Can proportional ventilation modes facilitate exercise in critically ill patients? A physiological cross-over study : Pressure support versus proportional ventilation during lower limb exercise in ventilated critically ill patients

Abstract

Background: Early exercise of critically ill patients may have beneficial effects on muscle strength, mass and systemic inflammation. During pressure support ventilation (PSV), a mismatch between demand and assist could increase work of breathing and limit exercise. A better exercise tolerance is possible with a proportional mode of ventilation (Proportional Assist Ventilation, PAV+ and Neurally Adjusted Ventilatory Assist, NAVA). We examined whether, in critically ill patients, PSV and proportional ventilation have different effects on respiratory muscles unloading and work efficiency during exercise.

Methods: Prospective pilot randomized cross-over study performed in a medico-surgical ICU. Patients requiring mechanical ventilation >48 h were enrolled. At initiation, the patients underwent an incremental workload test on a cycloergometer to determine the maximum level capacity. The next day, 2 15-min exercise, at 60% of the maximum capacity, were performed while patients were randomly ventilated with PSV and PAV+ or NAVA. The change in oxygen consumption (ΔVO₂, indirect calorimetry) and the work efficiency (ratio of ΔVO₂ per mean power) were computed.

Results: Ten patients were examined, 6 ventilated with PSV/PAV+ and 4 with PSV/NAVA. Despite the same mean inspiratory pressure at baseline between the modes, baseline VO₂ (median, IQR) was higher during proportional ventilation (301 ml/min, 270-342) compared to PSV (249 ml/min, 206-353). Exercise with PSV was associated with a significant increase in VO₂ (ΔVO₂, median, IQR) (77.6 ml/min, 59.9-96.5), while VO₂ did not significantly change during exercise with proportional modes (46.3 ml/min, 5.7-63.7, p < 0.05). As a result, exercise with proportional modes was associated with a better work efficiency than with PSV. The ventilator modes did not affect patient's dyspnea, limb fatigue, distance, hemodynamics and breathing pattern.

Conclusions: Proportional ventilation during exercise results in higher work efficiency and less increase in VO₂ compared to ventilation with PSV. These preliminary findings suggest that proportional ventilation could enhance the training effect and facilitate rehabilitation.

Keywords: Assisted mechanical ventilation; Critically ill patients; Exercise; Oxygen consumption; Proportional ventilation; Work efficiency.

Fig. 1

Example of a patient ventilated with Neurally Adjusted Ventilator Assist (NAVA) while performing exercise with the cycle ergometer (MOTOmed Letto 2, RECK-Technik GmbH and Co. Betzenweiler, Germany). Oxygen consumption is measured through indirect calorimetry

Fig. 2

Recordings of Flow, airway pressure (Paw) and electrical activity of the diaphragm (EAdi) during exercise in the same patient during two examined sessions: one with Neurally Adjusted Ventilator Assist (NAVA) and one with pressure support ventilation (PSV). During NAVA, one can observe the great variability of Flow due to changes in neural effort (reflected by changes in EAdi) which led to changes in delivered Paw. In contrast, despite similar changes in EAdi during PSV, delivered Paw and Flow remained stable

Fig. 3

Changes in oxygen consumption (ΔVO₂) and ΔVO₂/Wmean before and after exercise between the modes tested. ΔVO₂; difference in VO₂ between the start and the end of exercise in absolute values. PROP proportional ventilation, PSV pressure support ventilation