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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 II second-degree AV block occurs when one or more P waves are blocked without a preceding progressive delay in AV transmission.

ECG Features

  • One or more P waves not followed by a QRS complex; PR interval is constant but may be either normal or consistently prolonged (Figure 9).
  • Ventricular rate-usually slow.
  • Fixed ratio of P waves to QRS complexes may occur (e.g., 2:1, 3:1, 4:1 AV block).
  • High-grade (advanced) second-degree AV block is characterized by two or more consecutive blocked P waves.
  • In second-degree AV block with a 2:1 conduction ratio or higher, it is impossible to observe prolongation of the PR interval before the block, so a designation of Mobitz is not appropriate.
  • QRS complexes may appear normal but may also be wide or have an abnormal morphology due to aberrant intraventricular conduction or to ventricular enlargement.
  • Abnormally wide QRS complexes may indicate serious, extensive cardiac disease.

Pathophysiology

  • Rare in healthy animals.
  • May be hemodynamically important when ventricular rate is abnormally slow.
  • Frequently progresses to complete AV block, particularly when accompanied by wide QRS complexes.
  • Typically this type of AV block results from conduction delay within the AV node itself (rather than delay in another segment of the AV conducting system) that is characterized by normal or prolonged AH intervals with intermittent block between A and H deflections on a His bundle electrogram).

Systems Affected

  • Cardiovascular.
  • Central nervous or musculoskeletal systems if inadequate cardiac output.

Genetics

May be heritable in pugs

Incidence/Prevalence

Unknown

Geographic Distribution

N/A

Signalment

Species

Dog and cat

Breed Predilections

American cocker spaniel, pug, dachshund, Airedale terrier, Doberman pinscher.

Mean Age and Range

Generally occurs in older animals

Predominant Sex

N/A

Signs

Historical Findings

  • Presenting complaint may be syncope, collapse, weakness, or lethargy.
  • Some animals are asymptomatic.
  • Animals may show signs of the underlying disease process.

Physical Examination Findings

  • ± weakness.
  • Bradycardia common.
  • May be intermittent pauses in the cardiac rhythm.
  • An S4 may be audible in lieu of the normally expected heart sounds (i.e., S1, S2) when the block occurs.
  • If associated with digoxin intoxication, there may be vomiting, anorexia, and diarrhea.
  • May be other abnormalities reflecting the underlying etiology.

Causes

  • Heritable in pugs.
  • Enhanced vagal stimulation from non-cardiac diseases.
  • Degenerative change within the cardiac conduction system-replacement of AV nodal cells and/or Purkinje fibers by fibrotic and adipose tissue in old cats and dogs.
  • Pharmacologic agents (e.g., digoxin, -adrenergic antagonists, calcium channel blocking agents, propafenone, 2-adrenergic agonists, muscarinic cholinergic agonists, or severe procainamide or quinidine toxicity).
  • Infiltrative myocardial disorders (neoplasia, amyloid).
  • Endocarditis (particularly involving the aortic valve).
  • Myocarditis (viral, bacterial, parasitic, idiopathic).
  • Cardiomyopathy (especially in cats).
  • Trauma.
  • Atropine administered intravenously may cause a brief period of first- or second-degree heart block before increasing the heart rate.

Risk Factors

Any condition or intervention that enhances vagal tone

Diagnosis ⬆ ⬇

Diagnosis

Differential Diagnosis

  • High-grade (advanced) form must be distinguished from complete AV block.
  • Non-conducted P waves arising from refractoriness of the conduction system during supraventricular tachycardias must be differentiated from pathologic conduction block.

CBC/Biochemistry/Urinalysis

  • Serum electrolytes-hypokalemia and hyperkalemia may predispose to AV.
  • Conduction disturbances.
  • Leukocytosis-may be noted with bacterial endocarditis or myocarditis.
  • Electrolyte abnormalities (e.g., severe hypokalemia, hyperkalemia, or hypercalcemia) may predispose to AV block.

