IS THE END-TIDAL PARTIAL-PRESSURE OF ISOFLURANE A GOOD PREDICTOR OF ITS ARTERIAL PARTIAL-PRESSURE

End-tidal partial pressure of isoflurane (PE'(iso)) may be used as a measure of anaesthetic depth. During uptake, an arterial partial pressure (Pa(iso)) which is considerably less than PE'(iso) (Pa(iso)/PE'(iso) << 1) leads to underestimation of depth of anaesthesia and, during elimination, PE'(iso)/Pa(iso) << 1 will lead to an overestimation of anaesthetic depth. We measured Pa(iso)/PE'(iso) during a 60-min uptake period of 1 % isoflurane and PE'(iso)/Pa(iso) during the subsequent 60-min elimination period in 26 patients (age 13-88 yr, ASA I-III) undergoing various surgical procedures. After 15 min of isoflurane uptake, Pa(iso)/PE'(iso) of 26 patients was mean 0.78 (SD 0.10) and this increased only marginally at 60 min (0.79 (0.09)), whereas during elimination, PE'(iso)/Pa(iso) was in the range 0.79 (0.14)-0.83 (0.11). Predictability of Pa(iso) in a given patient is hindered by the high SD of Pa(iso)/PE'(iso) and PE'(iso)/Pa(iso), but it may be improved by taking into account age, ASA physical status category, vital capacity, inspired minus end-tidal isoflurane partial pressure and arterial minus end-tidal carbon dioxide partial pressure during uptake; and obesity, end-tidal isoflurane partial pressure and arterial minus end-tidal carbon dioxide partial pressure during elimination. However, even with multiple regression analysis (to account for the various possible variables), clinically useful prediction of Pa(iso)/PE'(iso) and PE'(iso)/Pa(iso) in a particular patient is not possible (residual SD 0.084 and 0.113, respectively).