Poor Reliability of Wrist Blood Pressure Self-Measurement at Home

ion (score) −0.80 (0.91) −2.58 to 0.97 0.4 −1.05 (0.60) −2.24 to 0.13 0.1 Digit span (score) 0.72 (1.89) −2.99 to 4.42 0.7 0.73 (1.26) −1.74 to 3.20 0.6 IPM (score) −0.48 (0.91) −2.27 to 1.31 0.6 0.11 (0.61) −1.08 to 1.31 0.9 Education, y 1.78 (1.19) −0.56 to 4.12 0.1 0.36 (0.80) −1.20 to 1.92 0.7 Multiple linear regression. Systolic and diastolic errors as dependent variable, respectively. Age, sex, hypertension, arm circumference, forearm length, systolic blood pressure, educational level, memory with interference at 10 s test (MI 10), immediate prose memory (IPM), praxic abilities, clock drawing, abstraction, and digit span tests as independent variables. Pulse pressure was used in alternative to systolic and diastolic blood pressure. CI indicates confidence interval. *Selectively in the 508 subjects keeping the wrist higher than the heart level. by gest on O cber 0, 2017 http://hyhajournals.org/ D ow nladed from Casiglia et al Unreliable Wrist Home BP 901 0.870; P=0.02) were negatively associated, and forearm length (odds ratio, 1.066; P=0.03) was positively associated with ±5 bad measurers (complete analysis is available in Table S7). Discussion The main finding of our study is that the relationship between BP measured at the upper-arm and at the wrist varied according to whether BP measurements were made in the office under a doctor’s supervision or at home in a real-life situation. When BP was taken in the office, the values measured at the wrist were, as shown by others, slightly lower than those measured at the upper-arm. In contrast, when BP was self-measured at home by the study participants, higher BP values were obtained at the wrist than at the arm. In the large majority of the participants classified as bad measurers, the home measurement error was because of a disproportionately high wrist BP. The discrepancy between the upper-arm−wrist BP difference obtained in the 2 settings is probably because of an error in home self-measurement despite appropriate training. It is likely that many subjects, when left free to measure their BP at home, did not follow the instructions received during the training session because of a deficit in memory or in executive functions, a limitation that persisted after adjustment for age and was not prevented by years of schooling. Inability to follow the instructions for poor memorization and carelessness were likely to affect wrist BP rather than upper-arm BP measurement because of the important effect of an incorrect forearm position on wrist BP. Accurate measurement of BP at the wrist requires that the heart and the wrist are kept at the same level to avoid the effects of hydrostatic pressure. If the forearm is kept horizontal on the supporting desk (Figure 1C), leaning or even vertical along the subject’s side (Figure 1D), the hydraulic pressure caused by the upper limb blood column mass is added to the hemodynamic pressure and their sum is recorded by the wrist device. Based on the difference between density of human blood and mercury, in the present study, the magnitude of the home measurement error would translate into an average level discrepancy between the heart and the wrist of 10±11 cm (confidence interval, 9.1– 11.5). According to this extrapolation, in our experience, the range of the error was from 10 over to 65 cm under the heart level. Obviously, part of the upper-arm−wrist BP difference may be because of random BP variability between 1 measurement and the other or to unreliable upper-arm BP measurements, and thus the differences in level reported above can only be considered as indicative. This interpretation was confirmed by significant effect of forearm length in multivariable linear analyses for the systolic home measurement error. When the wrist is kept at a lower level than the elbow (Figure 1D), a longer forearm magnifies the hydrostatic effect of the wrong arm position,34 an effect that is notoriously more pronounced for systolic than for diastolic BP. The reason for the wrong position of forearm in a high number of subjects was probably because of a worse cognitive pattern, as shown by the inverse association of the measurement error with memory, praxic abilities, visuospatial, and executive functions, as shown for instance by the clock drawing test. It is presumable that subjects having worse cognition were those more prone to make a mistake in wrist self-measurement. This interpretation is corroborated by previous observations that wrist devices provided with a position sensor, helping subjects to keep the wrist at heart level, usually give lower values than those without a sensor. In the present study, it was decided to use wrist monitors without a sensor because these are the devices mostly used in the real world. The home measurement error was also associated with higher BP levels or the diagnosis of hypertension. A higher BP is likely to affect the magnitude of the measurement error. In addition, in keeping with previous study, the hypertensive participants had a much worse cognitive pattern, as shown in Table S2. However, the measurement error was also present among the normotensives.