The patient should be supine on an examination table undressed from the waist down. If the patient has presented within 4–6 h of the commencement of the erection, a simple intracavernosal injection (one or two) is likely all that will be needed. If longer than this, the chance of needing aspiration increases significantly. When aspiration is expected, a penile block should be administered. While the definitive means of differentiating between ischemic and nonischemic states is a STAT cavernosal blood gas, for the patient in your office, the clinical history will usually suffice in defining the cause. Most of the patients will have undergone intracavernosal injection of a vasoactive agent for ED or for the purpose of a penile duplex Doppler ultrasound .