A quantitative study to investigate the relationship between a toe-brachial index in a supine position versus a seated position

Background: Research suggests that toe-brachial index (TBI) can be significantly more reliable than ankle-brachial index (ABI) when arterial rigidity due to medial artery calcification (MAC) is suspected. The need to place patients in a supine position to calculate both indices can be difficult in certain patient groups, including the elderly, those with respiratory impairment or who are wheelchair-bound. Given a prior study had validated the use of a hydrostatic pressure formula to correct ABI measured in a seated to a supine position, a similar investigation into TBI was considered to be worthwhile for application to diabetic patients or where MAC is suspected. The findings, if clinically validated, would offer healthcare professionals an affordable and reliable tool to identify peripheral arterial disease (PAD) when supine postures render a traditional approach to TBI infeasible. Aim: The primary objective of the study was to investigate whether there is an association between the values of TBI measured with participants in a seated and supine position, and to explore the feasibility of predicting TBI from the blood pressure measurements obtained in a seated position. Method: The TBI of 35 healthy volunteers was measured in both seated and supine positions using an automated photoplethysmography (PPG) device (BASIC 3.4, Atys Medical). Data analysis included a Pearson’s correlation test to determine the linear correlation between TBI in each position. Hydrostatic pressure elimination was then applied to infer the supine values of toe blood pressure from a seated position. Regression tests were undertaken to predict supine TBI. Bland-Altman analysis was conducted to evaluate the agreement between predicted TBI and measured TBI. Results: According to the results of Pearson’s correlation test, a positive and strong linear correlation (r = 0.598, p < 0.001) existed between the value of TBI measured in the two positions. In terms of predicting supine TBI in regression analysis, the approach of eliminating hydrostatic effect from toe systolic blood pressure (SBP) returned a larger coefficient of determination (R2 = 0.467) than the other without elimination (R2 = 0.358). With this approach, supine TBI can be approximated from the inferred supine TBI multiplied by a coefficient of 0.614 plus 0.309. Conclusion: The study results and analyses indicated a positive correlation whereby the greater the seated TBI, the higher the corresponding supine TBI. TBI can be predicted from the seated position with the use of a hydrostatic pressure formula. However, the predicted values should be applied with caution due to the limited sample size involved and potential influence of other parameters; therefore further investigation is recommended. A supine position should still be recommended for measuring the index where possible given that the predicted TBI from a seated position is not sufficiently agreeable with a supine TBI to be clinically reliable.