Measuring Blood Pressure

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Blood pressure is a vital sign. Because high blood pressure can lead to heart failure, stroke, and kidney disease, no doctor's visit would be complete without an assessment of a patient's blood pressure. This video will explain the difference between systolic and diastolic blood pressures; define pulse pressure; explain the calculation for mean arterial pressure (or MAP); provide a sample calculation of MAP; describe Korotkoff sounds and why they occur; and explain how hypertension is determined. Blood pressure is defined as the force per unit area exerted on a blood vessel wall by the blood contained within the vessel. Blood pressure fluctuates with the alternating contraction and relaxation of the ventricles. The pressure during contraction will be greater than the pressure during relaxation. A period of contraction is referred to as systole, and a period of relaxation is called diastole. In systemic circulation, the highest arterial pressure reached is a result of ventricular systole and therefore called systolic pressure. During diastole, the arterial pressure drops to its lowest level, diastolic pressure. Blood pressure is expressed in millimeters of mercury (abbreviated mm Hg) and is reported as systolic pressure over diastolic pressure. In this graph, the blood pressure is recorded as 120 over 80. The pulse pressure is the difference between the systolic and diastolic pressures. It reflects the additional pressure in the arteries when ventricles are contracting. For example, if blood pressure is 120 over 80, pulse pressure is 120 minus 80, which equals 40 millimeters of mercury. The pulse pressure does generate a pulse, which is a throbbing rhythmic sensation that can be felt at superficial locations in the body. Pulse rate, sometimes shortened to simply "pulse," is not the same as pulse pressure. The pulse rate is the number of pulsations per minute. Let's check your understanding. What is pulse pressure: the same value as pulse rate; systolic pressure divided by diastolic pressure; diastolic pressure minus systolic pressure; or systolic pressure minus diastolic pressure? >> Pulse pressure is calculated by subtracting diastolic pressure from systolic pressure. The driving force for moving blood through the tissues is the mean arterial pressure, or MAP. It is not calculated as a simple average, because the ventricles spend more time in diastole than systole during one heartbeat. The mean arterial pressure equation must account for this difference. Mean arterial pressure equals diastolic pressure plus one-third the pulse pressure, or diastolic pressure plus one-third the difference between systolic and diastolic pressures. Using our equation, a person whose blood pressure is measured as 140 over 80 would have an MAP of 100 millimeters of mercury. An MAP of greater than 60 millimeters of mercury is enough pressure to maintain the organs of an average person. The normal range for MAP is between 70 and 110 millimeters of mercury. Let's check your understanding. What is the mean arterial pressure for a person whose blood pressure (or BP) is 130 over 70: 60, 90, 100, or 130 millimeters of mercury? >> The mean arterial pressure for a person whose blood pressure is 130 over 70 is equal to 70 plus one-third the difference between 130 and 70, which is equal to 70 plus 20, or 90 millimeters of mercury. The most common indirect method for measuring blood pressure involves the use of a stethoscope and a sphygmomanometer, more commonly called a blood pressure cuff. The sphygmomanometer has an inflatable cuff with a pressure gauge. The cuff is connected to an inflation bulb with rubber tubing. When the release valve is open, air in the cuff is released. The deflated cuff is wrapped around the arm about one inch above the fold of the elbow and secured. The gauge should be facing the observer. Position the chestpiece of the stethoscope over the brachial artery, avoiding placement between the arm and the cuff. Using the inflation bulb, the cuff will be inflated to a pressure above systolic pressure. When the cuff is inflated to a pressure above the systolic pressure, the brachial artery is closed. As the pressure in the cuff is gradually released, the blood flowing through the partially compressed artery will generate tapping sounds that can be heard with a stethoscope. The pressure read on the gauge when these sounds, called Korotkoff sounds, first appear is recorded as the systolic pressure. As the pressure continues to be released, the artery will open further, resulting in louder sounds. The pressure read on the gauge when the sounds disappear is recorded as the diastolic pressure. The American Heart Association (or AHA) recommends that blood pressure be consistently below 120 over 80. Hypertension is diagnosed when an individual's blood pressure is elevated for three months or more with a systolic pressure greater than 139 millimeters of mercury or a diastolic pressure greater than 89 millimeters of mercury. >> Let's check your understanding. Here are three months of blood pressure readings for three individuals. Who would be considered to have hypertension? Individual 1 with pressure readings of 140 over 80, 145 over 90, and 130 over 80; individual 2 with pressure readings of 120 over 90, 130 over 90, and 140 over 95; or individual 3 with pressure readings of 135 over 80, 135 over 90, and 135 over 80? >> None of the individuals had systolic pressures that indicated hypertension, but individual 2 has three months of blood pressure readings that are consistently above the recommended diastolic values. Individuals 1 and 3 may be monitored for prehypertension because they had readings that were above the AHA recommendations. Careful monitoring of blood pressure is vital to health. As you head into the lab, think about the consequences of hypertension and what measures can be taken to lead a healthy lifestyle.