Crossing the ankles in the sitting position has no effect on blood pressure (BP), but BP increases when legs are crossed at the knee level. The physiological mechanism responsible for this BP increase is a higher cardiac output (CO), but not a higher total peripheral resistance (TPR).
The purpose of our study was to determine the effect and physiological mechanism of crossing the ankle over the knee on BP.
Finger BP was measured continuously and noninvasively by the use of the Nexfin monitor by one trained investigator. Twenty five participants crossed their ankle over the knee with the lateral malleolus on the suprapatellar bursa in the sitting position. Differences in systolic BP (SBP), diastolic BP (DBP), mean BP (MAP), heart rate (HR), stroke volume (SV), CO and TPR were determined in the crossed position versus feet flat on the floor.
Except TPR, all hemodynamic parameters were significantly higher with the ankle crossed over the knee versus the uncrossed position: SBP 11.4 ± 5.3 mmHg (mean ± SD) (9.2–13.6; 95% CI), DBP 3.8 ± 2.5 mmHg (2.8 – 4.9), MAP 7.0 ± 3.9 mmHg (5.4 – 8.6), HR 1.7 ± 3.2 beats/min (0.4 – 3.0), SV 5.7 ± 3.7 ml (4.2 - 7.3), CO 0.5 ± 0.3 l/min (0.4 – 0.7).
Our study shows that ankle crossing over the knee causes increases in BP, CO and SV, while TPR does not change. The working mechanism for the BP increase when the ankle is crossed over the knee seems to be comparable with the working mechanism of the BP increase when legs are crossed at knee level.
All persons who measure BP should be aware of the influence of leg position and instruct patients to keep their feet flat on the floor during BP measurements. The position of legs should be mentioned in all guidelines and publications regarding BP.
Radboud University Medical Centre, Nijmegen, The Netherlands