The Science Journal of the American Association for Respiratory Care
Background: Virtually all weaning algorithms supported by large clinical trials use a threshold in the rapid shallow breathing index (frequency to tidal volume ratio, f/VT) to triage patients. Based on the work of Yang and Tobin this threshold is usually
Methods: f/VT was measured under 3 experimental conditions (C1,2,3) in 21 patients, who fulfilled standard weaning screening criteria. During C1 and C2 patients received pressure support of 3 and CPAP of 5 cm H2O and O2 enriched air from a Nellcor-Puritan Bennett 7200 ventilator. During C3 patients were disconnected and breathed room air. During C1 f/VT was derived from the expired volume output of the ventilator. During C2 and C3 f/VT was derived using a precision calibrated Novametrics Ventcheck mechanics monitor. One minute averages in f, VT and f/VT and the associated biases due to ventilator settings (C2 vsC3) and measurement device (C1 vs. C2) were compared using t statistics.
Results: Under C1, 2&3 the average f/VT rose from 63 to 71 & 106, respectively. These differences across experimental conditions are largely explained by an instrument bias in VT (C1-C2=0.05L) and by a true change in breathing pattern when pressure support and O2 supplementation are withdrawn.
Conclusion: We conclude that the choices of ventilator settings and measurement equipment have profound effects on the f/VT and that the predictive values reported by Yang and Tobin should only be adopted if the measurement is performed during unsupported breathing of room air.