Diffusional shunting of oxygen in saline-perfused isolated rabbit heart is negligible

Diffusional shunting of oxygen in the saline-perfused heart was studied by comparing the time course of the coronary venous concentrations of oxygen and an intravascular indicator following a simultaneous step-like change in their arterial concentrations. To this end 7 rabbit hearts were perfused according to Langendorff with Tyrode solution at a perfusion flow rate of 3.8 +/- 1.4 ml.min-1.g-1 (wet weight) at 37 degrees C. In the reference situation arterial (Pao2) and venous oxygen tensions (Pvo2) were about 610 and 290 mmHg, respectively. Step changes in Pao2 were made to a 60 mmHg lower level and back. Simultaneously the arterial concentration of albumin-bound indocyanine green, an intravascular indicator, was changed. No deflection in PvO2 was detected before the venous dye concentration changed. The venous dye concentration crossed 5% of its step amplitude 4 s after the arterial change, on average 2.3 s before Pvo2 crossed its 5% level. We conclude that shunt diffusion of oxygen from arterioles to venules and from arterial to venous ends of the capillary bed is negligible in saline-perfused hearts and thus cannot explain the high value of Pvo2 in these preparations.