A 24-year-old man is brought to the emergency department after a high-speed motorcycle collision. He complains of severe pain in his left leg. Physical examination reveals a deformed left knee with an obvious posterior dislocation. The dislocation is promptly reduced in the emergency department. Afterward, palpation of the left pedal pulses reveals they are diminished compared to the right side. The patient remains hemodynamically stable. To evaluate for potential popliteal artery injury, the physician measures systolic blood pressures using a continuous-wave Doppler ultrasound and a sphygmomanometer. The following systolic pressures are recorded:
| Location | Artery | Systolic Pressure (mmHg) |
|---|---|---|
| Right Arm | Brachial | 130 |
| Left Arm | Brachial | 140 |
| Right Leg | Dorsalis Pedis | 135 |
| Right Leg | Posterior Tibial | 138 |
| Left Leg | Dorsalis Pedis | 60 |
| Left Leg | Posterior Tibial | 70 |
What is the Ankle-Brachial Index (ABI) of the injured extremity?
Answer: 0.5
Answer
0.5
The correct Ankle-Brachial Index (ABI) is 0.5. The ABI is a crucial non-invasive test used to screen for peripheral vascular injury, highly relevant after a posterior knee dislocation which carries a high risk of popliteal artery injury. To properly calculate the ABI, you must divide the highest systolic blood pressure at the ankle of the affected leg by the highest systolic blood pressure measured in either arm. In this patient, the highest left ankle pressure is the posterior tibial artery (). The highest brachial pressure out of both arms is the left arm (). Therefore, the ABI is . An ABI in the setting of trauma strongly suggests an arterial injury and requires immediate further evaluation, typically with a CT angiography. Notably, if the patient were hemodynamically unstable with hard signs of vascular injury (e.g., pulsatile bleeding, expanding hematoma, absent pulses), delaying for an ABI would be contraindicated, and the patient would require immediate surgical exploration.
Step-by-Step Solution
Key Concept
Calculation and interpretation of the Ankle-Brachial Index (ABI) in lower extremity trauma to assess for vascular injury.
Estimated Time:1m 30s