Epidemiology
Incidence
In the US, Junctional rhythms are common in patients with sick sinus syndrome or in patients who have significant bradycardia that allows the AV nodal region to determine the heart rate. Incidence of paroxysmal supraventricular tachycardia is approximately 1-3 per 1000.
Gender
Junctional escape rhythms, which are common in younger and/or athletic individuals during periods of increased vagal tone (eg, sleep), occur equally in males and females.
Age
Episodes of atrioventricular nodal re-entrant tachycardia may occur at any age. It commonly presents for the first time in childhood or adolescence, although it may appear at any age. Junctional rhythms during sleep are common in children and in athletic adults.
Risk factors
- Previous myocardial infarction
- Mitral valve prolapse
- Rheumatic heart disease
- Pericarditis
- Pneumonia
- Chronic lung disease
- Current alcohol intoxication
- Digoxin toxicity
Pathogenesis
The two types of junctional tachycardias are:
- Atrioventricular nodal re-entrant tachycardia (AVNRT) in which the entire tachycardia circuit is confined to the AV node and surrounding myocardium.
- Atrioventricular re-entrant tachycardia (AVRT) in which there is an additional abnormal pathway between the atrium and ventricle completing the circuit.
Atrioventricular nodal re-entrant tachycardia
The most common cause of paroxysmal regular narrow complex tachycardia. Usually affects young, healthy individuals with no organic heart disease.
They are caused by having two distinct pathways in the region of the atrioventricular node.
The 'fast' pathway conducts more rapidly, but has a longer refractory period.
The 'slow' pathway has slower conduction properties but a shorter refractory period. During sinus rhythm, AV node conduction occurs via the fast pathway with a normal PR interval. If a premature atrial extrasystole arises, conduction in the fast pathway is blocked, but slow pathway conduction may continue. Conduction down the slow pathway may be sufficiently slow to allow the fast pathway to recover and allow retrograde activation to occur via the fast pathway, causing a re-entry circuit with anterograde conduction via the slow pathway and retrograde conduction via the fast pathway.
Anterograde activation of the ventricles and retrograde activation of the atria occur virtually simultaneously, resulting in the P-wave being obscured within the QRS complex, or producing a very small distortion of the terminal QRS. The tachycardia is readily initiated by atrial premature stimulation, and terminated by appropriately timed external stimuli or by overdrive pacing.
A less common variant of AV node tachycardia may arise where anterograde conduction during tachycardia is via the fast pathway with retrograde conduction via the slow pathway. The atrium is activated well after the QRS complex, producing an inverted P wave with the RP interval greater than the PR interval during tachycardia.
Occasionally, slow/fast and fast/slow tachycardias may coexist in the same patient.
Atrioventricular re-entrant tachycardia
Occurs as a result of an accessory conduction pathway, allowing the atrial impulse to bypass the AV node and activate the ventricles prematurely. A re-entry circuit forms and paroxysmal atrioventricular re-entrant tachycardias occur. The commonest kind of atrioventricular re-entrant tachycardia occurs as part of the Wolff-Parkinson-White syndrome.
Orthodromic tachycardia
The common form of atrioventricular re-entry tachycardia. A premature atrial extrasystole may find the accessory pathway refractory, but be conducted through the atrioventricular node to the ventricles. Retrograde activation may then occur from the ventricle to atrium, causing a re-entry circuit. The QRS complex during re-entry tachycardia is normal, unless a rate-related, bundle-branch block develops. Retrograde atrial activation causes a characteristic inverted P wave early in the ST segment.
Antidromic tachycardia
A rarer form of atrioventricular tachycardia. Anterograde conduction occurs via the accessory pathway and retrograde conduction via the atrioventricular node. The QRS complexes are abnormal. The appearance depends on the site of the accessory pathway.
