IBHRE - International Board of Heart Rhythm Examiners Electrophysiology Principles Questions and Answers

Question 1

Which of the following conditions is essential for the initiation and maintenance of a re-entrant tachyarrhythmia?

Accepted Answer

A region of unidirectional conduction block.

Answer

Re-entry requires three conditions: 1) a circuit, 2) a region of slow conduction within that circuit, and 3) a unidirectional block. This block forces the impulse to travel in one direction around the circuit, allowing the tissue proximal to the block to recover excitability before the wavefront returns, thus sustaining the arrhythmia.

Question 2

A 65-year-old patient is undergoing an electrophysiology study. Programmed electrical stimulation reveals that a premature impulse delivered during the last two-thirds of the T-wave on the surface ECG is able to initiate a ventricular tachyarrhythmia. This premature impulse most likely fell during which period of the ventricular action potential?

Accepted Answer

Relative Refractory Period (RRP)

Answer

The Relative Refractory Period (RRP) corresponds to the latter part of Phase 3 of the cardiac action potential, which aligns with the mid-to-end portion of the T-wave. During the RRP, most voltage-gated Na+ channels have recovered from inactivation, and a stronger-than-normal stimulus can elicit a new action potential, potentially leading to arrhythmias like the R-on-T phenomenon.

Question 3

The spontaneous Phase 4 depolarization characteristic of pacemaker cells in the sinoatrial (SA) node is primarily mediated by which ionic current?

Accepted Answer

Inward 'funny' current (If)

Answer

The 'funny' current (If) is a unique inward current carried primarily by sodium ions that is activated by hyperpolarization at the end of Phase 3. This slow influx of positive charge gradually depolarizes the cell membrane during Phase 4, moving it towards the threshold potential to initiate the next action potential.

Question 4

Anisotropy in cardiac tissue refers to the property where conduction velocity is dependent on the direction of wavefront propagation relative to the myocardial fiber orientation. In healthy ventricular tissue, conduction velocity is typically:

Accepted Answer

Fastest in the direction longitudinal (parallel) to the fiber orientation.

Answer

Anisotropy is the directional dependence of conduction velocity. Conduction is fastest along the longitudinal axis of cardiac muscle fibers due to the elongated shape of the myocytes and the preferential alignment of gap junctions at the intercalated discs. This facilitates rapid and coordinated ventricular activation.

Question 5

During which phase of the ventricular myocyte action potential is the cell membrane absolutely refractory, meaning it cannot be depolarized to generate another action potential regardless of stimulus strength?

Accepted Answer

Phase 0, Phase 1, Phase 2, and the initial part of Phase 3

Answer

The Absolute Refractory Period (ARP) lasts from the onset of Phase 0 until the cell has repolarized to approximately -50mV to -60mV, which occurs during Phase 3. Throughout Phases 0, 1, and 2, the voltage-gated sodium channels are in an open or inactivated state and cannot be reopened by a new stimulus, rendering the cell unexcitable.

IBHRE - International Board of Heart Rhythm Examiners Practice Test

IBHRE - International Board of Heart Rhythm Examiners Practice Test

IBHRE - International Board of Heart Rhythm Examiners Electrophysiology Principles Questions and Answers