Blood flow in an artery is related to:

• pressure gradient across the segment;

• diameter of the vessel;

• viscosity of the blood.

In patients with occlusive arterial disease, stenoses or occlusions proximal or distal to the area being assessed reduce the pressure gradient either by reducing the perfusion pressure or increasing the peripheral resistance. Low-pressure gradients across a graft or diseased arterial segment reduce the flow and therefore predispose to thrombosis.

Inflow pressure will also be reduced in patients with a reduced cardiac output and therefore patients who have stenotic arterial disease are particularly at risk of acute thrombosis during periods of reduced cardiac output from whatever cause.

Increased blood viscosity due to polycythaemia or dehydration can reduce blood flow. Malignancy or chemotherapy

Figure 15.2. Cold injury.

Figure 15.1. Marginal artery of the colon demonstrating the collateral circulation in a patient with superior mesenteric artery occlusion.

can induce hypercoagulability. Both can increase risk of thrombosis in the arterial or venous circulation.

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