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°C. In the reference situation arterial (Pa _{O 2}) and venous oxygen tensions (Pv _{O 2}) were about 610 and 290 mmHg, respectively. Step changes inPa _{O 2} 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 inPvO₂ 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 beforePv _{O 2} 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 ofPv _{O 2} in these preparations.