Cardiovascular responses in the seated position - impact of four anesthetic techniques

To elucidate the responses of the cardiovascular and renin angiotensin systems to assuming the seated position during general anesthesia, 24 healthy patients undergoing elective neurosurgical procedures were studied using radial arterial and thermistor-tipped pulmonary arterial catheter monitoring. Hemodynamic variables and plasma renin activity were measured in patients in a supine position and also while seated for 5 min while awake, and again after induction of general anesthesia (60% N₂O supplemented with pancuronium 0.1 mg/kg, intravenously). The patients were randomly divided into four groups of six subjects according to the anesthetic supplement administered - group 1: enflurane, 0.7% end-tidal; group 2: halothane, 0.4% end-tidal; group 3: Innovar, 0.1 ml/kg, intravenously; group 4, morphine, 0.5 mg/kg, intravenously. When the conscious subjects were placed in the seated position, heart rate (HR) increased 12%, mean systemic arterial pressure (SAP) increased 11%, systemic vascular resistance (SVR) increased 12%, and stroke volume indices (SVI) and pulmonary capillary wedge pressure (PCWP) decreased 11% and 22%, respectively. In groups 1 and 2, SAP decreased 27% and 22%, respectively, with induction of anesthesia but did not change in the seated position. Cardiac index (CI) decreased 20% and 19%, respectively; SVI decreased 30% and 33% with anesthesia, with further decrease in those functions in the seated position. Surgical stimulation returned SAP to control values, but CI and SVI remained below control values. In group 3, induction of anesthesia and placement in the seated position caused 22% and 25% reductions in SAP, respectively, 25% and 29% reductions in SVI, and 14% and 22% reductions in CI. In the seated position, PCWP and right atrial pressure (RAP) decreased 54% and 63%, respectively. Surgical stimulation returned HR, SAP, SVI, and SVR to control values, but PCWP and RAP remained unchanged. In group 4, stroke volume index decreased 20% with induction of anesthesia, but SAP, CI, and SVR remained unchanged. In the seated position, only SVI and CI decreased below control values (32% and 25%, respectively). Surgical stimulation resulted in a 15% increase in SAP and a 62% increase in SVR while CI remained depressed by 25%. No significant change in plasma renin activity was observed during the study. We conclude that among the anesthetic techniques studied, morphine-N₂O resulted in the least impairment of cardiovascular performance when patients were placed in the seated position before surgical stimulation.