There are two common types of blood pressure testing machines, or sphygmomanometers: digital machines (like the ones you can buy in stores) and the kind that is found in most doctors' offices, in which the doctor or nurse inflates the cuff while listening with a stethoscope and watching the needle on a dial. Both types measure the pressure exerted by the blood against the walls of the blood vessels, with the top number being the pressure during a heartbeat and the bottom number being the pressure in the time between heartbeats. The unit of measurement is millimeters of mercury, or mmHg, based on how far the levels of mercury rise in a pressure gauge. Neither type of blood pressure machine actually contains mercury, however; that is merely the standard unit of measurement for pressure.
How the Doctor Measures Blood Pressure
To use a standard sphygmomanometer, the healthcare practitioner places the cuff around the patient's upper arm and finds the pulse in one of the blood veins near the elbow. The practitioner then places the end of a stethoscope over the vein and inflates the cuff by squeezing a rubber ball that causes air to flow into the cuff while watching the needle on the pressure gauge. The practitioner inflates the gauge to a level above the expected top number for the patient's blood pressure, then stops inflating while listening for the patient's pulse in the stethoscope. As soon as the practitioner stops inflating the cuff, the needle on the gauge begins to fall. At the point that the patient's pulse becomes audible in the stethoscope, the practitioner notes the number on the gauge. This is the systolic blood pressure (the top number). When the pulse stops being audible, the number when the pulse was last heard indicates the diastolic blood pressure (the bottom number).
Why This Method Works
When the practitioner inflates the cuff, the pressure in the air cuff cuts off the blood flow in the veins being used to measure the blood pressure. When the practitioner begins deflating the cuff and blood flows through the cuff again, the pulse becomes audible because the pressure against the blood vessels is equal to the pressure shown on the dial. As the pressure continues to fall in the cuff, the cuff will eventually be exerting less pressure on the blood vessels than the normal blood flow during the rest between heartbeats--so the pulse is no longer audible when the cuff exerts less pressure than the diastolic blood pressure number. When blood pressure is high, the blood will exert more force against the air, and thus the pulse will become audible even when the cuff is exerting higher levels of pressure against the blood vessels.
Digital Blood Pressure Machines
Digital blood pressure machines work in a similar manner to the ones doctors use, but the difference is that the digital blood pressure machine automatically calculates the top and bottom numbers, whereas the standard blood pressure testing machine relies on the practitioner to watch the dial.
Tags: blood pressure, blood pressure, blood vessels, , against blood, against blood vessels