Clinical review: Update on neurally adjusted ventilatory assist--report of a round-table conference

Abstract

Conventional mechanical ventilators rely on pneumatic pressure and flow sensors and controllers to detect breaths. New modes of mechanical ventilation have been developed to better match the assistance delivered by the ventilator to the patient's needs. Among these modes, neurally adjusted ventilatory assist (NAVA) delivers a pressure that is directly proportional to the integral of the electrical activity of the diaphragm recorded continuously through an esophageal probe. In clinical settings, NAVA has been chiefly compared with pressure-support ventilation, one of the most popular modes used during the weaning phase, which delivers a constant pressure from breath to breath. Comparisons with proportional-assist ventilation, which has numerous similarities, are lacking. Because of the constant level of assistance, pressure-support ventilation reduces the natural variability of the breathing pattern and can be associated with asynchrony and/or overinflation. The ability of NAVA to circumvent these limitations has been addressed in clinical studies and is discussed in this report. Although the underlying concept is fascinating, several important questions regarding the clinical applications of NAVA remain unanswered. Among these questions, determining the optimal NAVA settings according to the patient's ventilatory needs and/or acceptable level of work of breathing is a key issue. In this report, based on an investigator-initiated round table, we review the most recent literature on this topic and discuss the theoretical advantages and disadvantages of NAVA compared with other modes, as well as the risks and limitations of NAVA.

Figure 1

Example of recording during neurally adjusted ventilatory assist and pressure-support ventilation. (a) Neurally adjusted ventilatory assist using the neural trigger: no asynchrony was observed. (b) Pressure-support ventilation: wasted efforts are underscored. Each wasted effort is identified by a blue rectangle.

Figure 2

Titration of the neurally adjusted ventilatory assist level according to Brander and colleagues' procedure. The neurally adjusted ventilatory assist (NAVA) level is increased step by step. V_T, tidal volume; Paw, airway pressure; cmH₂O/AU, cmH₂O per arbitrary unit (the amount of microvolts recorded from the electrical activity of the diaphragm signal).

Figure 3

Change in neurally adjusted ventilatory assist according to maximum diaphragmatic electrical activity during spontaneous breathing. Electrical activity of the diaphragm (EAdi) values during 1 hour, each point representing the mean value over 1 minute. EAdi variations occurred before, during, and after a spontaneous breathing trial (SBT). Maximum EAdi was 21 μV after a SBT of 3 minutes and allowed a reduction in the neurally adjusted ventilatory assist (NAVA) level from 2.4 to 2.2 cmH₂O/μV in order to obtain EAdi values after the SBT of about 13 μV (60% of maximum EAdi). Arterial blood gases were not changed by the NAVA level modification.