From the Guidelines
Mobitz Type I second-degree AV block is a cardiac conduction abnormality that rarely requires permanent pacing, but may be considered in highly symptomatic patients who don't respond to medication adjustments, as recommended by the 2019 ACC/AHA/HRS guideline 1.
Diagnosis and Treatment
Mobitz Type I second-degree AV block, also known as Wenckebach phenomenon, is characterized by progressive prolongation of the PR interval on ECG until a beat is completely blocked, followed by a reset of the cycle. This condition occurs due to gradual impairment of conduction through the AV node, typically resulting from increased vagal tone, medication effects, or ischemia.
Key Considerations
- Most patients with Mobitz Type I are asymptomatic and require no specific treatment if they remain hemodynamically stable.
- For symptomatic patients, atropine (0.5-1 mg IV) may be administered to increase heart rate temporarily, as suggested by the 1996 ACC/AHA guidelines 1.
- If medications are causing the block, they should be discontinued or their dosages reduced, particularly beta-blockers, calcium channel blockers, or digoxin.
- Underlying causes such as myocardial ischemia should be addressed.
- Unlike Mobitz Type II, Mobitz Type I rarely progresses to complete heart block and generally has a benign prognosis.
Management and Monitoring
- Patients should be monitored periodically with ECGs to ensure the condition doesn't worsen.
- Permanent pacemaker implantation is rarely needed for Mobitz Type I, but may be considered in highly symptomatic patients who don't respond to medication adjustments, as stated in the 2019 ACC/AHA/HRS guideline 1.
- The guideline recommends permanent pacing for patients with marked first-degree or second-degree Mobitz type I (Wenckebach) atrioventricular block with symptoms that are clearly attributable to the atrioventricular block, with a class IIa recommendation and level of evidence C-LD 1.
From the Research
Diagnosis of Mobitz Type I Second-Degree AV Block
- The diagnosis of Mobitz Type I second-degree atrioventricular (AV) block is characterized by a visible, differing, and generally decremental AV conduction 2.
- It is also known as Wenckebach second-degree AV block, which describes a lengthening PR interval consistent with second-degree AV block - Mobitz Type I 3.
- The site of the block in Mobitz Type I is usually within the AV node 4.
Treatment of Mobitz Type I Second-Degree AV Block
- The treatment of Mobitz Type I second-degree AV block depends on the underlying cause and the presence of symptoms.
- In some cases, aminophylline may be effective in improving AV conduction in atropine-resistant late advanced AV block complicating acute inferior myocardial infarction, including Mobitz Type I AV block 5.
- However, the use of aminophylline is not a standard treatment for Mobitz Type I second-degree AV block, and its effectiveness may vary depending on the individual case.
- In general, pacing is not required for Mobitz Type I second-degree AV block unless it is associated with symptoms or other underlying conditions that require pacing 2.
Differential Diagnosis
- Second-degree AV block must be distinguished from other causes of pauses, including nonconducted premature atrial contractions and atrial tachycardia with block 4.
- Concealed (nonpropagated) His bundle or ventricular extrasystoles may mimic both type I and/or type II block (pseudo AV block) 2.
- A pattern resembling a narrow QRS type II block in association with an obvious type I structure in the same recording effectively rules out type II block because the coexistence of both types of narrow QRS block is exceedingly rare 2.