A water-filled body plethymograph for the measurement of pulmonary capillary blood flow during changes of intrathoracic pressure.

A water-filled body plethysmograph was constructed to measure gas exchange in man. As compared to an air-filled plethysmograph, its advantages were greater sensitivity, less thermal drift, and no change from adiabatic to isothermal conditions after a stepwise change of pressure. When five subjects were completely immersed within it and were breathing to the ambient atmosphere, they had a normal heart rate, oxygen consumption, CO(2) output, and functional residual capacity. Pulmonary capillary blood flow ([unk]Qc) during and after Valsalva and Mueller maneuvers was calculated from measurements of N(2)O uptake. Control measurements of [unk]Qc were 2.58 liters/min per m(2) at rest and 3.63 liters/min per m(2) after moderate exercise. During the Valsalva maneuver at rest (intrapulmonary pressure: 24, SD 3.0, mm Hg), [unk]Qc decreased from a control of 2.58, SD 0.43, liters/min per m(2) to 1.62, SD 0.26, liters/min per m(2) with a decrease in pulmonary capillary stroke volume from a control of 42.4, SD 8.8, ml/stroke per m(2) to 25.2, SD 5.5, ml/stroke per m(2). After release of the Valsalva, there was an overshoot in [unk]Qc averaging +0.78, SD 0.41, liter/min per m(2) accompanied by a significant increase in heart rate. Similar changes occurred during and after the Valsalva following moderate exercise. During the Mueller maneuver at rest and after exercise, [unk]Qc, heart rate, and central stroke volume did not change significantly.