Relate the surface ECG to the events of the cardiac cycle (60% of mark).

Describe how the PR, QRS and QT intervals may be prolonged by the action of drugs.

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College Answer

The first part of the question is best answered by a labelled and annotated diagram of the
ECG with the pressure events of the cardiac cycle. Common errors included mistiming of the
ECG with the pressure waveform. The second part of the question could be answered in a
tabular format such as:
 

Interval Drug Mechanism
PR Digoxin

Increases refractory period
of AV node probably by
increased vagal activity

QRS

Tricyclic antidepressants

Quinidine like effect,
decreasing sodium influx
into cells

     


References:
Power and Kam 2nd edition p129 – 131
Stoelting and Hillier 4th edition p403, 409, 415