Physiologic response to changing positive end-expiratory pressure during neurally adjusted ventilatory assist in sedated, critically ill adults.
BACKGROUND
Neurally adjusted ventilatory assist (NAVA) delivers airway pressure (Paw) in proportion to neural inspiratory drive as reflected by electrical activity of the diaphragm (EAdi). Changing positive end-expiratory pressure (PEEP) impacts respiratory muscle load and function and, hence, EAdi. We aimed to evaluate how PEEP affects the breathing pattern and neuroventilatory efficiency during NAVA.


METHODS
In 20 adult patients, adequate assist (NAVAal) was first identified based on Paw and tidal volume (Vt) responses to systematic increases in NAVA level while using preset PEEP (PEEPbl). Thereafter, using NAVAal, PEEP was increased to 20 cm water (H(2)O) (PEEPhigh) and then lowered stepwise to 1 cm H(2)O (PEEP1). EAdi, Paw, and Vt were recorded.


RESULTS
Median NAVAal was 2.7 (interquartile range, 2.3-3.5) cm H(2)O/muV and was similar to NAVAal identified post hoc by 17 independent physicians (2.5 [2.0-3.4] cm H(2)O/muV; P = NS). Reducing PEEPhigh to PEEP1 increased inspiratory EAdi by 34% (2-67; P = .046) and was associated with an increase in mean Paw above PEEP from 8.5 (6.7-11.4) cm H(2)O to 12.2 (8.8-16.7) cm H(2)O (P = .008), whereas Vt and respiratory rate remained unchanged. The response pattern in Vt/EAdi, indicating changes in neuroventilatory efficiency, differed among patients. Tidal breathing occurred at the lowest EAdi cost in seven patients with PEEP1 or half PEEPbl, in six patients with PEEPbl, and in seven patients with PEEPhigh.


CONCLUSIONS
During NAVAal, increasing PEEP reduces respiratory drive. Patients adapt their neuroventilatory efficiency such that the individual ventilatory pattern is preserved over a wide range of PEEP levels. Monitoring Vt/EAdi during PEEP changes allows identification of a PEEP level at which tidal breathing occurs at minimal EAdi cost.


TRIAL REGISTRATION
clinicaltrials.gov; Identifier: NCT00529347.