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 transducer
• 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 SBP

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 re-inflated 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.