Prior to sleeping, the unit is secured to the patient’s thigh in a pouch worn on the leg, and the penile loops are fitted to the penis , one toward the base and the other toward the tip (Fig. 3.2). We typically place these at the junction of the mid-proximal one third of the penis (base loop) and at the junction of the mid and distal one thirds of the penis (tip loop). The penile circumference is measured by the machine at baseline every 15 s and saved into memory. The penile rigidity is checked every 3 min; if there is a greater than 3 mm increase, the sampling rate increases to every 30 s until the rigidity returns to baseline, and all is saved into memory. The device is worn for at least two nights at home. Once the unit is returned to the office, it may be connected to a computer where the data can be downloaded to show rigidity and circumference readings.

The limitation of this device is that the majority of patients even with normal erectile function will have an equivocal result; that is, they will fail to get 3–5 erections of 60–70 % rigidity lasting at least 10–15 min in duration. The most recent iteration of this device uses rigidity activity units (RAU) to quantify more accurately the duration of the achieved rigidity. The number of minutes recorded at any given rigidity level is printed on the data sheet. Despite the advent of this innovation , there is little evidence to suggest that this approach is anymore accurate than the use of number or erections , degree of rigidity, and duration on any given night.