The Electrocardiographic Footprints of Wenckebach Block
Harry G. Mond, MD, PhD a,b,c*, Jitendra Vohra, MD, FRCP a,b
A Department of Cardiology, The Royal Melbourne Hospital, Melbourne, Vic, Australia;
B University of Melbourne, Melbourne, Vic, Australia;
B Cardioscan Pty Ltd, Melbourne, Vic, Australia
Read full article: Originally published in in Heart, Lung and Circulation (2017) 26, 1252–1266 https://www.heartlungcirc.org/article/S1443-9506(17)31272-6/fulltext
In 1899, Karel Frederik Wenckebach described a cardiac arrhythmia with periodic dropped beats now referred to as a Wenckebach sequence. This was later shown to be due to a block in the atrioventricular node, but today, we identify Wenckebach sequences throughout the heart with most being recognised on the surface electrocardiograph as characteristic footprints. This manuscript will revisit Wenckebach atrioventricular block, the typical features of which only occur in about 15% of cases, with the remainder atypical.
Earlier reports regarded Wenckebach atrioventricular sequences as rare as they are only occasionally seen on the surface 12-lead electrocardiograph. Today, however, with the increased use of ambulatory Holter monitoring, Wenckebach atrioventricular sequences occur in 4–6% of all traces and are particularly common at night in the young. Most, but not all cases are benign and the clinical spectrum will be reviewed. Atypical Wenckebach atrioventricular sequences are a complex group which will be analysed in detail with a broad range of illustrations. Outside the atrioventricular conducting system, such as in the sinus node, Wenckebach sequences may not be obvious as they are partially hidden from the electrocardiographic tracing.
However, by understanding the sequence footprints, clues are available in interpreting tracing with periodic pauses. Dual chamber paced rhythms may show Wenckebach sequences due to electronic control of the atrioventricular delay. Rarely exit blocks at the cellular level in the atrium, ventricle or at the pacing electrode-tissue interface can demonstrate Wenckebach sequences recognised on the surface electrocardiograph.
Keywords: Wenckebach Block, Electrocardiography, Mobitz type 1
Excerpt: The Clinical Features of Wenckebach AV Block
Most examples of Wenckebach AV block demonstrate a narrow QRS and are regarded as AV nodal. With His bundle electrograms, there is a progressive increase in the AH interval until the H spike is blocked. With atrial rates greater than 200 bpm, Wenckebach AV block is both common and cardioprotective and generally regarded as physiologic. Wenckebach sequence patterns can occur sporadically and in ratios such as 5:4, 4:3, 3:2 and 2:1, often within the same patient. With 2:1 sequences, there is no gradual AV prolongation and this appearance can occur with both Wenckebach and Mobitz type II block. In order to differentiate between these, it is necessary to monitor the patient and if lesser degrees of block such as 3:2 are seen, then this confirms a Wenckebach sequence.
Patients with conventional Wenckebach AV block without higher levels of block are rarely symptomatic unless the sequence occurs during a period of bradycardia and the pause excessive. High levels of block such as 3:1 and 4:1 may also create significant symptomatic pauses. Wenckebach AV sequences can be modified or aborted with exercise, increased sympathetic tone and atropine, whereas carotid sinus pressure will exacerbate the block [6,7]. These manoeuvres have been suggested as means to differentiate between Wenckebach AV and Mobitz type II blocks. However, they will also affect sinus node function and this may result in confusing results. Apart from Holter monitoring and exercise testing, these tests are rarely used today.
On occasion, the Wenckebach block is at the intra-Hisian level. In this situation, Wenckebach AV sequences become worse with exercise and atropine, better with carotid sinus massage and patients may experience syncopal episodes due to higher levels of AV block. Wenckebach AV sequences may also occur in the presence of a wide QRS . The Wenckebach AV block is usually distal or infranodal, syncope is common and permanent pacing is frequently indicated . However in about 20% of these patients, the AV block is nodal and an electrophysiology study may be required in the asymptomatic patient .
In the older literature, the incidence of asymptomatic Wenckebach AV block was always quoted as very low [10–12], even with long-term follow-up . This is not surprising as Wenckebach AV block is infrequently seen on the resting 12-lead ECG. However, using ambulatory Holter monitoring, the incidence of Wenckebach AV block in asymptomatic subjects has ranged from 4 to 6% [14,15].
Narrow QRS Wenckebach AV block occurs across a wide age range, but is most common in the younger patient (<35 years), particularly during nocturnal hours [14,15]. Episodes may also occur in athletes outside the time of intensive training  and will reverse with detraining or administering sympathomimetic and vagolytic drugs , suggesting that Wenckebach AV block can in certain circumstances be a vagal mediated suppression of AV node conduction and therefore a normal variation of cardiac conduction. Similarly, vagal induced Wenckebach AV block may occur in otherwise normal subjects during sleep, vomiting, swallowing, micturition and lumbar epidural anaesthesia . Transient Wenckebach AV block may also occur during an acute inferior myocardial infarction  and hyperkalaemia .
Although digitalis toxicity is usually associated with complete heart block and atrial fibrillation, not surprisingly it can also cause Wenckebach AV block . Although asymptomatic Wenckebach AV block is generally regarded as benign, the reported natural history is confounded by a number of variables including the inability to always exclude Mobitz type II patients, the age and fitness of the patient, the location of the block and the recognition of underlying pathology including the spectrum
of congenital AV block. Some children or adolescents demonstrate Wenckebach AV block at all hours together with a first degree AV block and a narrow QRS and may represent significant progressive disease or developmental disruption of the cardiac conducting system and hence has a guarded prognosis . When evaluating a young patient with Wenckebach AV block, these differences should be taken into account as the prognosis in the two groups is very different. Not all cases of Wenckebach AV block occur in the young.
In older patients (69 years), the mortality is similar to the more clinically significant Mobitz type II block, whereas with permanent pacing, the five-year outlook is the same as the general population . Wenckebach AV block has been shown not to be benign in a 45-year cohort with those patients being paced having a better prognosis . Patients with Wenckebach AV block and an average age of 75 years
do better with cardiac pacing , whereas those with no heart disease have a more benign course than those with concomitant heart disease . The benign course of Wenckebach AV block is reflected in the North American guidelines for pacemaker implantation. In asymptomatic patients, shown not to be intra- or infra-Hisian, the indication for permanent cardiac pacing is class III C (from limited clinical evidence, the consensus opinion is no benefit and pacemaker implantation not recommended) . However in keeping with the findings in older patients, the European guidelines differ with the recommendations also including a class II b C (usefulness/efficacy is less well established by evidence/opinion).
Although Wenckebach sequences are most commonly seen and recognised in the AV conducting system, similar sequences with pauses can occur in many areas of the heart and even though some appear ‘‘invisible” on the surface ECG, nevertheless they leave characteristic footprints which allow recognition.
Full paper originally published in Heart, Lung and Circulation (2017) 26, 1252–1266. Read full article: http://dx.doi.org/10.1016/j.hlc.2017.06.718