AF Anatomy & Physiology Fundamentals 3 — Questions and Answers
Question 1: During atrial fibrillation, the ventricular rate is irregular primarily because of which property of the AV node?
- Enhanced automaticity
- Concealed conduction causing variable refractoriness (Correct answer)
- Increased conduction velocity
- Retrograde Wenckebach block
Correct answer: Concealed conduction causing variable refractoriness
Multiple atrial impulses penetrate the AV node to varying depths (concealed conduction), leaving it in varying states of refractoriness and producing an irregular ventricular response.
Question 2: The 'atrial kick' contributes approximately what percentage of ventricular filling under normal sinus rhythm?
- 5–10%
- 15–30% (Correct answer)
- 40–50%
- 60–70%
Correct answer: 15–30%
Active atrial contraction contributes roughly 15–30% of ventricular end-diastolic volume, a contribution lost during AF.
Question 3: Which ion channel is primarily responsible for the plateau (phase 2) of the cardiac action potential in ventricular cardiomyocytes?
- Fast sodium (Nav1.5)
- Inward rectifier potassium (IK1)
- L-type calcium (Cav1.2) (Correct answer)
- Transient outward potassium (Ito)
Correct answer: L-type calcium (Cav1.2)
L-type calcium channels carry the slow inward Ca²⁺ current that sustains the plateau phase and triggers excitation-contraction coupling.
Question 4: The crista terminalis in the right atrium is clinically significant in AF because it:
- Is the site of the AV node
- Represents a zone of anisotropic conduction that can anchor reentrant circuits (Correct answer)
- Connects the right atrium to the coronary sinus
- Separates the tricuspid valve from the pulmonary valve
Correct answer: Represents a zone of anisotropic conduction that can anchor reentrant circuits
The crista terminalis is a muscular ridge with marked anisotropy (directional conduction differences) that can serve as a line of functional block anchoring atrial flutter and AF circuits.
Question 5: Increased intracellular calcium overload in atrial myocytes during rapid firing can cause spontaneous depolarizations via which mechanism?
- INa block
- Delayed afterdepolarizations through NCX forward mode (Correct answer)
- Enhanced IK1 outward current
- Hyperpolarization-activated funny current (If) suppression
Correct answer: Delayed afterdepolarizations through NCX forward mode
Calcium overload causes spontaneous sarcoplasmic reticulum Ca²⁺ release, which is extruded by the Na⁺/Ca²⁺ exchanger (NCX) in forward mode, generating a transient inward current and delayed afterdepolarizations.
Question 6: Why do patients with hypertrophic left ventricles have an especially high risk for hemodynamic compromise during AF?
- Their AV nodes conduct faster
- They are more dependent on the atrial kick due to impaired diastolic filling (Correct answer)
- Their pulmonary veins contain more ectopic foci
- They have longer atrial refractory periods
Correct answer: They are more dependent on the atrial kick due to impaired diastolic filling
A stiff, hypertrophied ventricle has impaired diastolic relaxation and relies heavily on the atrial kick for adequate preload, so losing atrial contraction during AF significantly reduces cardiac output.
Question 7: The right atrial appendage differs anatomically from the left atrial appendage in that it is:
- Smooth-walled and tube-like
- Trabeculated with a broad triangular base (Correct answer)
- The primary site of thrombus formation in AF
- Connected directly to the coronary sinus
Correct answer: Trabeculated with a broad triangular base
The right atrial appendage has a broad, triangular base and prominent pectinate muscle trabeculae, whereas the left atrial appendage is narrow and finger-like with a higher thrombus risk in AF.
AF Anatomy & Physiology Fundamentals 3
During atrial fibrillation, the ventricular rate is irregular primarily because of which property of the AV node?