[The relationship between respiratory function and the change in end-tidal nitrous oxide concentration]

The aim of the present study is to clarify the relationship between respiratory function and the rate of change in alveolar anesthetic concentration. We measured the concentration of end-tidal nitrous oxide (N2O) when 50% N2O was administered to 15 patients of ASA I possessing normal respiratory function during the course of propofol-100% oxygen anesthesia. All patients were ventilated at a rate of 8-10 ml.kg-1 x 8 times per minute using a conventional anesthetic ventilator with semi-closed circuit and 4 l.min-1 inflow of fresh gas. Arterial CO2 partial pressure was maintained at 36.2 +/- 1.8 mmHg and no significant circulatory change was observed while N2O was administered. The rate of increase of end-tidal N2O concentration in poor FEV1.0/FVC% group was significantly slower than that in high FEV1.0/FVC% group, while there was no relation between %VC and the end-tidal N2O concentration change. Since N2O is an inhaled anesthetic, it is well considered that the effect of FEV1.0/FVC% may be observed in other inhaled anesthetic although the magnitude of the effect may vary. The present result suggests that respiratory function, especially FEV1.0/FVC%, is an important factor affecting the rate of change in alveolar anesthetic concentration and, in lower FEV1.0/FVC% group, it takes more time to achieve the intended alveolar concentration.