Objective: To compare the effects of 2 fractions of inspired oxygen, 50% and > 95%, on ventilation, ventilatory rhythm, and gas exchange in isoflurane-anesthetized horses.
Animals: 8 healthy adult horses.
Procedures: In a crossover study design, horses were assigned to undergo each of 2 anesthetic sessions in random order, with 1 week separating the sessions. In each session, horses were sedated with xylazine hydrochloride (1.0 mg/kg, IV) and anesthesia was induced via IV administration of diazepam (0.05 mg/kg) and ketamine (2.2 mg/kg) Anesthesia was subsequently maintained with isoflurane in 50% or > 95% oxygen for 90 minutes. Measurements obtained during anesthesia included inspiratory and expiratory peak flow and duration, tidal volume, respiratory frequency, end-tidal CO(2) concentration, mixed expired partial pressures of CO(2) and O(2), PaO(2), PaCO(2), blood pH, arterial O(2) saturation, heart rate, and arterial blood pressure. Calculated values included the alveolar partial pressure of oxygen, alveolar-to-arterial oxygen tension gradient (PaO(2) - PaO(2)), rate of change of PAO(2) - PaO(2), and physiologic dead space ratio. Ventilatory rhythm, based on respiratory rate and duration of apnea, was continuously observed and recorded.
Results: Use of the lower inspired oxygen fraction of 50% resulted in a lower arterial oxygen saturation and PaO(2) than did use of the higher fraction. No significant difference in PaCO(2), rate of change of PAO(2) - PaO(2), ventilatory rhythm, or other measured variables was observed between the 2 sessions.
Conclusion and clinical relevance: Use of 50% inspired oxygen did not improve the ventilatory rhythm or gas exchange and increased the risk of hypoxemia in spontaneously breathing horses during isoflurane anesthesia. Use of both inspired oxygen fractions requires adequate monitoring and the capacity for mechanical ventilation.