PDS aims to examine the cavernosal arteries and the response of their spectral Doppler waveforms following intracavernosal injection of a pharmacostimulant agent, commonly a prostaglandin E1 derivative such as alprostadil. The fundamental principle is repeated sampling of these waveforms in a stepwise manner until maximal peak systolic and minimal diastolic velocities have been reached [7
Owing to the nature of the problem under investigation, a quiet, private and comfortable environment is essential for satisfactory PDS. Many patients will be anxious, and a detailed explanation of the procedure is required prior to commencing. Informed consent should be obtained, especially with regard to the low risk of priapism following intracavernosal injection [11
]. The patient is positioned supine upon the examination bed and an initial injection of alprostadil is tailored according to the patient. If the patient is pharmacologically naïve, then a small dose (5 μg of alprostadil) is initially given. If there has been a poor response to the PDE5 agents previously, then up to the full dose (20 μg) may be given at the outset. The injection is gently massaged into one of the corpora cavernosa. The authors' standard approach is dorsal, although ventral scanning may also be undertaken. Initially transverse views are obtained, utilising a high-frequency linear probe, and once tumescence commences an oblique–longitudinal approach may be necessary. Angulation of 20–30° cephalad in the transverse plane enables visualisation of the cavernosal artery at its root, running towards the probe, and the artery can be insonated at a Doppler angle of 0°. As usual, angle correction is essential and should be implemented. A velocity gradient exists within the artery from the base to the tip, and reproducible and accurate measurements are best obtained at the penile base towards the peno-scrotal junction [12
]. 2–3 min after injection, the cavernosal arteries should become more visible, and spectral measurement and image acquisition should begin at this stage. In addition, the quality of the erection should be assessed both objectively by the operator and subjectively by the patient, and recorded. If the quality of erection is insufficient, repeated injection can be made. Other authors advocate self- or visual stimulation [13
Repeated Doppler measurements should occur at 5-min intervals until the maximal peak systolic velocity (PSV) and end-diastolic velocity (EDV) are judged to have been reached. The PSV is normal if it is >35 cm s –1 and EDV is usually normal if negative or close to 0 cm s–1.
The PDS waveform during erection is multiphasic ( and ). Following injection of pharmacostimulant, initial waveforms display elevation of velocities, especially the diastolic velocity, which reflects smooth muscle relaxation (Phase 2, ). The PSV then usually stabilises after this and is sustained for at least 5 min. After this, intracavernosal pressure (IP) increases, reflected by a steady reduction in the end-diastolic velocity as increasing sinusoidal distension elevates IP and impedes venous outflow (Phase 3, ). As the IP continues to rise, the diastolic velocity diminishes and the systolic waveform narrows. When IP exceeds diastolic pressure, the diastolic waveform will reverse (Phase 4, ). As maximal penile rigidity is achieved in the final phase, the IP is equal to or greater than systolic, producing further narrowing of the systolic peak. In some cases the systolic velocity may reduce to such an extent that there is a transient interruption to systolic flow.
Figure 7 Diagrammatic representation of multiphasic Doppler waveforms encountered during normal erection in the cavernosal arteries. Phase 1—initial low flow arterial waveform. Phase 2—elevation of systolic and diastolic velocities approximately (more ...)
Figure 8 (a) Longitudinal Doppler image showing cavernosal artery at base of penis (seen in longitudinal section) prior to stimulation. Note the barely recordable arterial flow. (b) The same cavernosal artery trace a few minutes following injection. Note the elevated (more ...)
When the minimal diastolic velocity is attained, the Doppler study is complete, although subsequent examination of the vessel course should be undertaken to exclude distal stenosis and appreciate any variations in the vascular supply.
Standard greyscale imaging is then used to examine for the presence of non-vascular abnormalities such as plaques, fibrosis or tunica albuginea defects.