28 Patho

Disorders of Cardiac Conduction and Rhythm

Cardiac Conduction System

The cardiac conduction system is responsible for controlling the rate and direction of electrical impulse conduction in the heart, ensuring coordinated contraction and efficient pumping of blood.

• Key Components:

  • SA node (Sinoatrial node): The primary pacemaker of the heart, generating impulses at the fastest rate.

  • AV node (Atrioventricular node): Delays the impulse before passing it to the ventricles.

  • Bundle of His, bundle branches, and Purkinje fibers: Specialized cells that rapidly conduct impulses throughout the ventricles.

• Function: Maintains a regular heart rate and the pumping efficiency of the heart.

Phases of Cardiac Potentials

Cardiac action potentials are essential for initiating and coordinating heart contractions. They occur in distinct phases:

1. Phase 0: Rapid Upstroke

   • Sodium channels open, allowing Na+ ions to enter the cell.

   • Membrane potential rapidly increases from negative to positive.

2. Phase 1: Early Repolarization

   • Sodium channels close; potassium channels open, allowing K+ ions to exit.

   • Slight decrease in membrane potential.

3. Phase 2: Plateau

   • Calcium channels open, Ca2+ ions enter the cell, counteracting K+ efflux.

   • Maintains a plateau in membrane potential, crucial for muscle contraction.

4. Phase 3: Final Repolarization

   • Calcium channels close; potassium channels remain open, K+ continues to exit.

   • Membrane potential returns to negative values.

5. Phase 4: Diastolic Repolarization

   • Cell is at rest; sodium and calcium channels closed, potassium channels open.

   • Membrane potential maintained by the sodium-potassium pump.

Electrocardiography (ECG)

Electrocardiography is a diagnostic tool that records the electrical activity of the heart using multiple leads.

• 12-lead ECG: Includes 6 chest leads and 6 limb leads, each providing a unique view of the heart's electrical forces.

• Diagnostic Criteria: Lead-specific; allows for advanced arrhythmia detection and early identification of ischemia/infarction-related changes.

• Importance of Lead Placement: Incorrect placement can alter QRS morphology and lead to misdiagnosis of arrhythmias or conduction defects.

Continuous Bedside Cardiac Monitoring

Continuous monitoring uses a 3-lead or 5-lead system to provide real-time data on heart rhythm and electrical activity.

• Main Goals:

  • Real-time detection of arrhythmias and myocardial ischemia

  • Identification of ST segment changes

  • Advanced arrhythmia identification

  • Diagnosis and treatment guidance

Disorders of the Cardiac Conduction System

Disorders can affect the heart's rhythm or impulse conduction, leading to abnormal heartbeats or impaired electrical signal transmission.

• Types:

  • Disorders of rhythm: Heart beats too fast, too slow, or irregularly (e.g., Atrial fibrillation, Tachycardia, Bradycardia).

  • Disorders of impulse conduction: Problems with how impulses travel (e.g., bundle branch block, heart block, long QT syndrome).

• Causes:

  • Congenital defects or degenerative changes

  • Myocardial ischemia and infarction

  • Fluid and electrolyte imbalances

  • Drug effects

Types of Arrhythmias

Sinus Node Arrhythmias

• Sinus bradycardia: Heart rate < 60 bpm; often due to vagal stimulation or medications.

• Sinus tachycardia: Heart rate > 100 bpm; caused by increased sympathetic stimulation or withdrawal of vagal tone.

• Sinus arrest: Failure of SA node to discharge; irregular pulse with prolonged periods of systole.

Arrhythmias of Atrial Origin

• Paroxysmal supraventricular tachycardia (SVT): Sudden onset, regular HR 140-240 bpm; symptoms include rapid HR, dizziness, fatigue, chest pain, palpitations.

• Atrial flutter: HR 240-340 bpm, sawtooth ECG pattern; rapid, regular atrial contractions.

• Atrial fibrillation: HR 400-600 bpm, uncoordinated atrial activity; irregular pulse, risk of thromboembolism.

Arrhythmias of Ventricular Conduction and Rhythm

• Premature ventricular contractions (PVCs): Wide QRS, early ventricular contraction.

• Ventricular tachycardia: HR 70-250 bpm, three or more PVCs in a row; can be stable or unstable.

• Ventricular fibrillation: Chaotic, no effective contraction; requires immediate defibrillation.

• Long QT syndrome: Prolonged QT interval, risk of torsades de pointes and sudden cardiac death.

Arrhythmias of Atrioventricular Conduction

• First-degree AV block: All signals reach ventricles but are delayed; usually asymptomatic.

• Second-degree AV block: Some signals dropped; irregular heart rate, possible dizziness.

• Third-degree AV block: No signals reach ventricles; very slow, irregular rhythm, serious symptoms, may require pacemaker.

Diagnostic Methods

• Signal-averaged electrocardiogram

• Holter monitoring

• Exercise stress testing

• Electrophysiologic studies

Pharmacologic Treatment of Arrhythmias

Antiarrhythmic drugs are classified by their mechanism of action:

Class	Mechanism
I	Block fast sodium channels
II	Beta-adrenergic blockers; inhibit sympathetic stimulation
III	Extend action potential and refractoriness
IV	Block slow calcium channels; depress phase 4, lengthen phases 1 and 2

Interventions for Conduction Defects, Bradycardias, and Tachycardias

• Electronic pacemaker: Device that regulates heart rhythm by sending electrical impulses. Can be temporary or permanent.

• Cardioversion: Procedure to restore regular rhythm using electrical shock; includes synchronized and unsynchronized (defibrillation).

• Ablation: Removal or destruction of abnormal tissue using heat or cold to treat rhythm disorders.

• Surgical interventions: Repair of structural defects or implantation of devices like pacemakers or implantable cardioverter-defibrillators (ICDs).

Example: ECG Interpretation

• Normal sinus rhythm: Regular rhythm, P wave before each QRS, normal PR interval.

• Atrial fibrillation: Irregularly irregular rhythm, no distinct P waves, variable ventricular rate.

• Ventricular tachycardia: Wide QRS complexes, rapid rate, may be life-threatening.

Additional info: Cardiac conduction and rhythm disorders are a major focus in both clinical medicine and biomedical research, with implications for emergency care, chronic disease management, and pharmacology.