This occurs when the sinus rate is slowed or when the junctional pacemaker increases its firing rate. The normal junctional pacemaker discharge rate is 40-60 bpm. Depending upon whether the conduction from the junctional pacemaker is anterograde or retrograde, inverted "P" waves (inferior leads) may be seen preceding, following or coinciding with the QRS complex.
The QRS complex will be identical to that resulting from normal sinus conduction because the origin of firing is above the bundle branch divisions.
Junctional rhythms occur commonly under anaesthesia, especially using halogenated agents. This may cause a fall in cardiac output and blood pressure, and may need treatment with an anti-cholinergic agent or vasopressor.
AV nodal tachycardia
This is manifested as a:
- Passive escape rhythm with a rate of 70-140 bpm, or as a
- Paroxysmal junctional tachycardia with a rate of 150-200 bpm
- The P waves may occur before, within or after the QRS complexes, or be unrelated to it.
- The QRS complex is usually narrow (may be wide with a bundle branch block).
This arrhythmia is relatively common under halothane anaesthesia. It rarely requires treatment.
In patients receiving digitalis, this arrhythmia may represent digitalis toxicity.