Indirect Blood Pressure Measurement

When the bladder is first placed around the arm and not inflated, the artery beneath the bladder functions normally (figure A).

When the bladder is inflated, the bladder squeezes the arm. If the bladder is inflated to a pressure greater than the systolic pressure of the artery, the artery beneath the bladder will collapse (figure B). The artery will remain collapsed, thus shutting off blood flow below the bladder, even when there is a heartbeat. Thus, when there is no blood flow in the artery below the bladder, you know that the air pressure in the bladder is greater than the systolic blood pressure.

When the bladder is inflated to a pressure that is less than the systolic pressure but greater than the diastolic pressure, blood will flow beneath the bladder only when the (blood) pressure within the artery is greater than the (air) pressure within the bladder. This occurs when the force of the heartbeat increases the pressure within the artery. Once the additional force of the heartbeat has passed (the artery returns to diastolic pressure), the artery will collapse again (figure C). Thus, when blood suddenly passes through the artery beneath the bladder, stops, starts again, and stops again, you know that the pressure within the bladder is less than the systolic pressure but more than the diastolic pressure.

A  Artery without the bladder--artery expands during heartbeat, returns to normal. B Air pressure in the bladder is greater than the systolic pressure--the artery stays collapsed. C Air pressure in bladder is between systolic pressure and diastolic pressure--artery collapsed except during heartbeat. D Air pressure in bladder is less than diastolic pressure--artery does not collapse.

Figure. Effects of an inflated bladder on an artery.

If the bladder is inflated, but the pressure within the bladder is less than the lowest level of pressure within the artery (diastolic pressure), then the bladder cannot collapse the artery. The pressure of the bladder may interfere somewhat with the blood flow, but it cannot stop the blood flow (figure D). Thus, when the blood continues to flow through the artery beneath the bladder without stopping, you know that the pressure within the bladder is less than the lowest (diastolic) pressure of the blood within the artery.

Use the following procedures when taking a patient's blood pressure.

Gather Materials. You will need the following items:

• Sphygmomanometer.

• Stethoscope.

• Disinfecting solution (70% alcohol solution).

• Watch or clock with second hand.

• Pen or pencil.

• Form or note pad on which to write.

Verify the Patient's Identity. Make sure that you are preparing to take the blood pressure of the proper person.

Set up Equipment. If you have a portable mercury sphygmomanometer, set up the gauge so that it will be at about eye level. If you are using an aneroid gauge, position yourself or the patient so that the gauge will be about eye level when you read it. You can read a gauge easier and more accurately when it is at eye level.

Clean Earpieces. Wet one swap with the alcohol disinfecting solution. Clean the inside of one plastic earpiece of the stethoscope with the swab and discard the swab. Wet another swab, clean the outside of the plastic earpiece, and discard that swab also. Repeat the procedure for the other plastic earpiece.

Position the Patient. Before measurement commences the patient should be seated for several minutes in a quiet room. The chair should provide comfortable back support.

• The blood pressure is normally taken using the patient's upper arm. The patient can stand, sit, or lie down. Normally, the patient will sit with his arm resting on a table or lie down with his arm resting on the bed, cot, or ground.

• Sometimes, the physician orders that the patient's blood pressure be taken on his thigh arm. (In the patients with hypertension. The patient's arms may be injured or amputated, for example.) If the blood pressure is to be taken using the thigh, have the patient to lie on his abdomen. If the patient cannot lie on his abdomen, have him to lie on his back with his knees flexed.

Expose the Site. Have the patient expose the site to be used (upper arm or thigh). Assist the patient as needed. The patient may need to remove a long sleeve shirt or lower his pajama bottoms. The bladder must be placed over the patient's flesh, not his clothing. Rolling a shirt-sleeve or pants leg up could create a tight area above the site where the bladder is applied. This extra tightening could cause the blood pressure readings to be inaccurate. Therefore, it is better to remove a long sleeve shirt or pants rather than rolling them up.

Explain Procedure to Patient. Briefly tell the patient what you are going to do. The explanation can be combined with instructing the patient to expose the site where the bladder will be applied. Warn the patient that his arm (leg) may be uncomfortable while the bag (bladder) around his arm (leg) is inflated, but reassure the patient that the discomfort will only last 1 or 2 minutes.

Prepare Bladder. Make sure that the bladder is completely deflated. If air is present in the bladder, open the release valve, force the air out of the bladder, and close the valve.

Prepare Gauge. If a mercury gauge is being used, place the gauge where it can easily read, yet not in the way. If an aneroid gauge is being used, attach it to the bladder.

Position Patient's Limb.

• If the bladder is to be applied to the patient's upper arm and there is a support for the patient's arm (bed, table, ground, etc.), have the patient to extend his arm in a palm up position. The arm should be about the same level as his heart.

• If the bladder is to be applied to the patient's upper arm but there is no support for his arm, tuck his wrist under your arm so that you will be supporting his arm and keeping it steady.

• If the bladder is to be applied to the patient's thigh, the patient should remain lying on his abdomen or lying on his back with his knee flexed.

Wrap Bladder Around Limb.

I f the upper arm is being used, wrap the bladder around the upper arm. The non-slip material or buckles of the bladder should be on the outside of the bladder, not next to the patient's skin. The bottom edge of the bladder should be one to two inches above the elbow (figure).