1. Overview and definition

Second-degree atrioventricular (AV) block, Mobitz type I—also called the Wenckebach phenomenon—is an intermittent failure of AV conduction in which the PR interval progressively lengthens over successive conducted beats until an atrial impulse is blocked and no corresponding QRS complex follows. After the non-conducted P wave, the cycle generally resets with a shorter PR interval.

The usual site is the AV node. The characteristic pattern reflects decremental conduction: successive impulses encounter progressively slower conduction through nodal tissue until one impulse fails to reach the ventricles.

Why recognition matters

Typical AV-nodal Wenckebach is often transient and relatively benign, but the clinical context matters. Symptoms, associated myocardial infarction, medication effects, structural heart disease, QRS width, and the possibility of intra-Hisian or infra-Hisian disease should influence risk assessment.

2. Electrophysiology and mechanism

In the common nodal form, the AV node has decremental conduction properties. Repeated atrial impulses arrive before complete recovery of nodal tissue, producing progressively longer AV conduction times. Eventually, an impulse is not conducted. The subsequent pause permits recovery, so the next conducted impulse has a shorter PR interval.

Common associated settings described in the supplied reference

• AV-nodal drugs: beta-blockers, non-dihydropyridine calcium-channel blockers, digoxin, and amiodarone.
• Increased vagal tone, including in trained athletes.
• Inferior myocardial infarction, through AV-nodal ischemia and/or increased vagal tone (including the Bezold–Jarisch reflex).
• Myocarditis.
• After cardiac surgery, including mitral valve or congenital heart surgery.

3. Characteristic ECG features

Assess the atrial rhythm and every P-to-QRS relationship across the entire rhythm strip. The diagnosis is based on the sequence, not on a single PR measurement.

Click to enlarge

PR and R–R behavior through a cycle

The P–P interval is usually relatively constant. The greatest PR increase often occurs between the first and second conducted beats of a cycle. The R–R intervals within the group may progressively shorten before the pause. Common conduction sequences include 3:2, 4:3, and 5:4; the ratio can vary.

Click to enlarge

The supplied 12-lead example on PDF page 3 is described as a repeating 5:4 P-to-QRS conduction pattern, with progressive PR prolongation and relatively constant P–P intervals despite irregular QRS timing.

Click to enlarge

The example on PDF page 4 shows grouped QRS complexes separated by non-conducted P waves, with the source describing variable conduction ratios of 5:4 to 6:5. Compare the PR interval at the beginning and end of each conducted group.

Click to enlarge

4. Systematic ECG interpretation

1. Check recording quality and calibration. Confirm standardization where shown (commonly 25 mm/s and 10 mm/mV); identify artifact that could obscure P waves.
2. Find the atrial activity first. Identify P waves in a lead where they are most visible (often lead II or V1). Do not assume every deflection near a QRS is a P wave.
3. Measure P–P intervals. Determine whether atrial timing is regular or relatively constant through the pause. A premature P wave suggests a different mechanism.
4. Map every P wave to the next QRS. Mark conducted P waves and the non-conducted P wave. Look for a consistent atrial sequence rather than judging only ventricular rhythm.
5. Measure successive PR intervals. Use the same lead and sweep speed. Look for progressive lengthening before the dropped beat and a shorter PR after the pause.
6. Assess the conduction ratio and cycle. Describe the observed pattern (e.g., 3:2, 4:3, or 5:4) without assuming that the ratio is fixed.
7. Assess QRS width and baseline conduction. A narrow QRS supports, but does not prove, AV-nodal block. Wide QRS or bundle-branch block warrants greater concern for distal disease.
8. Review the clinical context. Check symptoms, blood pressure, medication exposure, ischemic symptoms, recent procedures, and reversible causes.
9. State the diagnosis and uncertainty. If only 2:1 conduction is captured, report second-degree AV block with indeterminate subtype rather than forcing a Mobitz I or II label.

5. Diagnostic criteria

Essential ECG pattern

• At least two consecutive conducted atrial impulses are available for comparison.
• Progressive PR-interval prolongation across conducted beats.
• A subsequent P wave is not followed by a QRS complex.
• The PR interval after the pause is typically shorter than the PR interval immediately before the blocked beat.

Supportive, not obligatory, features

• Relatively constant P–P intervals.
• Grouped beating and a repeating Wenckebach sequence.
• Progressive shortening of R–R intervals within a group.
• Narrow QRS complexes in the usual AV-nodal form.

There is no single PR-duration threshold that defines Mobitz I. The diagnosis depends on the relative change in PR intervals and the presence of a non-conducted P wave. A single long PR interval is not sufficient.

6. Differential diagnosis

Grouped QRS complexes separated by pauses are a useful first clue, but the supplied reference explicitly notes that this appearance may also represent second-degree AV block, non-conducted premature atrial complexes, or occasionally sinoatrial exit block.

7. Clinical presentation and significance

Patients may be asymptomatic, particularly with transient AV-nodal Wenckebach. When symptoms occur, they may include light-headedness, presyncope, syncope, fatigue, exercise intolerance, or dyspnea, depending on the ventricular rate, frequency of dropped beats, underlying cardiac disease, and hemodynamic reserve.

