A blood pressure monitor works by temporarily cutting off blood flow in an artery (typically in your arm) and then measuring the pressure as the blood flow returns.
Here's a breakdown of the process:
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Cuff Inflation: The inflatable cuff is wrapped around your upper arm and inflated to a pressure high enough to completely stop blood flow in the brachial artery. This is why you feel your arm tightening.
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Artery Occlusion: When the cuff is fully inflated, the pressure exerted on the artery is greater than the blood pressure inside it, preventing any blood from flowing through.
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Deflation and Measurement: The cuff slowly deflates, gradually reducing the pressure on the artery.
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Systolic Pressure: As the cuff pressure decreases below your systolic pressure (the pressure when your heart beats), blood starts to flow through the artery again. This turbulent flow creates vibrations in the artery wall. The monitor detects this either through a stethoscope (in manual monitors) or a pressure sensor (in digital monitors). The pressure at which this initial blood flow starts is recorded as your systolic pressure.
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Diastolic Pressure: The cuff continues to deflate. Eventually, the pressure reaches a point where the blood flow becomes smooth and silent. This occurs when the cuff pressure drops below your diastolic pressure (the pressure when your heart is at rest between beats). The point where the sound disappears (in manual monitors) or the pressure sensor detects smooth flow (in digital monitors) is recorded as your diastolic pressure.
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Digital Monitors: Digital monitors use an oscillometric method. They detect small pressure oscillations in the artery as the cuff deflates. An algorithm then determines the systolic and diastolic pressures based on these oscillations.
In summary, a blood pressure monitor measures the pressure in your arteries by first stopping the blood flow, and then listening for (or detecting) the return of blood flow as the cuff is deflated. The pressures at which blood flow starts (systolic) and becomes smooth (diastolic) are recorded.
