BackgroundThe purpose of this study was to compare the effects of 0.5 fraction of inspired oxygen (FiO2) and >0.95 FiO2 on pulmonary gas exchange, shunt fraction and oxygen delivery (DO2) in dorsally recumbent horses during inhalant anesthesia. The use of 0.5 FiO2 has the potential to reduce absorption atelectasis (compared to maximal FiO2) and augment alveolar oxygen (O2) tensions (compared to ambient air) thereby improving gas exchange and DO2. Our hypothesis was that 0.5 FiO2 would reduce ventilation-perfusion mismatching and increase the fraction of pulmonary blood flow that is oxygenated, thus improving arterial oxygen content and DO2.ResultsArterial partial pressures of O2 were significantly higher than preanesthetic levels at all times during anesthesia in the >0.95 FiO2 group. Arterial partial pressures of O2 did not change from preanesthetic levels in the 0.5 FiO2 group but were significantly lower than in the >0.95 FiO2 group from 15 to 90 min of anesthesia. Alveolar to arterial O2 tension difference was increased significantly in both groups during anesthesia compared to preanesthetic values. The alveolar to arterial O2 tension difference was significantly higher at all times in the >0.95 FiO2 group compared to the 0.5 FiO2 group. Oxygen delivery did not change from preanesthetic values in either group during anesthesia but was significantly lower than preanesthetic values 10 min after anesthesia in the 0.5 FiO2 group. Shunt fraction increased in both groups during anesthesia attaining statistical significance at varying times. Shunt fraction was significantly increased in both groups 10 min after anesthesia but was not different between groups. Alveolar dead space ventilation increased after 3 hr of anesthesia in both groups.ConclusionsReducing FiO2 did not change alveolar dead space ventilation or shunt fraction in dorsally recumbent, mechanically ventilated horses during 3 hr of isoflurane anesthesia. Reducing FiO2 in dorsally recumbent isoflurane anesthetized horses does not improve oxygenation or oxygen delivery.

Rectal administration of morphine did not increase bioavailability above that reported for oral administration of morphine in dogs. Low bioavailability and plasma concentrations limit the clinical usefulness of morphine administered per rectum in dogs.

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Horses are the most difficult of the common companion animals to anaesthetise. Hypoxaemia or inadequate oxygen delivery to peripheral tissues during anaesthesia would seem a potential cause of increased mortality, but no direct link has been established. A number of methods of increasing oxygenation and oxygen delivery have been reported, with varying results and potential applicability. The purpose of this article is to review the literature with regard to oxygenation, oxygen delivery and methods to improve each and to make recommendations for clinical application.

Midazolam produces muscle relaxation but not sedation in adult horses.

The anesthetic, hemodynamic, and respiratory effects of an intravenously administered 1:1 combination of tiletamine and zolazepam were evaluated in dogs. Each dog received tiletamine-zolazepam (6.6, 13.2, 19.8 mg/kg) on two occasions, once when awake and a second time with residual isoflurane anesthesia while instrumented for the recording of hemodynamic data. Tiletamine-zolazepam administered to conscious dogs resulted in good, rapid induction of anesthesia. Time to sternal recumbency (recovery) was dose-dependent. Character of recovery tended to be better with the lower dose. Tiletamine-zolazepam caused significant increases in heart rate after all doses and significant increases in cardiac output after the two larger doses. All doses caused significant decreases in arterial blood pressure at 1 minute. Arterial blood pressures returned to baseline and then increased significantly above baseline values. The rate of development of left ventricular pressure was significantly decreased 1 minute after the two higher doses, returned to normal, and then was significantly increased above baseline for all doses. Peripheral vascular resistance increased transiently 1 minute after the 6.6 mg/kg dose. Peripheral vascular resistance decreased significantly after the 13.2 and 19.8 mg/kg doses. Minute ventilation was significantly decreased only after the 19.8 mg/kg dose.

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Anesthesia was induced in 14 greyhounds with a mixture of diazepam or midazolam (0.28 mg/kg) and ketamine (5.5 mg/kg), and maintained with halothane. There were no significant differences in weight, age, or duration of anesthesia between the treatment groups. Time to intubation with diazepam-ketamine (4.07 +/- 1.43 min) was significantly longer than with midazolam-ketamine (2.73 +/- 0.84 min). Heart rate, respiratory rate, PaCO2, and arterial pH did not vary significantly during anesthesia in either treatment group. Arterial blood pressures, PaO2, halothane vaporizer setting, and body temperature changed significantly from baseline values in both treatment groups during anesthesia. Times to sternal recumbency and times to standing were not significantly different. These data suggest that both diazepam-ketamine and midazolam-ketamine are useful anesthetic combinations in greyhounds. In combination with ketamine, midazolam offers little advantage over diazepam.