Arterial blood gas tensions during recovery in horses anesthetized with apneustic anesthesia ventilation compared with conventional mechanical ventilation.

Objective: To compare PaO and PaCO in horses recovering from general anesthesia maintained with either apneustic anesthesia ventilation (AAV) or conventional mechanical ventilation (CMV).

Study Design: Randomized, crossover design.

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

Methods: Dorsally recumbent horses were anesthetized with isoflurane in oxygen [inspired oxygen fraction = 0.3 initially, with subsequent titration to maintain PaO ≥ 85 mmHg (11.3 kPa)] and ventilated with AAV or CMV according to predefined criteria [10 mL kg tidal volume, PaCO 40-45 mmHg (5.3-6.0 kPa) during CMV and < 60 mmHg (8.0 kPa) during AAV]. Horses were weaned from ventilation using a predefined protocol and transferred to a stall for unassisted recovery. Arterial blood samples were collected and analyzed at predefined time points. Tracheal oxygen insufflation at 15 L minute was provided if PaO < 60 mmHg (8.0 kPa) on any analysis. Time to oxygen insufflation, first movement, sternal recumbency and standing were recorded. Data were analyzed using repeated measures anova, paired t tests and Fisher's exact test with significance defined as p < 0.05.

Results: Data from 10 horses were analyzed. Between modes, PaO was significantly higher immediately after weaning from ventilation and lower at sternal recumbency for AAV than for CMV. No PaCO differences were noted between ventilation modes. All horses ventilated with CMV required supplemental oxygen, whereas three horses ventilated with AAV did not. Time to first movement was shorter with AAV. Time to oxygen insufflation was not different between ventilation modes.

Conclusions: Although horses ventilated with AAV entered the recovery period with higher PaO, this advantage was not sustained during recovery. Whereas fewer horses required supplemental oxygen after AAV, the use of AAV does not preclude the need for routine supplemental oxygen administration in horses recovering from general anesthesia.