Ankle to arm index response to exercise and heat stress in healthy subjects

Ankle to arm index (AAI) defined as the ratio of ankle systolic blood pressure (ASBP), to brachial systolic blood pressure is largely used in the study of lower extremity arterial disease (LEAD). To study the hypothesis of the shunt of blood away from the skin as the explanation of AAI decrease in exercise, we studied the AAI and ASBP responses to an increase in cardiac output originating from an increase either in muscle blood flow (exercise) or in cutaneous blood flow (thermal stress). Brachial systolic pressure, ankle systolic pressure and heart rate (HR) were measured in 9 healthy subjects at rest, during heart thermal stress and following maximal exercise on a cycle ergometer. Compared to resting values, AAI decreased in all subjects from 1.05 +/- 0.07 to 0.75 +/- 0.07 (P < 0.05) 1 min following exercise and from 1.08 +/- 0.07 to 0.94 +/- 0.05 (P < 0.05) during heat stress. On the other hand, HR increased from 72.8 +/- 12.2 to 112.4 +/- 19.6 (P < 0.05) min following exercise and from 75.5 +/- 13.6 to 96.8 +/- 15.3 (P < 0.05) during heat stress. Since a comparable relation exists between AAI and HR in thermal stress and exercise, we suggest that the decrease in AAI in normal subjects following exercise is due to turbulences at high flow levels, rather than the shunting of blood to active muscles in exercise.