Blood Pressure

Definition

Blood pressure measurement is the non-invasive measurement of the pressure exerted by the circulating blood on the walls of the body's arteries.

Purpose

The purpose of non-invasive blood pressure measurement is to detect any changes from normal values, which may indicate disease. Measurement is also performed to monitor the effectiveness of medication and other methods used to control elevated blood pressure.

Blood pressure should be routinely checked every one to two years and may be monitored more closely during illnesses that affect blood pressure or during medical treatments which may change blood pressure. Measurement can be taken as often as every few minutes.

Precautions

As there may be no prior knowledge of the patient's previous blood pressure for comparison, a wide range of normal values apply to patients of different ages. The inflated cuff can cause discomfort, and this should be taken into account when dealing with very ill patients. Patients with a history of sickle cell anemia should not have non-invasive blood pressure measurements made with a typical blood pressure cuff, because the sickling process can be initiated by the pressure on the arm. Blood pressure measurements should occur on a limb free of intravascular catheters and arterial venous fistulas(joined artery and vein) used for chronic dialysis.

Description

Blood pressure is usually recorded by measuring the force of the blood during the contraction of the ventricles (lower chambers of the heart) as blood is pumped from the heart to the rest of the body (systolic pressure), and during the period when the heart is relaxed between beats and pressure is lowest (diastolic pressure).

The cardiac output, resistance, quality, and quantity of blood circulating through the heart, and the condition of the arterial walls are all factors that influence the blood pressure. Hypertension is an elevation in the blood pressure above normal values, with the diastolic pressure being the indicator most commonly used.

Hypotension is a reduction in the blood pressure below normal values. If a very high or very low pressure is taken, the blood pressure reading may be inaccurate and should be repeated immediately, prior to the initiation of medical treatment.

The non-invasive blood pressure is taken using a sphygmomanometer, a hand bulb pump, and a cuff.

The sphygmomanometer may be electronic or mercury-based. The mercury-based unit has a manually inflatable cuff attached by tubing to the unit that contains mercury and is calibrated in millimeters of mercury. The electronic unit is similar, but is mercury free and inflates and deflates automatically with the reading displayed digitally. The electronic units are also calibrated to display the measurement in millimeters of mercury. Blood pressure can be measured with either unit, although electronic units are becoming more commonplace in both home care and clinical use.

Children and adults with smaller or larger than average-sized limbs require special sized cuffs appropriate for their needs. The blood pressure cuff is usually placed on the arm, but can also be used on the leg.

To record blood pressure, the patient may be seated or lying down. The cuff will be positioned so that it is level with the heart. With an electronic unit the cuff is placed in accordance with manufacturer instructions on the bare upper arm, on the bare wrist, or on the bare index finger.

If the blood pressure is monitored with a manual system, a cuff is placed level with the heart and wrapped firmly but not too tightly around the bare arm 1 in (2.5 cm) above the elbow, with any creases in the cuff smoothed out. Leg measurements, require the cuff to be positioned below the groin on the bare leg over the femoral artery.

Following the manufacturer's guidelines (electronic models), the cuff is inflated and then deflated automatically. The reading is displayed and recorded by the user. The results are charted with the systolic pressure first, then by the diastolic pressure in the following manner, xxx/xx (e.g., 120/70). A manual system requires a stethoscope be placed over the artery, the cuff is then inflated until the artery is occluded and no sound is heard through the stethoscope.

The cuff is then inflated a further 10 mm Hg above the last sound heard. The valve in the pump is slowly opened no faster than 5 mm Hg per second to deflate the pressure in the cuff to the point where a tapping sound is heard over the artery. This point is noted as the systolic pressure. The sounds continue as the pressure in the cuff is released and the artery is no longer occluded. At this point, the noises are no longer heard and this is noted as the diastolic pressure.

With children, the tapping noise changes to a soft muffled sound. That point is noted as the diastolic pressure, as commonly in children, sounds continue to be heard as the cuff deflates to zero.