It is not reasonable to use pressure recorders that require needle insertion into an artery for making routine pressure measurements in human patients, although these are used on occasion when special studies are necessary. Instead, the clinician determines systolic and diastolic pressures by indirect means, usually by the auscultatory method.
Auscultatory Method. Figure 15-7 shows the auscultatory method for determining systolic and diastolic arterial pressures. A stethoscope is placed over the antecubital artery and a blood pressure cuff is inflated around the upper arm. As long as the cuff continues to compress the arm with too little pressure to close the brachial artery, no sounds are heard from the antecubital artery with the stethoscope. However, when the cuff pressure is great enough to close the artery during part of the
arterial pressure cycle, a sound then is heard with each pulsation. These sounds are called Korotkoff sounds.
The exact cause of Korotkoff sounds is still debated, but they are believed to be caused mainly by blood jetting through the partly occluded vessel. The jet causes turbulence in the vessel beyond the cuff, and this sets up the vibrations heard through the stethoscope.
In determining blood pressure by the auscultatory method, the pressure in the cuff is first elevated well above arterial systolic pressure. As long as this cuff pressure is higher than systolic pressure, the brachial artery remains collapsed so that no blood jets into the lower artery during any part of the pressure cycle. Therefore, no Korotkoff sounds are heard in the lower artery. But then the cuff pressure gradually is reduced. Just as soon as the pressure in the cuff falls below systolic pressure, blood begins to slip through the artery beneath the cuff during the peak of systolic pressure, and one begins to hear tapping sounds from the antecubital artery in synchrony with the heartbeat. As soon as these sounds begin to be heard, the pressure level indicated by the manometer connected to the cuff is about equal to the systolic pressure.
As the pressure in the cuff is lowered still more, the Korotkoff sounds change in quality, having less of the tapping quality and more of a rhythmical and harsher quality. Then, finally, when the pressure in the cuff falls to equal diastolic pressure, the artery no longer closes during diastole, which means that the basic factor causing the sounds (the jetting of blood through a squeezed artery) is no longer present. Therefore, the sounds suddenly change to a muffled quality, then disappear entirely after another 5- to 10-millimeter drop in cuff pressure. One notes the manometer pressure when the Korotkoff sounds change to the muffled quality; this pressure is about equal to the diastolic pressure. The auscultatory method for determining systolic and diastolic pressures is not entirely accurate, but it usually gives values within 10 per cent of those determined by direct catheter measurement from inside the arteries.
Normal Arterial Pressures as Measured by the Auscultatory Method. Figure 15-8 shows the approximate normal systolic and diastolic arterial pressures at different ages. The progressive increase in pressure with age results from the effects of aging on the blood pressure control mechanisms. We shall see in Chapter 19 that the kidneys are primarily responsible for this long-term regulation of arterial pressure; and it is well known that the kidneys exhibit definitive changes with age, especially after the age of 50 years.
A slight extra increase in systolic pressure usually occurs beyond the age of 60 years. This results from hardening of the arteries, which itself is an end-stage result of atherosclerosis. The final effect is a bounding systolic pressure with considerable increase in pulse pressure, as previously explained.
Mean Arterial Pressure. The mean arterial pressure is the average of the arterial pressures measured millisecond
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This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.