Did you ever wonder how your automatic blood pressure cuff actually works? No? Just me? Well…no surprise there.
When the machine inflates the cuff, its goal is to get it above the patient’s systolic blood pressure (table how it knows that for one second). It then uses this as its estimate for the next time it cycles. Incidentally, this is why the first pressure cycled on a new patient can be remarkably uncomfortable sometimes, or take a really long time.
While the cuff is cycling, a pressure sensor in the machine is looking for variance of the oscillations of the pressure in the cuff. When the cuff pressure exceeds the patient’s SBP there is no flow in the limb, and when the cuff pressure is below the patient’s DBP there is smooth flow in the limb, and the cuff pressure oscillations don’t vary much. In between, there are variable amplitude micro-oscillations measurable in the cuff with each beat as some blood is squished through the artery.
As it turns out, these oscillation amplitudes as maximal at the patient’s MAP. The SBP and DBP are calculated by proprietary, trade-secret algorithms. Each company has its own, and different machines applied to the same patient might give you different answered.
What does this mean? The MAP your machine gives you is the most reliable number. Bradycardic patients or patients with other arrhythmia can be relatively confusing to these machines. Two different BPs in each limb of the same patient should be a lot less scary if the MAPs are the same. I’m sure you can think of plenty more implications.
Knowing how your machines make your numbers is part of being a good user of your machines!