The sphygmomanometer, often recognized by its inflatable arm cuff, is the specialized medical instrument designed solely to measure blood pressure. Its name combines the Greek word sphygmos (pulse) with manometer (a device for measuring pressure). This device remains the standard for the non-invasive quantification of the circulatory system’s mechanical performance.
Defining Blood Pressure
Blood pressure represents the force that circulating blood exerts outward against the walls of the body’s arteries. This mechanical force is generated primarily by the heart’s pumping action as it propels blood through the vascular network. When the term is used without qualification, it refers to the systemic arterial pressure, typically measured in the brachial artery of the upper arm. Arterial pressure is considerably higher than venous pressure because the arteries are muscular, high-resistance vessels that receive blood directly from the ventricles of the heart. The sphygmomanometer is designed to accurately capture the higher, pulsatile pressures within the arterial system.
Interpreting the Two Numbers
A blood pressure reading is always expressed as two distinct numbers, written as a fraction, such as 120/80. Both values are measured in millimeters of mercury (mmHg). The top number represents the systolic pressure, and the bottom number indicates the diastolic pressure.
Systolic pressure is the maximum pressure reached in the arteries during the cardiac cycle. This peak occurs when the heart’s ventricles contract, forcefully ejecting a volume of blood into the aorta and pulmonary artery. It reflects the force of the heart’s pump and the elasticity of the major arteries. A typical reading is around 120 mmHg in a healthy adult.
The diastolic pressure reflects the minimum pressure in the arteries. This occurs when the heart is at rest, specifically when the ventricles relax and refill with blood between beats. It is a measure of the resting pressure within the vessels and reflects the constant resistance in the circulatory system. This number is generally lower than the systolic pressure, often around 80 mmHg.
How the Sphygmomanometer Works
The process begins when the inflatable cuff is wrapped around the upper arm and inflated to a pressure high enough to completely stop the flow of blood through the underlying brachial artery. Air is then slowly released from the cuff, allowing the pressure to drop gradually.
In a manual device, a healthcare provider listens with a stethoscope over the artery for the Korotkoff sounds, which are vibrations caused by the turbulent return of blood flow. The pressure reading on the gauge the moment the first sound is heard marks the systolic pressure.
As the cuff continues to deflate, the sounds change and then eventually disappear completely. The pressure reading at the exact moment the sounds vanish marks the diastolic pressure, indicating that blood flow has returned to normal and is no longer turbulent.
Digital sphygmomanometers, which do not require a stethoscope, use a method called oscillometry. Instead of listening for sounds, an electronic sensor detects and analyzes the tiny pressure fluctuations, or oscillations, in the cuff caused by the pulse. An internal algorithm then uses the amplitude of these oscillations to calculate and display the systolic and diastolic pressures.