Dissociation of mixed venous oxygen saturation and cardiac index during opioid induction

Abstract

Study Objective: To evaluate the independent effects of opioid induction and paralysis on changes in mixed venous oxygen saturation (Sv̄O 2). Design: Prospective, with each patient serving as his or her own control. Setting: Cardiac operating rooms of a major U.S. teaching hospital. Patients: 15 elective patients for aortocoronary bypass graft surgery. Interventions: Prior to induction with sufentanil 2 to 3 μg/kg (t 1), after recovery from succinylcholine 1 to 1.5 mg/kg (t 2), and after subsequent paralysis with vecuronium (t 3), we recorded cardiac index (CI), arterial oxygen saturation (SaO 2), and Sv̄O 2, then calculated oxygen consumption (V̇O 2) and arterial oxygen content (C aO 2). Measurements and Main Results: Sv̄O 2 increased from 75% at t 1 to 82% at t 2 ( p = 0.03) and to 83% at t 3 ( p = 0.006). CI decreased from 3.1 L/min/m 2 at t 1 to 2.5 L/min/m 2 at t 2 ( p = 0.023) and to 2.1 L/min/m 2 at t 3 (t 1 vs. t 3, p = 0.019; t 2 vs. t 3 p = NS). Calculated V̇O 2 decreased from 2.88 ml O 2/min/kg at t 1 to 2.09 ml O 2/min/kg at t 2 ( p = 0.03) and to 1.87 ml O 2/min/kg at t 3 (t 1 vs. t 3, p = 0.002; t 2 vs. t 3 p = 0.027). C aO 2 remained unchanged. Conclusions: Opioid anesthesia, not paralysis, increases Sv̄O 2. Most of the decrease in V̇O 2 occurs from anesthesia, not paralysis. The direct relationship between Ci and Sv̄O 2 no longer holds upon induction of anesthesia. Parallel changes in CI cannot be inferred based on Sv̄O 2 alone.