Automated noninvasive blood pressure (NIBP) cuffs do NOT directly measure systolic or diastolic blood pressures (SBP and DBP, respectively).
Instead, the automatic NIBP cuffs only directly measure the mean arterial pressure (MAP) through an oscillometric technique. Let’s assume we’re taking an NIBP on the arm. Initially, the cuff inflates to occlude the brachial artery (no flow). As the cuff pressure drops, turbulent flow is generated through the vessel, creating oscillations against the arterial wall. As the pressure keeps dropping, these oscillations reach a point of maximal amplitude. The cuff pressure at this point is the MAP. The cuff then fully deflates opening up the artery, promoting more laminar flow, and reducing oscillations.
When we’re first taught to perform manual blood pressure with auscultation and Korotkoff sounds, we can discern the systolic and diastolic pressures based on when oscillations begin and end. However, keep in mind that automated cuffs don’t auscultate. They can only feel oscillations, and therefore, only directly measure a MAP.
This is extremely important since most patients in the hospital do NOT have invasive arterial lines for blood pressure monitoring. Therefore, hold/treatment parameters for antihypertensive medications should be written knowing that the automated NIBP cuff only measures a MAP.
For example, it’s not appropriate to write labetalol 10 mg IV q4 hour PRN for SBP > 160 mmHg in a patient with an automatic NIBP cuff. Instead, consider the treatment parameter as a MAP > 100 mmHg. 🙂