The Science Journal of the American Association for Respiratory Care

1996 OPEN FORUM Abstracts

IN-VIVO COMPARISON OF ESTIMATED WITH MEASURED INTRATRACHEAL PEAK PRESSURE.

Kelly Jager, RRT, Martin Tweeddale, MB, PhD. Vancouver Hospital and Health Sciences Center, Vancouver, British Columbia, Canada.

BACKGROUND: High peak airway pressures (Paw) have been associated with ventilator induced lung injury. However, in intubated patients, a significant pressure drop can occur across the endotracheal tube (ETT) thereby making intratracheal peak airway pressure (Ptrach) more accurate than proximal (Paw) for the proper determination of lung mechanics. Gottfried et al. have reported in-vitro constants to assist in the estimation of ETT resistance (RETT). The purpose of this study was to determine whether a simple equation based on their constants could be used at the bedside to accurately estimate Ptrach (ePtrach) in ventilated critical care patients.

Methods: The equation used was: ePtrach = (estimated Rpt x flowrate) + Pplateau, (where Rpt is patient resistance determined as respiratory system resistance minus RETT, estimated from Gottfried's constants). Estimated Ptrach was compared to Ptrach measured (mPtrach) using a small bore catheter placed distal to the tip of the ETT. In 10 patients (group 1) proximal Paw, Ptrach and Pplateau were measured and averaged over 3 breaths at the patients' set flowrate. In a second set of 10 patients (group 2), the same measurement were made at 4 different flowrates. In both groups, bias ± precision were used to compare ePtrach to mPtrach using the methods of Bland and Altman. Bias was plotted against the mean of ePtrach and mPtrach. We defined limits of agreement of ± 3.0 cm H_{2}O as clinically acceptable. In group 2 patients, Friedman's non-parametric test was used to compare the bias and precision of the 4 different flowrates.

Results: In group 1, bias ± precision was 5.3 ± 4.24 cm H_{2}O. The calculated limits of agreement were -3.1 to 13.7 cm H_{2}O. All data points for flowrates < 85 L/min were within the clinically acceptable limits of agreement. Results for group 2 are shown in the table.

Flow (L/min) 60 80 100 120

mPtrach* 29.7 ± 5.43 30.2 ± 4.05 31.6 ± 3.44 31.1 ± 2.77

ePtrach* 31.0 ± 4.70 31.2 ± 3.68 28.4 ± 4.09 26.2 ± 5.79

bias ± precision* -1.3 ± 1.64 -1.0 ± 2.71 3.2 ± 4.08? 4.9 ± 4.67?

* cm H_{2}O

? p < 0.05 compared to flowrates of 60 and 80 L/min.

Conclusions: The results of this study indicate that the equation based on Gottfrieds' constants can be used to estimate Ptrach in patients with ventilator flowrates < = 85 L/min, however, with flowrates above this level, Ptrach should be measured directly.

Reference: OF-96-010

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