To evaluate the accuracies of the respiratory rate/tidal volume ratio (rate/volume ratio), minute volume, and negative inspired force in predicting weaning outcome in postoperative mechanically ventilated patients.
A prospective, observational study.
Surgical intensive care unit of a 270-bed community teaching hospital.
One hundred eighty-three postoperative, mechanically ventilated patients.
Measurements and Main Results
The spontaneous minute volume, unassisted respiratory rate/tidal volume ratio, and negative inspired force were measured just before weaning. The rate/volume ratio was remeasured after 30 to 60 mins of weaning. Weaning was conducted by the patients' primary physicians. Weaning success was defined as unassisted breathing for >24 hrs. Predictive characteristics were computed using threshold values of 100 breaths/min/L, 10 L/min, and -20 cm H2 O for the rate/volume ratios, minute volume, and negative inspired force, respectively. Receiver operating characteristic curves were also constructed to assess each parameter.
Sensitivities for the initial rate/volume ratio, rate/volume ratio after 30 mins, minute volume, and negative inspired force were 0.97, 0.96, 0.76, and 0.96, respectively. Specificities were 0.33, 0.31, 0.40, and 0.07, respectively. Areas (+/- SD) for receiver operating characteristic curves were 0.76 +/- 0.08, 0.75 +/- 0.06, 0.54 +/- 0.08, and 0.62 +/- 0.07, respectively. The rate/volume ratio after 30 mins correlated with the initial rate/volume ratio; the rate/volume ratio after 30 mins did not add significant, additional predictive information.
The rate/volume ratio measured at the beginning and after 30 mins of weaning is more highly predictive of weaning outcome than the negative inspired force and minute volume. The principal weakness of the rate/volume ratio is false-positive results. (Crit Care Med 1997; 25:253-257)