Distribution of myocardial blood flow and capillary diffusion capacity across the canine heart wall.

Myocardial capillary permeability was determined in 20 dogs by applying a new method which resembles the tissue-uptake technique. The method consisted of a bolus injection of 51Cr-EDTA into the left atrium, determination of the average arterial tracer concentration and subsequent assay of myocardial tissue activity 10, 20 and 30 s after injection. Under the fundamental assumption of no back-diffusion of tracer to capillary blood, uptake of 51Cr-EDTA into myocardium allowed calculation of the transfer constant, Kin, independent of blood flow. Regional plasma flow, fpl, was simultaneously determined from tissue content and average arterial concentration of radioactive microspheres and haematocrit. From estimates of Kin and fpl the fractional extraction, E, of 51Cr-EDTA was calculated as E = Kin/fpl. The capillary permeability-surface area product, PS, was calculated as PS = -fpl X k X 1n (1 - E). Constant fractional extraction of 51Cr-EDTA indicates that the method can be employed 10 to 20 s after injection without risk of back-diffusion. From tissue samples taken from subendocardial and subepicardial layers of the left and right ventricular walls and from left and right side parts of the septal region we measured similar PS values. Letting capillary surface area, S, equal 500 cm2 X -1 the permeability coefficient for 51Cr-EDTA was 1.39 X 10(-5) cm X s-1.