One of the most common mistakes I’ve heard on rounds, in the ICU, on the floor, or just in general conversation is that the “patient’s blood pressure is XXX over XXX.” The overwhelming majority of clinics and hospitals rely on automatic noninvasive blood pressure (NIBP) cuffs – monitors which do NOT measure systolic or diastolic blood pressures.
Instead, the automatic NIBP cuffs only measure the mean arterial pressure (MAP) through an oscillometric technique. Let’s assume we’re taking a NIBP on 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. Keep in mind that automated cuffs don’t auscultate. They can only feel for oscillations, and therefore, only measure a MAP.
This is extremely important to know since most patients in the hospital do NOT have invasive arterial lines for blood pressure monitoring. If you’re writing a antihypertensive, make sure you place appropriate hold/treatment parameters knowing that the 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 who has an automatic NIBP cuff. Instead, consider the treatment parameter as a MAP > 100 mmHg. 🙂