- Penile blood flow with vasoactive stimuli of prostaglandin (VSP), measured by color echo-doppler. After applying a standard dosage of VSP, an exam measures blood flow in the cavernous and dorsal arteries at the base of the penis. This measurement is performed periodically every 5 minutes, as of the application of the medication until obtaining the best possible flow. This determines normal flow when above 35 cm/sec , undetermined when between 25 and 35 cm/sec and abnormal flow when under 25 cm/sec.