RECOVERY OF HORSES FROM INHALATION ANESTHESIA

To study behavioral and cardiopulmonary characteristics of horses reco vering from inhalation anesthesia, 6 nonmedicated horses were anesthet ized under laboratory conditions on 3 different days, with either halo thane or isoflurane in O2. Anesthesia was maintained at constant dose (1.5 times the minimum alveolar concentration [MAC]) of halothane in O 2 for 1 hour (H1), halothane in O2 for 3 hours (H3), or isoflurane in O2 for 3 hours (13). The order of exposure was set up as a pair of Lat in squares to account for horse and trial effects. Circulatory (arteri al blood pressure and heart rate) and respiratory (frequency, Pa(CO2), Pa(O2), pHa) variables were monitored during anesthesia and for as lo ng as possible during the recovery period. End-tidal percentage of the inhaled agent was measured every 15 seconds by automated mass spectro metry, then by hand-sampling after horses started moving. Times of rec overy events, including movement of the eyelids, ears, head, and limbs , head lift, chewing, swallowing, first sternal posture and stand atte mpts, and the number of sternal posture and stand attempts, were recor ded. The washout curve or the ET ratio (end-tidal percentage of the in haled agent at time t to end-tidal percentage of the inhaled agent at the time the anesthesia circuit was disconnected from the tracheal tub e) plotted against time was similar for Hl and H3. The slower, then fa ster (compared with halothane groups) washout curve of isoflurane was explainable by changes in respiratory frequency as horses awakened and by lower blood/gas solubility of isoflurane. The respiratory depressa nt effects of isoflurane were marked and were more progressive than th ose for halothane at the same 1.5 MAC dose. During the first 15 minute s of recovery, respiratory frequency for group-I3 horses increased sig nificantly (P < 0.05), compared with that for the halothane groups. Fo r all groups, arterial blood pressure increased throughout the early r ecovery period and heart rate remained constant. Preanesthesia tempera ment of horses and the inhalation agent used did not influence the tim e of the early recovery events (movement of eyelids, ears, head, and l imbs), except for head lift. For events that occurred at anesthetic en d-tidal percentage < 0.20, or when horses were awake, temperament was the only factor that significantly influenced the nature of the recove ry (chewing P = 0.04, extubation P = 0.001, first stand attempt P = 0. 008, and standing P = 0.005). The quality of the recoveries did not di ffer significantly among groups (H1, H3, I3) or horses; however 5 of 6 horses recovering from the H1 exposure had ideal recovery. During rec overy, the anesthetic end-tidal percentage did not differ significantl y among groups. However, when concentrations were compared on the basi s of anesthetic potency (ie, MAC multiple) a significantly (P < 0.05) lower MAC multiple of isoflurane was measured for the events ear movem ent, limb movement, head lift, and first attempt to sternal posture, c ompared with that for horses given halothane, indicating that isoflura ne may be a more-potent sedative than halothane in these horses.