EFFECT OF ISOPROTERENOL ON PERIPHERAL AIRWAY PRESSURE-DIAMETER BEHAVIOR

I radiographically evaluated the pressure-diameter relationships of pe ripheral airways (average diameter almost-equal-to 2 mm at total lung capacity) in a collaterally ventilating lung segment in 22 caudal dog lung lobes. Eleven lobes were studied when freshly excised, and 11 lob es were studied after being rinsed with 0.5% isoproterenol. Airways wi thin the segment (intrasegmental) and a control region of similarly si zed airways external to the segment were studied. Radiographs were tak en when airway opening pressure (Pao) equaled pressure in the segment (Ps) (homogeneous case) and when Pao was 5, 10, or 15 CmH2O greater th an Ps (nonhomogeneous case). Airway diameter was calculated as a fract ion of the diameter at Pao = Ps = 25 cmH2O. The response of intrasegme ntal airways to increases in pressure in the homogeneous case (Ps = Pa o) and nonhomogeneous case (Ps > Pao) differed between freshly excised and isoproterenol-rinsed lobes. I conclude that dilating peripheral a irways by decreasing bronchomotor tone limits the extent to which they can respond to a regional increase in alveolar pressure.