ECG criteria
- The PQ interval progressively lengthens from cycle to cycle followed by a non-conducted P wave; no minimum number of lengthenings is established, but repeating groups are preferred for reliable diagnosis
- The increment in PQ prolongation is typically maximal on the second conducted complex of the cycle and then decreases (decrement) — a characteristic but not mandatory feature of classic periodicity
- The RR interval often progressively shortens before the pause with a stable PP interval — an auxiliary, non-mandatory sign
- In typical periodicity with a stable PP interval, the R–R interval that includes the dropped beat is usually shorter than the sum of two adjacent expected R–R intervals (equivalently, shorter than 2 PP); with variable PP or atypical periodicity, this rule may not hold
- Following the QRS dropout, the cycle begins with the shortest PQ interval, and the sequence repeats
- The QRS complex is usually narrow (less than 120 ms) when the block is localized in the AV node, but may be wide due to concomitant bundle branch block or infra-Hissian localization; wide complexes warrant caution
- The ratio of conducted to blocked P waves is written as number of P : number of QRS (e.g., 4:3, 3:2) per cycle
What it looks like on ECG
On a long recording, preferably in lead II, a repeating group of cycles is visible. In each group, the first PQ interval after the pause is the shortest, and then the PQ interval progressively lengthens from beat to beat before a non-conducted P wave. Typically, the increment in PQ prolongation is greatest between the first and second complexes of the group, and subsequently the increment becomes smaller with each successive cycle (decrement conduction) — this is a characteristic feature of classic periodicity, although the strict criterion is the very fact of progressive PR prolongation prior to dropout.
With a stable atrial rhythm (PP), the RR interval often gradually shortens before the pause because the absolute PQ increment decreases faster than the underlying interval grows. This is a frequent but non-mandatory finding: with sinus arrhythmia or fluctuations in the atrial rate, the RR may not shorten. The group ends with the dropout of a single QRS complex — a P wave is present, but is not followed by a QRS.
In typical periodicity with a stable PP interval, the R–R interval that includes the dropped beat is usually shorter than the sum of two adjacent expected R–R intervals (equivalently, shorter than 2 PP). This is a mathematical consequence of the decreasing PQ increment, rather than a sign of an additional block. With a variable PP or atypical form of periodicity, this rule may not hold and is not strictly required for the diagnosis.
| Parameter | Typical picture |
|---|---|
| PQ | progressively lengthens, the increment frequently decreases |
| RR | often shortens before the pause (auxiliary sign) |
| QRS pause | usually shorter than 2 expected RRs at stable PP |
| QRS | narrow if the block is in the AV node |
| P:QRS ratio | 3:2, 4:3, 5:4, etc. |
With long groups (e.g., 8:7 vs 3:2), PR changes may be subtle, and the periodicity can mimic second-degree AV block without obvious grouping — precise interval measurements and a sufficiently long recording are required.
Mechanism
Periodicity arises due to decremental conduction in the AV node: the next impulse reaches the node in a state of incomplete recovery (functional refractoriness) following the previous conducted excitation, and therefore is conducted more slowly. The degree of slowing for each successive impulse is generally less than the previous one, as the refractoriness of the node gradually recovers — hence the typical decreasing PQ increment. When the next atrial impulse reaches the AV node during a phase of insufficient recovery, conduction fails and the P wave is not conducted to the ventricles. After the pause, the AV node has time to recover, and the cycle begins anew with the shortest PQ. Most often, the block is localized in the AV node, where decremental conduction is typical. However, Mobitz I can have an infra-Hissian localization, especially with a wide QRS and organic disease of the conduction system; in such cases, additional evaluation, sometimes an electrophysiology study, is required.
Causes
- Increased vagal tone (during sleep, in young healthy individuals, athletes)
- Inferior myocardial infarction, most often due to right coronary artery occlusion, less commonly left circumflex artery occlusion in left-dominant AV nodal blood supply
- Myocarditis, rheumatic fever
- Use of drugs that slow AV conduction: beta-blockers, calcium channel blockers, digoxin, amiodarone
- Electrolyte disturbances, hypothyroidism
- Age-related degenerative changes of the conduction system
- State after cardiac surgery, catheter ablation near the AV node
Differential diagnosis
- Second-degree AV block, Mobitz II: PQ is constant prior to dropout, without progressive lengthening; sudden dropout, QRS is often wide due to block at the bundle of His or its branches level.
- 2:1 AV block: with a stable 2:1 ratio, PQ progression cannot be observed (there is only one conducted complex before the blocked one), so reliably typing Mobitz I or II on an ECG is usually impossible; QRS width and clinical context are assessed, with exercise or pharmacological testing under monitoring or electrophysiological study performed if necessary.
- Sinus arrhythmia with dropped beats: in sinus arrhythmia, the PP interval varies while the PQ interval remains constant; in Wenckebach periodicity, conversely, PP is relatively stable while PQ systematically varies.
