COMPARISON OF THE RESPIRATORY AND SYSTEMIC KINETICS OF NITROUS-OXIDE IN THE SHEEP

Background. To determine whether discrepancies in views on the kinetic s of nitrous oxide (N2O) may have a methodological basis, we compared its kinetics, simultaneously, in the respiratory system and systemic c irculation. Methods. Six merino ewes (40-50 Kg) were previously prepar ed with catheters in the pulmonary artery and aorta. The animals were anaesthetised with thiopentone then ventilated on a mixture of 70% N2O , 1% halothane in oxygen for 4 h. Simultaneous serial arterial and pul monary arterial blood samples were assayed for N2O by gas chromatograp hy and respiratory gases were monitored continuously by mass spectrome try. Results. Marked differences were observed between the respiratory and systemic kinetics of N2O uptake. While the expired I inspired N2O concentration ratio rose within 30 min to a value close to unity, the pulmonary arterial/arterial blood N2O concentration ratio did not rea ch unity during the 4 h of each study, but approached a constant rate of uptake shown by the mean ratio of 0.94 (SD 0.01) from about 2 h onw ard. Conclusions. Discrepancies in fluid flow between respiratory gas and the cardiovascular systems, a concentration effect of N2O in the l ungs, the relative solubility of N2O in blood and tissues, and ventila tion/perfusion inequalities all may contribute to the observed differe nces. The ongoing uptake is consistent with persistent extrapulmonary losses. There remains a need for experimental data on the pharmacokine tics of N2O. Unequivocal studies on the disposition of N2O can be unde rtaken only by using direct measurement of fluxes of N2O across releva nt organs or tissues.