Influence of neurally adjusted ventilatory assist and positive end-expiratory pressure on breathing pattern in rabbits with acute lung injury

Objective. To evaluate the influence of neurally adjusted ventilatory assist (NAVA) and positive end-expiratory pressure (PEEP) on the control of breathing in rabbits with acute lung injury. Design: Prospective animal study. Setting. Experimental laboratory in a university hospital. Subjects. Male White New Zealand rabbits (n = 18). Intervention: Spontaneously breathing rabbits with hydrochloric acid-induced lung injury were ventilated with NAVA and underwent changes in NAVA gain and PEEP (six nonvagotomized and five vagotomized). Seven other nonvagotomized rabbits underwent 4 hrs of ventilation with hourly titration of PEEP, FIO2, and NAVA gain. Measurements and Main Results. We studied diaphragm electrical activity, respiratory pressures, and breathing pattern. After lung injury, 0 cm H2O of PEEP resulted in high tonic and no discernible phasic diaphragm electrical activity in the nonvagotomized rabbits; stepwise increases in PEEP (up to 11.7 +/- 2.6 cm H2O) reduced tonic but increased phasic diaphragm electrical activity. Increasing the NAVA gain reduced phasic diaphragm electrical activity to almost half and abolished esophageal pressure swings. Tidal volume remained at 4-5 mL/kg, and respiratory rate did not change. In the vagotomized group, lung injury did not induce tonic activity, and phasic activity and tidal volume were several times higher than in the nonvagotomized rabbits. Four hours of breathing with NAVA restored breathing pattern and neural and mechanical breathing efforts to pre-lung injury levels. Conclusions. Acute lung injury can cause a vagally mediated atypical diaphragm activation pattern in spontaneously breathing rabbits. Modulation of PEEP facilitates development of phasic diaphragm electrical activity, whereupon implementation of NAVA can efficiently maintain unloading of the respiratory muscles without delivering excessive tidal volume in rabbits with intact vagal function.