THE UNASSISTED RESPIRATORY RATE TIDAL VOLUME RATIO ACCURATELY PREDICTS WEANING OUTCOME IN POSTOPERATIVE-PATIENTS/

Objective: To evaluate the accuracies of the respiratory ratel tidal v olume ratio (rate/volume ratio), minute volume, and negative inspired force in predicting weaning outcome in postoperative mechanically vent ilated patients. Design: A prospective, observational study. Setting: Surgical intensive care unit of a 270-bed community teaching hospital. Patients: One hundred eighty-three postoperative, mechanically ventil ated patients. Interventions: None. Measurements and Main Results: The spontaneous minute volume, unassisted respiratory rate/tidal volume r atio, and nega tive inspired force were measured just before weaning. The ratel volume ratio was remeasured after 30 to 60 mins of weaning. Weaning was conducted by the patients' primary physicians. Weaning suc cess was defined as unassisted breathing for >24 hrs. Predictive chara cteristics were computed using threshold values of 100 breaths/min/L, 10 L/min, and -20 cm H2O for the rate/volume ratios, minute volume, an d negative inspired force, respectively. Receiver operating characteri stic curves were also constructed to assess each parameter. Sensitivit ies 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, an d 0.96, respectively. Specificities were 0.33, 0.31, 0.40; and 0.07, r espectively. Areas (+/-SD) for receiver operating characteristic curve s 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 no t add significant, additional predictive information. Conclusions: The rate/volume ratio measured at the beginning and after 30 mins of wean ing is more highly predictive of weaning outcome than the negative ins pired force and minute volume. The principal weakness of the rate/volu me ratio is false-positive results.