Ankle Brachial Indices Revisited
Your next patient is a 67 year-old male who presents for painful right foot. He notes that the foot hurts more with ambulation than with rest, although he sometimes dangles the foot off the edge of the bed to help decrease the symptoms. He denies trauma.
On physical examination, his feet are both slightly cool, but the right more so than the left. A palpable pulse on the right is present but thready. He has no erythema, no significant swelling, and no tenderness to palpation.
The most likely diagnosis for this gentleman is that he has some vascular occlusion and is suffering from an intermittently ischemic limb. The gravity dependent nature of his symptoms particularly point to this diagnosis, as lower positioning often makes other causes of painful extremity (fracture, cellulitis, veinous obstruction) worse.
The appropriate next test is a calculation of his ankle-brachial index. Traditionally, ankle-brachial indices are measured as follows:
1) Manual blood pressure is taken on bilateral upper and lower extremities, with doppler used to distinguish the systolic blood pressure. Both dorsalis pedis and posterior tibialis arteries should be assessed in the lower extremities.
2) The highest systolic blood pressure in the affected lower extremity is divided by the highest systolic blood pressure in the upper extremity.
The resulting number is the ankle-brachial index. An ankle-brachial index of less than 0.9 is indicative of potential arterial insufficiency and requires further evaluation. In the setting of penetrating injuries, such as gunshot wounds, this is also an effective strategy of evaluation.
This approach is time-tested and validated. However, it is also time-consuming and requires items that are often used but difficult to find in the average emergency department, such as manual blood pressure cuffs and dopplers. Fortunately, studies have demonstrated that automatic blood pressure machines do a good job of estimating ABIs. Ankle-brachial indices calculated using automatic blood pressure cuffs had, when compared to manually-derived ABIs, a sensitivity of 92%, specificity of 98%, positive predictive value of 86, and negative predictive value of 99% in the determination of ABI < 0.9.
Benchimol D, Pillois X, Benchimol A, Houitte A, Sagardiluz P, Tortelier L, Bonnet J. Accuracy of ankle-brachial index using an automatic blood pressure device to detect peripheral artery disease in preventive medicine. Arch Cardiovasc Dis. 2009 Jun-Jul;102(6-7):519-24.