Energy Expenditure Associated with CPAP and T-Piece Spontaneous Ventilatory Trials: Changes Following Prolonged Mechanical Ventilation

Abstract

The use of portable metabolic carts to assess energy expenditure (EE) by measuring oxygen consumption (VO2) and carbon dioxide production (VCO2) has recently been applied to patients undergoing weaning from mechanical ventilation. The VO2 and EE can be used to estimate changes in the work of breathing (WOB) associated with different weaning strategies. The purpose of this study was to use VO2 and EE to assess changes in the WOB when assisted mechanical ventilation (AMV) was replaced with two spontaneous ventilatory trial (SVT) techniques: continuous positive airway pressure (CPAP) and T-piece. Nine difficult-to-wean patients were studied during the initial weaning period following 26 +/- 18 days (mean +/- SD) of mechanical ventilatory support. The VO2 and EE during all AMV were 296 +/- 75 ml/min and 2069 +/- 519 kcal/day, respectively. Compared to the baseline AMV levels, during CPAP overall VO2 and EE increased 14 percent and 13 percent, respectively, and during T-piece overall VO2 and EE increased 20 percent and 19 percent, respectively. Respiration rate (f) increased and tidal volume (VT) decreased during both SVTs compared to AMV although no significant change in minute ventilation was seen. The WOB, as judged from changes in VO2, was only 5 percent higher during T-piece compared to CPAP; however, patients tolerated an average of only 141 +/- 45 min on T-piece vs 165 +/- 29 minutes on CPAP. We conclude that during the initial weaning stages in patients who have received prolonged mechanical ventilatory support, the WOB associated with SVTs is increased compared to AMV but that the WOB associated with T-piece is not significantly greater than that for CPAP.