MYOCARDIAL DEPRESSANT EFFECT OF NITROUS-OXIDE AFTER VALVE SURGERY

We have studied the cardiovascular effects of 50% nitrous oxide after cardiopulmonary bypass in 14 patients under-going valve surgery. All p atients received morphine as the principal anaesthetic. Nitrous oxide administration for 5 min caused a decrease in mean arterial pressure f rom 82+/-10 to 71+/-12.7 mmHg (P<0.001), cardiac index (2.8+/-0.5 to 2 .4+/-0.5 litres min-1 m-2, P<0.01), heart rate (104+/-17 to 99+/-18 be ats min-1, P<0.05), left ventricular stroke work index (29.4+/-8.1 to 22+/-8.7 gm-m beat-1 mm-1, P<0.001), stroke volume (45.3+/-11.6 to 40/-12.8 ml beat-1, P<0.05) and an increase in pulmonary vascular resist ance from 106.4+/-53.9 to 143.9+/-81.0 dynes s cm-5 (P<0.01) and right atrial pressure (1.42+/-2.09 to 1.71+/-2.21 mmHg, P<0.05). There was no change in systemic vascular resistance. When nitrous oxide was disc ontinued all the parameters started to recover within 3 min. Mean arte rial pressure returned to control value in 5 min, but cardiac index an d pulmonary vascular resistance returned to control value in 10 min. O ur results suggest a direct myocardial depression and that the use of nitrous oxide is not recommended immediately after valve surgery and c ardiopulmonary bypass.