This interesting paper notes that the difference in blood pressure assessed in left and right limbs correlates with measures arterial stiffening, but perhaps only in the legs rather than the arms. This is a consequence of human physiology, the structure of the circulatory system, and how stiffening affects control of blood pressure. The debate that the authors address is whether or not arms and legs are both relevant measurement sites on the body for this correlation. Their evidence points to ankle measurement of blood pressure as the more relevant site, and they theorize as to why this might be the case. It is all interesting! But as is true of a great deal of present day biometrics relating to aging, the purpose of rejuvenation research, such as those parts of the field that might ultimately eliminate arterial stiffening, is to render all of these measurements and considerations irrelevant.
In this population-based study using a simultaneous measurement of blood pressure and arterial stiffness, we evaluated the association between contralateral systolic blood pressure (SBP) differences and arterial stiffness by pulse wave velocity (PWV). We found that the prevalence of interankle differences in SBP of ≥10 mmHg and ≥15 mmHg were common, namely 25% and 12%. Our findings showed that higher body mass index (BMI), and lower ankle-brachial index (ABI) were significantly correlated to greater interarm SBP differences, while increased age, higher BMI, lower ABI, and greater contralateral differences in PWV were significantly correlated to greater interankle SBP differences.
Previous studies indicated SBP differences between arms carry prognostic information and that patients should have evaluation of blood pressure in both arms. In addition, the ankle has been suggested as an alternative and/or additional site for noninvasive blood pressure measurement. In the present study, blood pressure was measured simultaneously, bilaterally at both limbs. Compared with sequentially repeated measurements of blood pressure with a single-cuff that is typically conducted, the simultaneous measurement may be more precise as beat-by-beat differences in blood pressure can be accounted for and can improve diagnostic accuracy. Although brachial SBP was significantly associated with ankle SBP, interarm differences in SBP were not related to interankle differences in SBP suggesting that these contralateral differences in blood pressures may be modulated by different factors. Indeed, arterial stiffness was associated with interankle, but not with interarm, differences in SBP in the present study.
What are the explanations for the contribution of arterial stiffness to interankle SBP difference but a lack thereof to interarm differences? Arterial stiffening is a principal determinant of SBP and has been independently associated with stroke, coronary disease severity, and cardiovascular outcome. As the arterial wall stiffens, arterial wave reflection is a primary mechanism responsible for augmenting SBP. In the arterial tree, branching points, areas of alteration in arterial elastance (from elastic artery to muscular artery), and high-resistance arterioles can all give rise to wave reflection and the lower body is believed to be an important site of wave reflection. This cumulation of reflected waves along with the longer distance of the arterial tree to the ankle versus the upper arm is the reason that SBP at the level of the ankles is elevated in comparison to pressures measured in the arms in healthy humans. It appears plausible that contralateral differences in SBP may be influenced to a greater extent by the stiffness of arteries in the ankle.