Utility of Measurements of Oxygen Cost of Breathing in Predicting Success or Failure in Trials of Reduced Mechanical Ventilatory Support

Abstract

Objective

Test whether a change in oxygen consumption produced by a reduction in level of mechanical ventilatory support predicts failure to tolerate the reduction in level of support.

Design

Prospective study of the sensitivity and specificity of increased oxygen cost of breathing as a predictor of failure to tolerate a reduction in ventilatory support in patients undergoing weaning, using a protocol that incrementally reduces the level of mechanical ventilatory support.

Setting

University medical center.

Methods

We studied 228 trials in 30 patients who had required mechanical ventilatory support for at least 72 hours and who were being weaned using a standardized protocol that provided for three 30-minute trials of reduced mechanical ventilatory support per day, followed by ventilatory muscle rest. Using a metabolic monitor, we monitored oxygen consumption ( ${\dot{V}}_O{}_{{}_2}$ ) prior to and during 228 incremental reductions in level of mechanical ventilatory support conducted as part of a standardized weaning protocol. Oxygen cost of breathing was defined as the difference in ${\dot{V}}_O{}_{{}_2}$ (Δ ${\dot{V}}_O{}_{{}_2}$ ) during the trial of reduced mechanical ventilatory support, compared to a 30-minute resting period immediately before the trial. A successful trial was defined as one that could be continued for 30 minutes without development of clinical signs of ventilatory failure. Changes in ${\dot{V}}_O{}_{{}_2}$ and the ratio of respiratory frequency to tidal volume (f/V_T) during a weaning trial were evaluated as predictors for failure of a 30-minute trial of reduced ventilatory support.

Results

A 15% increase in oxygen cost of breathing predicted failure in the trial, with a sensitivity of 96.6%, specificity of 85.7%, positive predictive value of 98.5%, and negative predictive value of 72.0%. Neither change in ${\dot{V}}_O{}_{{}_2}$ measured early in the trial nor f/V_T proved to be as successful in predicting failure to tolerate an incremental reduction in ventilatory support.

Conclusion

Change in ${\dot{V}}_O{}_{{}_2}$ following an incremental reduction in level of mechanical ventilatory support may be a useful predictor for determining which patients will rapidly fail to tolerate that level of reduction.