Neurally adjusted ventilatory assist

Abstract

Purpose of review: Compared with the conventional forms of partial support, neurally adjusted ventilatory assist was repeatedly shown to improve patient-ventilator synchrony and reduce the risk of overassistance, while guaranteeing adequate inspiratory effort and gas exchange. A few animal studies also suggested the potential of neurally adjusted ventilatory assist in averting the risk of ventilator-induced lung injury. Recent work adds new information on the physiological effects of neurally adjusted ventilatory assist.

Recent findings: Compared with pressure support, neurally adjusted ventilatory assist has been shown to improve patient-ventilator interaction and synchrony in patients with the most challenging respiratory system mechanics, such as very low compliance consequent to severe acute respiratory distress syndrome and high resistance and air trapping due to chronic airflow obstruction; enhance redistribution of the ventilation in the dependent lung regions; avert the risk of patient-ventilator asynchrony due to sedation; avoid central apneas; limit the risk of high (injurious) tidal volumes in patients with acute respiratory distress syndrome of varied severity; and improve patient-ventilator interaction and synchrony during noninvasive ventilation, irrespective of the interface utilized.

Summary: Several studies nowadays prove the physiological benefits of neurally adjusted ventilatory assist, as opposed to the conventional modes of partial support. Whether these advantages translate into improvement of clinical outcomes remains to be determined.

FIGURE 1

Publications on neurally adjusted ventilatory assist (NAVA) from 1999 to 2013. The studies related to NAVA yearly published from 1999 (first description of the technique) to 2013 are shown as a whole and divided according to the type of study: animal (black), adult (light gray) and pediatric patients (dark gray), and others (white), including reviews, editorials, and investigations on healthy individuals. After the introduction of NAVA in clinical use in 2008, the studies related to this mode progressively increased every year, either overall or considering the studies performed on adult and pediatric patients.

Box 1

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FIGURE 2

Relation between patient's demand and ventilator support with different modes of partial assistance. The figure depicts the composite interplay among respiratory drive, pressure generated by the respiratory muscles (P_mus), and ventilatory output (i.e., volume and flow) during partial ventilator assist with different modes of ventilation. The output of the respiratory centers is modulated by stimuli from mechanoreceptors and chemoreceptors, and the cortical or limbic system. Sedatives also affect, directly or indirectly, the output of the respiratory centers. The amount of assistance delivered by the ventilator with the conventional modes (single dotted line), such as pressure support (PSV), assist control (A/C), and synchronized intermittent mandatory ventilation (SIMV), is not influenced by either drive or effort or ventilator output, which exposes this mode to the risk of overassistance. In contrast, with the proportional modes, the delivered support is affected by patient's demand indirectly, by the ventilatory output in proportional assist ventilation (PAV) (dashed-dotted line), or directly, by the neural output of the respiratory centers, as obtained by the electrical activity of the diaphragm (EAdi), in neurally adjusted ventilatory assist (NAVA) (dashed line). With NAVA, moreover, a changed neuromechanical coupling, changes in respiratory mechanics, or air leaks may not disturb the relation between neural output and mechanical support. See text for further explanations. Modified with permission from [2].