Assessing the influence of mechanical ventilation on blood gases and blood pressure in rattlesnakes.

ABSTRACT

OBJECTIVE: To characterize the impact of mechanical positive pressure ventilation on heart rate (HR), arterial blood pressure, blood gases, lactate, glucose, sodium, potassium and calcium concentrations in rattlesnakes during anesthesia and the subsequent recovery period. STUDY DESIGN: Prospective, randomized trial. ANIMALS Twenty one fasted adult South American rattlesnakes (Crotalus durissus terrificus). METHODS: Snakes were anesthetized with propofol (15 mg kg(-1)) intravenously, endotracheally intubated and assigned to one of four ventilation regimens Spontaneous ventilation, or mechanical ventilation at a tidal volume of 30 mL kg(-1) at 1 breath every 90 seconds, 5 breaths minute(-1), or 15 breaths minute(-1). Arterial blood was collected from indwelling catheters at 30, 40, and 60 minutes and 2, 6, and 24 hours following induction of anesthesia and analyzed for pH, PaO2, PaCO2, and selected variables. Mean arterial blood pressure (MAP) and HR were recorded at 30, 40, 60 minutes and 24 hours. RESULTS: Spontaneous ventilation and 1 breath every 90 seconds resulted in a mild hypercapnia (PaCO2 22.4 ± 4.3 mmHg [3.0 ± 0.6 kPa] and 24.5 ± 1.6 mmHg [3.3 ± 0.2 kPa], respectively), 5 breaths minute(-1) resulted in normocapnia (14.2 ± 2.7 mmHg [1.9 ± 0.4 kPa]), while 15 breaths minute(-1) caused marked hypocapnia (8.2 ± 2.5 mmHg [1.1 ± 0.3 kPa]). Following recovery, blood gases of the four groups were similar from 2 hours. Anesthesia, independent of ventilation was associated with significantly elevated glucose, lactate and potassium concentrations compared to values at 24 hours (p < 0.0001). MAP increased significantly with increasing ventilation frequency (p < 0.001). HR did not vary among regimens. CONCLUSIONS AND CLINICAL RELEVANCE Mechanical ventilation had a profound impact on blood gases and blood pressure. The results support the use of mechanical ventilation with a frequency of 1-2 breaths minute(-1) at a tidal volume of 30 mL kg(-1) during anesthesia in fasted snakes.