The supplied reference characterizes typical Mobitz I as often benign, with minimal hemodynamic disturbance and a low risk of progression to complete heart block. This should not be generalized to every patient: the level of block and clinical setting matter.

Clinical contexts requiring closer evaluation

• Acute inferior myocardial infarction: AV-nodal ischemia and increased vagal tone can contribute to AV block. Evaluate the entire ECG and the patient for acute coronary syndrome and right ventricular involvement.
• Drug exposure: review dose, recent changes, renal function where relevant, and combinations of AV-nodal suppressing agents.
• Wide QRS, bundle-branch disease, unexplained syncope, or suspected intra-/infra-Hisian block: consider specialist assessment and further localization.
• Postoperative or inflammatory settings: correlate with timing, operative details, and evidence of myocardial or conduction-system injury.

Click to enlarge

The supplied postoperative example (PDF page 5) shows atrial pacing after mitral valve surgery. The source describes progressive lengthening of the interval between pacing spikes until a pacing impulse is not conducted. This illustrates a Wenckebach-like conduction pattern in a paced context; interpret the pacing markers and device context rather than treating it as an ordinary spontaneous sinus rhythm tracing.

8. Clinical management principles

Management is determined by symptoms, hemodynamic stability, the suspected level of block, reversible causes, and the underlying disease—not by the ECG label alone.

Initial assessment

• Assess airway, breathing, circulation, blood pressure, perfusion, mental status, and symptoms of ischemia or heart failure.
• Obtain a 12-lead ECG and continuous rhythm monitoring when clinically indicated.
• Review medications and evaluate potentially reversible contributors, guided by the clinical presentation (for example, ischemia, electrolyte disturbance, or drug toxicity).

Management considerations

• Asymptomatic, typical AV-nodal Mobitz I: observation and clinical follow-up are commonly appropriate; permanent pacing is generally not indicated in the absence of symptom correlation or a higher-risk conduction disorder.
• Symptomatic bradycardia: manage according to the applicable adult bradycardia/emergency protocol. Atropine may improve AV-nodal block, but response is not guaranteed and is less reliable when block is infranodal. Do not delay escalation in an unstable patient.
• Persistent instability or failure of initial therapy: urgent expert support and pacing capability may be required, according to the clinical scenario and local protocol.
• Suspected infranodal disease, syncope, wide QRS, or progressive conduction disease: obtain cardiology/electrophysiology assessment; further testing may be needed to establish the level of block and pacing indication.
• Reversible cause: correct the cause where possible and reassess conduction before considering permanent device therapy.

9. Common pitfalls and diagnostic limitations

• Calling every dropped QRS Mobitz I: identify the P wave and prove progressive PR lengthening.
• Missing a P wave buried in a T wave: inspect the T-wave contour and use a longer rhythm strip or another lead. The supplied inferior STEMI/RV infarction example highlights a non-conducted P wave deforming the downslope of a T wave.
• Classifying 2:1 block as Mobitz I or II: a 2:1 segment alone is indeterminate.
• Confusing a blocked PAC with AV block: check whether the atrial deflection is premature and whether it resets the sinus cycle.
• Assuming a narrow QRS is mandatory: it is common in nodal Wenckebach, not an absolute diagnostic requirement.
• Assuming all Mobitz I is benign: evaluate symptoms, QRS width, associated conduction disease, and the clinical setting.
• Relying on automated ECG interpretation: manually inspect P waves, PR intervals, and the full rhythm sequence.

10. ECG interpretation practice

Case 1 — Classic Wenckebach sequence

A rhythm strip shows regular P–P intervals. The PR interval lengthens over successive conducted beats, followed by a P wave without a QRS. The next conducted beat has a shorter PR interval.

Case 2 — Predominantly 2:1 conduction in inferior STEMI

The supplied source describes a tracing in which most of the rhythm strip shows 2:1 AV conduction. This portion cannot independently distinguish Mobitz I from Mobitz II. A short 3:2 cycle is visible in the middle of the strip, with progressive PR prolongation and a non-conducted P wave partly hidden in the downslope of a T wave. Continuous rhythm recording subsequently supported Mobitz I.

Click to enlarge

Case 3 — A tracing that challenges pattern recognition

The source's PDF page 6 presents a rhythm strip originally shown as Wenckebach and asks the reader to identify a deliberate diagnostic error. The supplied extract does not provide the answer or explanation. Therefore, this page preserves it as a critical-thinking exercise and does not assert a definitive reinterpretation unsupported by the supplied material.

Click to enlarge

11. Clinical pearls and key takeaways

• The defining feature is progressive PR prolongation culminating in a non-conducted P wave.
• The PR interval is usually longest immediately before the dropped beat and shortest immediately after it.
• P–P intervals are often relatively constant; R–R intervals may shorten within a Wenckebach group.
• Grouped beating is a clue, not a diagnosis. Exclude blocked PACs and other causes of pauses.
• A 2:1 AV block cannot be reliably typed as Mobitz I or Mobitz II from that segment alone.
• Typical AV-nodal Mobitz I is often benign, but symptoms, QRS width, and suspected infranodal disease change the clinical assessment.
• Permanent pacing decisions depend on symptom correlation, block location, and associated conduction-system disease.

12. References