Impact of prolonged assisted ventilation on diaphragmatic efficiency: NAVA versus PSV

Background: Prolonged controlled mechanical ventilation depresses diaphragmatic efficiency. Assisted modes of ventilation should improve it. We assessed the impact of pressure support ventilation versus neurally adjusted ventilator assist on diaphragmatic efficiency. Method: Patients previously ventilated with controlled mechanical ventilation for 72 hours or more were randomized to be ventilated for 48 hours with pressure support ventilation (n = 12) or neurally adjusted ventilatory assist (n = 13). Neuro-ventilatory efficiency (tidal volume/diaphragmatic electrical activity) and neuro-mechanical efficiency (pressure generated against the occluded airways/diaphragmatic electrical activity) were measured during three spontaneous breathing trials (0, 24 and 48 hours). Breathing pattern, diaphragmatic electrical activity and pressure time product of the diaphragm were assessed every 4 hours. Results: In patients randomized to neurally adjusted ventilator assist, neuro-ventilatory efficiency increased from 27 +/- 19 ml/mu V at baseline to 62 +/- 30 ml/mu V at 48 hours (p < 0.0001) and neuro-mechanical efficiency increased from 1 +/- 0.6 to 2.6 +/- 1.1 cmH(2)O/mu V (p = 0.033). In patients randomized to pressure support ventilation, these did not change. Electrical activity of the diaphragm, neural inspiratory time, pressure time product of the diaphragm and variability of the breathing pattern were significantly higher in patients ventilated with neurally adjusted ventilatory assist. The asynchrony index was 9.48 [6.38-21.73] in patients ventilated with pressure support ventilation and 5.39 [3.78-8.36] in patients ventilated with neurally adjusted ventilatory assist (p = 0.04). Conclusion: After prolonged controlled mechanical ventilation, neurally adjusted ventilator assist improves diaphragm efficiency whereas pressure support ventilation does not.