Aside from the differences in location, venous cannulation is performed similarly to arterial cannulation. Since calcification of the venous system is less common, the inspection or use of an ultrasound for calcification at the cannulation sites is unnecessary. Also, because the venous system is under much less pressure than the arterial system, only a single suture is required to hold the cannula in place. If only a single cannula is to be used (dual-stage cannulation), it is passed through the right atrial appendage, through the tricuspid valve, and into the inferior vena cava. If two cannula are required (single-stage cannulation), the first one is typically passed through the superior vena cava and the second through the inferior vena cava. The femoral vein may also be cannulated in select patients.