How do you measure blood pressure?
Blood pressure is an expression of the strength of arterial circulation in a human or animal. To measure the flow, one uses a pressure meter or 'sphygmomanometer', comprising an inflatable cuff to restrict blood flow, and a mercury or electronic manometer to measure the pressure.
The pressure is stated as two numbers, the highest pressure that blood reaches (pushes past the cuff) and the lowest (the flow is unimpeded and flows without being pushed by the heart contraction). The higher pressure is the systolic (heart pushing), the lower the diastolic. Blood pressure is stated as both of these numbers, e.g. 115/75 spoken "115 over 75" is a systolic pressure of 115 (mm Hg), a diastolic of 75 (mm Hg).
*For SI conversion, the mm of mercury, also known as the Torr (for Evangelista Torricelli), is equal to approximately 133.3 Pascals.
BP can be measured using palpation in emergency situations. This only gives a rough estimate of systolic pressure. It can be used with the carotid, the femoral, or radial pulse.
Normally, manual sphygmomanometers are used in conjunction with a stethoscope to determine the two pressures. You use a BP cuff, pump it up until the pressure in the cuff is 20 mm Hg above the pressure in the artery. Using a stethoscope you can hear when this happens. No sound is heard in the artery.Then the pressure is released and when the pressure is heard again the second reading is made.
Another method of measurement is invasive, intra arterial pressure monitoring. This is where an arterial line is inserted into an artery and taped into place, with a bag of pressurised fluid preventing the backflow of blood from the artery (as it is under a lot of pressure in an artery). The line is used to sample arterial blood flow to measure dissolved gases in it and measure arterial blood pressures continuously.