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Basics ⬇

Basics

Definition

Second-degree AV block refers to failure of one or more P waves but not all P waves to be conducted. Mobitz Type I second-degree AV block occurs when AV transmission is progressively delayed prior to a blocked P wave.

ECG Features

  • PR interval-becomes progressively longer prior to the appearance of a P wave that is not followed by a QRS complex (Figure 8).
  • Heart rate and QRS morphology-usually normal.
  • Often cyclical.

Pathophysiology

  • Frequently associated with high resting vagal tone and sinus arrhythmia in dogs.
  • Generally not pathologic or hemodynamically significant.
  • This type of AV block usually results from conduction delay within the AV node itself (rather than delay in other segments of the AV conducting system) and is characterized by a progressive increase in AH interval with eventual block between the A and H deflections on a His bundle recording.

Systems Affected

Cardiovascular

Genetics

N/A

Incidence/Prevalence

Radiotelemetry studies have shown that this arrhythmia occurs in 64% of healthy adult dogs and 100% of healthy puppies 8–12 weeks of age.

Geographic Distribution

N/A

Signalment

Species

Dog; uncommon in cat

Breed Predilections

N/A

Mean Age and Range

  • Usually occurs in young, otherwise healthy dogs as a manifestation of high vagal tone.
  • Occasionally occurs in older dogs with abnormally strong vagal tone.
  • Rarely noted in old dogs with degenerative conduction system disease.

Signs

Historical Findings

  • Most animals are asymptomatic.
  • If drug-induced, owner may report signs of drug toxicity-anorexia, vomiting, and diarrhea with digoxin; weakness with calcium channel blockers or -adrenergic antagonists.
  • If heart rate is abnormally slow, syncope or weakness may occur.

Physical Examination Findings

  • May be normal unless signs of more-generalized myocardial disease or non-cardiac disease are present.
  • Intermittent pauses in the cardiac rhythm.
  • First heart sound may become progressively softer, followed by a pause.
  • An audible S4 may be heard unaccompanied by S1 and S2 when block occurs.

Causes

  • Occasionally noted in normal animals.
  • Enhanced vagal stimulation resulting from non-cardiac diseases-usually accompanied by sinus arrhythmia, sinus arrest.
  • Pharmacologic agents-digoxin, -adrenergic antagonists, calcium channel blocking agents, propafenone, amiodarone, 2-adrenergic agonists, opioids.

Risk Factors

Any condition or intervention that enhances vagal tone.

Diagnosis ⬆ ⬇

Diagnosis

Differential Diagnosis

  • Non-conducted P waves from supraventricular premature impulses or supraventricular tachycardias should be distinguished from second-degree AV block.
  • Type II second-degree AV block (no variation in PR intervals).

CBC/Biochemistry/Urinalysis

Hypokalemia may predispose to AV conduction disturbances

Other Laboratory Tests

Serum digoxin concentration-may be high

Imaging

N/A

Diagnostic Procedures

  • May be necessary to identify specific causes of enhanced vagal tone (e.g., upper airway disease, cervical and thoracic masses, gastrointestinal disorders, and high intraocular pressure).
  • Atropine response test-administer 0.04 mg/kg atropine IM and repeat ECG in 20–30 minutes; may be used to determine whether AV block is due to vagal tone; resolution of AV block with atropine supports vagal cause.
  • Electrophysiologic studies are generally unnecessary but will confirm this type of second-degree AV block if surface ECG is equivocal.

Pathologic Findings

Generally, no gross or histopathologic findings

Treatment ⬆ ⬇

Treatment

Appropriate Health Care

  • Treatment usually unnecessary
  • Treat or remove underlying cause(s)

Nursing Care

Generally unnecessary

Activity

Unrestricted

Diet

Modifications or restrictions only to manage an underlying condition.

Client Education

Explain that any treatment is directed toward reversing or eliminating an underlying cause.

Surgical Considerations

N/A except to manage an underlying condition

Medications ⬆ ⬇

Medications

Drug(s)

Only as needed to manage an underlying condition

Contraindications

Drugs with vagomimetic action (e.g., digoxin, bethanechol, physostigmine, pilocarpine) may potentiate block.

Precautions

Hypokalemia increases the sensitivity to vagal tone and may potentiate AV conduction delay.

Possible Interactions

N/A

Follow-Up ⬆ ⬇

Follow-Up

Patient Monitoring

Typically not necessary

Prevention/Avoidance

N/A

Possible Complications

N/A

Miscellaneous ⬆

Miscellaneous

Associated Conditions

N/A

Age-Related Factors

N/A

Pregnancy/Fertility/Breeding

N/A

Synonyms

  • Wenckebach periodicity
  • Wenckebach phenomenon

Abbreviations

  • AV = atrioventricular
  • ECG = electrocardiogram

Suggested Reading

Branch CE, Robertson BT, Williams JC. Frequency of second-degree atrioventricular heart block in dogs. Am J Vet Res 1975, 36:925–929.

Kittleson MD. Electrocardiography. In: Kittleson MD, Kienle RD, eds., Small Animal Cardiovascular Medicine. St. Louis, MO: Mosby, 1998, pp. 72–94.

Mangrum JM, DiMarco JP. The evaluation and management of bradycardia. N Engl J Med 2000, 342:703–709.

Tilley LP. Essentials of Canine and Feline Electrocardiography, 3rd ed. Baltimore, MD: Williams & Wilkins, 1992.

Tilley LP, Smith FWK, Jr. Electrocardiography. In: Smith FWK, Tilley LP, Oyama MA, Sleeper MM, eds., Manual of Canine and Feline Cardiology, 5th ed. St. Louis, MO: Saunders Elsevier, 2015 (in press).

Authors Francis W.K. Smith, Jr. and Larry P. Tilley

Consulting Editors Larry P. Tilley and Francis W.K. Smith, Jr.

Acknowledgment The authors and editors acknowledge the prior contribution of Janice McIntosh Bright.

Client Education Handout Available Online