The initial information given to the candidates was -
You were asked by an intensive care nurse whether you would like to start some antihypertensive
treatment for an ICU patient who had a very high arterial blood pressure
(systolic 180mmHg, diastolic 60mmHg) displayed on the invasive haemodynamic monitor at
the bedside. The ICU nurse said that the non-invasive oscillometric blood pressure monitor
(systolic 140mmHg and diastolic 80mmHg) was much lower than the invasive blood
pressure.
Can you explain how a non-invasive oscillometric blood pressure monitor works?
This viva explored the candidates’ knowledge in relation to the following points
Principles of oscillometric blood pressure measurements, factors affecting accuracy
Principles of invasive blood pressure measurement, factors affecting accuracy,
resonance and damping
Examination feedback: All candidates had a good understanding of the principles behind
oscillometric blood pressure monitoring and its pitfalls. A good understanding on invasive
blood pressure monitoring was also essential to pass this viva.
The common weaknesses were 1) the effects of the intra-arterial catheter size or bore,
catheter length, and compliance of the tubing on resonance and damping; 2) how to assess
the damping factor at the bed side; and 3) what is the minimal requirement in terms of
natural frequency of the measuring system in relation to the frequency of the blood
pressure waveform to prevent resonance. Most candidates found it difficult to convert a
heart rate of 60 per minute into frequency in Hz.