ECG criteria
- The PQ interval progressively lengthens from complex to complex in conducted cycles, followed by a dropped QRS; the more such cycles are present on the tracing, the more reliable the assessment
- In typical periodicity, the absolute PR increment is usually maximal at the beginning of the cycle and then decreases; this is a supportive, but not mandatory feature
- With a regular atrial rhythm, RR usually gradually shortens from cycle to cycle despite PQ lengthening; this is an ancillary sign, not a mandatory condition
- With a regular PP, the R–R interval between complexes surrounding the dropped complex is usually less than twice the preceding R–R; this is an ancillary, not a mandatory feature
- After the dropped beat — a P wave with the shortest (baseline) PQ, and the cycle begins anew
- A P wave precedes each conducted QRS, P morphology is unchanged
- The QRS complex is usually narrow if the block is localized in the AV node
What it looks like on ECG
The classical Wenckebach phenomenon is a series of P-QRS cycles in which the PQ interval increases from complex to complex until a given P wave is blocked and the ventricular complex drops out. After the pause, conduction recovers typically with a shorter PR, and the sequence repeats — this repeating structure is called the Samoyloff–Wenckebach periodicity. A 3:2 periodicity may correspond to Mobitz I if the PR increase is clear and repetitive, but classification based on a single cycle has limited reliability; in this case, a longer tracing and other features should be evaluated.
Key patterns are assessed by measuring PQ and RR in each consecutive cycle, preferably on a long tracing and in the lead where P and PR are most clearly visible (often II or V1):
| Parameter | Cycle changes |
|---|---|
| PQ | progressively lengthens in conducted complexes, the absolute increment is usually maximal at the beginning of the cycle and then decreases |
| RR | with regular PP, it usually gradually shortens despite the increase in PQ (ancillary feature) |
| Pause (dropped complex) | with regular PP, the interval between QRS before and after the drop is usually less than twice the preceding RR (ancillary feature) |
| QRS | usually narrow (AV nodal block) |
The shortening of RR against the background of increasing PQ during a regular atrial rhythm is explained by the fact that the absolute PQ increment from cycle to cycle decreases, although the PQ itself continues to increase — this creates the characteristic "grouping" of complexes on the tracing. In sinus arrhythmia or other atrial rhythm variability, this pattern may not be observed, and the diagnosis is still established based on PR dynamics.
The Wenckebach phenomenon can manifest not only as second-degree AV block Mobitz I, but also in a broader sense — as any periodic decremental conduction with increasing delay, including in the sinoatrial node (Wenckebach-type sinoatrial block); similar patterns are possible in certain decremental accessory conduction pathways, but this is a rare and specialized situation.
Mechanism
The phenomenon is based on decremental conduction in tissue with a relatively long refractory period, most commonly in the AV node. Each successive impulse finds the nodal cells in varying degrees of recovery from the previous excitation, and therefore is conducted more slowly than the previous one. In a typical cycle, the absolute increase in delay gradually decreases due to the decremental conduction properties of the AV node; the exact shape of this relationship is not a diagnostic criterion, the key factor is the decreasing PQ increment with each cycle. When the next impulse reaches insufficiently recovered, functionally refractory AV nodal tissue, it is not conducted — resulting in a dropped QRS. After the pause, the node manages to recover better, and the cycle starts over, typically with a shorter PR.
Causes
- physiological variant in healthy individuals with high vagal tone (during sleep, in young and athletic individuals)
- use of drugs that slow AV conduction: beta-blockers, non-dihydropyridine calcium channel antagonists, digoxin, amiodarone
- inferior wall myocardial ischemia and infarction (the right coronary artery typically supplies the AV node)
- myocarditis, infiltrative and degenerative diseases of the conduction system
- increased intracranial pressure, vagotonia in other diseases
- age-related degenerative lesion of the conduction system (especially the His-Purkinje system); in elderly patients with a wide QRS, such a pattern requires evaluation for possible infrasodal block
Differential diagnosis
- Second-degree AV block, Mobitz type II — PR interval in conducted beats is constant, without progressive prolongation before dropped beats; the block is usually infrasodal, QRS is often wide
- 2:1 AV block — a single isolated cycle is insufficient to determine whether the PR interval is lengthening, so the type (Mobitz I or II) cannot be classified here without additional data
- Sinus arrhythmia with irregular PR — PR fluctuations lack a regular progressive pattern and do not culminate in a dropped complex
- Concealed conduction and parasystole — may mimic rhythm irregularity but do not produce true progressive PR prolongation
Clinical significance and management
The Wenckebach phenomenon in the form of second-degree AV block Mobitz I most often reflects impaired conduction at the AV node level and in most cases has a benign course, especially if caused by vagotonia in a young, healthy individual without organic heart disease. In contrast to infrasodal block (Mobitz II), the risk of progression to complete AV block and symptomatic bradycardia is lower.
