Last Updated : 1/5/2011
N C Joshi
SINUS RHYTHM: Rhythm originating in the sinus node

Definition: Irregular rhythm originating in the sinus node.
(P-P interval varies > 0.08 sec)

They are seen in the following conditions:

Arrhythmia with narrow QRS,
- Wandering atrial pacemaker
- Aberrant atrial pacemaker
- Accelerated junctional rhythm

Arrhythmia with wide QRS
- Accelerated ventricular rhythm
- Sinus with aberrancy
- Accelerated junctional rhythm with aberrancy

QRS COMPLEX: Consider the following when looking at the QRS complex
- Regular or irregular
- How fast?
- P-waves: Narrow or wide
- P-wave axis
- Association of P-waves to QRS

Narrow QRS : Infant < 80 msec, Child < 85 msec. It is generally supraventricular in origin. Generally requires activation through bundle of His

Wide QRS: It is often of ventricular origin. Also seen with Aberrant conduction (Rate-related versus present in sinus). There may be presence of accessory connection. Associated with drug effects. Seen with electrolyte imbalance.

Wide QRS of Ventricular origin : The following is supportive of ventricular origin:
- Positive concordance (If no RVH)
- AV dissociation

Wide QRS of Aberrancy
Fixed bundle branch block
- Common in post-op hearts
- Reflects conduction system disease in unoperated hearts
- Ashman phenomenon
- Failure of bundle to consistently conduct at higher rates
- Usually refractory period of right bundle is more than left bundle

Wide QRS of Accessory connection
- Bundle Branch morphology: opposite location of accessory connection
- May be present intermittently in sinus rhythm

Wide QRS of Drug effects
- Type IA anti-arrythmics
- Tricyclic antidepressants

Wide QRS of Electrolytes
- Hyperkalemia
- Hypoxia and acidosis

It is calculated from the lowest heart rate for 6 sec in normal individuals. It is defined as less than
- 68 BPM in Infants
- 42 BPM in Child
- 26 BPM in Adolescent

Causes of bradycardia:
- Sinus bradycardia
- Atrial escape
- Junctional escape
- Ventricular escape
- Asystole

Causes of Bradycardia with narrow QRS:
- Sinus arrhythmia or bradycardia
- Wandering atrial pacemaker (Atrial escape)
- Junctional escape (40 - 60 BPM)

Causes of Bradycardia with wide QRS:
- Ventricular Escape (30 - 40 BPM)
- Escape above bifurcation of His with aberrancy

Definition: Three successive beats faster than normal for age.

- Sinus tachycardia
- Atrial tachycardia
- Junctional tachycardia
- Ventricular tachycardia

Tachycardia with Narrow QRS:
- Flutter
- Atrial Fibrillation
- Junctional tachycardia

Tachycardia with wide QRS:
- Ventricular tachycardia
- SVT with aberrancy
- Junctional tachycardia with aberrancy
- Ventricular Fibrillation

Mechanism of formation of tachycardia:
Disorders of impulse formation
- Abnormal automaticity
- Triggered dysrhythmia
Disorders of impulse propagation
- Re-entry
- Reflection
Abnormal Automaticity

Anatomical Malformations causing reentry:
- Bypass tracts
- Dual AV Nodal Pathways
- Damaged tissue
- Anatomical obstruction
- Anisotropy

Figure 1: Mechanism for re-entry

Definition: Tachycardia resulting from an abnormal mechanism dependent upon structures above the bifurcation of the bundle of His for propagation and excluding atrial flutter or fibrillation.

Characteristics in Children
- 90% narrow QRS
- May be difficult to see P wave
- Orientation and location of P waves suggest mechanism of tachycardia
- BPM usually around 300

Mechanisms of Supraventricular Tachycardia:
Probable Re-entrant Mechanism
- Orthodromic reciprocating
- Antidromic reciprocating
- AV nodes re-entry
- Permanent junctional reciprocating tachycardia (PJRT)
- Sinus node reentry
- Atrial muscle re-entry
Presence of Bypass Tracts
- Atrioventricular or accessory connections
- Wolff-Parkinson-White
- Unidirectional retrograde accessory pathway (URAP)
- intranodal pathways
- Dual AV nodal pathways
Probable Abnormal Automaticity
- Atrial ectopic tachycardia(AET)
- Chaotic atrial tachycardia s
- Junctional ectopic tachycardia

- Typical saw-toothed P waves
- Rate 280-450 BPM (Infants may have even 500BPM!)
- Atypical in Post-op hearts
- May be modified by drugs

Characteristics :
- Irregularly irregular rate
- Intermittent P waves
- If WPW, may have variable QRS morphology

Definition: Wide QRS tachycardia originating in the ventricle

- Dissociation from atria proves Ventricular tachycardia
- Change in morphology of QRS (particularly if abnormal in sinus)

- To determine if tachycardia is present
- To determine the mechanism of tachycardia
- For drug testing if tachycardia is present
- To locate the origin of critical structures
- Ablation of the origin of critical structures

Contributor Information and Disclosures

N C Joshi
Consultant Pediatrician, Nanavati Hospital, Mumbai, India

First Created : 1/18/2001


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