Estimation of regional pleural surface expansile forces in intact dogs

Distances between percutaneously inserted apical and basal lung markers determined by biplane X-ray, computer-based videometry (J. Appl. Physiol. 34: 544, 1973) were calibrated against dependent percutaneously recorded pleural liquid pressures (J. Appl. Physiol. 31: 277, 1971) in five 10-12 kg mongrel dogs under morphine-pentobarbital anesthesia studied without thoracotomy. At the same apical pleural-liquid pressure values, the apical intermarker distances were uniformly greater when in the head-up rather than head-down position. This finding suggests that the expansile forces acting on the apical regions of the lung, in the head-up position, are greater (-30 +/- 2 cmH2O) than would be predicted from "pleural liquid" pressures (-15 +/- 1 cmH2O) measured at this nondependent site in the thorax, and much greater than the "pleural surface" pressures measured by the generally accepted balloon and counter-pressure techniques. In contrast, in the head-down body position, expansile forces acting on the nondependent basal regions of the lung estimated by the intermarker distance technique were variable to either side of the pressures measured by open-ended liquid-filled catheters, and thus were not significantly different. Mean values measured by the catheters and predicted by the markers were -14 +/- 1 and -10 +/- 3 cmH2O, respectively.