Comparison of alveolar ventilation, oxygenation, pressure support, and respiratory system resistance in response to noninvasive versus conventional mechanical ventilation in foals

Abstract

SUMMARY Objective To compare the efficacy of positive pressure ventilation applied through a mask versus an endotracheal tube, using anesthetized/paralyzed foals as a model for foals with hypoventilation. Animals Six 1-month-old foals. Procedure A crossover design was used to compare the physiologic response of foals to 2 ventilatory techniques, noninvasive mask mechanical ventilation (NIMV) versus endotracheal mechanical ventilation (ETMV), during a single period of anesthesia and paralysis. Arterial pH, Pao2, Paco2, oxygen saturation, end-tidal CO2 tension, airway pressures, total respiratory system resistance, resistance across the upper airways (proximal to the midtracheal region), and pos-itive end-expiratory pressures (PEEP) were measured. Only tidal volume (VT; 10, 12.5, and 15 ml/kg of body weight) or PEEP (7 cm of H2O) varied. Results Compared with ETMV, use of NIMV at equivalent VT resulted in Paco2 and pH values that were significantly higher, but Pao2 was only slightly lower. Between the 2 methods, peak airway pressure was similar, but peak expiratory flow was significantly lower and total respiratory resistance higher at each VT for NIMV. Delivery of PEEP (7 cm of H2O) was slightly better for ETMV (7.1 ± 1.3 cm of H2O) than for NIMV (5.6 ± 0.6 cm of H2O). Conclusion These data suggest that use of NIMV induces similar physiologic effects as ETMV, but the nasal cavities and mask contribute greater dead space, manifesting in hypercapnia. Increasing the VT used on a per kilogram of body wieght basis, or the use of pressure-cycled ventilation might reduce hypercapnia during NIMV. Clinical Relevance Use of NIMV might be applicable in selected foals, such as those with hypoventilation and minimal changes in lung compliance, during weaning from endotracheal mechanical ventilation, or for short-term ventilation in weak foals. (Am J Vet Res 1997;58:1463–1467)