Blood/gas partition coefficients of halothane, isoflurane and sevoflurane in horse blood.

BACKGROUND

Blood/gas partition coefficients (lambda(b/g)) for volatile agents in horse blood are reported for halothane but not for isoflurane and sevoflurane. We measured the lambda(b/g) of halothane, isoflurane and sevoflurane in the blood of fasted horses. The correlation with age, weight and some haematological and biochemical variables was studied. The temperature correction factor for isoflurane solubility was calculated.

METHODS

Twenty-four horses were randomly allocated to halothane (n=8), isoflurane (n=8) or sevoflurane (n=8). Blood samples were taken after 10 h' fasting. Calculation of lambda(b/g) was based on the measurement of anaesthetic partial pressures in blood at 37 degrees C, which was achieved with tonometer equilibration and headspace gas chromatography.

RESULTS

Mean lambda(b/g) was 1.66 (SD 0.06) for halothane, 0.92 (0.04) for isoflurane, and 0.47 (0.03) for sevoflurane. The lambda(b/g) values were all significantly lower than in humans (P<0.001). No correlation was found between lambda(b/g) and weight, age, haematocrit, plasma triglycerides, cholesterol or total bilirubin. The change in isoflurane solubility per 1 degrees C temperature increase was -2.63 (0.13)%.

CONCLUSION

The lambda(b/g) values of halothane, isoflurane and sevoflurane in fasted horses are significantly lower than those reported in humans. The lambda(b/g) for halothane in this study agrees with values reported in the literature but a positive correlation with plasma triglycerides could not be confirmed. Knowledge of lambda(b/g) can refine models of anaesthetic uptake.