I. Arrhythmias Associated with Impaired Automaticity

Pathophysiology

A. Sick Sinus Syndrome (SSS) / Sinus Node Dysfunction

Diagnostic Criteria

ECG Findings

• Persistent sinus bradycardia (HR < 40–50 bpm)
• Long, inappropriate sinus pauses
• Slow junctional escape rhythm (40–60 bpm) or ventricular escape (20–40 bpm) when sinus fails
• Post-tachycardia pauses in tachy-brady syndrome

B. AV Junctional Escape Rhythm

C. Idioventricular Escape Rhythm

D. General ECG Approach (Harrison's, p. 6937)

"The ECG is the cornerstone and most important diagnostic test… evidence suggesting underlying cardiac disease, such as prior MI, LVH, atrial disease, or baseline conduction system disease may suggest a diagnosis."

• Rate below expected escape rate for the pacemaker level in question
• P wave absence or abnormality (inverted, retrograde, absent)
• Escape morphology (narrow = junctional; wide = ventricular)
• AV dissociation without block (competing slow rhythms)

II. Diagnostic Criteria for an Attack of Atrial Paroxysmal Tachycardia (APT)

Clinical Diagnostic Criteria

ECG Diagnostic Criteria

1. P Wave Morphology

• P waves present and precede each QRS (long RP tachycardia pattern typical)
• P wave morphology differs from sinus P wave — the axis depends on the focus location:
  • High right atrium (near SA node): P wave resembles sinus; tall, upright in II, III, aVF
  • Low right atrium / coronary sinus: Negative P in II, III, aVF (inferiorly directed focus)
  • Left atrium: Negative P in V1 reversal, positive in inferior leads

2. RP/PR Relationship

• Typically long RP interval (RP > PR); P wave falls clearly before the next QRS
• RP > 70 ms (P wave visible before QRS, not buried within or immediately after it)

3. Rate and AV Conduction

• Atrial rate 150–250 bpm
• 1:1 AV conduction is common; variable block (2:1, Wenckebach) may occur at faster rates
• AV block during tachycardia does not terminate it — the atria continue at the same rate (hallmark distinguishing AT from AVNRT/AVRT)

4. QRS Complex

• Narrow (< 120 ms) unless aberrant conduction or pre-existing BBB
• Wide-complex APT can occur with BBB — must differentiate from VT

5. Warm-Up / Cool-Down Phenomenon

• Gradual acceleration at onset and gradual slowing at termination (distinguishes from AVNRT/AVRT, which start/stop abruptly)

Differential Diagnosis of SVT (Harrison's, p. 6993)

Summary Table

Diagnostic Criteria for Arrhythmias Associated with Impaired Automaticity

Background: Mechanism

1. Sinus Bradycardia

Diagnostic Criteria

• Rate: < 60 bpm (clinically significant < 50 bpm)
• P wave: Normal morphology (upright in I, II, aVF; biphasic or inverted in V1), positive in inferior leads — confirming sinus origin
• PR interval: Normal (120–200 ms)
• QRS: Normal narrow complex, each preceded by a P wave
• Rhythm: Regular

Clinical significance

2. Sinus Node Dysfunction (Sick Sinus Syndrome)

"Intrinsic sinus node disease is sometimes referred to as sick sinus syndrome or sinus node dysfunction (SND) and can manifest as fatigue, exercise intolerance, or syncope resulting from either reduced heart rate or pauses. Electrocardiographic recording plays a central role in the diagnosis and management of SA node dysfunction. The correlation between symptoms and slow heart rate or pauses is essential in determining whether bradycardia may be considered pathologic and necessitating intervention."

Diagnostic Criteria

3. Sinus Arrest vs. Sinoatrial Exit Block

"Sinus arrest results from failure of impulse formation within the sinus node. Sinoatrial exit block results from failure of sinus node activity to propagate to the atrium."

