Low-flow Direct By-pass
Grafts may be low flow, moderate flow, or high flow. Per The Handbook of Neurosurgery, low flow is 15-25 ml/min with corresponding graft material involving direct anastomosis of the superficial temporal artery (STA) to the middle cerebral artery (MCA), a major branch of the anterior (Carotid) circulation of the brain. The STA is a pedicled arterial graft and the most common direct by-pass method. It has 95% graft patency. This provides immediate improvement of circulation to the brain. Occipital to PICA is another type of direct low-flow by-pass bringing blood to the posterior (Vertebrobasilar) circulation of the brain.
High-flow Direct By-Pass
The next category is moderate to high flow (40-70 ml/min), typically radial artery graft. It is easy to harvest form the arm, and has a lumen adaptable to the lumen of M2 (a branch of the Middle Cerebral artery) and P1 (a Brain of the Posterior Cerebral Artery) but carrying a risk for vasospasm. Lastly, saphenous vein graft form the leg, is a high flow graft with a flow rate of 70-140 ml/min that is easily accessible and has long length but has risk of thrombosis and distal anastomosis due to flow mismatch and turbulence and lower overall graft patency rates. Radial artery grafts have > 90% graft patency at 5 years, while saphenous vein grafts only have 82% patency at 5 years.