Assessing the deformation of pulmonary capillaries produced by ventilation.

We used a Fourier transform method to compute from the density fluctuation of aortic blood the changes in the blood volume of pulmonary capillaries and the transit time from the capillaries to the aorta. First the time constants of a lagged normal density function capable of simulating the saline dilution through the central circulation were established. Then we decomposed the Fourier transform of the lagged normal density function to determine an attenuation for the estimation of the density fluctuation in blood leaving the capillaries. Regarding the low hematocrit blood in the capillaries as a density indicator, we deduced from the density fluctuation the percentage change of capillary volume for dogs ventilated mechanically. This change was found to range from 3.8 to 5.8% for frequencies within 6 to 20 cycles per minute. Based on this simulation and the delay time of the density fluctuation from the tracheal pressure, we estimated that the transit time for the density indicator being released from the pulmonary capillaries to the aorta was 45% of the mean transit time of the central circulation, suggesting that the releasing sites are midway of the pulmonary capillaries.