Differentiating the Two

ECG Example: Sinus Arrest with Sinus Bradycardia

4. Escape Rhythms (Subsidiary Pacemaker Activation)

A. AV Junctional Escape Rhythm

• Rate: 40–60 bpm
• QRS: Narrow (< 120 ms), normal morphology
• P waves: Absent, retrograde (negative in II, III, aVF), or dissociated from QRS
• Regularity: Regular

B. Idioventricular Escape Rhythm

• Rate: 20–40 bpm
• QRS: Wide (> 120 ms), LBBB or RBBB morphology
• P waves: Absent or AV dissociation present
• Regularity: Regular
• Indicates failure of both SA and AV nodal automaticity

5. Enhanced Vagotonia / Drug-Induced Impaired Automaticity

• Beta-blockers, calcium channel blockers, digoxin, amiodarone — suppress SA automaticity
• Vasovagal / carotid sinus hypersensitivity — transient excessive parasympathetic tone
• Hypothyroidism, hypothermia, hypoxia, hyperkalemia — metabolic suppression

Diagnostic Algorithm Summary

Bradyarrhythmia suspected
        │
        ▼
12-lead ECG + rhythm strip
        │
  ┌─────┴──────┐
Sinus       Escape rhythm
bradycardia  (junctional or ventricular)
  │
  ├─ Asymptomatic → likely physiologic (athlete, sleep)
  │
  └─ Symptomatic → Holter / MCT monitoring
               │
     ┌─────────┴──────────┐
  Sinus arrest        SA exit block
  (pause ≠ multiple    (pause = exact
  of P–P)              multiple of P–P)
               │
        Tachycardia-bradycardia?
               │
          Diagnose SSS / SND

Key Diagnostic Pearls

• Symptom-rhythm correlation is mandatory — incidental sinus bradycardia alone is not diagnostic of pathologic SND (Harrison's, p. 6949)
• A pause > 3 seconds while awake is generally considered pathologic
• Junctional escape during a pause is a normal protective response, not an additional pathology
• Distinguish sinus arrest (non-multiple pause) from SA exit block (multiple of P–P cycle) — both represent failure at different points of impulse generation/propagation
• Resting ECG is insufficient alone — prolonged monitoring is required for intermittent dysfunction

Diagnostic Criteria for Arrhythmias Associated with Impaired Automaticity

Pathophysiological Basis

Classification of Arrhythmias from Impaired Automaticity

1. Sinus Bradycardia

• Heart rate < 60 bpm (clinically significant when < 50 bpm)
• P wave: Normal sinus morphology — upright in I, II, aVF; biphasic/inverted in V1
• P–QRS relationship: Each P wave is followed by a QRS; PR interval 120–200 ms
• QRS: Narrow (< 120 ms), normal morphology
• Rhythm: Regular

2. Sinus Node Dysfunction (Sick Sinus Syndrome — SSS)

"Intrinsic sinus node disease is sometimes referred to as sick sinus syndrome or sinus node dysfunction (SND) and can manifest as fatigue, exercise intolerance, or syncope resulting from either reduced heart rate or pauses. The correlation between symptoms and slow heart rate or pauses is essential in determining whether bradycardia may be considered pathologic and necessitating intervention."

3. Sinus Arrest vs. Sinoatrial Exit Block — Key Distinction

ECG Illustration: Abnormal Sinus Rhythms

4. Escape Rhythms (Subsidiary Pacemaker Activation)

A. AV Junctional Escape Rhythm

• Rate: 40–60 bpm
• QRS: Narrow (< 120 ms), normal morphology
• P waves: Absent, retrograde (negative in II, III, aVF), or dissociated from QRS
• Rhythm: Regular
• Significance: Indicates SA node failure with intact AV junctional automaticity

B. Idioventricular Escape Rhythm

• Rate: 20–40 bpm
• QRS: Wide (> 120 ms), LBBB or RBBB morphology
• P waves: Absent or AV dissociation present
• Rhythm: Regular
• Significance: Failure of both SA node and AV junctional automaticity; hemodynamically unstable

