Management of Heart Blocks: A Practical Guide
Understanding Heart Block Types
Heart blocks are classified into three main categories based on ECG findings and clinical significance:
First-Degree AV Block
- PR interval >200 ms with 1:1 AV conduction 1
- Generally benign when asymptomatic and isolated 2
- No treatment required for asymptomatic patients with PR <300 ms and normal QRS duration 2
- Permanent pacing is NOT recommended for persistent first-degree AV block with bundle-branch block of old or indeterminate age 1
- Exception: Permanent pacing is reasonable when marked first-degree block (PR ≥240-300 ms) causes symptoms clearly attributable to AV delay, such as pacemaker syndrome-like symptoms 1
Second-Degree AV Block
Mobitz Type I (Wenckebach):
- Progressive PR prolongation until a P wave fails to conduct 1
- Usually occurs at the AV node level 3
- Management approach:
Mobitz Type II:
- Periodic nonconducted P waves with constant PR intervals in conducted beats 1
- Almost always occurs below the AV node (infranodal) 3
- High risk of progression to complete heart block 3
- Permanent pacing is indicated regardless of symptoms 1
2:1 AV Block:
- Every other P wave conducts 1
- Determine site of block using clinical context, QRS width, and response to atropine 1
High-Grade/Advanced AV Block:
- ≥2 consecutive nonconducted P waves with some evidence of AV conduction 1
- Permanent pacing indicated regardless of symptoms 1
Third-Degree (Complete) AV Block
- No atrial impulses reach the ventricles 1, 5
- Immediate management required:
- Permanent pacing indicated regardless of symptoms when not due to reversible causes 1
Acute Management Algorithm
Step 1: Assess Hemodynamic Stability
Signs of instability include: 1
- Systolic BP <90 mmHg
- Altered mental status
- Ischemic chest pain
- Dyspnea
- Syncope
Step 2: Immediate Pharmacologic Intervention
For symptomatic bradycardia or hemodynamically unstable AV block:
- Atropine 0.6-1.0 mg IV bolus 1, 4
- Repeat every 3-5 minutes up to maximum 2 mg total 1, 4
- Atropine is more effective for bradycardia (49% response) than AVB (27% response) 6
- In acute inferior MI with AV block: aminophylline IV may be considered to improve AV conduction 1
Step 3: Temporary Pacing
If atropine fails or contraindicated:
- Transcutaneous pacing may be considered as bridge to transvenous or permanent pacing 1
- Transvenous pacing is reasonable for second-degree or third-degree AV block with symptoms/hemodynamic compromise refractory to medical therapy 1
- For prolonged temporary pacing needs: externalized permanent active fixation lead is reasonable over standard passive fixation temporary lead 1
Permanent Pacing Indications
Class I (Indicated - Must Do)
Permanent pacing is recommended for: 1
- Acquired Mobitz type II second-degree AV block (regardless of symptoms)
- High-grade AV block (regardless of symptoms)
- Third-degree AV block (regardless of symptoms)
- All above when NOT due to reversible causes
Symptomatic permanent atrial fibrillation with bradycardia 1
Symptomatic AV block caused by necessary guideline-directed medical therapy when no alternative treatment exists 1
Class IIa (Reasonable to Do)
Permanent pacing is reasonable for: 1
- Marked first-degree or Mobitz type I with symptoms clearly attributable to AV block
- Infiltrative cardiomyopathy (sarcoidosis, amyloidosis) with second-degree Mobitz II, high-grade, or third-degree AV block (with ICD capability if needed) 1
Class III (Should NOT Do)
Permanent pacing should NOT be performed for: 1
- Asymptomatic vagally mediated AV block
- AV block that completely resolves after treatment of reversible cause
- Transient AV block without intraventricular conduction defects 1
- Transient AV block with isolated left anterior fascicular block 1
Special Situations
Post-Myocardial Infarction
Permanent pacing indicated for: 1
- Persistent second-degree AV block in His-Purkinje system with bilateral bundle-branch block
- Third-degree AV block within or below His-Purkinje system
- Transient advanced infranodal AV block with bundle-branch block
- Persistent symptomatic second- or third-degree AV block
Important: Temporary pacing requirement during STEMI does NOT automatically indicate need for permanent pacing 1, 7
Neuromuscular Diseases
Permanent pacing (with ICD capability if needed) indicated for: 1
- Myotonic dystrophy type 1, Kearns-Sayre syndrome with second-degree, third-degree AV block, or HV interval ≥70 ms
- Lamin A/C mutations with PR >240 ms and LBBB 1
Diagnostic Testing for Chronic Management
When to Obtain Additional Testing
Ambulatory ECG monitoring is reasonable for: 1
- Symptoms (lightheadedness, dizziness) of unclear etiology with first-degree or Mobitz type I AV block
Exercise treadmill test is reasonable for: 1
- Exertional symptoms with first-degree or Mobitz type I at rest
Electrophysiology study may be considered for: 1
- Selected patients with second-degree AV block to determine level of block
Critical Pitfalls to Avoid
Do not assume first-degree AV block is always benign - marked PR prolongation (≥240-300 ms) can cause hemodynamic compromise requiring pacing 1
Do not delay pacing for Mobitz type II - this has high progression risk to complete heart block regardless of symptoms 3
Do not place permanent pacemaker for reversible AV block that completely resolves with treatment 1
Do not give atropine for asystole without chest compressions - prompt resuscitative measures including compressions, atropine, vasopressin, epinephrine, and temporary pacing should be administered together 1
Atropine may worsen infranodal block - use with caution in wide-complex bradycardia suggesting infranodal pathology 5
Transcutaneous pacing causes significant pain - high-risk patients likely to require pacing should receive transvenous pacemaker rather than prolonged transcutaneous pacing 1