Indirect Measurement of Blood Pressure
Blood pressure should ideally be assessed in both arms. The patient is positioned so that the elbow is flexed and perhaps supported on a pillow. The cuff of the mercury or aneroid manometer is placed so that the lower edge is about an inch above the antecubital fossa. Do not wrap the cuff over clothing or the reading will be falsely high. A 5 inch cuff is used in adults. The bladder of the cuff should encircle at least 2/3 of the arm. In an obese
person, it is wise to use a large or leg cuff 8 inches wide or the reading will be 10-15 mmHg higher than the actual value. In infants and small children, a 1 1/2 inch cuff is used. Ages 2-5 years require a 3 inch cuff.
Palpate the radial artery and inflate the cuff until pulsations disappear. Note the pressure. Deflate the cuff, palpate and place the stethoscope on the brachial artery at heart level. An elevated position above the heart will give a falsely low reading; one below the heart will give a falsely high reading. Inflate the cuff 20-30 mmHg above where radial artery pulsations disappear, then lower the pressure slowly until sounds appear. These are the sounds of Korotkoff and are divided into several phases.
Phase I is when they are first heard, and is the systolic pressure. Phase II corresponds to Phase I with a swishing sound, Phase III occurs when the sounds are loudest, Phase IV is
the muffling of sounds and Phase V is when sounds are no longer heard. Controversy exists over whether Phase IV or V is the true diastolic pressure. The point at which sounds are no longer heard is usually recorded.
If the difference between the muffling and disappearance of sounds is less than 10 mmHg, it makes little difference which is recorded. If greater than 10, it is prudent to record both (i.e., 160/80/60). Sometimes sounds are heard to 0. Obviously a diastolic pressure of 0 is impossible, so the muffling of sounds is the diastolic pressure. This is seen in such conditions as hyperthyroidism and aortic regurgitation.
Occasionally, while deflating the cuff, sounds appear, then disappear, then reappear. This auscultatory gap, caused by diminished blood flow to the extremity, is seen in conditions such as arteriosclerotic disease, hypertension and aortic stenosis. Sounds are actually present, but inaudible. The gap has important consequences for the blood-pressure taker. If the cuff is only inflated to the gap, the systolic pressure will be falsely low. If the systolic pressure is noted correctly and the first disappearance of sounds is taken as the diastolic pressure, the diastolic will be falsely high. This is overcome by
palpating the radial artery while the cuff is deflated, making sure sounds are auscultated for a sufficient amount of time. The gap is usually within 40 mmHg of the systolic pressure.
The opposite arm is used if an IV is running, and the affected arm is not used in trauma and in the postmastectomy and renal patient with an arteriovenous fistula. Lower extremity arteries are used when trauma, including burns, is present in both upper extremities. Each time blood pressure is taken the cuff should be completely deflated. Multiple attempts at blood pressure taking without deflating the cuff not only irritates the patient but produces a falsely elevated reading.
Blood-Pressure by Palpation
In some critically ill patients, it is occasionally not possible to auscultate a blood pressure. Palpation may be performed, similar to auscultation: the cuff is inflated to 20 mmHg above the level at which a palpable brachial pulse disappears and deflated until it appears. The result is an estimated systolic pressure.
Forearm and Leg Blood-Pressures
If the brachial artery is unavailable, the cuff may be placed around the forearm and the radial artery auscultated. The systolic pressure is 10 mmHg lower than the brachial. If the arm is unavailable, the thigh or leg may be used. The person lies on his stomach, or on his back with the knee flexed, and an 8 inch cuff is wrapped around the thigh. Inflate the cuff as the popliteal artery is auscultated (a difficult artery to palpate). The systolic pressure is 20 mmHg higher than the brachial systolic pressure. The diastolic is the same. The cuff may be wrapped around the leg just above the malleoli. Either the dorsal pedis or posterior tibial artery may be used. Systolic and diastolic values are the same as for the brachial artery.
The Doppler Stethoscope is a transducer with a high-frequency output that measures flow. A weak nonpalpable pulse may be auscultated, and systolic pressures as low as 30 mmHg may be detected (the diastolic cannot be measured). The cuff is placed around the arm and inflated, the Doppler is placed over the artery and the systolic pressure is noted when pulsatile sounds are first heard. Because it evaluates flow rather than pulsations, the Doppler is more sensitive than a regular stethoscope.
Approximation of Blood-Pressure from Pulse
In the past, it was alleged but never substantiated by invasive monitoring that a palpable radial, femoral and carotid pulse meant a systolic pressure of >80 mmHg, a carotid and femoral pulse represented a pressure between 70 and 80 mmHg and a palpable carotid pulse indicated a pressure between 60 and 70 mmHg. Since invasive confirmation was never obtained and since pressures seemed to vary widely the concept faded.
Recently, a British anesthetist performed invasive monitoring on 30 patients with hypotension secondary to hypovolemic shock, grouping them as follows: group 1—radial, femoral and carotid pulses, group 2—femoral and carotid pulses, and group 3—carotid pulse only. In group 1 the high pressure was 88 mmHg with a mean of 72.5 mmHg, group 2 had a high of 78 mmHg with a mean of 66.4 mmHg, and group 3 had a high of 56 mmHg with a mean of 50 mmHg. A fourth group showed no palpable
pulses, but had systolic pressures of 52, 54 and 76 mmHg. The results indicate a lower than expected correlation, with a wide variation in predicted pressures from pulses. In summary, in this study a palpable radial pulse indicated a pressure between 53 and 88 mmHg, a femoral indicated a pressure between 48 and 78 mmHg, and a carotid indicated a pressure between 38 and 58 mmHg. The interesting facet of this investigation is that a substantial blood pressure existed in the absence of any palpable pulse.
Later, after development of the blood-pressure cuff, it was found that a decrease in blood-pressure accompanied the weak pulse. In a normal person a slight waxing and waning of blood pressure exists during inspiration—up to 10 mmHg. Today a pulsus paradoxus is defined as an inspiratory fall in systolic blood-pressure greater than 10 mmHg. It is measured as follows: using normal blood-pressure protocol, the cuff is inflated above systolic pressure, then deflated to Korotkoff sounds during expiration.
While the pressure is maintained the person inhales. The cuff pressure is lowered until Korotkoff sounds reappear. If this exceeds 10 mmHg, a paradoxical pulse is present. It is seen in conditions such as cardiac tamponade, severe asthma, COPD and heart failure. Its usefulness in a busy emergency setting is limited. Palpation of the radial or femoral pulse during inspiration is more practical, particularly if cardiac tamponade is suspected.