Management strategy: evaluate symptoms (dizziness, presyncope), rule out reversible causes — review medications that slow AV conduction, check for signs of inferior wall ischemia and electrolyte disturbances. Asymptomatic Wenckebach phenomenon detected, for example, during Holter monitoring in sleep usually does not require treatment. In case of symptomatic bradycardia, progression to high-grade AV block or complete AV block, pacing should be considered according to general indications for AV block.
Common interpretation errors
- Evaluating only a single PR-PR cycle instead of a series of several complexes — the pattern is more reliably observed over a longer segment, whereas classification based on a short pattern (e.g., 3:2) is less reliable.
- Confusing the Wenckebach phenomenon with any simply lengthening PR interval without a subsequent dropped complex.
- Considering progressive RR shortening as a mandatory diagnostic criterion, whereas the defining sign is PR dynamics, while RR changes are only auxiliary.
- Misclassifying 2:1 AV block as Mobitz I or II without additional data on PR dynamics.
- Forgetting that the term applies not only to the AV node, but also to the sinoatrial node.
"The PR interval progressively increases, then a P wave is blocked; after the pause, the PR interval is usually shorter" — this is the basis of Wenckebach periodicity diagnosis, whereas RR shortening and pause ratios are only auxiliary signs.
1. Which sign confirms the Wenckebach phenomenon in a series of P-QRS cycles?
The defining sign is progressive PR lengthening up to a non-conducted P wave; RR shortening with a regular atrial rhythm additionally supports the diagnosis.
2. At which level of the conduction system is the block most frequently localized in the typical Wenckebach phenomenon?
Decrement conduction with progressive PQ prolongation is a typical property of AV nodal tissue.
3. Why is the R–R interval surrounding the dropped complex often less than twice the preceding R–R during a regular atrial rhythm?
The absolute increment in conduction delay decreases with each cycle, so with a regular PP, the resulting pause is usually less than twice the preceding R–R — this is a classic auxiliary sign.
Frequently asked questions
How to distinguish the Wenckebach phenomenon from Mobitz II?
The dynamics of PR are primarily compared: in the Wenckebach phenomenon (Mobitz I), the PR interval progressively increases in conducted complexes before a dropped beat, whereas in Mobitz II, the PR in conducted complexes is usually constant (minor variability is possible), and the drop occurs suddenly. A wide QRS in Mobitz II increases the likelihood of an infranodal block, but is not a mandatory sign.
Is the Wenckebach phenomenon dangerous?
In itself, it is usually benign, especially in young individuals with high vagus nerve tone and no structural heart disease. The risk increases with symptomatic bradycardia, progression to high-grade block, or if it is associated with acute myocardial ischemia.
Can the Wenckebach phenomenon occur in a healthy person?
Yes, this is a frequent finding during sleep or with high vagal activity in trained and young individuals, typically without clinical significance.
How does Samoilov–Wenckebach periodicity differ from the Wenckebach phenomenon?
These are synonymous terms: Samoilov–Wenckebach periodicity describes the same repeating cycle of progressive PQ prolongation followed by a dropped complex, named after two researchers who independently described this phenomenon.
Does the Wenckebach phenomenon occur outside the AV node?
Yes, a similar pattern of decremental conduction can be observed in the sinoatrial node (type I sinoatrial block) and, in rare cases, in certain decremental accessory pathways.
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Related terms
More in the «Phenomena and Eponyms» section
Sources
- Hampton J.R. The ECG in Practice
- Orlov V.N. Guide to Electrocardiography
- Surawicz B., Knilans T. Chou's Electrocardiography in Clinical Practice
- AHA/ACCF/HRS Recommendations for the Standardization of the ECG, 2009
Educational material for medical students and physicians. Does not replace clinical guidelines and physical examination; the ECG must always be interpreted in conjunction with the clinical picture.