COMPARISON OF ALVEOLAR VENTILATION, OXYGENATION, PRESSURE SUPPORT, AND RESPIRATORY SYSTEM RESISTANCE IN RESPONSE TO NONINVASIVE VERSUS CONVENTIONAL MECHANICAL VENTILATION IN FOALS

Objective-To compare the efficacy of positive pressure ventilation app lied through a mask versus an endotracheal tube, using a anesthetized/ paralyzed foals as a model for foals with hypoventilation. Animals-Six 1-month-old foals. Procedure-A crossover design was used to com pare the physiologic response of foals to 2 ventilatory techniques, noninva sive mask mechanical ventilation (NIMV) versus endotracheal mechanical ventilation (ETMV), during a single period of anesthesia and paralysi s. Arterial pH, Pa-O2, Pa-CO2, oxygen saturation. end-tidal CO2 tensio n, airway pressures, total respiratory system resistance, resistance a cross the upper airways (proximal to the midtracheal region), and posi tive end-expiratory pressures (PEEP) were measured. Only tidal volume (V-T,; 10, 12.5, and 15 ml/kg of body weight) or PEEP (7 cm of H2O) va ried. Results-Compared with ETMV, use of NIMV at equivalent V-T result ed in Pa-CO2 and pH values that were significantly higher, but Pa-O2 w as only slightly lower. Between the 2 methods, peak airway pressure wa s similar, but peak expiratory flow was significantly lower and total respiratory resistance higher al each V-T 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 hyper capnia. Increasing the V-T used on a per kilogram of body weight basis , or the use of pressure-cycled ventilation might reduce hypercapnia d uring NIMV. Clinical Relevance-Use of NIMV might be applicable in sele cted foals, such as those with hypoventilation and minimal changes in lung compliance, during weaning from endotracheal mechanical ventilati on, or for short-term ventilation in weak foals.