5. Extrinsic Causes of Impaired Automaticity (Must Be Excluded)

Diagnostic Algorithm

Bradyarrhythmia / pauses suspected
              │
              ▼
       12-lead ECG + rhythm strip
              │
    ┌─────────┴──────────┐
Sinus bradycardia      Escape rhythm present
    │                  (junctional or ventricular)
    │
    ├─ Asymptomatic → Likely physiologic
    │
    └─ Symptomatic → Holter / MCT monitoring
                  │
         ┌────────┴─────────┐
     Sinus arrest        SA exit block
     (pause ≠ multiple    (pause = exact
      of P–P)              multiple of P–P)
                  │
        Tachy-brady alternation?
                  │
           Diagnose SSS / SND

Key Diagnostic Pearls

• Symptom-rhythm correlation is mandatory — asymptomatic sinus bradycardia alone does not constitute pathologic SND (Harrison's, p. 6949)
• A pause > 3 seconds while awake is generally considered pathologic and warrants investigation
• Sinus arrest (failure of impulse formation) is the purest expression of impaired automaticity
• SA exit block involves intact node firing but conduction failure — a distinction important for ablation and pacemaker planning
• Junctional escape during a pause is a normal protective response, not an independent pathology
• Resting ECG alone is insufficient for intermittent dysfunction — prolonged ambulatory monitoring is the diagnostic standard

Diagnostic Criteria for Arrhythmias Associated with Impaired Automaticity

Pathophysiological Basis

Classification and Diagnostic Criteria

1. Sinus Bradycardia

• Heart rate < 60 bpm (clinically significant when persistently < 50 bpm)
• P wave: Normal sinus morphology — upright in I, II, aVF; biphasic or inverted in V1
• P–QRS relationship: Every P wave followed by a QRS; PR interval 120–200 ms
• QRS: Narrow (< 120 ms), normal morphology
• Rhythm: Regular

Pathologic only when symptomatic. Physiologic in athletes and during sleep.

2. Sinus Node Dysfunction (Sick Sinus Syndrome — SSS)

"Intrinsic sinus node disease… can manifest as fatigue, exercise intolerance, or syncope resulting from either reduced heart rate or pauses. The correlation between symptoms and slow heart rate or pauses is essential in determining whether bradycardia may be considered pathologic and necessitating intervention."

3. Sinus Arrest vs. Sinoatrial Exit Block — Critical Distinction

"Type II second-degree SA block can be inferred on the ECG if the sinus rate abruptly transitions to a rate that is half the previous rate… Sinoatrial Wenckebach can be inferred from progressive shortening of the P–P interval leading up to a sinus pause." — Harrison's, p. 6956

ECG Illustration: Abnormal Sinus Rhythms Associated with Impaired Automaticity

4. Escape Rhythms (Subsidiary Pacemaker Activation)

A. AV Junctional Escape Rhythm

• Rate: 40–60 bpm
• QRS: Narrow (< 120 ms), normal morphology
• P waves: Absent, retrograde (negative in II, III, aVF), or dissociated from QRS
• Rhythm: Regular
• Significance: SA node failure with intact AV junctional automaticity

B. Idioventricular Escape Rhythm

• Rate: 20–40 bpm
• QRS: Wide (> 120 ms), LBBB or RBBB morphology
• P waves: Absent or AV dissociation present
• Rhythm: Regular
• Significance: Failure of both SA and AV junctional automaticity; often hemodynamically unstable

5. Extrinsic Causes to Exclude Before Diagnosing Intrinsic SND

Key Diagnostic Pearls

• Symptom-rhythm correlation is mandatory — incidental sinus bradycardia alone is not pathologic SND (Harrison's, p. 6949)
• Pauses > 3 seconds while awake are generally considered pathologic
• Sinus arrest is the purest expression of impaired automaticity (failure of impulse formation)
• SA exit block involves intact node firing with conduction failure — an important mechanistic distinction for management
• Junctional escape during a pause is a normal protective response, not an independent pathology
• Resting ECG alone is insufficient — prolonged ambulatory monitoring is the diagnostic standard for intermittent dysfunction
• Other SA block types require invasive electrophysiological study (EPS) for precise characterization (Harrison's, p. 6956)

