3.2 Blood Pressure Basics
Open Resources for Nursing (Open RN)
What is Blood Pressure?
A blood pressure reading is the measurement of the force of blood against the walls of the arteries as the heart pumps blood through the body. It is reported in millimeters of mercury (mmHg). This pressure changes in the arteries when the heart is contracting compared to when it is resting and filling with blood. Blood pressure is typically expressed as the reflection of two numbers, systolic pressure and diastolic pressure. The systolic blood pressure is the maximum pressure on the arteries during systole, the phase of the heartbeat when the ventricles contract. Systole causes the ejection of blood out of the ventricles and into the aorta and pulmonary arteries. The diastolic blood pressure is the resting pressure on the arteries during diastole, the phase between each contraction of the heart when the ventricles are filling with blood.[1]
To obtain a manual blood pressure reading, the blood pressure cuff is placed around a patient’s extremity, and a stethoscope is placed over an artery. For most blood pressure readings, the cuff is usually placed around the upper arm, and the stethoscope is placed over the brachial artery. The cuff is inflated to constrict the artery until the pulse is no longer palpable, and then it is deflated slowly. The American Heart Association (AHA) recommends that the blood pressure cuff be inflated at least 30 mmHg above the point at which the radial pulse is no longer palpable. The first appearance of sounds, called Korotkoff sounds, are noted as the systolic blood pressure reading. Korotkoff sounds are named after Dr. Korotkoff, who first discovered the audible sounds of blood pressure when the arm is constricted.[2] The blood pressure cuff continues to be deflated until Korotkoff sounds disappear. The last Korotkoff sounds reflect the diastolic blood pressure reading.[3] It is important to deflate the cuff slowly at no more than 2-3 mmHg per second to ensure that the absence of pulse is noted promptly and that the reading is accurate. Blood pressure readings are documented as systolic blood pressure/diastolic pressure, for example, 120/80 mmHg.
Abnormal blood pressure readings can signify an area of concern and a need for intervention. Normal adult blood pressure is less than 120/80 mmHg. Hypertension is the medical term for elevated blood pressure readings of 130/80 mmHg or higher. See Table 4.2 for blood pressure categories according to the 2017 American College of Cardiology and American Heart Association Blood Pressure Guidelines.[4] Prior to diagnosing a person with hypertension, the health care provider will calculate an average blood pressure based on two or more blood pressure readings obtained on two or more occasions.
Hypotension is the medical term for low blood pressure readings less than 90/60 mmHg.[5] Hypotension can be caused by dehydration, bleeding, cardiac conditions, and the side effects of many medications. Hy