2017A Question 10
Draw both aortic root and radial artery pressure waveforms on the same axes. Explain the differences between them.
Examiner Report
56% of candidates achieved a pass in this question.
Most candidates correctly drew arterial waveforms on appropriately labelled axes, as requested in the question. A large number of axes however were unlabelled, and had no values attached to them. Candidates were not penalised if graphs were drawn on two identical axes rather than a single graph with superimposed waveforms.
Identification of the difference between the waveforms and elucidating the physics behind these differences were awarded marks. Most candidates managed to identify at least the gross differences between the waveforms and the better candidates were able to accurately describe the causes of the changes in the individual features correctly.
Marks were also awarded for discussion of changes caused by aging or other pathological states if these were correctly related to the changes in the waveforms.
Model Answer
Structure:
- Waveforms
- Aetiology
- Difference by location
- Difference with ageing
Waveforms
Aetiology
| Factor | Mechanism |
|---|---|
| Aortic wave | - Due to change in intra-aortic volume and pressure across the cardiac cycle |
| Radial wave | - Due to transmission of the waveform - Not due to blood flow (but a small amount of forward flow is required for propagation of the waveform) |
| Pressure wave propagation | - Wave has fast moving (10m.s-1) and slow moving (0.5m.s-1 components) - Role of resistance: - Some forward flow required to distend the vessel and propagate the wavefront - Pressure drop ∝ arterial resistance - Role of compliance: - Arterial compliance ∝ volume increment ∝ 1/speed |
Difference by Location
| Radial waveform (cf. aortic) | Reason |
|---|---|
| Delayed start 0.06 seconds | - 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 |
Difference with Ageing
| Factor | Mechanism |
|---|---|
| ↓ Large artery elasticity | - ↓ Compliance - ↓ Elastic return and Windkessel effect - ↓ Waveform variation (aortic similar to radial) - ↑ Waveform propagation velocity |