ECG of Arrhythmias

Overview: Multi-Arrhythmia Comparison Strip

1. Bradyarrhythmias (Impaired Automaticity)

2. Tachyarrhythmias

A. Supraventricular Tachycardias (SVT)

B. Ventricular Arrhythmias

3. Conduction Disorders

ECG Reading Checklist for Any Arrhythmia

1. Rate         → Atrial rate? Ventricular rate? Same or different?
2. Rhythm       → Regular / irregular / irregularly irregular?
3. P waves      → Present? Normal morphology? Relationship to QRS?
4. PR interval  → Normal (120–200 ms)? Short? Long? Constant?
5. QRS width    → Narrow (< 120 ms) = supraventricular
                  Wide (> 120 ms) = ventricular or aberrant SVT
6. QT interval  → Prolonged? (risk of Torsades)
7. Axis         → Normal / left / right deviation?
8. ST / T waves → Ischemia? Repolarization abnormality?

Diagnostic Criteria for an Attack of Atrial Paroxysmal Tachycardia (APT)

Definition and Mechanism

Clinical Diagnostic Criteria

ECG Diagnostic Criteria

12-Lead ECG — Paroxysmal SVT (Focal AT Pattern)

1. Rate

• Atrial rate 150–250 bpm (typically 150–200 bpm)
• Ventricular rate depends on AV conduction ratio (1:1 most common; 2:1 or Wenckebach block at faster rates)

2. Rhythm

• Regular (may be slightly irregular at onset — warm-up phenomenon)

3. P Wave — Most Diagnostically Important Feature

"It can be distinguished from sinus tachycardia by the P-wave morphology, which usually differs from sinus P waves depending on the location of the focus. Focal AT tends to originate in areas of complex atrial anatomy, such as the crista terminalis, valve annuli, atrial septum, and atrial muscle extending along cardiac thoracic veins."

4. RP / PR Relationship

• Long RP tachycardia: RP interval > PR interval (P wave falls well before the next QRS)
• PR interval shorter than RP interval — especially with rapid AV conduction from sympathetic tone (Harrison's, p. 7008)
• RP > 70 ms (P wave not buried within QRS — distinguishes from AVNRT)

5. QRS Complex

• Narrow (< 120 ms) — normal ventricular conduction
• Wide QRS possible with pre-existing bundle branch block or aberrant conduction — must differentiate from VT

6. AV Conduction Behavior

• 1:1 AV conduction at moderate rates; 2:1 or Wenckebach AV block at faster atrial rates
• AV block during tachycardia does NOT terminate it — the atria continue firing independently (hallmark distinguishing AT from AVNRT/AVRT where the AV node is part of the reentry circuit)

7. Warm-Up / Cool-Down Phenomenon

• Gradual acceleration at onset (first few beats slightly slower before reaching full rate)
• Gradual slowing at termination
• Indicates enhanced automaticity or triggered activity as the mechanism (absent in AVNRT/AVRT which start/stop abruptly)

Differential Diagnosis of SVT

Summary: Diagnostic Checklist for APT Attack

✔ Abrupt onset and offset (paroxysmal)
✔ Rate 150–250 bpm, regular
✔ P wave present, morphology differs from sinus
✔ Long RP pattern (RP > PR)
✔ Narrow QRS (< 120 ms)
✔ AV block with adenosine/vagal — tachycardia persists
✔ Warm-up at onset / cool-down at termination
✔ P wave axis localizes ectopic focus

Key Pearls

• The P wave morphology is the single most important ECG feature — it both confirms atrial origin and localizes the ectopic focus (Harrison's, p. 7008)
• Left atrial AT: positive monophasic P in V1 + negative P in I/aVL is a reliable localization pattern
• Septal AT: narrower-than-normal P wave duration is a subtle but useful clue
• Persistence of tachycardia despite AV block (spontaneous or induced) is the pathognomonic hallmark separating AT from all reentrant SVTs involving the AV node
• Incessant APT (> 10–15% of day) can cause tachycardia-induced cardiomyopathy — a reversible cause of dilated cardiomyopathy if detected and treated