Non-Invasive Blood Pressure

Describe the invasive and non-invasive measurement of blood pressure and cardiac output including calibration, sources of errors and limitations

Non-invasive blood pressure measurements is performed with either a:

  • Device for Indirect Non-invasive Automatic Mean Arterial Pressure (DINAMAP)
    Automatic blood pressure cuff.
  • Von Recklinghausen's oscillotonometer
    "Manual" blood pressure cuff.
    • Uses two cuffs, and therefore two tubes

DINAMAP

Components:

  • One cuff
    Performs both arterial occlusion and measurement.
  • Tubing
  • Device for inflating the occlusive cuff and gradually deflating it
  • Pressure tranducer
  • Display

Method:

  • Cuff is inflated above SBP
  • Cuff deflates at a rate of 2-3mmHg.s-1
    When cuff pressure equals:
    • SBP
      Turbulent flow occurs past the cuff, creating pressure oscillations. The pressure at which these are first detected is the SBP.
    • MAP
      The pressure at which amplitude of oscillations is maximal.
  • DBP is calculated from MAP and DBP

Cons

  • Requires an appropriately sized cuff
    Cuff should be ~20% greater than arm diameter.
    • Cuffs that are too small will over-read
    • Cuffs that are too wide will under-read
  • Requires a regular rhythm
  • Inaccurate at extremes of blood pressure
  • Inaccurate when used more frequently than once per minute
  • Inaccurate when the vessel is incompressible
    • Heavily calcified vessels
    • When applied to forearm/foreleg
  • May cause neuropraxia

Von Recklinghausen's Oscillotonometer

Components:

  • Two cuffs
    • Occlusive cuff
    • Measurement cuff
  • Tubing
  • Device for inflating the occlusive cuff and gradually deflating it
  • Aneroid barometer for transducing pressure
  • Display

Process:

  • Cuff is inflated until the radial pressure is no longer palpable
    This is approximates SBP.
  • Cuff is deflated, and reinflated to 20mmHg above the estimated SBP
  • Cuff is deflated at a rate of 2-3mmHg.s-1 whilst auscultating the brachial artery
    When cuff pressure equals:
    • SBP
      Turbulent flow occurs past the cuff, turbulent flow causes the first of the Korotkoff sounds (clear tapping pulsations) to be heard.
    • DBP
      The cuff no longer compresses the vessel at all, so no turbulent flow occurs and nothing is auscultated.

References

  1. ANZCA July/August 2000
  2. Aston D, Rivers A, Dharmadasa A. Equipment in Anaesthesia and Intensive Care: A complete guide for the FRCA. Scion Publishing Ltd. 2014.
  3. Leslie RA, Johnson EK, Goodwin APL. Dr Podcast Scripts for the Primary FRCA. Cambridge University Press. 2011.
Last updated 2017-09-21

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