Comparison of six methods to calculate airway resistance during mechanical ventilation in adults.

OBJECTIVE

A variety of methods are used to calculate indices of lung mechanics. We conducted this study to compare 6 methods of calculating airway resistance.

METHODS

Data were recorded from 20 adult mechanically ventilated patients. All were relaxed and breathing in synchrony with the ventilator, and an end-inspiratory pause sufficient to produce a pressure plateau (0.5-1.5 s) was used. Pressure and flow rate were measured at the proximal airway using a calibrated lung mechanics analyzer (VenTrak, Med Science, St Louis, MO). Flow rate, pressure, and volume were printed simultaneously. Airway resistance was calculated using 6 methods: Suter, Krieger, Neergard, Bergman, Comroe, and Jonson.

RESULTS

Mean calculated resistances (+/- SD) (cm H2O/L/s) were 11.7 +/- 4.8 (Suter method), 13.3 +/- 5.0 (Krieger method), 14.9 +/- 5.3 (Neergard method), 25.0 +/- 6.6 (Bergman method), 24.7 +/- 6.4 (Comroe method), and 26.9 +/- 4.8 (Jonson method). By repeated measures analysis of variance, these differences were significant (p < 0.001). Using Scheffe analysis, no difference was found between the calculations using the Bergman, Comroe, and Jonson methods; these were significantly greater than the other 3 methods (p < 0.05).

CONCLUSIONS

Methods that evaluate expiratory resistance (Comroe, Bergman, and Jonson) produce higher values than methods that evaluate inspiratory resistance (Suter and Neergard) or a combination of inspiratory and expiratory resistance (Krieger). Because of these differences, investigators should clearly describe their calculations when reporting airway resistance values.