Measurement of Arterial Blood Pressure:

By Todd Hoagland, Ph.D.

Review the gross anatomy of the upper extremity to the right..  

Note the axillary artery becomes the brachial artery at the inferior border of the teres major muscles and continues through the anteromedial portion of the arm before bifurcating into the radial and ulnar arteries, in the cubital fossa.

When a stethoscope is placed on the skin over the brachial artery in the cubital fossa in a normal person (without arterial disease), no sound should be audible. As the heart beats, these pulses are transmitted smoothly throughout the arteries and no sound is produced.

If the cuff of a sphygmomanometer is placed around a patient's upper arm and inflated to a pressure above the patient's systolic blood pressure, there will similarly be no sound audible (#1). This is because the pressure in the cuff is high enough such that it completely occludes the blood flow.

If the pressure is dropped to just slightly below the patient's systolic blood pressure, the first Korotkoff sound (#2, named after the Russian physician who first described them in 1905) will be heard. As the pressure in the cuff is slightly below the pressure produced by the heart, some blood will be able to pass through the upper arm when the pressure in the artery rises during systole. This blood flows in spurts as the pressure in the artery rises above the pressure in the cuff and then drops back down, resulting in turbulence that results in audible sound.  

As the pressure in the cuff is allowed to fall further, thumping sounds continue to be heard as long as the pressure in the cuff is between the systolic and diastolic pressures, as the arterial pressure keeps on rising above and dropping back below the pressure in the cuff (#3 & 4).  

Eventually, as the pressure in the cuff drops further, the sounds change in quality, then become muted, then disappear altogether (#5). As the pressure in the cuff drops below the diastolic blood pressure, the cuff no longer provides any restriction to blood flow allowing the blood flow to become smooth again with no turbulence and thus produce no further audible sound.

Blood pressure is the pressure exerted by the blood on the walls of the blood vessels. Unless indicated otherwise, blood pressure refers to systemic arterial blood pressure, e.g., the pressure in the large arteries delivering blood to body parts other than the lungs, such as the brachial artery (in the arm). The pressure of the blood in other vessels is lower than the arterial pressure. Blood pressure values are universally stated in millimeters of mercury (mm Hg). The systolic pressure is defined as the peak pressure in the arteries during the cardiac cycle; the diastolic pressure is the lowest pressure (at the resting phase of the cardiac cycle). The mean arterial pressure and pulse pressure are other important quantities.

Typical values for a resting, healthy adult are approximately 120 mm Hg systolic and 80 mm Hg diastolic (written as 120/80 mm Hg), with large individual variations. These measures of blood pressure are not static, but undergo natural variations from one heartbeat to another or throughout the day (in a circadian rhythm); they also change in response to stress, nutritional factors, drugs, or disease.

Major Arteries and Pulse Points:

At certain points along the pathway of the systemic arterial circulation, large and medium-sized arteries lie near the body’s surface and can be used to take a pulse (via compressing an artery against a hard underlying structure, usually a bone).  The most distal pulse (farthest from the heart) is usually that taken by using the dorsalis pedis artery on the dorsum of the foot or the posterior tibial artery medial to the ankle.