IBHRE - International Board of Heart Rhythm Examiners Electrophysiology Principles Questions and Answers — Questions and Answers
Question 1: Which of the following conditions is essential for the initiation and maintenance of a re-entrant tachyarrhythmia?
- Homogeneous conduction velocity throughout the myocardium.
- A long refractory period relative to the conduction time around the circuit.
- A region of unidirectional conduction block. (Correct answer)
- Enhanced automaticity in a focal group of cells.
Correct answer: A region of unidirectional conduction block.
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?
- Effective Refractory Period (ERP)
- Absolute Refractory Period (ARP)
- Relative Refractory Period (RRP) (Correct answer)
- Supranormal Period
Correct answer: Relative Refractory Period (RRP)
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?
- Inward fast sodium current (INa)
- Inward L-type calcium current (ICa-L)
- Outward potassium current (IKr)
- Inward 'funny' current (If) (Correct answer)
Correct answer: Inward 'funny' current (If)
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:
- Fastest in the direction transverse (perpendicular) to the fiber orientation.
- Equal in all directions, demonstrating isotropic conduction.
- Fastest in the direction longitudinal (parallel) to the fiber orientation. (Correct answer)
- Slowest at the epicardial surface compared to the endocardium.
Correct answer: Fastest in the direction longitudinal (parallel) to the fiber orientation.
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?
- Phase 4
- Phase 0, Phase 1, and the early part of Phase 2
- The latter half of Phase 3
- Phase 0, Phase 1, Phase 2, and the initial part of Phase 3 (Correct answer)
Correct answer: Phase 0, Phase 1, Phase 2, and the initial part of Phase 3
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.
Question 6: A patient with a history of myocardial infarction develops a stable monomorphic ventricular tachycardia. The underlying mechanism is most likely a re-entrant circuit that has formed around the infarct scar. Which electrophysiologic property of the scar border zone is critical for sustaining this arrhythmia?
- Enhanced automaticity
- A zone of slow conduction (Correct answer)
- A shortened effective refractory period
- Absence of anisotropic conduction
Correct answer: A zone of slow conduction
For a re-entrant circuit to be sustained, the wavelength of the impulse (conduction velocity x refractory period) must be shorter than the path length of the circuit. A zone of slow conduction, often found in the heterogeneous tissue at the border of an infarct scar, lengthens the transit time of the impulse around the circuit. This delay allows the tissue at the start of the circuit to recover from its refractory period before the wavefront returns, permitting the arrhythmia to continue.
IBHRE - International Board of Heart Rhythm Examiners Electrophysiology Principles Questions and Answers
Which of the following conditions is essential for the initiation and maintenance of a re-entrant tachyarrhythmia?