Other Laboratory Tests

  • Serum digoxin concentration-may be high.
  • High T4 in cats-if associated with hyperthyroidism.
  • High arterial blood pressure-if associated with hypertensive heart disease.
  • Positive Borrelia, Rickettsia, or Trypanosoma cruzi titers-if associated with one of these infectious agents.
  • Blood cultures may be positive in patients with vegetative endocarditis.

Imaging

Echocardiographic examination may reveal structural heart disease (e.g., endocarditis, neoplasia, or cardiomyopathy).

Diagnostic Procedures

  • 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 high vagal tone.
  • Electrophysiologic testing is generally unnecessary but can be done to confirm this type of AV block if surface ECG findings are equivocal.

Pathologic Findings

  • Variable-depend on underlying cause.
  • Old animals with degenerative change of the conduction system may have focal mineralization of the interventricular septal crest visible grossly; chondroid metaplasia of the central fibrous body and increased fibrous connective tissue in the AV bundle is noted histopathologically.

Treatment ⬆ ⬇

Treatment

Appropriate Health Care

  • Treatment-may be unnecessary if heart rate maintains adequate cardiac output.
  • Positive dromotropic interventions are indicated for symptomatic patients.
  • Treat or remove underlying cause(s).

Nursing Care

Generally unnecessary

Activity

Cage rest advised for symptomatic patients.

Diet

Modifications or restrictions only to manage an underlying condition.

Client Education

  • Need to seek and specifically treat underlying cause.
  • Pharmacologic agents may not be effective long term.

Surgical Considerations

Permanent pacemaker may be required for long-term management of symptomatic patients.

Medications ⬆ ⬇

Medications

Drug(s) Of Choice

  • Atropine (0.02–0.04 mg/kg IV, IM) or glycopyrrolate (5–10 µ–g/kg IV, IM) may be used short term if positive atropine response.
  • Chronic anticholinergic therapy (propantheline 0.5–2 mg/kg PO q8–12h or hyoscyamine 3–6 µg/kg q8h)-indicated for symptomatic patients if improved AV conduction with atropine response test.
  • Isoproterenol (0.04–0.09 µg/kg/minute IV to effect) or dopamine (2–5 µg/kg/minute IV to effect) may be administered in acute, life-threatening situations to enhance AV conduction and/or accelerate an escape focus.

Contraindications

  • Drugs with vagomimetic action (e.g., digoxin, bethanechol, physostigmine, pilocarpine) may potentiate block.
  • Avoid drugs likely to impair impulse conduction further or depress a ventricular escape focus (e.g., procainamide, quinidine, lidocaine, calcium channel blocking agents, -adrenergic blocking agents).

Precautions

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

Possible Interactions

N/A

Alternative Drug(s)

N/A

Follow-Up ⬆ ⬇

Follow-Up

Patient Monitoring

Frequent ECG because often progresses to complete (third-degree) AV block.

Prevention/Avoidance

N/A

Possible Complications

Prolonged bradycardia may cause secondary congestive heart failure or inadequate renal perfusion.

Expected Course and Prognosis

Variable-depends on cause. If degenerative disease of the cardiac conduction system, often progresses to complete (third-degree) AV block.

Miscellaneous ⬆

Miscellaneous

Associated Conditions

May be noted in cats with primary or secondary myocardial disease.

Age-Related Factors

N/A

Zoonotic Potential

N/A

Pregnancy/Fertility/Breeding

N/A

Abbreviations

  • AV = atrioventricular
  • ECG = electrocardiogram
  • T4 = thyroxine

Suggested Reading

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.

Podrid PJ, Kowey PR. Cardiac Arrhythmia-Mechanisms, Diagnosis, and Management. Baltimore, MD: Williams & Wilkins, 1995.

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

Tilley LP, Smith FW. Essentials of Electrocardiography. Interpretation and Treatment, 4th ed. Ames, IA: Wiley Blackwell Publishing, 2016 (in preparation).

Tilley LP, Smith FWKJr. 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 Larry P. Tilley and Francis W.K. Smith, Jr.

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

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

Client Education Handout Available Online