Flow-volume relationships during spontaneous breathing through endotracheal tubes

Abstract

Breathing spontaneously through an endotracheal tube is often equated with "breathing through a straw." The objective of this study was to examine whether endotracheal tubes functionally act as tubular resistors in patients during tidal breathing, machine-initiated breaths, and vital capacity maneuvers. Descriptive study. University hospital surgical ICU. Eighteen mechanically ventilated, critically ill, postoperative patients (34 to 87 yrs). Flow-volume loops were recorded during tidal and machine breaths, as well as during vital capacity maneuvers. The loops were inspected visually for evidence of air flow limitation. Expiratory characteristics were examined using the ratio of expiratory volume in 1 sec to the expiratory volume in 0.5 sec and the ratio of expiratory volume in 1 sec to peak expiratory flow rate. The ratio of expiratory volume in 1 sec to that in 0.5 sec (greater than 1.5) and the ratio of expiratory volume in 1 sec to peak expiratory flow rate (greater than 10 mL/L/min) are seen with upper airway obstruction. Mean spontaneous tidal volume was 361 +/- 92 (SD) mL and vital capacity was 1104 +/- 16 mL, while the ratio of expiratory volume in 1 sec to that in 0.5 sec was 1.57 +/- 0.17 and 1.7 +/- 0.1 (p less than .05), respectively. The ratio of expiratory volume in 1 sec to peak expiratory flow rate was 9.9 +/- 1.7 during tidal breathing and 12.9 +/- 1.2 (p less than .05) during vital capacity maneuvers. Endotracheal tubes of the diameters used in these adult postoperative patients (7 and 8 mm) cause only minimal limitation to air flow during the small volumes and low-flow rates observed during tidal breathing. Evidence of greater air flow limitation was seen during vital capacity maneuvers.