Arterial to inspired partial pressure ratio of halothane, isoflurane, sevoflurane and desflurane in rats

The inspired partial pressure of an anaesthetic is often used as an index o f arterial partial pressure in small animal experiments. We have investigat ed the influence of anaesthetic solubility on the ratio of arterial to insp ired partial pressure in 24 rats, allocated randomly to receive halothane, isoflurane or desflurane at four different inspired concentrations. The art erial partial pressure of the volatile agent was measured by two-stage head space analysis using a gas chromatograph calibrated with the same gas used to calibrate the Datex Capnomac that measured the inspired concentration. M ean values of arterial to inspired ratio at the lowest concentrations were 0.60 (95% confidence intervals 0.50, 0.71) for 0.8% halothane, 0.54 (0.38, 0.69) for 0.8% isoflurane, 0.72 (0.59, 0.86) for 1.5% sevoflurane and 0.71 (0.54, 0.87) for 4% desflurane. Analysis of variance showed a significant e ffect of anaesthetic agent (P = 0.008) on the arterial to inspired ratio. T hus volatile anaesthetic agents do not demonstrate a fixed arterial to insp ired ratio in rats.