Blood Pressure Measurements Affected by Location

January 28, 2021
Bob Kronemyer

Freelance writer for Contemporary OB/GYN

Blood pressure (BP) measurements are higher when taken at alternative locations like the wrist and forearm, according to a prospective study of pregnant patients.

This article is on based on information presented at the Society for Maternal-Fetal Medicine’s (SMFM) 2021 Virtual Annual Meeting, which is being held Jan. 25 to Jan. 30.

For more information and registration details, visit SMFM.org.

The observational study, which was presented virtually at SMFM’s annual meeting, also found that forearm BP measurements are higher than upper arm measurements with each body mass index (BMI) unit.

“It is a common scenario for providers to be notified when there is an elevated BP blood pressure taken at one location, but a normal reading when taken at another location. Clinically, which blood pressure is real?” said principal investigator Michelle Lende, DO,a first-year fellow in maternal-fetal medicine at Albany Medical Center in New York. “Unfortunately, there is limited data in pregnancy to guide us. We were concerned that inaccurate BPs could lead to overtreatment or under-recognition of hypertensive disorders in pregnancy.”

The investigators hypothesized that BP measurements on the forearm and wrist would differ from the upper arm and that this difference would be greater at higher BMIs.

The study comprised 100 pregnant patients seen at Albany Medical Center from July to August 2020. All patients had biometric measurements and three BP measurements, both systolic BP (SBP) and diastolic (DBP), taken at the upper arm, forearm and wrist that were 1 minute apart.

Measurements at the forearm and wrist were considered alternative locations and compared to the upper arm. Patients were also equally stratified into one of five BMI classes.

For the forearm, there was an upward bias of 11.5 mm Hg (limits of agreement [LOA]: +30.7 to -7.8) for SBP and 11.2 mm Hg (LOA: +25.9 to -2.9) for DBP as compared to the upper arm.

For the wrist, there was an upward bias of 7.1 mm Hg (LOA: +35.1 to -0.9) for SBP and 7.3 mm Hg (LOA: +26.2 to -11.7) for DBP versus the upper arm.

Overall, there was a larger discrepancy in BP measurements with increasing BMI. For instance, for the forearm compared to the upper arm, the discrepancy of SBP increased by 0.43 mm Hg (P < 0.001) and of DBP increased by 0.18 mm Hg (P = 0.02) for each unit of BMI.

But there was no statistically significant change in the discrepancy of SBP or DBP in the upper arm versus the wrist measurements based on BMI.

“Existing data in non-pregnant patients indicates that BPs are comparable, but different, between the upper arm, lower arm and wrist. Arm circumference also influences BP,” Lende told Contemporary OB/GYN. “Thus, it was not surprising that our results showed differences between the upper arm, lower arm and wrist in pregnant patients.”

However, the investigators were surprised that the wrist measurements did not differ across BMI, whereas the lower arm measurements did. “This finding could be secondary to the distribution of subcutaneous tissue and edema in pregnancy,” Lende said.

The study demonstrates that BP at the lower arm and wrist in pregnancy cannot be reliably used to treat or diagnose hypertensive disorders.

Concerning at-home BP monitors, “they are often automatic arm or wrist cuffs, so caution should be used before making hypertensive diagnoses based on home measurements alone in pregnancy,” Lende said.

The authors anticipate undertaking similarly designed studies to expand understanding of alternative BP sites. “Because radial and wrist cuffs are not readily available at all institutions, we plan to evaluate differences in BP when a thigh cuff is used on the upper arm or when a brachial cuff is used on the lower arm,” Lende said. “Additionally, we are interested in investigating how gestational age impacts subcutaneous tissue distribution and blood pressure, to determine if there is a difference between trimesters.”

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Lende reports no relevant financial disclosures.