Regional gas trapping during bronchoconstriction in the prone and supine positions [using PET and radio-labeled tracers]

Airway closure and gas trapping are thought to occur during bronchoconstriction, but the magnitude and location of trapped gas regions are unknown. To examine this phenomenon, we imaged regional lung function in (12 (12/12) supine, 5/12 supine and prone) mechanically ventilated supine sheep (FIO₂=0.5) before and one hour after intravenous infusion of methacholine (2.2-6.6 mg/hr) using positron emission tomography (PET). During steady state ventilatory conditions, mechanical ventilation was stopped at end-exhalation, a bolus of ¹³ NN was infused intravenously, and intrapulmonary regional tracer content, proportional to perfusion, was imaged for 30 seconds. At that point, ventilation was restarted and regional specific ventilation was assessed from the washout rate of the radio-labeled tracer. The fraction of perfusion into trapped regions was expressed as the ratio of the mean tracer content remaining after 1.5 minutes of ventilation to the initial tracer before the washout. Peak airway pressures increased from a mean of 20 to 36 cm H₂O with methacholine infusion. Trapped tracer was always measured in the dorsal lung regions and ranged from 0.111 to 0.405 in the supine position (N=12) and from 0.108 to 0.3505 (N=5) in the prone position. We conclude that gas trapping with methacholine infusion occurs preferentially in the dorsal lung regions and appears to be minimally affected by changes in regional lung inflation caused by body position