Different fresh gas flows produce similar rebreathing and regional ventilation during helmet continuous positive airway pressure in sedated Beagles

Objective: To evaluate the effects of 4 oxygen driving flows on carbon dioxide rebreathing and ventilation in healthy sedated dogs receiving helmet continuous positive airway pressure (CPAP). Methods: This prospective experimental crossover study included 7 healthy adult Beagles sedated using a standardized protocol. Dogs received helmet CPAP (approx 5 cm H2O) through a Venturi valve system. Following stabilization at 15 L/min, dogs underwent evaluation at 4 oxygen driving flows (5, 10, 15, and 20 L/min), with flow conditions administered in randomized order. Primary outcomes included inspired carbon dioxide (PiCO2), PaCO2, and end-tidal carbon dioxide. Secondary outcomes included oxygenation, hemodynamics, and electrical impedance tomography-derived regional ventilation. Data were analyzed using generalized linear mixed-effects models with flow as a fixed effect and dog as a random effect. Results: Flow significantly affected PiCO2, although the magnitude of the difference was small. Median PiCO2 differed by approximately 1 mm Hg between 5 and 15 L/min, and maximum observed PiCO2 remained low (2 mm Hg). No differences were detected in PaCO2, end-tidal carbon dioxide, respiratory rate, surrogate minute ventilation, or regional ventilation distribution. Arterial oxygen tension differed among flows, but the ratio of arterial oxygen tension to inspired oxygen fraction (P:F ratio) was unchanged. Conclusions: Carbon dioxide rebreathing during helmet CPAP remained minimal across the tested oxygen driving flows and was not associated with measurable changes in PaCO2 or regional ventilation. Clinical Relevance: Lower oxygen driving flows may provide adequate helmet washout in healthy sedated dogs using a Venturi-based helmet CPAP system.