Flow-volume relationships during spontaneous breathing through endotracheal tubes.
OBJECTIVE
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.


METHODS
Descriptive study.


METHODS
University hospital surgical ICU.


METHODS
Eighteen mechanically ventilated, critically ill, postoperative patients (34 to 87 yrs).


METHODS
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.


RESULTS
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.


CONCLUSIONS
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.
Weissman
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Department of Anesthesiology, College of Physicians and Surgeons, Columbia University, New York, NY 10032.
D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000403:Airway Resistance; D016638:Critical Illness; D005069:Evaluation Studies as Topic; D005541:Forced Expiratory Volume; D006785:Hospitals, University; D006801:Humans; D007320:Inspiratory Capacity; D007362:Intensive Care Units; D007442:Intubation, Intratracheal / Q000009:adverse effects / Q000295:instrumentation / Q000592:standards*; D008875:Middle Aged; D010366:Peak Expiratory Flow Rate; D011184:Postoperative Period; D013990:Tidal Volume; D014797:Vital Capacity; D014939:Work of Breathing