BP Apparatus

Sphygmomanometers

What is blood pressure?

Blood pressure is the force exerted by the blood on blood vessel walls. Today, it's measured in millimeters of mercury, units that refer to the height to which a column of mercury is raised by an equivalent pressure.

Recording blood pressure leads to the discovery of hypertension

Although physiologists who studied animals knew about the phenomenon of blood pressure in the 1700s, it was many years before physicians figured out how to measure it in humans. As soon as doctors had an accurate device and a simple procedure for measuring blood pressure, it became a normal part of a medical exam. Physicians could detect and monitor blood pressure over time, and they soon discovered hypertension, or chronic high blood pressure, a widespread and life-threatening condition.

Unfortunately, doctors had few options for treating hypertension and little understanding of its causes. Sometimes kidney disease was associated with hypertension, but in most cases, no cause could be identified.

This 1863 sphygmograph was an early attempt to measure blood pressure that proved to be cumbersome and inaccurate. A spring pressed one end of a lever onto the wrist, compressing an artery. The lever rose and fell with each pulse wave. A recording device traced the movements on smoked paper.

This 1898 sphygmomanometer had a cuff that was too narrow. Later models with wider cuffs gave more accurate readings.

This 1901 model had a wide cuff, but doctors used their fingers to detect arterial blood flow as they watched for changes in pressure.

	In 1905, Russian surgeon Nikolai Korotkoff developed the modern technique of using a stethoscope to listen for the sounds of blood flowing through the artery. His method proved to be extremely accurate and led to the discovery of hypertension.

1920 mercury sphygmomanometer, the "Pilling Midget"

How to take a blood pressure reading, based on Korotkoff sounds

A modern sphygmomanometer

1. Place cuff snugly around the middle of the upper arm.
2. Place the bell of your stethoscope over the pulse point just below the cuff.
3. Pump up the cuff until it's tight enough to stop blood flow.
4. Slowly deflate the cuff and listen.
5. Note the pressure reading when you hear the first faint sounds—that means some blood has started to pass through the vessels under the cuff. This is the maximum, or systolic, pressure.
6. When all sounds stop, note the pressure reading again. Now blood is flowing freely. This is the minimum, or diastolic, pressure.
7. Record the blood pressure as systolic over diastolic.