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.