Question 13

Relate the surface electrocardiogram (ECG) to the events of the cardiac cycle (60% of marks). Briefly describe the mechanism of the effects of digoxin, and the mechanism of the effects of amiodarone, on the ECG (40% of marks)

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College Answer

Candidates were expected to provide sufficient detail in answers. Extra marks were
awarded for diagrams relating the ECG accurately to pressure events during the
cardiac cycle. Time intervals, units of measurement and clear labels were essential
for diagrams.
Mechanisms pertaining to ion flux and ion channels needed to be specifically
explained. Discussion of mechanisms needed to be accurate and relevant to the
effect on the ECG. For example, better answers noted that AV conduction was
depressed by Digoxin, predominantly due to an increase in Vagal tone
Syllabus: Cib2c C2c2b
Recommended sources: Principles of Physiology for the Anaesthetist, Power and
Kam, pages 107-110, Pharmacology and Physiology in Anaesthetic Practice,
Stoelting

Discussion

 Relationship of  cardiac cycle events and the ECG:

relationship of the surface ECG to the events of the cardiac cycle

If one needed to expand on this:

  • The SA node fires well into late diastole, and this is not represented on the surface ECG, nor is the propagation of signal along the internodal tracts.
  • The P wave is produced as the atrial muscle depolarises. The right atrium contracts first.
  • At the end of the P wave, the left atrium finally contracts. The end of diastole occurs during the PR interval.
  • The R wave is generated by ventricular depolarisation, and its peak corresponds to the beginning of systole (specifically, of isovolumetric contraction). 
  • The T-wave represents ventricular repolarisation, and corresponds to the phase of decreased contraction (slow ejection).  The peak of the T-wave correlates reasonably well with the onset of diastole, i.e the closure of the aortic valve.

Effects of digoxin on the ECG:

  • Rate:  decreased (vagal tone increase)
  • P wave:  unchanged
  • PR interval: increased (slowed AV nodal conduction)
  • QRS complex:  unchanged
  • QT interval:  Shortened
  • ST segment: Downsloping "sagging" ST depression
  • T waves: flattened, biphasic, inverted ("reverse tick")

Effects of amiodarone on the ECG

  • Rate:  decreased (β-blocker effect)
  • P wave:  unchanged
  • PR interval: increased (slowed AV nodal conduction)
  • QRS complex:  unchanged
  • QT interval:  Prolonged
  • ST segment: Unchanged
  • T waves: flattened, biphasic, inverted ("reverse tick")

References

Yates, Christopher, and Alex F Manini. "Utility of the electrocardiogram in drug overdose and poisoning: theoretical considerations and clinical implications." Current cardiology reviews 8.2 (2012): 137-151.

Antoni, H. "Electrophysiology of the heart at the single cell level and cardiac rhythmogenesis.Comprehensive Human Physiology. Springer, Berlin, Heidelberg, 1996. 1825-1842.

Pinnell, Jeremy, Simon Turner, and Simon Howell. "Cardiac muscle physiology." Continuing Education in Anaesthesia, Critical Care and Pain 7.3 (2007): 85-88.

Harley, Alexander, C. Frank Starmer, and Joseph C. Greenfield. "Pressure-flow studies in man. An evaluation of the duration of the phases of systole.The Journal of clinical investigation 48.5 (1969): 895-905.

Braunwald, Eugene, ALFRED P. FISHMAN, and AndrÉ Cournand. "Time relationship of dynamic events in the cardiac chambers, pulmonary artery and aorta in man." Circulation research 4.1 (1956): 100-107.

Singh, V., et al. "Amiodarone Induced Variety of ECG Changes-A Beginners ECG Delight." J Clin Exp Cardiolog 6.2 (2015).