Educational

Heart Rate and Rhythm

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by

Saim Khalid
in

Introduction

Electrocardiography (ECG) is a non-invasive, rapid, and invaluable tool for evaluating heart rate and rhythm. It allows clinicians to visualize the heart’s electrical activity and identify normal sinus rhythm, arrhythmias, conduction abnormalities, and rate disorders. Accurate interpretation of ECG is critical in both acute and chronic cardiac care, guiding treatment decisions for conditions ranging from tachycardia and bradycardia to atrial fibrillation and life-threatening ventricular arrhythmias.

Heart rate and rhythm analysis relies on understanding waveforms, intervals, and the relationship between atrial and ventricular depolarization. This article provides a comprehensive, structured, and in-depth discussion on ECG-based heart rate and rhythm analysis, including measurement techniques, types of rhythms, clinical significance, and advanced considerations.

1. Basic Principles of Heart Rate Determination

Heart rate is determined by the frequency of ventricular depolarizations, represented by the R wave of the QRS complex.

1.1 Normal Heart Rate

  • Adults: 60–100 beats per minute (bpm).
  • Children and neonates have higher rates:
    • Neonates: 100–160 bpm
    • Infants: 90–150 bpm
    • Children: 70–120 bpm

1.2 Bradycardia and Tachycardia

  • Bradycardia: <60 bpm. May be physiological (athletes, sleep) or pathological (sinus node dysfunction, AV block).
  • Tachycardia: >100 bpm. Causes include fever, hypovolemia, hyperthyroidism, sympathetic activation, or arrhythmias.

1.3 ECG-Based Measurement Methods

1.3.1 6-Second Method

  • Count QRS complexes in a 6-second strip (30 large squares).
  • Multiply by 10 to obtain bpm.
  • Simple but approximate; used in irregular rhythms.

1.3.2 Large Box Method

  • ECG paper speed: 25 mm/s (1 large square = 0.2 s).
  • Count the number of large boxes between two consecutive R waves:
    • HR = 300 ÷ number of large boxes.
  • Most accurate in regular rhythms.

1.3.3 Small Box Method

  • Count the number of small squares between R waves:
    • HR = 1500 ÷ number of small squares.
  • Provides higher precision; useful in borderline tachycardia or bradycardia.

1.3.4 Instantaneous Heart Rate

  • Useful for continuous monitoring or ambulatory ECG (Holter).
  • HR = 60 ÷ RR interval (in seconds).

2. Rhythm Analysis Fundamentals

2.1 Steps for Rhythm Evaluation

  1. Assess regularity: Are R–R intervals consistent?
  2. Identify P waves: Are they present before each QRS?
  3. Measure PR interval: Normally 0.12–0.20 seconds (3–5 small boxes).
  4. Measure QRS duration: Normally <0.12 seconds (≤3 small boxes).
  5. Evaluate T waves and ST segment: Look for abnormalities associated with rhythm disorders.
  6. Compare atrial and ventricular rates: Identify AV dissociation or conduction blocks.

2.2 Categories of Cardiac Rhythms

  1. Normal Sinus Rhythm (NSR):
    • Originates in the SA node.
    • Rate: 60–100 bpm.
    • P wave: upright in lead II, precedes each QRS.
    • Regular R–R intervals.
  2. Sinus Arrhythmia:
    • Variation in RR intervals with respiration.
    • Common in young adults; usually benign.
  3. Ectopic Rhythms:
    • Originate outside the SA node (atria, AV node, ventricles).
    • Includes premature atrial contractions (PACs), premature ventricular contractions (PVCs), junctional rhythms.
  4. Bradyarrhythmias:
    • Sinus bradycardia: slow sinus rhythm.
    • AV blocks: first, second, third degree.
    • Junctional escape rhythm: AV node as pacemaker.
  5. Tachyarrhythmias:
    • Sinus tachycardia: physiological response.
    • Supraventricular tachycardia (SVT): atrial tachycardia, AVNRT, AVRT.
    • Atrial fibrillation/flutter: irregularly irregular rhythm.
    • Ventricular tachycardia (VT): wide complex, life-threatening.

3. P Wave Analysis

3.1 Significance

  • Represents atrial depolarization.
  • Provides clues about sinus node function, atrial enlargement, and ectopic atrial rhythms.

3.2 Normal P Wave

  • Duration: ≤0.12 s
  • Amplitude: ≤2.5 mm
  • Shape: upright in leads I, II, aVF; inverted in aVR

3.3 Abnormal P Waves

  • Peaked P waves: Right atrial enlargement.
  • Notched or broad P waves: Left atrial enlargement.
  • Absent or inverted P waves: Ectopic atrial rhythm or junctional rhythm.

4. PR Interval and AV Conduction

  • PR interval: onset of P wave → beginning of QRS.
  • Normal: 0.12–0.20 seconds.

4.1 Prolonged PR Interval (>0.20 s)

  • First-degree AV block.

4.2 Progressive PR Lengthening

  • Second-degree AV block type I (Wenckebach).

4.3 Dropped QRS Without PR Lengthening

  • Second-degree AV block type II (Mobitz II).

