Assessment of airway function during assisted ventilation.

Abstract

The purpose of this study was to evaluate three bedside methods for measuring respiratory system resistance to airflow (Rrs). To this end, we studied 19 intubated patients, 6 of whom had diseases characterized predominantly by acute airway obstruction (Group I) and 13 in whom respiratory failure was attributable to extensive pneumonia or edema (Group II). For two of the methods, pressure and flow were measured during assisted inhalation. In one, designated RrsIP, driving pressure was taken as the difference between peak airway pressure and airway pressure recorded with the lung held at inflation after a tidal breath. Flow was measured during assisted inhalation. Method 2, designated RrsIC, differed from RrsIP in that driving pressure was calculated from the airway pressure measured at mid-tidal inhalation, with a correction for dynamic compliance. In contrast, for Method 3, driving pressure was derived from the static pressure-volume curve of the respiratory system and flow was measured during passive exhalation (RrsEX). RrsEX was significantly higher in Group I than in Group II patients, the values (mean±SD) being 21.3±11.5 and 10.3±7.3 cm H2O/(l/s) respectively (P<0.01). The corresponding values of driving pressures were 10.7±5.7 cm H2O in Group I and 5.3±3.9 cm H2O in Group II. Both inspiratory methods yielded lower values for Rrs, especially in Group I patients with obstructive disease. Neither RrsIP nor RrsIC were significantly different in the two patient groups. We conclude that Rrs measurement by the RrsEX method may provide information of clinical value. We reason that inspiratory methods for measurement of Rrs fail to identify expiratory limitation of flow through airways of narrowed caliber, because the inspiratory pressure in intubated mechanically ventilated patients tends to open rather than compress the flow-limiting regions.