Contributor: Aaron Lessen, MD
Educational Pearls:
How does an automated blood pressure cuff work?
Automated blood pressure cuffs work differently than taking a manual blood pressure.
While taking a manual blood pressure, one typically listens for Korotkoff sounds (turbulent flow) while slowly deflating the cuff.
An automatic blood pressure cuff only senses the pressure in the cuff itself and specifically pays attention to oscillations in the pressure caused by when the pressure of the cuff is between the systolic (heart squeezing) and diastolic (heart relaxed) pressures.
These oscillations are at a maximum when the pressure in the cuff matches the mean arterial pressure (MAP) and therefore the machines are most accurate at reporting the MAP.
The machines then use the MAP and other information about the oscillations to estimate the systolic and diastolic pressures, which are less accurate.
What should you do if you need more accurate systolic and diastolic blood pressures?
Take a manual blood pressure.
Get an arterial-line (a-line), which provides continuous data for the blood pressure at the end of a catheter.
What happens if the cuff is too big or too small for the patient?
If the cuff is too small it will overestimate the pressure.
If the cuff is too large it will underestimate the pressure.
What should you do if the cuff cycles a bunch of times before reporting a blood pressure?
It probably isn’t very accurate so consider another method.
Bonus fact!
The MAP is not directly in the middle of the systolic and diastolic pressures but is weighted towards the diastolic pressure. The MAP can be calculated by adding two-thirds of the diastolic pressure to one third of the systolic pressure. For example if the BP is 120/90 the MAP is 100 mmHg.
References
Benmira, A., Perez-Martin, A., Schuster, I., Aichoun, I., Coudray, S., Bereksi-Reguig, F., & Dauzat, M. (2016). From Korotkoff and Marey to automatic non-invasive oscillometric blood pressure measurement: does easiness come with reliability?. Expert review of medical devices, 13(2), 179–189. https://doi.org/10.1586/17434440.2016.1128821
Liu, J., Li, Y., Li, J., Zheng, D., & Liu, C. (2022). Sources of automatic office blood pressure measurement error: a systematic review. Physiological measurement, 43(9), 10.1088/1361-6579/ac890e. https://doi.org/10.1088/1361-6579/ac890e
Vilaplana J. M. (2006). Blood pressure measurement. Journal of renal care, 32(4), 210–213. https://doi.org/10.1111/j.1755-6686.2006.tb00025.x
Summarized by Jeffrey Olson, MS3 | Edited by Meg Joyce, MS1 & Jorge Chalit, OMS3