Blood Pressure Assessment The Principles

Blood pressure can be measured directly with an intra-arterial catheter or indirectly with a sphygmomanometer. The sphygmomanometer consists of an inflatable rubber bladder within a cloth cover, a rubber bulb to inflate the bladder, and a manometer to measure the pressure in the bladder. Indirect measurement of blood pressure involves the auscultatory detection of the appearance and disappearance of the Korotkoff sounds over the compressed artery. Korotkoff sounds are low-pitched sounds originating in the vessel that are related to turbulence produced by partially occluding an artery with a blood pressure cuff. Several phases occur in sequence as the occluding pressure drops. Phase 1 occurs when the occluding pressure falls to the systolic blood pressure. The tapping sounds are clear and gradually increase in intensity as the occluding pressure falls. Phase 2 occurs at a pressure about 10 to 15 mm Hg lower than that in phase 1 and consists of tapping sounds followed by murmurs.{ Phase 3 occurs when the occluding pressure falls enough to allow a large amount of volume to cross the partially occluded artery. The sounds are similar to the sounds of phase 2, except that only the tapping sounds are heard. Phase 4 is the abrupt muffling and decreased intensity of the sounds as the pressure approaches the diastolic blood pressure. Phase 5 is the complete disappearance of the sounds. The vessel is no longer compressed by the occluding cuff. Turbulent flow is no longer present.

The normal blood pressure for adults is up to 140 mm Hg systolic and up to 85 mm Hg diastolic. For the diastolic blood pressure reading, the point of disappearance of the Korotkoff sounds is probably more accurate than the point of muffling. However, if the point of disappearance is more than 10 mm Hg lower than the point of muffling, the point of muffling is probably more accurate. Recording both the point of muffling and disappearance frequently helps in communication. A blood pressure might be recorded as 125/75-65: the systolic blood pressure is 125;the point of muffling is 75;the point of disappearance is 65 (the diastolic blood pressure).

Blood pressure should be recorded only to the nearest 5 mm Hg because there is a ± 3 mm Hg limit of accuracy for all sphygmomanometers. In addition, normal blood pressure changes occur from moment to moment, and measuring to less than 5 mm Hg provides a false sense of accuracy.

The size of the cuff is important for the accurate determination of blood pressure. It is recommended that the cuff be snugly applied around the arm, with its lowest edge 1 inch above the antecubital fossa. The cuff should be approximately 20% wider than the diameter of

*Short stature, retarded sexual development, and webbed neck in a female patient, associated with an abnormality of the sex chromosomes (45,XO). {Male Turner's syndrome (46,XY).

{A murmur is a blowing auscultatory sign produced by turbulence in blood flow. These vibrations can originate in the heart or in blood vessels as a result of hemodynamic changes.

Figure 14-20 Technique for blood pressure assessment by palpation.

the extremity. The bladder should overlie the artery. The use of a cuff that is too small for a large arm results in an erroneously high reading of blood pressure.

Another cause of falsely elevated blood pressure readings is lack of support of the patient's arm. To obtain an accurate measurement, the cuff must be at heart level. If the arm is not supported, the patient is performing isometric exercise, which raises the recorded pressure. In contrast, excessive pressure on the diaphragm of the stethoscope produces a spuriously lower reading of the diastolic blood pressure without any significant alteration of systolic pressure. If the arm is held correctly, no skin indentations should occur.

The auscultatory gap is the silence that occurs between the disappearance of the Korotkoff sounds after the initial appearance and their reappearance at a lower pressure. The auscultatory gap is present when there is a decreased blood flow to the extremities, as is found in hypertension and in aortic stenosis. Its clinical importance lies in the fact that the systolic blood pressure may be mistaken for the lower blood pressure, the point of reappearance.

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