Question 10

College Answer

This question required the integration and understanding of cardiovascular monitoring with what influences the changes as the arterial pulse moves through the vascular tree. In general, the waveforms drawn were often of a poor standard, with little attention given to accurately representing the different shapes, correct axis (pressure and timing) and labelling. The description of the differences was of a better standard however, there were often contradictions between what was drawn and described. For example, several answers correctly described a delay in the systolic peak seen in the radial artery, but this was not illustrated in the diagram. The windkessel effect was often correctly mentioned but not further explained to help describe the changes seen in the waveforms. Interruptions to the arterial traces
from the anacrotic and diacrotic notches, the origins of the diacrotic hump, and insuria were often incorrect and poorly explained. The examiners reported that answer template structure was not a major issue, but a lack of integration of the waveform and translation of that knowledge into a description of the cardiac cycle was often poorly done. 

Discussion

This is a repeat of Question 10 from a 2017 ANZCA primary exam paper, lovingly answered by ketaminenightmares.com and jennysjamjar.com.  

Here's a diagram annotated with the main ways in which the radial waveform is different from the aortic:

The differences, and the factors that account for them, are:

• Widened pulse pressure:
  • The lower systolic and higher diastolic in the aortic root are due to the windkessel effect of the aorta
  • The higher systolic and lower diastolic in the radial artery are also due to the effects of distal pulse amplification
• Steeper anacrotic upstroke
  • ​​​​​​​Lower compliance of the (more muscular) distal arteries
  • Less elastic tissue than the aorta
• Delayed systolic peak:
  • ​​​​​​​The pressure wave travels at aboout 100m/sec and arrives to the radial artery with a delay of about 60 milliseconds.
• Dicrotic notch: 
  • Reflected waves from the distal arterial tree create this artifact
• Slightly lower MAP: 
  • The increased compliance in the aorta maintains a higher MAP mainly by sustaining a higher diastolic pressure due to the windkessell effect, but the difference is not massive: Pauca et al (1992) determined that it was about 0.8 mmHg on average, and mostly under 3mmHg​​​
• Loss of the incisura and other high frequency components
  • ​​​​​​​Damping by blood vessel walls "smoothes" the contour of the arterial waveform, removing detail

Nielsen, P. E., J-P. Barras, and P. Holstein. "Systolic pressure amplification in the arteries of normal subjects." Scandinavian journal of clinical and laboratory investigation 33.4 (1974): 371-377.

Wemple, R. R. "Origin of the dicrotic notch and wave." Biomedical sciences instrumentation 9 (1972): 45.

Sabbah, Hani N., and Paul D. Stein. "Valve origin of the aortic incisura." American Journal of Cardiology 41.1 (1978): 32-38.

Esper, Stephen A., and Michael R. Pinsky. "Arterial waveform analysis." Best Practice & Research Clinical Anaesthesiology28.4 (2014): 363-380.

Thiele, Robert H., and Marcel E. Durieux. "Arterial waveform analysis for the anesthesiologist: past, present, and future concepts." Anesthesia & Analgesia 113.4 (2011): 766-776.