Longitudinal distribution of pulmonary vascular resistance

We have modified the low viscosity bolus technique described by Brody et al. (1968) for determining the longitudinal distribution of pulmonary vascular resistance. A bolus of saline is introduced into the pulmonary artery of an isolated cat lung. As this low viscosity bolus passes through the lung, we record a fall in the inflow pressure which has a characteristic shape depending on the changing shape and position of the low viscosity bolus and the longitudinal distribution of resistance. To determine the shape and position of the bolus within the lung at a given time, we measure the change in viscosity of the blood as the bolus enters and as it leaves the lung. We then mathematically model the dispersion process occurring between arterial and venous sampling sites. Assuming that the hemodynamic resistance of a segment is proportional to the product of the viscosity and its geometric factor, we employ an optimization procedure which gives us the best longitudinal distribution of geometric factors compatible with the position and dispersion of the bolus at any time and the arterial pressure curve. We have used these procedures to study how inflation influences the distribution of resistance. At low lung volumes, the calculated resistance was concentrated near the proximal part of the bed. As the lung was inflated, the resistance became more evenly distributed and localized more centrally.