The principle of blood pressure measurement by sphygmomanometer is mainly through the change of external pressure, indirectly responding to the pressure range of human arteries, arterial blood pressure is commonly used cuff type mercury sphygmomanometer. The cuff is wrapped around the upper arm at 2 cm above the elbow fossa of the brachial artery, and the stethoscope is placed on the brachial artery, which is first inflated and pressurized until no arterial pulsation is heard, indicating that blood flow is blocked at the brachial artery, and then slowly deflated. When the cuff pressure is less than the systolic pressure for a moment, the arterial pulsation sound can be heard with the stethoscope, and the pressure in the blood can exceed the pressure in the cuff for a moment, which is the systolic pressure. Continuing to deflate slowly, the arterial pulsation sound will continue, and the moment the pressure within the blood completely exceeds the pressure within the cuff, the stethoscope will hear the last arterial pulsation sound, which is the diastolic pressure, also called low pressure. The principle of non-invasive blood pressure measurement is indirect, while invasive blood pressure measurement, on the other hand, involves sending a catheter directly into the body’s arterial system and measuring it directly through an external connecting tube to a pressure receptor.