title: Is the end-tidal partial pressure of isoflurane a good predictor of its arterial partial pressure?
abstract: End-tidal partial pressure of isoflurane (PE'iso) may be used as a measure of anaesthetic depth. During uptake, an arterial partial pressure (Paiso) which is considerably less than PE'iso (Paiso/PE'iso much less than 1) leads to underestimation of depth of anaesthesia and, during elimination, PE'iso/Paiso much less than 1 will lead to an overestimation of anaesthetic depth. We measured Paiso/PE'iso during a 60-min uptake period of 1% isoflurane and PE'iso/Paiso 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, Paiso/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/Paiso was in the range 0.79 (0.14)-0.83 (0.11). Predictability of Paiso in a given patient is hindered by the high SD of Paiso/PE'iso and PE'iso/Paiso, 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 Paiso/PE'iso and PE'iso/Paiso in a particular patient is not possible (residual SD 0.084 and 0.113, respectively).
authors.affiliation: Department of Anaesthesia, University of Basel/Kantonsspital, Switzerland.
authors.forename: F J
authors.initials: FJ
authors.lastname: Frei
authors.forename: A M
authors.initials: AM
authors.lastname: Zbinden
authors.forename: D A
authors.initials: DA
authors.lastname: Thomson
authors.forename: H U
authors.initials: HU
authors.lastname: Rieder
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000762:Anesthesia Recovery Period; D000769:Anesthesia, Inhalation; D001158:Arteries; D002245:Carbon Dioxide / Q000378:metabolism; D005260:Female; D006801:Humans; D007530:Isoflurane / Q000097:blood / Q000493:pharmacokinetics*; D008297:Male; D008875:Middle Aged; D010313:Partial Pressure; D013990:Tidal Volume / Q000502:physiology
chemical_list: D002245:Carbon Dioxide; D007530:Isoflurane