Question 11.1

Created on Mon, 05/18/2015 - 17:43
Last updated on Tue, 08/11/2015 - 10:25
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The following set of questions relate to invasive arterial blood pressure monitoring.

11.1.   The above series of figures represents  waveforms  obtained simultaneously  from different arterial sites from the same patient.

Assuming optimal dynamic responses, list the likely sites A-E. 

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College Answer

A- central aorta
B- proximal UL
C- Distal UL or LL
D- Proximal LL
E- Distal UL or LL

Discussion

Normal arterial line waveform variations are discussed in greater detail elsewhere.

In brief:

Difference in arterial waveforms according to site of insertion

The further you get from the aorta,

  • The taller the systolic peak (i.e. a higher systolic pressure)
  • The further the dicrotic notch
  • The lower the end-diastolic pressure (i.e. the wider the pulse pressure)
  • The later the arrival of the pulse (its 60msec delayed in the radial artery)


But, the MAP doesn't change very much.

This is because, from the aorta to the radial artery, there is little change in the resistance to flow.

MAP only really begins to change once you hit the arterioles.

This is called Distal systolic pulse amplification:

The systolic peak is steeper the further down the arterial tree you travel because of “reflected waves”.

References

From Bersten and Soni's" Oh's Intensive Care Manual", 6th Edition; plus McGhee and Bridges Monitoring Arterial Blood Pressure: What You May Not Know (Crit Care Nurse April 1, 2002 vol. 22 no. 2 60-79 )

For those who like hardcore physics, this excellent resource will be an enormous source of amusement. It appears to be a free online textbook of anaesthesia. Nowhere else was this topic covered with a greater depth, or with a greater attention to mathematical detail.