3. How hard is it to measure?
Blood pressure is normally measured by inflating a flexible cuff placed around the upper arm. As it inflates, the cuff blocks blood flow in the arm’s main artery. When the air is gradually let out again, blood flow resumes. The measurement is non-invasive, and hence indirect. What is actually measured is the pressure inside the cuff at this point.
The restored blood flow makes a series of characteristic sounds (the Korotkoff sounds ), which can be heard through a stethoscope held over the artery just below the cuff. These are not to be confused with the normal sounds of the heartbeat, which are heard through a stethoscope placed over the heart. When the pressure in the cuff falls just below the highest (systolic ) pressure, blood flow resumes in spurts, but is stopped again during each heartbeat when it falls again to the lower (diastolic ) pressure. As the cuff continues to slowly deflate the sounds change as the turbulent blood flow gradually settles back into a normal, silent pulse. When the external pressure falls below the internal diastolic pressure, the blood sounds stop altogether.
In the most commonly used devices, the operator listens for these sounds (auscultation ), and uses them to mark the systolic and diastolic pressures. In the mercury sphygmomanometer, they are read off the scale on a mercury column. This is a skill acquired through training, and individual interpretation of the Korotkoff sounds is one source of error.
Other potential sources of error include poor calibration or maintenance of the equipment, selecting the cuff of an incorrect size, allowing the patient to move, and positioning the arm incorrectly in relation to the heart. It is also important to keep the mercury manometer vertical. More than one reading is normally taken to produce a valid measurement. Even so, readings in a doctor’s office or in the clinic may differ from those taken at home. These can arise from variations in an individual’s blood pressure over the course of a day, or from anxiety induced by the medical setting.