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An unusual hyperkalaemia induced block
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The tracing below was recorded from a patient with a DDD pacemaker and hyperkalaemia (7.8 mEq/1) caused by acute renal failure. The atrial mechanism is sinus at a rate of 110/min. Every sinus P wave is followed by a ventricular spike, but several of these fail to depolarise the myocardium. The paced QRS complexes are extremely wide, measuring about 0.38 seconds. There is a basic 2:1 block of artificial stimuli; following a ventricular capture, the ensuing spike falls at the apex of the T wave during the refractory period. This is caused by the extreme prolongation of the ventricular depolarisation and repolarisation process. In addition, spike-to-QRS intervals show a gradual lengthening, ending in block of two consecutive stimuli.

 The association of 2:1 block with progressive prolongation of the conduction intervals up to block of two or three consecutive impulses is known as “alternating Wenckeback periodicity”. This phenomenon is well recognised in atrioventricular conduction, particularly in atrial flutter, but has not been hitherto observed in conduction between an artificial pacemaker and the ventricular myocardium.

 Correction of the hyperkalaemia was followed by normal ventricular capture and narrowing of the QRS complexes.