Frequency Function of Transit Times through Dog Pulmonary Circulation

The frequency function of transit times through the pulmonary vascular system in dogs was obtained by deconvolution of a pair of simultaneous tracer dilution curves. One tracer dilution curve represented transit times from pulmonary artery to aorta; the other represented transit times from left atrium to aorta. Theory of and requirements for deconvolution are presented. Two preparations were used: closed chest with recirculation, open chest with only myocardial recirculation. The frequency function of pulmonary vascular transit times was skewed markedly to the right. At constant cardiac output and left atrial pressure it was unaffected by change in heart rate or by respiratory movements. Frequency functions of transit time obtained by deconvolution of observed tracer concentration curves, including recirculation, were compared with those obtained by monoexponential extrapolation of the same tracer concentration curves to "eliminate" recirculation. If recirculation seemed to appear before the observed concentration curve (aortic sampling following pulmonary arterial injection) fell to 20% of peak, monoexponential extrapolation led to a 10% overestimate of mean transit time and to greater errors in estimate of higher moments. If recirculation did not seem to occur until the concentration fell to 10% or less of peak, there was no difference between the frequency functions of transit times calculated by the two methods. Increased coefficients of variance and skewness were associated with large mean transit times produced by hypovolemia and reduced cardiac output secondary to withdrawal of blood.