4.4 Complete AV Block

  • Third-degree AV block; atrial and ventricular activity independent.

5. QRS Complex and Ventricular Conduction

  • Represents ventricular depolarization.
  • Duration: 0.06–0.12 s.

5.1 Narrow QRS (<0.12 s)

  • Supraventricular origin of rhythm.

5.2 Wide QRS (>0.12 s)

  • Ventricular origin (VT, PVC).
  • Bundle branch block (RBBB or LBBB).

5.3 Morphology Considerations

  • Monomorphic vs. polymorphic ventricular tachycardia.
  • QRS axis deviation suggests chamber hypertrophy or infarction.

6. Sinus Rhythms and Variants

6.1 Normal Sinus Rhythm

  • Rate: 60–100 bpm
  • Regular R–R intervals, upright P waves in II

6.2 Sinus Tachycardia

  • Rate >100 bpm
  • Physiological: exercise, stress
  • Pathological: fever, anemia, hyperthyroidism

6.3 Sinus Bradycardia

  • Rate <60 bpm
  • Common in athletes
  • May indicate sinus node disease or increased vagal tone

6.4 Sinus Arrhythmia

  • Respiratory variation in R–R interval
  • Benign, often in young adults

7. Supraventricular Tachyarrhythmias (SVT)

7.1 Atrial Fibrillation (AF)

  • Most common sustained arrhythmia
  • Characteristics:
    • Irregularly irregular R–R intervals
    • Absent distinct P waves
    • Risk: stroke, heart failure

7.2 Atrial Flutter

  • Regular atrial rate 250–350 bpm
  • Sawtooth flutter waves (best in II, III, aVF)
  • Ventricular response may be regular (2:1, 3:1)

7.3 Paroxysmal Supraventricular Tachycardia (PSVT)

  • Sudden onset and termination
  • Narrow QRS, regular rhythm
  • Includes AVNRT, AVRT

8. Ventricular Tachyarrhythmias

8.1 Premature Ventricular Contractions (PVCs)

  • Wide, bizarre QRS complexes
  • Usually followed by compensatory pause
  • May be benign or precede VT

8.2 Ventricular Tachycardia (VT)

  • Rate: 100–250 bpm
  • Wide QRS, monomorphic or polymorphic
  • Life-threatening; can lead to VF

8.3 Ventricular Fibrillation (VF)

  • Chaotic, irregular electrical activity
  • No effective cardiac output
  • Requires immediate defibrillation

9. Heart Blocks

9.1 First-Degree AV Block

  • PR interval >0.20 s
  • All P waves conducted

9.2 Second-Degree AV Block

  • Type I (Wenckebach): progressive PR prolongation, dropped QRS
  • Type II (Mobitz II): sudden dropped QRS, PR constant

9.3 Third-Degree (Complete) AV Block

  • Atrial and ventricular activity independent
  • Ventricular escape rhythm often <50 bpm

9.4 Bundle Branch Blocks

  • RBBB: rsR’ in V1, wide S in V6
  • LBBB: broad, notched QRS in V5–V6, absent Q in V5–V6

10. Stepwise Approach to Heart Rate and Rhythm Analysis

  1. Determine rate: Use small box, large box, or 6-second method.
  2. Assess regularity: Measure multiple R–R intervals.
  3. Evaluate P waves: Presence, morphology, relationship to QRS.
  4. Measure PR interval: Detect AV conduction abnormalities.
  5. Assess QRS duration and morphology: Narrow vs. wide complex.
  6. Check for ST-T abnormalities: Ischemia, electrolyte disturbances.
  7. Determine rhythm type: Sinus, supraventricular, or ventricular.
  8. Identify axis deviation: Correlate with chamber enlargement or MI.

11. Clinical Significance

  • Tachycardia: Can precipitate ischemia, heart failure, or syncope.
  • Bradycardia: May indicate sinus node dysfunction or high-grade AV block.
  • AF/flutter: Stroke risk, heart failure risk; anticoagulation indicated.
  • VT/VF: Life-threatening; immediate resuscitation needed.
  • Heart blocks: Guide pacemaker implantation or pharmacological management.

Accurate rhythm analysis is critical in emergency care, perioperative monitoring, chronic disease management, and pharmacological decision-making.

12. Advanced Considerations

12.1 Holter Monitoring

  • Continuous 24–48 hour ECG recording
  • Detects intermittent arrhythmias, rate variability, pauses

12.2 Event Monitors

  • Patient-activated devices for symptomatic events

12.3 Implantable Loop Recorders

  • Long-term arrhythmia monitoring for unexplained syncope or palpitations

12.4 Rate vs. Rhythm Control in AF

  • Determines treatment strategy
  • Rate control: beta-blockers, calcium channel blockers
  • Rhythm control: antiarrhythmic drugs, cardioversion, ablation

13. Common Pitfalls

  • Misinterpreting artifacts as arrhythmias
  • Electrolyte disturbances mimicking tachyarrhythmias
  • Lead misplacement affecting P wave or QRS morphology
  • Ignoring correlation with clinical symptoms

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