BLOOD PRESSURE
Defined as the lateral force exerted by blood on the arterial walls, blood pressure depends on the force of ventricular contractions, arterial wall elasticity, peripheral vascular resistance, and blood volume and viscosity. Systolic, or maximum, pressure occurs during left ventricular contraction and reflects the integrity of the heart, arteries, and arterioles. Diastolic, or minimum, pressure occurs during left ventricular relaxation and directly indicates blood vessel resistance.
Pulse pressure—the difference between systolic and diastolic pressures—varies inversely with arterial elasticity. Rigid vessels, incapable of distention and recoil, produce high systolic pressure and low diastolic pressure. Normally, systolic pressure exceeds diastolic pressure by about 40 mm Hg. Narrowed pulse pressure—a difference of less than 30 mm Hg—occurs when systolic pressure falls and diastolic rises. These changes reflect reduced stroke volume, increased peripheral resistance, or both. Widened pulse pressure—a difference of more than 50 mm Hg between systolic and diastolic pressures—occurs when systolic pressure rises and diastolic pressure remains constant, or when systolic pressure rises and diastolic pressure falls. These changes reflect increased stroke volume, decreased peripheral resistance, or both.
Blood pressure is measured in millimeters of mercury with a sphygmomanometer and a stethoscope, usually at the brachial artery (less often at the popliteal or radial artery). Lowest in the neonate, blood pressure rises with age, weight gain, prolonged stress, and anxiety. (See Effects of age on blood pressure.)
Frequent blood pressure measurement is critical after serious injury, surgery, or anesthesia, and during any illness or condition that threatens cardiovascular stability. (Frequent measurement may be done with an automated vital signs monitor.) Regular measurement is indicated for patients with a history of hypertension or hypotension, and yearly screening is recommended for all adults.
Equipment
Mercury or aneroid sphygmomanometer • stethoscope • alcohol pad • automated vital signs monitor (if available).
The sphygmomanometer consists of an inflatable compression cuff linked to a manual air pump and a mercury manometer or an aneroid gauge. The mercury sphygmomanometer is more accurate and requires calibration less frequently than the aneroid model but is larger and heavier. To obtain an accurate reading, you must rest its gauge on a level surface and view the meniscus at eye level; you can rest an aneroid gauge in any position but must view it directly from the front.
Hook, bandage, snap, or Velcro cuffs come in six standard sizes ranging from neonate to extra-large adult. Disposable cuffs are available.
The automated vital signs monitor is a noninvasive device that measures pulse rate, systolic and diastolic pressures, and mean arterial pressure at preset intervals. (See Using an electronic vital signs monitor.)
Preparation of equipment
Carefully choose a cuff of appropriate size for the patient. An excessively narrow cuff may cause a false-high pressure reading; an excessively wide one, a false-low reading. If you aren't using your own stethoscope, disinfect the earpieces with an alcohol pad before placing them in your ears to avoid cross-contamination.
To use an automated vital signs monitor, collect the monitor, dual air hose, and pressure cuff. Then make sure the monitor unit is firmly positioned near the patient's bed.
Implementation
  • Tell the patient that you're going to take his blood pressure.
  • The patient can lie supine or sit erect during blood pressure measurement. His arm should be extended at heart level and be well supported. If the artery is below heart level, you may get a false-high reading. Make sure the patient is relaxed and comfortable when you take his blood pressure so it stays at its normal level.
  • Wrap the deflated cuff snugly around the upper arm.
  • If necessary, connect the appropriate tube to the rubber bulb of the air pump and the other tube to the manometer. Then insert the stethoscope earpieces into your ears.
  • Locate the brachial artery by palpation. Center the bell of the stethoscope over the part of the artery where you detect the strongest beats, and hold it in place with one hand. The bell of the stethoscope transmits low-pitched arterial blood sounds more effectively than does the diaphragm. (See Positioning the blood pressure cuff.)
  • Using the thumb and index finger of your other hand, turn the thumbscrew on the rubber bulb of the air pump clockwise to close the valve.
