An implementation of apneustic anesthesia ventilation in the horse: comparison with conventional mechanical ventilation.

OBJECTIVE

To compare the cardiopulmonary effects of apneustic anesthesia ventilation (AAV) and conventional mechanical ventilation (CMV) in dorsally recumbent anesthetized horses.

STUDY DESIGN

Randomized, crossover design.

ANIMALS

A total of 10 healthy adult horses from a university-owned herd.

METHODS

Following xylazine, midazolam and ketamine administration, horses were orotracheally intubated and positioned in dorsal recumbency. Anesthesia was maintained with isoflurane in oxygen [inspired oxygen fraction (FiO) = 0.3 initially, with subsequent titration to maintain PaO ≥ 85 mmHg (11.3 kPa)]. Horses were instrumented and ventilated with AAV or CMV for 1 hour according to predefined criteria [10 mL kg tidal volume (V), PaCO of 40-45 mmHg (5.3-6.0 kPa) during CMV and <60 mmHg (8.0 kPa) during AAV]. Dobutamine was administered to maintain mean arterial pressure (MAP) >65 mmHg. Cardiopulmonary data were collected at baseline, 30 and 60 minutes. The effects of ventilation mode and time were analyzed using repeated-measures anova with significance defined as p < 0.05.

RESULTS

Data from nine horses were analyzed. A significant effect of mode at one or more time points was found for respiratory rate, arterial and end-tidal CO tensions, arterial pH, mean airway pressure (P), respiratory system dynamic compliance index (CI), venous admixture (Q˙/Q˙), mean pulmonary artery pressure and systemic vascular resistance. No significant differences between modes were found for V, FiO, PaO, arterial hemoglobin saturation, alveolar dead space, heart rate, MAP, cardiac index, stroke volume index, oxygen delivery index, oxygen extraction ratio and dobutamine administration.

CONCLUSIONS AND CLINICAL RELEVANCE

In dorsally recumbent anesthetized horses, both ventilation modes supported adequate oxygenation with minimal supplemental oxygen. Compared with CMV, AAV resulted in higher CI and lower Q˙/Q˙. Despite higher mean P with AAV, the cardiovascular effects of each mode were not different. Further trials of AAV in anesthetized horses are warranted.