Blood pressure measurement and nocturnal dipping patterns are heavily affected by body posture through changes in hydrostatic pressure between the arm and the heart

Abstract

Nocturnal blood pressure (BP) shows the highest predictive power for cardiovascular events. However, there is a poor reproducibility of personalized dipping patterns in single individuals. We hypothesize that changes in body position during sleep cause variations in hydrostatic pressure,leading to incorrect BP values and dipping classifications. 26 subjects aged 18-30 years, as well as 25 participants aged 50 years and older underwent ambulatory BP measurements on the left arm, as well as determination of the hydrostatic pressure difference between the cuff and heart level during BP measurement. We observed that the BP measurement cuff was above the heart level (negative hydrostatic pressure) mostly through the night. Laying on the right side revealed the largest hydrostatic pressure difference and maximum incorrect BP measurement, with a mean of -9.61 mmHg during sleep. Correcting for hydrostatic pressure led to reclassification of nocturnal hypertension in 14 subjects (27.5%). Dipping patterns changed in 19 participants (37.3%). In total, 25 subjects (49.0%) changed either their nocturnal hypertension and/or their dipping classification. Our findings underscore the importance of accounting for hydrostatic pressure in ambulatory BP monitoring. Changes in body posture during sleep provide a plausible reason for the variability seen in nocturnal dipping patterns. Further research should focus on incorporating hydrostatic pressure compensation mechanisms in 24-h BP measurement. Limiting the noticeable effect of hydrostatic pressure differences could greatly improve hypertension diagnosis, classification, and treatment monitoring.

Keywords: Blood pressure monitoring, Ambulatory; Circadian rhythm; Hydrostatic pressure; Hypertension; Posture.

Fig. 1

Effect of body position on measured blood pressure: The upper-left panel (a) depicts the measurement setup. The ECG (blue), BP (grey), body position (yellow), and hydrostatic pressure (red) sensors are shown. The upper-right panel shows the recommended position for BP measurement with cuff at the heart level. We calibrated the measured hydrostatic pressure in that position to 0 mmHg (upper right panel b). The lower panel (c) shows examples of the effect of changes in body position on the measured blood pressure. The measured BP is lower than the true (heart level) BP when the arm is above the heart level and higher when below

Fig. 2

Hydrostatic pressure difference during measurement vs. at Heart Level: This figure delineates the deviation in hydrostatic pressure between the cuff and the heart level during BP measurement. We compared the hydrostatic pressure difference over the whole measurement as well as while awake and asleep. Distinctions are depicted between the whole data set, the younger (18–30 years) and older subgroup (50 years and above). Asterisks next to boxed indicate a significant effect compared to zero. Asterisks on top horizontal bars indicate differences between groups. * = p < 0.05

Fig. 3

Hydrostatic pressure difference and postural changes: Boxplots represent hydrostatic pressure differences at cuff versus heart level for various axial body postures, stratified by awake state (a), and sleep state (b). * = p < 0.05, SD standard deviation

Fig. 4

Systolic blood pressure and hydrostatic pressure difference correction: The figure depicts the comparison of mean systolic blood pressure values for all measurements compared to hydrostatic corrected values. Asterisks (*) indicate a statistical significance of p < 0.005

Fig. 5

Effect of correcting nocturnal hydrostatic pressure differences on clinical classification: the figure depicts changes of nocturnal hypertension (a) and dipping pattern (b) classifications. A yellow background indicates a classification change in the respective patient. SYS systolic blood pressure, corr. corrected, DIA diastolic blood pressure, HT Hypertension, unc. uncorrected, cor. corrected, RD reverse dipper, ND non dipper, D dipper, ED extreme dipper

Fig. 6

Effect of Mean Nocturnal Hydrostatic Pressure on Dipping: The left panel (a) depicts the connection between the mean nocturnal hydrostatic pressure difference between the cuff and the heart and the nocturnal dipping. The right panel (b) shows the difference in mean hydrostatic pressure difference between the four different dipping pattern classifications. RD reverse dipper, ND non dipper, D dipper, ED extreme dipper, Avg. average, N number of samples, SD standard deviation