  • Then pump air into the cuff while auscultating for the sound over the brachial artery to compress and, eventually, occlude arterial blood flow. Continue pumping air until the mercury column or aneroid gauge registers 160 mm Hg or at least 10 mm Hg above the level of the last audible sound.
  • Carefully open the valve of the air pump, and then slowly deflate the cuff—no faster than 5 mm Hg per second. While releasing air, watch the mercury column or aneroid gauge and auscultate for the sound over the artery.
  • When you hear the first beat or clear tapping sound, note the pressure on the column or gauge. This is the systolic pressure. (The beat or tapping sound is the first of five Korotkoff
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    sounds. The second sound resembles a murmur or swish; the third sound, crisp tapping; the fourth sound, a soft, muffled tone; and the fifth, the last sound heard.)
  • Continue to release air gradually while auscultating for the sound over the artery.
  • Note the diastolic pressure—the fourth Korotkoff sound. If you continue to hear sounds as the column or gauge falls to zero (common in children), record the pressure at the beginning of the fourth sound. This is important because, in some patients, a distinct fifth sound is absent.
  • Rapidly deflate the cuff. Record the pressure, wait 15 to 30 seconds, and then repeat the procedure and record the pressures to confirm your original findings. After doing so, remove and fold the cuff, and return it to storage.

Special considerations
  • If you can't auscultate blood pressure, you may estimate systolic pressure. To do this, first palpate the brachial or radial pulse. Then inflate the cuff until you no longer detect the pulse. Slowly deflate the cuff and, when you detect the pulse again, record the pressure as the palpated systolic pressure. When measuring blood pressure in the popliteal artery, position the patient so that you can assess the area, wrap a cuff around the middle of his thigh, and proceed with blood pressure measurement.
  • Palpation of systolic blood pressure may also be important to avoid underestimating blood pressure in patients with an auscultatory gap. This gap is a loss of sound between the first and second Korotkoff sounds that may be as great as
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    40 mm Hg. You may find this in patients with venous congestion or hypotension.
  • Another alternative to auscultating with a stethoscope is to use Doppler ultrasound to hear Korotkoff sounds. This is useful when the blood pressure is diminished or when the pulse is nonpalpable.
  • If the patient is crying or anxious, delay blood pressure measurement, if possible, until he becomes calm to avoid falsely elevated readings.
  • If your facility considers the fourth and fifth Korotkoff sounds as the first and second diastolic pressures, record both pressures.
  • Remember that malfunction in an aneroid sphygmomanometer can be identified only by checking it against a mercury manometer of known accuracy. Be sure to check your aneroid manometer this way periodically. Malfunction in a mercury manometer is evident in abnormal behavior of the mercury column. Don't attempt to repair either type yourself; instead, send it to the appropriate service department. (For information on other situations that can cause false-high or false-low readings, see Correcting problems of blood pressure measurement.)
  • Occasionally, blood pressure must be measured in both arms or with the patient in two different positions (such as lying and standing or sitting and standing). In such cases, observe and record any significant difference between the two readings and record the blood pressure and the extremity and position used.
  • Measure the blood pressure of a patient taking antihypertensive medication while he's in a sitting position to ensure accurate measurements.
Complications
Don't take a blood pressure in the arm on the affected side of a mastectomy patient because it may decrease already compromised lymphatic circulation, worsen edema, and damage the arm. Likewise, don't take a blood pressure on an arm with an arteriovenous fistula or hemodialysis shunt because blood flow through the vascular device may be compromised.
Documentation
On the patient's chart, record blood pressure as systolic over diastolic pressures, such as 120/78 mm Hg; if necessary, record systolic pressure over the two diastolic pressures such as 120/78/20 mm Hg. Chart an auscultatory gap if present. If required by your facility, chart blood pressures on a graph, using dots or checkmarks. Document the extremity used and the patient's position. If the blood pressure was palpated or auscultated using a Doppler device, record this as well.