Neurally adjusted ventilatory assist in patients recovering spontaneous breathing after acute respiratory distress syndrome: Physiological evaluation*

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Abstract
Objective: Pressure-support ventilation is widely used during the weaning phase in patients with acute respiratory distress syndrome. The pressure-support level is adjusted to prevent ventilator-induced lung injury while limiting the patient's work of breathing. Neurally adjusted ventilatory assist is an assist mode that applies a positive pressure proportional to the integral of the electrical activity of the diaphragm. The objective was to assess the physiologic response to varying pressure-support ventilation and neurally adjusted ventilatory assist levels in selected acute respiratory distress syndrome patients and to evaluate the effect of neural triggering. Methods: We prospectively assessed 11 consecutive patients with acute respiratory distress syndrome attributable to pulmonary diseases. Pressure-support ventilation and neurally adjusted ventilatory assist were used in random order. Neurally adjusted ventilatory assist was used with a low electrical activity of the diaphragm trigger (neurally adjusted ventilatory assist–electrical activity of the diaphragm) and with a high electrical activity of the diaphragm trigger that led to rescue triggering by inspiratory flow (neurally adjusted ventilatory assist–inspiratory flow). With each ventilation modality, four levels of assistance (100%, 120%, 140%, and 160%) were used in random order. Statistical analysis was performed using analysis of variance for repeated measurements and mixed models. Main Results: Contrary to pressure-support ventilation, neurally adjusted ventilatory assist–electrical activity of the diaphragm and neurally adjusted ventilatory assist–inspiratory flow were associated with stable tidal volume levels despite increasing assistance. For the asynchrony index, an interaction was present between ventilation mode and assist level (p = .0076) because asynchrony index increased significantly with the pressure-support ventilation level (p = .004), but not with the neurally adjusted ventilatory assist–electrical activity of the diaphragm or neurally adjusted ventilatory assist–inspiratory flow level. The lowest asynchrony index was obtained with neurally adjusted ventilatory assist–electrical activity of the diaphragm. Conclusion: Compared to pressure-support ventilation, neurally adjusted ventilatory assist in acute respiratory distress syndrome patients holds promise for limiting the risk of overassistance, preventing patient–ventilator asynchrony, and improving overall patient–ventilator interactions. Neural triggering (neurally adjusted ventilatory assist–electrical activity of the diaphragm) considerably decreased patient–ventilator asynchrony. (Crit Care Med 2010; 38:1830–1837)