Pressure index in hypotensive or hypertensive patients

Pressure index (brachial/ankle systolic pressure) has been used in vascular surgery for many years and it has been assumed that its value is independent of the systemic pressure. The purpose of this study was to determine whether this assumption is true. Two hundred patients having cardiac catheterisation were studied by measurements of aortic or brachial pressure and ankle systolic pressure using Doppler ultrasound. These measurements were done during catheterisation for cardiac investigation, during operation and in the intensive care unit. None of these patients had peripheral arterial disease. A non-linear relationship was found between the systemic pressure and the systolic ankle pressure. For systemic pressure less than 100 mmHg or greater than 200 mmHg, the ankle pressure was on average 25% lower. For systemic pressure 100 mmHg to 200 mmHg, the ankle pressure was the same or slightly higher. In conclusion, in normal limbs the PI is 1.0-1.2 when the systolic pressure is between 100 and 200 mmHg, but in patients with hypotension or hypertension, it may be less than 1.0. This is an important observation which will prevent vascular surgeons from making erroneous diagnoses of arterial disease in the presence of hypertension or hypotension.