CRAT Cardiac Anatomy and Physiology 2 — Questions and Answers
Question 1: Which structure is the primary pacemaker of the heart under normal conditions?
- Sinoatrial (SA) node (Correct answer)
- Atrioventricular (AV) node
- Bundle of His
- Purkinje fibers
Correct answer: Sinoatrial (SA) node
The SA node in the right atrium is the dominant pacemaker, firing at 60-100 bpm due to its fastest intrinsic automaticity.
The SA node has the highest intrinsic rate (60-100 bpm), so it normally overrides all other potential pacemakers. It generates spontaneous action potentials through If (funny) current channels. Autonomic nervous system input modulates its rate: sympathetic stimulation increases rate, parasympathetic decreases it.
Question 2: What is the intrinsic firing rate of the AV node if it serves as the escape pacemaker?
- 40-60 bpm (Correct answer)
- 60-100 bpm
- 20-40 bpm
- 100-150 bpm
Correct answer: 40-60 bpm
The AV junction has an intrinsic escape rate of 40-60 bpm, slower than the SA node but faster than ventricular escape rhythms.
The AV junction serves as a backup pacemaker when the SA node fails. Its intrinsic rate of 40-60 bpm maintains adequate cardiac output in many patients. Junctional rhythms produce narrow QRS complexes (unless aberrant conduction) with inverted or absent P waves in inferior leads.
Question 3: During ventricular depolarization, which ECG component is produced?
- QRS complex (Correct answer)
- P wave
- T wave
- U wave
Correct answer: QRS complex
The QRS complex represents ventricular depolarization — the electrical activation that spreads through the ventricular myocardium triggering contraction.
The QRS complex is generated as the electrical impulse travels from the bundle branches through the Purkinje fibers into the ventricular myocardium. The sequence is: septal depolarization (Q wave), left ventricular free wall (R wave), and basal areas (S wave). Normal QRS duration is less than 120 ms.
Question 4: The refractory period of the AV node serves which primary function?
- Limiting ventricular rate during rapid atrial arrhythmias (Correct answer)
- Initiating spontaneous depolarization
- Conducting impulses faster to the ventricles
- Generating the P wave
Correct answer: Limiting ventricular rate during rapid atrial arrhythmias
The AV node's inherent delay and refractory period act as a gatekeeper, preventing excessively rapid atrial rates from being conducted to the ventricles at dangerous speeds.
The AV node intentionally slows conduction (the PR interval delay) and its refractory period prevents conduction of every atrial impulse during rapid atrial arrhythmias. In atrial flutter at 300 bpm, the AV node typically conducts at 2:1 or lower ratios, protecting the ventricles from dangerously rapid rates.
Question 5: Which coronary artery most commonly supplies the SA node?
- Right coronary artery (RCA) (Correct answer)
- Left anterior descending artery (LAD)
- Left circumflex artery
- Left main coronary artery
Correct answer: Right coronary artery (RCA)
In approximately 60% of individuals, the SA node is perfused by the right coronary artery, which is why inferior MI (RCA territory) often causes sinus bradycardia or AV block.
The SA nodal artery arises from the RCA in approximately 60% of people and from the left circumflex in 40%. This explains why right coronary territory infarcts (inferior MI) frequently present with sinus bradycardia, SA nodal dysfunction, or AV block. The AV node is also supplied predominantly by the RCA.
Question 6: The T wave on the ECG represents which electrophysiological event?
- Ventricular repolarization (Correct answer)
- Atrial repolarization
- Ventricular depolarization
- AV nodal conduction
Correct answer: Ventricular repolarization
The T wave represents ventricular repolarization — the recovery phase of the ventricular cells preparing for the next depolarization.
Ventricular repolarization generates the T wave. Atrial repolarization also occurs but is hidden within the QRS complex. T wave changes (inversion, peaked, flattened) provide important clinical information about ischemia, electrolyte disturbances (peaked T in hyperkalemia), and myocardial injury.
CRAT Cardiac Anatomy and Physiology 2
Which structure is the primary pacemaker of the heart under normal conditions?