Longitudinal distribution of vascular resistance in the lung

We have modified the low-viscosity bolus technique for determining the longitudinal distribution of pulmonary vascular resistance. A bolus of saline was introduced into the pulmonary artery of an isolated cat lung. As this low-viscosity bolus passed through the lung, a fall in inflow pressure was recorded which had a characteristic shape depending on the changing shape and position of the low-viscosity bolus and the longitudinal distribution of resistance. The shape and position of the bolus within the lung at a given time were calculated using the change in viscosity of the blood measured as the bolus entered the pulmonary artery and as it emerged from the left atrium. Assuming that the hemodynamic resistance of a small segment of the vasculature is proportional to the product of the viscosity and its geometric factor, we employed a sequential-pattern search technique to calculate the "best" longitudinal distribution of the geometric factor, compatible with the position and dispersion of the bolus at any time and the arterial pressure curve.