The ankle brachial pressure index is a measure and a test used to check for peripheral vascular disease. Peripheral vascular disease is an affliction that happens over time and could be because of some kind of blockage of the arteries. Usually, the problem is also one of calcification of the arteries in the leg because of age and several other factors. When a blood vessel calcifies, then it loses its ability to stretch under pressure. This means that the patient has severe peripheral vascular disease.
The ankle brachial pressure index is calculated using the ankle brachial index formula, which is the blood pressure at the leg divided by the pressure in the arm. Ideally, and in a normal patient, this index should give a reading of just 1 with some variations. This means that the blood pressure measure at the leg and the blood pressure measured in the arm should be equal. When this ratio goes above 1.3, this means that the blood pressure in the leg is more than in arm. This means that the vessels of the leg are not able to stretch adequately to accommodate the pressure difference and therefore are increasing the blood pressure to the leg.
Values that are below normal like between 0.9 and 0.8 would indicate that some artery is blocked and therefore not enough blood is flowing to the leg. The way that this pressure is measure is by taking the blood pressure in both arms with the highest systolic pressure being taken as the denominator of the formula. The pressures in the legs are then taken from behind the knee where there are two major arteries. Again, both legs are measured and the higher reading is used as a base. There is a ready table of values to interpret the index and is called ankle brachial index formula.
Calcification of arteries is usually a consequence of age. Low values in the ankle brachial index are usually indications of venous ulcers. Veins are designed in such a way as to prevent the back flow of blood. This is required to prevent pooling of blood and increased blood pressure in a vein. When a venous ulcer occurs, the inner lining of the vein does not function to prevent backflow. This causes proteins in the blood to leak and spill into the surrounding tissue causing a venous ulcer.