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Measuring blood pressure

German: Blutdruckmessung

1 Overview

Blood pressure can be assessed using different methods for different parts of the vascular system. In everyday clinical practice blood pressure measurements usually refer to arterial blood pressure.

2 Classification

Blood pressure can be measured "directly" (invasively) and "indirectly" (noninvasively).

2.1 Measuring blood pressure invasively

Invasive blood pressure measurement (synonym: direct blood pressure measurement) uses a sensor that is inserted using an intra-arterial catheter. This method is only used in intensive care units, specially equipped wards, and in operating rooms.

2.2 Measuring blood pressure noninvasively

Indirect or noninvasive blood pressure measurement (NIBP) uses an electronic monitor or a blood pressure cuff and a stethoscope. The resulting measurements are less accurate than those of invasive methods. Noninvasive measuring is far more common in clinical practice and can also be performed by patients themselves at home.

2.2.1 Explanation

  • Blood flow in the brachial artery is stopped by compressing it using a cuff that has been inflated either manually or electromechanically to a pressure that is greater than systolic blood pressure. After the blood flow has come to a stop, no more flow sounds are audible.
  • The valve is opened (not releasing more than 3 mmHg per second), air can escape
    • When cuff pressure equals systolic pressure, blood begins flowing through the artery
    • Diastolic pressure, however is still less than cuff pressure causing the blood not to flow continually
  • As a result there's a rhythmic change in blood flow: Blood flows (systole) <-> Blood does not flow (diastole).
  • This change causes the typical flow sound that are synchronous with pulse (Korotkoff sounds).
  • The valve remains open, cuff pressure decreases: Once the cuff pressure is less than diastolic pressure the artery persistently remains open and no more flow sounds are audible.

2.2.2 Procedure

Blood pressure should always be taken under the same circumstances and with the patient resting. The measurement should invariably use the same arm and the patient can be either lying down, sitting or standing up. During measurement there should be no audible noise and the patient is not supposed to talk. The blood pressure cuff must be suitable to the patient: smaller cuffs for children, wider cuffs for large circumferences of the upper arm. The procedure is as follows:

  • Exposing the upper arm, being careful that tight clothing is not constricting
  • Squeezing the cuff, pressing the air out; closing the valve and then putting on the cuff
  • Positioning the stethoscope in the crook of the arm
  • Quickly pumping up the cuff until the radial pulse subsides and then another 30 mmHg
  • Sound begins: systolic value
  • Sound subsides: diastolic value
  • Noting the measurements
  • Wiping stethoscope and cuff with disinfectant

2.2.3 Contraindications

Relative contraindications on the affected arm for taking the blood pressure using a cuff:

Absolute contraindications on the affected arm for taking the blood pressure using a cuff:

3 Variants

A variant of noninvasive blood pressure measurement is the 24h blood pressure measurement. This method uses an automatic blood pressure cuff that measures and records blood pressure over one day in programmable time intervals.

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