Junctional premature beats

Originate from the His bundle or the AV junction. The impulse may activate the ventricles via the normal pathway. Alternatively the atria may also be activated by retrograde conduction. This is where the impulse travels back up towards the atria instead of down towards the ventricles activating the atria. Junctional pranature beats can be seen on the ECG as they occur before the next normal sinus beat. Often there is no P wave present if the atria and ventricles are depolarised simultaneously, as this is masked by the QRS complex (Fig. 6.3). Alternatively the P wave may be inverted (Fig. 6.4) and can occur either before, or after the QRS complex (Fig. 6.5). 

Normal beat Atrial premature beat Normal beat 

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A tachycardia made up of three or more Premature Junctional Beats. Junctional Tachycardia is usually caused by the AV node taking over as the dominant pacemaker due to enhanced automaticity. As in junctional escape beats the atria are retrogradely depolarized, resulting in inverted P waves that can occur after the QRS complex. 

Vomiting is common in patients with digitalis overdose. Hyperkalemia may be caused by acute digitalis overdose or severe poisoning, whereas hypokalemia may be present in patients as a result of long-term diuretic treatment. (Digitalis does not cause hypokalemia.) A variety of cardiac rhythm disturbances may occur, including sinus bradycardia, AV block, atrial tachycardia with block, accelerated junctional rhythm, premature ventricular beats, bidirectional ventricular tachycardia, and other ventricular arrhythmias. 

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