- Second-degree sinoatrial block: there, the entire PQRST complex drops out (block prior to the atria), whereas in AV periodicity, the P wave is present but not conducted to the ventricles.
- Blocked atrial extrasystole: a single non-conducted P differs in morphology and occurs earlier than expected, without preceding PQ progression.
Clinical significance and management
Samoylov–Wenckebach periodicity (Mobitz I) is benign in the majority of cases, especially in young individuals with high vagal tone and during sleep — specific treatment is not required, observation suffices. The physician needs to: determine the P:QRS ratio and pattern stability on a long recording or Holter monitoring, assess QRS width and clinical context (a narrow QRS points to a nodal level of block and a favorable prognosis), find and correct a reversible cause if possible (discontinue or adjust the dose of drugs slowing AV conduction, evaluate for ischemia in inferior infarction). If typing the level of the block is necessary, an exercise or pharmacological test under monitoring is used, and if uncertainty persists, an electrophysiology study; vagal maneuvers are not recommended as a routine typing method, especially in acute infarction and ischemia. With a wide QRS, the width itself is not an indication for pacing — it merely increases the likelihood of an infra-Hissian block level (including due to pre-existing bundle branch block) and requires further evaluation. In acquired Mobitz II, high-grade, or complete AV block, permanent cardiac pacing is indicated regardless of symptoms after reversible causes have been excluded. In Mobitz I, pacing is considered in the presence of symptoms (syncope, presyncope, pronounced bradycardia with clinical manifestations), significant bradycardia, or proven infra-Hissian localization of the block.
Common interpretation errors
- Conclude Mobitz I is present based on a single short pause without evaluating several consecutive PQ cycles.
- Consider the maximum PQ increment between the first and second complexes and decrement to be mandatory signs, even though this is only a typical, but not strictly mandatory, presentation.
- Attempt to type the block in a 2:1 ratio from a single tracing without assessing QRS width and clinical context or without prolonged monitoring.
- Fail to measure QRS width, which helps indirectly judge the level of the block.
- Mistake normal sinus arrhythmia with an accidental dropped beat for true Wenckebach periodicity.
"PQ progressively increases, pause is shorter than two expected RRs" — a simplified rule for recognizing typical periodicity.
1. Which ECG sign is most specific for Samoylov–Wenckebach periodicity?
Specifically, the progressive prolongation of the PQ interval before a non-conducted P wave and the subsequent dropout of QRS is the defining feature of Wenckebach periodicity (Mobitz I).
2. How frequently does the RR interval change before the pause during typical Wenckebach periodicity?
With a stable PP interval, the absolute increment of PQ from cycle to cycle decreases; therefore, the RR interval often gradually shortens before the pause—this is a typical, but not strictly mandatory finding.
3. At what level of the conduction system is the block most frequently localized in Samoilov–Wenckebach periodicity?
Decrement conduction with a decreasing PQ increment is most characteristic of the AV node; however, Mobitz I can also have an infra-Hissian localization, especially with a wide QRS.
Frequently asked questions
How to distinguish Wenckebach periodicity from Mobitz II AV block?
In Wenckebach periodicity (Mobitz I), the PQ interval progressively prolongs before QRS dropout, whereas in Mobitz II, the PQ interval remains constant and dropout is sudden. Additionally, in Mobitz II, the QRS complex is often wide because the block is localized below the AV node.
Is Samoilov–Wenckebach periodicity dangerous?
Usually no: with a narrow QRS and nocturnal/vagal origin, this is generally benign second-degree AV block Mobitz I. The risk increases with symptomatic bradycardia, a wide QRS, or progression to a higher-degree block—in which case further evaluation and potentially cardiac pacing are required.
Why is the pause in Wenckebach periodicity shorter than the sum of two normal RR intervals?
Because the increment of the PQ interval from cycle to cycle usually decreases. Although one P wave is blocked, the interval between the QRS complexes before and after the dropout is determined not only by the skipped cycle but also by the decreasing PQ increment, which makes this pause shorter than the sum of two normal RR intervals at a stable atrial rhythm.
Can Wenckebach periodicity be observed with a 2:1 ratio?
With a persistent 2:1 block on a given tracing, it is not possible to reliably see PR progression and classify it as Mobitz I or II, because only one conducted complex is visible before the blocked P wave. However, if the conduction ratio changes or longer groups are present on neighboring cycles, the signs of Wenckebach may become visible.
Where is Wenckebach periodicity best visualized on an ECG?
It is best observed on a long recording in lead II or on a rhythm strip, where it is convenient to compare consecutive PQ and RR intervals within a single group of cycles.
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Related terms
More in the «AV Blocks» section
Sources
- Orlov, V.N. «Electrocardiography Guide»
- Hampton J.R. The ECG in Practice
- Surawicz B., Knilans T.K. Chou's Electrocardiography in Clinical Practice
- AHA/ACCF/HRS Recommendations for the Standardization and Interpretation of the Electrocardiogram, 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.