Canonical Question
Draw a labelled diagram of both the aortic root and radial artery pressure waveforms in a young adult using the same axis. Explain the factors that account for the differences betw
Master answer
The arterial pulse waveform:
- The arterial pulse waveform can be separated into three distinct components
- The systolic phase, characterised by a rapid increase in pressure to a peak, followed by a rapid decline. This phase begins with the opening of the aortic valve and corresponds to the left ventricular ejection
- The dicrotic notch, which represents the closure of the aortic valve
- The diastolic phase, which represents the run-off of blood into the peripheral circulation.
Windkessel effect
is a term used in medicine to account for the shape of the arterial blood pressure waveform in terms of the interaction between the stroke volume and the compliance of the aorta and large elastic arteries (Windkessel vessels) and the resistance of the smaller arteries and arterioles.
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)
Waveform Difference: Radial vs Aortic
| Radial waveform (vs Aortic) | Reason |
|---|---|
| Delayed start 60msec | · Time taken for wave front to travel (note much faster than blood flow) |
| Narrower and higher systolic component Steeper upstroke = ↑dp/dt max | · Less elastic tissue · Relatively more muscular tissue · ↓ compliance (δv/δp) |
| Lower diastolic component | · Less elastic tissue · Less Windkessel effect (storage and release of energy) |
| Slightly lower mean | · Above factors |
| Widened pulse pressure | · Above factors |
| Disappearance of high freq components | · Damping affects high > low freq · Damping ∝ frictional tissue interactions per length or time |
| Appearance of dicrotic notch | · Reflection and resonance. (peripheral dicrotic notch owes more of its shape to the vascular resistance of peripheral vessels than to the closing of the aortic valve) |
Source: Deranged Physiology, Ketamine nightmares
Exam appearances
| Exam | Exact wording | Relationship | Success |
|---|---|---|---|
| 2016B Q17 | Draw a labelled diagram of both the aortic root and radial artery pressure waveforms in a young adult using the same axis (60% of marks). Explain the factors that account for the differences between these two waveforms (40% of marks). | historical_member | — |
| 2022B Q10 | Draw a labelled diagram of both the aortic root and radial artery pressure waveforms in a young adult using the same axis (60% marks). Explain the factors that account for the differences between these two waveforms (40% marks). | historical_member | — |