American journal of veterinary research1993; 54(10); 1693-1702;

Recovery of horses from inhalation anesthesia.

Whitehair, KJ
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Steffey, EP
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Willits, NH
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Woliner, MJ

Abstract: To study behavioral and cardiopulmonary characteristics of horses recovering from inhalation anesthesia, 6 nonmedicated horses were anesthetized under laboratory conditions on 3 different days, with either halothane or isoflurane in O2. Anesthesia was maintained at constant dose (1.5 times the minimum alveolar concentration [MAC]) of halothane in O2 for 1 hour (H1), halothane in O2 for 3 hours (H3), or isoflurane in O2 for 3 hours (I3). The order of exposure was set up as a pair of Latin squares to account for horse and trial effects. Circulatory (arterial blood pressure and heart rate) and respiratory (frequency, PaCO2, PaO2, pHa) variables were monitored during anesthesia and for as long as possible during the recovery period. End-tidal percentage of the inhaled agent was measured every 15 seconds by automated mass spectrometry, then by hand-sampling after horses started moving. Times of recovery events, including movement of the eyelids, ears, head, and limbs, head lift, chewing, swallowing, first sternal posture and stand attempts, and the number of sternal posture and stand attempts, were recorded. The washout curve or the ET ratio (end-tidal percentage of the inhaled agent at time t to end-tidal percentage of the inhaled agent at the time the anesthesia circuit was disconnected from the tracheal tube) plotted against time was similar for H1 and H3. The slower, then faster (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 depressant effects of isoflurane were marked and were more progressive than those for halothane at the same 1.5 MAC dose. During the first 15 minutes of recovery, respiratory frequency for group-I3 horses increased significantly (P < 0.05), compared with that for the halothane groups. For all groups, arterial blood pressure increased throughout the early recovery period and heart rate remained constant. Preanesthesia temperament of horses and the inhalation agent used did not influence the time of the early recovery events (movement of eyelids, ears, head, and limbs), except for head lift. For events that occurred at anesthetic end-tidal percentage < 0.20, or when horses were awake, temperament was the only factor that significantly influenced the nature of the recovery (chewing P = 0.04, extubation P = 0.001, first stand attempt P = 0.008, and standing P = 0.005).(ABSTRACT TRUNCATED AT 400 WORDS)

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Publication Date: 1993-10-01 PubMed ID: 8250395

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The study focused on analyzing the recovery behavior and cardiopulmonary features of horses after inhalation anesthesia. A total of six horses were anesthetized using different gas mixtures, and the researchers closely monitored parameters such as circulatory and respiratory variables, along with the recovery times for various events like movements and regaining consciousness.

Methodology and Experiment Setup

The experiment involved six nonmedicated horses. They were anesthetized on three different occasions under laboratory conditions, using either halothane or isoflurane in oxygen.
There were three possible anesthesia scenarios: using halothane in oxygen for one hour (H1), halothane in oxygen for three hours (H3), or isoflurane in oxygen for three hours (I3).
A Latin square design was implemented to handle the effects of different horses and trial runs efficiently.
Circulatory and respiratory parameters were monitored regularly, as was the end-tidal percentage of the inhaled agent through automated mass spectrometry and manual sampling when the horses began moving.
Times of recovery events, from eyelid movement to stand attempts, were recorded meticulously.

Results and Findings

The researchers discovered that the washout curve or the ET ratio – comparing the end-tidal percentage of the inhaled agent at time t to the end-tidal percentage of the inhaled agent at the time the anesthesia circuit was disconnected from the horse’s tracheal tube – demonstrated similar patterns for H1 and H3.
The slower, then faster washout curve for isoflurane as compared to halothane could be attributed to changes in respiratory frequency as the horses awakened and the lower blood/gas solubility of isoflurane.
Isoflurane had a considerably pronounced and gradual respiratory depressant effect compared to halothane, evident by the significant increase in respiratory frequency for group-I3 horses during the initial 15 minutes of recovery.
The arterial blood pressure demonstrated progressive increment throughout the initial recovery period while the heart rate remained largely constant for all the groups.
The type of inhalation agent used or the preanesthesia temperament of the horses didn’t significantly affect the early recovery events (motion of eyelids, ears, head, and limbs), except for head lift.
However, for the events occurring at anesthetic end-tidal percentage lower than 0.20, or when the horses were awake, the temperament of the horses was found to substantially influence the recovery nature.