Left ventricular performance and pulmonary circulation following addition of nitrous oxide to morphine during coronary-artery surgery

The effects of nitrous oxide on ventricular performance and pulmonary circulation were studied in 12 patients with angiographically demonstrated coronary-artery disease and normal ventricular contractility who had received 2 mg/kg morphine intravenously. Seventeen studies were performed intraoperatively, five before and 12 after cardiopulmonary bypass and myocardial revascularization. Recordings were obtained during oxygen breathing and during nitrous oxide administration. Fifty per cent nitrous oxide significantly decreased mean arterial pressure (P less than 0.05), cardiac index (P less than 0.01), stroke index (P less than 0.01), left ventricular stroke work index (P less than 0.01), peak left ventricular dP/dt (P less than 0.05) and dP/dt/P (P less than 0.01), and heart rate-systolic arterial pressure product (P less than 0.01). Mean pulmonary arterial pressure (P less than 0.05), pulmonary artery occluded pressure (P less than 0.01), left ventricular end-diastolic pressure (P less than 0.01) and pulmonary vascular resistance (P less than 0.05) increased. Heart rate, right atrial pressure and systemic vascular resistance remained unchanged. When nitrous oxide was discontinued, all variables returned to control except mean pulmonary arterial pressure and pulmonary vascular resistance. Responses were similar before and after cardiopulmonary bypass and myocardial revascularization. These findings suggest that nitrous oxide depresses left ventricular performance when administered intraoperatively to patients who have received large doses of morphine for coronary-artery surgery. Nitrous oxide also increases pulmonary vascular resistance, possibly via alpha-adrenergic stimulation.