Response to Tubular Airway Resistance in Normal Subjects and Postoperative Patients

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Abstract
Critically ill patients must often breathe spontaneously through an endotracheal tube that acts as a fixed inspiratory and expiratory tubular airway resistor. Although this practice is common, its effect on the pattern of breathing is not known. The mean breathing patterns of seven normal, healthy male subjects and eight male patients who had undergone upper abdominal surgery 2-4 days previously were studied breathing through a mouthpiece fitted in random order with a 5, 6, 7, 8, or 15 mm diameter (17 mm long) resistor. These diameters were selected because they simulate the pressure-flow relationships of adult endotracheal tubes. With the 15 mm aperture, the patients had a greater breathing frequency (f) than did the normal subjects (21 .+-. 5 [SD] vs. 14 .+-. 4 breaths/min, P < 0.01) as well as a smaller mean tidal volume (VT). In both groups, minute ventilation (.ovrhdot.VE) and f progressively decreased as resistance was increased by decreasing the aperture size from 15 to 6 mm. In the normal subjects, but not the patients, VT also progressively decreased. When the diameter was decreased from 6 mm to 5 mm, there were increases in VT and decreases in f that were more marked in the normal subjects. In both groups, the changes in .ovrhdot.VE were accompanied by decreases in mean and peak inspiratory and expiratory flow rates. Throughout the study, oxygen consumption (.ovrhdot.VO2) and carbon dioxide production (.ovrhdot.VO2) did not change. This, coupled with the decreases in .ovrhdot.VE resulted in decreases in the ventilatory equivalents to CO2 and O2 (.ovrhdot.VE/.ovrhdot.VO2,.ovrhdot.VE/.ovrhdot.VOO2). This study demonstrates that increasing tubular airway resistance significantly alters respiratory pattern without affecting gas exchange. When interpreting the breathing pattern of patients whose trachea is intubated, the clinician should consider the effects of the fixed tubular resistance. These results also reinforce previous admonitions that in patients with respiratory dysfunction, it is important to use as large an endotracheal tube as possible and allow spontaneous respirations only once substantial clinical improvement has occurred.