This is relevant to Question 13 from the first paper of 2001, "The nurse notes a marked difference between blood pressure recorded via an arterial  line in one arm and non-invasive pressure recorded from the other arm.  What may be causing this difference?  Which reading will you use to guide management?"

Non-invasive blood pressure measurement

LITFL have some good references for this.

  • Method of oscillometric measurement
    • The cuff is coupled to an oscillometer which measures the pulse of the brachial artery transmitted through the air-filled tubing
    • As it inflates, the oscillometer stores the maximal amplitude of oscillations.
    • The cuff inflates above the systolic (i.e. when the oscillometer no longer sees any oscillations)
    • The cuff is gradually deflated until oscillations are 25-50% of their maximal amplitude: this is the systolic.
    • The cuff is gradually deflated until maximal amplitude is reached: this is the MAP
    • The cuff is deflated until the amplitude decreases again by 80% or more; this is the diastolic.
  • Sources of inaccuracy in the oscillometric measurement of blood pressure
    • Wildly inaccurate: 95% CI for NIBP within the normal range is 15mmHg
    • Even more inaccurate in the extremes of blood pressure; over-estimates low blood pressure and under-estimates high blood pressure
    • Impossible to calibrate
    • Cuff size is a major influence of measurement
    • Oscillometer is confused by arrhythmia, shivering, or tremor.

References

Crul, J. F. "Measurement of arterial pressure." Acta Anaesthesiologica Scandinavica 6.s11 (1962): 135-169.