augmenting cardiac indices, leaving the systemic vascular resistance unaffected. The norepinephrine dose needed to keep the blood pressure high had to be continuously increased, reflecting the reduced vascular responsiveness to catecholamines during sepsis. Renal blood flow remained unaffected by all treatment forms. Norepinephrine and L-NMMA led to a dramatic increase in urine production. Blocking the nitric oxide synthase with L-NMMA did not interfere with the host's pulmonary ability to clear bacteria, nor did treatment with norepinephrine.ConclusionsBlocking nitric oxide synthase had a marked vasoconstrictive effect. Both norepinephrine and L-NMMA increased arterial pressure without reducing renal blood flow, leading to an improved renal function.(Crit Care Med 1996; 24:835-844)