Serial measurements of the rapid-shallow-breathing index as a predictor of weaning outcome in elderly medical patients

Study objectives: To determine the usefulness of serial measurements of the rapid-shallow-breathing index (f/VT) as a predictor for successfully weaning elderly medical patients from mechanical ventilator support using a threshold value (less than or equal to 130) derived specifically for this population. Design: Prospective observational study using parameters suggested from retrospective analysis. Setting: Medical ICUs of a university-affiliated private teaching hospital. Patients: Using data obtained from a retrospective analysis of 10 medical patients greater than or equal to 70 years old who had failed weaning, 49 additional medical patients older than 70 years were studied prospectively. Interventions: Standard weaning parameters were determined using a hand-held spirometer. Respiratory rate (f, breaths/min) and tidal volume (VT, liters) were measured at the beginning of a spontaneous breathing trial and hourly thereafter for up to 5 h using the same hand-held spirometer. Measurements ann results: Retrospective analysis showed that the published threshold value for f/VT (less than or equal to 105) had poor predictability for weaning success when measured at the beginning of the weaning trial. In the 9 of 10 patients who failed to wean in the retrospective review, the f/VT increased to >130 as the trial progressed over 2 to 3 h. Using an f/VT less than or equal to 130 as the threshold value for prospectively predicting successful weaning, the diagnostic accuracy, sensitivity, specificity, positive predictive value, and negative predictive value increased from 84%, 92%, 57%, 87%, and 67%, respectively, when measured at the beginning of the weaning trial to 92%, 93%, 89%, 97%, and 80%, respectively, when measured 3 h later. The area under the receiver operating characteristic curve for f/VT also improved from 0.81 to 0.93. Conclusions: Serial measurements of the rapid-shallow-breathing index in medical elderly patients during a period of spontaneous breathing can accurately predict the ability to be successfully weaned from mechanical ventilator support.