The Science Journal of the American Association for Respiratory Care

1999 OPEN FORUM Abstracts

VENTILATOR SETTINGS AND VOLUME MEASUREMENT DEVICE AFFECT THE RAPID SHALLOW BREATHING INDEX.

Steven Holets RRT, CCRA, Rolf Hubmayr MD, and Curtis F Buck CRNA, RRT. Mayo Medical Center, Rochester MN.

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 ยป 100 min-1 L-1. We tested the hypothesis that the choices of ventilator settings and volume measurement device affect the f/VT and by inference its predictive value in weaning decisions making.

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bias in VT (C1-C2=0.05L) and by a true change in breathing pattern when pressure support and O2 supplementation are withdrawn.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.

OF-99-092

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