The effect of intracavernosal haemodilution on the radionuclide quantification of penile vascular changes during pharmacologically induced penile erections.

Radionuclide phallography was performed using 99Tcm-labelled red blood cells in conjunction with pharmacologically induced penile erections. The radioactivity content of aliquots of systemic venous blood taken from the antecubital vein was compared with aliquots of blood withdrawn from the penile cavernosa 20 min after an intracavernosal injection of 10 micrograms prostaglandin E1. The cavernosal samples showed significantly lower counts per unit of blood compared to the systemic venous blood indicating a comparatively lower concentration of tagged red blood cells. On haematological analysis, a significant difference in the haematocrit of the two samples was also established. The fall in the cavernosal haematocrit was found to result from dilution of the cavernosal blood pool by the injected volume of the drug since larger volumes of injection produced a greater fall in the haematocrit. Restriction of the cavernosal venous outflow in response to the injection of the vasoactive drug causes sequestration of the diluted blood in the cavernosal compartment. This factor might affect the quantification of penile blood volume using radiotracer methods.