The purpose of this study was to compare the cardiopulmonary effects of 0.9% end-tidal halothane (HA) with a total intravenous anesthetic (TIVA) technique. Seven healthy adult horses weighing 507 ± 28 kg were anesthetized on 2 separate occasions in random sequence. Anesthesia was induced with romifidine (0.1 mg/kg IV) and ketamine (2.2 mg/kg IV) and maintained with either HA in oxygen or TIVA. Horses in the TIVA group received a bolus of 50 mg/kg of guaiphenesin (GG) immediately after induction. TIVA was maintained with 82.5 ug/kg/hr of romifidine, 6.6 mg/kg/hr of ketamine, and 100 mg/kg/hr of GG for the first 30 minutes. The rate of GG administration was then reduced to 50 mg/kg/hr for the remaining 45 minutes. Infusion rates of romifidine and ketamine remained the same. Horses in both groups were intubated and attached to a large animal anesthetic circuit with an oxygen flow of 1 ml/kg/min. Cardiac output, stroke distance, heart rate, maximal aortic blood flow velocity, aortic blood flow acceleration, pre-ejection period (PEP), and systolic ejection time (ET) were measured by transesophageal Doppler echocardiography at 20, 40, 60, and 70 minutes after the start of drug infusion or HA anesthesia. Systemic and pulmonary arterial pressure, peak inspiratory and expiratory flows, tidal volume, minute ventilation, and respiratory frequency were also measured. Arterial and mixed venous blood were collected for blood gas analysis and measurement of hemoglobin concentration. Venous admixture was calculated using the shunt equation. Data were analyzed with ANOVA for repeated measures and individual means of interest were compared using a student’s t-test. P < 0.05 was considered significant.

Heart rate and pulmonary arterial pressures were higher with HA vs TIVA at all measurement times. Mean PAP for the HA group at 20, 40, 60, and 70 minutes were 45 ± 8.4, 45 ± 8.5, 45 ± 8.4, and 45 ± 8.2 mmHg compared with those for TIVA of 34 ± 2.7, 33 ± 2.8, 33 ± 3.4, and 34 ± 4.3 mmHg. Systolic ET was shorter with HA at 40 and 70 minutes and PEP was longer with HA at 60 minutes when compared to TIVA. PaCO2 was higher with HA at 60 minutes only. Respiratory frequency was higher and inspiratory time was longer with TIVA.

We conclude that, with the exception of pulmonary arterial pressures, significant differences in cardiopulmonary measurements between these two anesthetic techniques were not apparent.

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