Eli5: how does the blood pressure cuff determine your blood pressure so accurately?
In: 200
There are actually multiple ways of determining blood pressure. Direct and indirect. Direct is when you have a catheter in an artery and it feels the blood pushing. Indirect is with the cuff.
When a human being is taking blood pressure with a cuff and stethoscope, what they’re listening for is turbulence in the artery caused when an artery collapses. So when the blood pressure cuff is inflated to a pneumatic pressure higher than what the heart is pumping, no blood can flow through an artery. Therefore, no sound. Then the operator will slowly release the blood pressure cuff. When the blood pressure coming from the heart matches the pneumatic pressure of the cuff, blood flows again. However, the flow of blood is NOT what you hear. When the blood has been pumped through the artery, the pressure in the blood drops, and the pneumatic cuff pressure is greater than the pressure in the artery again. This causes the artery to collapse against the cuff. This collapse causes a pop that the operator hear. The action of blood overcoming the cuff pressure to collapsing against the cuff happens very fast. Almost instantaneous. The first pop that the operator hear, is the systolic blood pressure. It effectively means the highest pneumatic cuff pressure that the heart can overcome.
Then, the operator keeps releasing the cuff pressure and the cycle repeats. Until, the last pop that the operator hear. The last pop signifies the lowest pneumatic cuff pressure that can still overcome the blood pressure causing the artery to collapse. Any cuff pressure lower than that, the artery is able to stay open, causing no further collapse, causing no further sound. This is the diastolic, or resting blood pressure.
For the machine, it’s actually more complicated than that. The machines don’t listen for the pop. What it does is first inflate the cuff. Then it measures the pressure from the artery pushing against the cuff. It puts it on a graph. The graph is upside down parabola shaped. Then, it deflates the cuff incrementally, then it measures the artery pressure again and charts it. It takes several of these readings at different pneumatic pressures, comparing to see if the graph appears similar. If it does, then it uses integral calculus to determine the area under the curve of each curves, then, average it out. This gives it something called “mean arterial pressure.” With that value, through different proprietary mathematical formula (depending on the manufacturer), calculates the systolic and diastolic blood pressure.
Latest Answers