What triggers the normal cardiac cycle?

The Normal Cardiac Cycle: Triggers and Conduction Pathway

The normal cardiac cycle is triggered by the spontaneous depolarization of pacemaker cells in the sinoatrial (SA) node, which serves as the primary pacemaker of the heart with an intrinsic firing rate of 60-100 beats per minute. 1 This automatic depolarization initiates electrical impulses that spread through the cardiac conduction system to coordinate the heartbeat.

Cardiac Conduction System Components

Primary Pacemaker - Sinoatrial Node

• Located in the right atrium at the junction of the superior vena cava
• Contains specialized cardiac myocytes with unique properties:
  • Faster rates of phase 4 diastolic depolarization
  • Longer action potentials
  • Unique ion channel and connexin expression profile that enables automaticity 1
• Surrounded by connective tissue that electrically insulates pacemaker cells from surrounding atrial myocardium

Conduction Pathway

1. Atrial Conduction: From the SA node, electrical impulses spread through both atria via specialized conduction pathways

   • Activation spreads centrifugally to both atria 2
   • Atrial depolarization causes atrial contraction

2. Atrioventricular Node:

   • Located in the inferior-posterior region of the interatrial septum
   • Provides a critical delay (approximately 200 ms) in impulse conduction from atria to ventricles 2
   • This delay allows atrial contraction to complete before ventricular contraction begins, increasing cardiac output by 25-30% 2

3. His-Purkinje System:

   • Bundle of His
   • Right and left bundle branches
   • Purkinje fiber network
   • Conducts impulses approximately twice as fast as atrial tissue 2
   • Enables near-simultaneous activation of ventricular myocardium

Backup Pacemakers

If the SA node fails, secondary pacemakers can take over:

• Secondary pacemaker (AV node): Fires at 40-60 beats per minute 1
• Tertiary pacemakers (His bundle, bundle branches, Purkinje fibers): Fire at 30-40 beats per minute 1

Electrophysiological Basis of Cardiac Cycle

The cardiac cycle involves precise timing of electrical and mechanical events:

1. Electrical activation: Proceeds from endocardium to epicardium
2. Electrical repolarization: Proceeds from epicardium to endocardium 2
3. Mechanical contraction: Follows electrical activation with synchronous inward movement
4. Mechanical relaxation: Follows repolarization with outward diastolic movement 2

Clinical Relevance and Potential Dysfunction

Disruptions in the normal cardiac conduction system can lead to various arrhythmias:

• Sinus node dysfunction: Can cause bradycardia, tachy-brady arrhythmias, and conduction abnormalities 1
• AV node dysfunction: Can result in various degrees of heart block
• Bundle branch blocks: Cause asynchronous ventricular contraction
• Fibrosis: Often affects both SA and AV nodes, causing broader conduction issues 1

ECG Representation of the Cardiac Cycle

The normal cardiac cycle is represented on ECG by:

• P wave: Atrial depolarization
• PR interval: Conduction through AV node (normal: 120-200 ms)
• QRS complex: Ventricular depolarization
• T wave: Ventricular repolarization
• QT interval: Total ventricular electrical activity (depolarization and repolarization)

Key Considerations in Cardiac Cycle Assessment

• Heart rate variability reflects autonomic influence on the SA node
• Patients with elevated left atrial pressure show less beat-to-beat variation in mitral inflow velocity 2
• Proper timing of atrial contraction is critical for optimal cardiac output
• PR intervals that are too short or too long can adversely affect ventricular filling and cardiac performance 2

Understanding the normal cardiac cycle and its triggers is essential for interpreting ECGs, diagnosing arrhythmias, and implementing appropriate treatment strategies for cardiac conduction disorders.