Chapter 1 Test — Electrical Basis of the ECG

The ST segment of the surface ECG corresponds to which phase of the ventricular action potential?

Which phase of the ventricular action potential coincides with the origin of the Q wave on the ECG?

Match each phase of the ventricular myocyte action potential with its underlying ionic event:
1. Phase 0   A. L-type voltage-gated Ca²⁺ channel opens
2. Phase 1   B. Fast voltage-gated Na⁺ channels open
3. Phase 2   C. L-type voltage-gated Ca²⁺ channel closes
4. Phase 3   D. K⁺ ions leak out

The trace shows a ventricular myocyte action potential. The arrow marks the prolonged plateau maintained for about 200 ms at slightly positive potentials. Which ion channel predominantly maintains the phase indicated by the arrow, and which drug class targets that channel to treat stable angina?

A 62-year-old man started on amiodarone for recurrent atrial fibrillation has a follow-up ECG showing a prolonged QT interval. Referring to the labelled cardiac action potential shown, the drug's primary electrophysiological action is to prolong which phase?

The recording shows spontaneous action potentials from a cardiac pacemaker cell: between spikes the membrane potential drifts slowly upward toward a threshold of about −50 mV, with no flat phase 4. A cardiologist wants to slow the heart rate in a patient with stable angina without reducing blood pressure or contractility. The drug chosen blocks the channel responsible for this slow drift. Which channel is it?

In the normal heart, spontaneous action potentials are generated at the fastest rate in the:

The upstroke (phase 0) of the sinoatrial node action potential is generated by the influx of:

A physiology student is mapping the location, blood supply and electrical behaviour of the sinoatrial node. Which of the following statements about the SA node is INCORRECT?

A cardiologist performing AV nodal slow-pathway ablation for AVNRT identifies the anatomical triangle that contains the AV node. All of the following form a boundary of that triangle EXCEPT:

If the sinoatrial node fails, the AV junction takes over as an escape pacemaker. Its intrinsic firing rate is approximately:

If both the SA node and the AV junction fail and the Purkinje fibres become the pacemaker of the heart, the expected heart rate is closest to:

The His-bundle electrogram shown is recorded alongside the surface ECG, with atrial (A), His (H) and ventricular (V) deflections. The HV interval measures conduction:

At one instant during ventricular activation, the mean electrical vector travels exactly perpendicular to the axis of a particular lead. What deflection will that lead record at that instant?

A 58-year-old man with exertional breathlessness has a completely normal resting 12-lead ECG. Echocardiography later shows an ejection fraction of 25%. Which statement best explains this combination?

A 72-year-old man has a P wave duration of 150 ms that is notched in leads II, III and aVF. Echocardiography shows a normal-sized left atrium. Damage to which structure best explains this, and what is the main associated risk?

A patient presents with a regular tachycardia at 170/min. The QRS duration is 90 ms. Which conclusion is most reliable?

A premature ventricular complex that falls on the peak of the preceding T wave can precipitate ventricular fibrillation. This 'vulnerable period' corresponds to which electrophysiological state of the ventricular myocardium?

The figure shows a ventricular myocyte at rest (−90 mV) and in the depolarised state (+10 mV). The rapid ion movement that reverses the membrane polarity between these two states generates which deflection on the surface ECG?

In the conduction system shown, the impulse is deliberately slowed at one point before it reaches the ventricles. On the surface ECG this physiological delay is measured as:

The figure aligns the ECG waveform with the mechanical phases of the cardiac cycle. Which statement about this relationship is correct?

A small wave is noted immediately after the T wave on a routine ECG. Which statement about this U wave is correct?