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Basics ⬇

Basics

Definition

Refers to a delay in conduction that occurs between atrial and ventricular activation.

ECG Features

  • Rate and rhythm-usually normal.
  • Usually there are regularly occurring normal P waves and QRS complexes (Figures 7a and 7b).
  • Prolonged, consistent PR intervals-dogs, >0.13 sec; cats, >0.09 sec (Figures 7a and 7b).

Pathophysiology

  • Virtually never causes clinical signs.
  • May become a more severe AV conduction disturbance in some animals.
  • Normally the PR interval tends to shorten with rapid heart rates.
  • May be the result of intra-atrial conduction delay (prolongation of the PA interval on surface ECG and simultaneous His bundle electrogram) or delay of conduction within the AV node itself (prolongation of the AH interval on His bundle electrogram).

Systems Affected

Cardiovascular

Genetics

N/A

Incidence/Prevalence

Common

Geographic Distribution

None

Signalment

Species

Dog and cat

Breed Predilections

American cocker spaniel, dachshund, brachycephalic dogs, Persian cats

Mean Age and Range

  • May occur in young, otherwise healthy dogs as a manifestation of high vagal tone.
  • Intra-atrial conduction delay involving the right atrium may be seen with congenital heart disease, especially atrioventricular septal defects.
  • May be noted in aged patients with degenerative conduction system disease, particularly cocker spaniels and dachshunds.
  • Persian cats of any age with high vagal tone and in cats of any age with hypertrophic cardiomyopathy.

Signs

Historical Findings

  • Most animals are asymptomatic.
  • If drug-induced, may have a history of clinical signs related to drug toxicity-anorexia, vomiting, and diarrhea with digoxin; weakness with calcium channel blockers or -adrenergic antagonists.

Physical Examination Findings

  • Normal-unless also signs of more generalized myocardial disease, drug toxicity, or non-cardiac disease.

Causes

  • May occur in normal animals.
  • Enhanced vagal stimulation resulting from non-cardiac diseases-usually accompanied by sinus arrhythmia, sinus arrest, and/or Mobitz type I second-degree AV block.
  • Pharmacologic agents (e.g., digoxin, -adrenergic antagonists, calcium channel blocking agents, propafenone, amiodarone, 2-adrenergic agonists, parasympathomimetic agents [bethanechol, physostigmine, pilocarpine] and severe procainamide or quinidine toxicity).
  • Degenerative disease of the conduction system.
  • Hypertrophic cardiomyopathy.
  • Myocarditis (especially Trypanosoma cruzi, Borrelia burgdorferi, Rickettsia rickettsii).
  • Infiltrative diseases (tumors, amyloid).
  • Atropine administered intravenously may briefly prolong the PR interval.

Risk Factors

Any condition or intervention that raises vagal tone

Diagnosis ⬆ ⬇

Diagnosis

Differential Diagnosis

P waves superimposed upon preceding T waves because of first-degree AV block should be differentiated from bifid T waves.

CBC/Biochemistry/Urinalysis

  • Serum electrolytes-hypokalemia and hyperkalemia may predispose to AV conduction disturbances.
  • Leukocytosis-may be noted with bacterial endocarditis or myocarditis.

Other Laboratory Tests

  • Serum digoxin concentration-may be high.
  • T. cruzi, B. burgdorferi, R. rickettsii titers-may be high.
  • T4-may be high in cats if associated with thyrotoxic myocardial disease.

Imaging

Echocardiographic examination-may reveal hypertrophic or infiltrative myocardial disorder.

Diagnostic Procedures

May be needed to identify causes of high vagal tone-upper airway disease, cervical and thoracic masses, gastrointestinal disorders, and high intraocular pressure.

Pathologic Findings

Variable-depend on underlying cause

Treatment ⬆ ⬇

Treatment

Appropriate Health Care

  • Remove or treat underlying cause(s).
  • Hospitalization may be necessary to manage the underlying cause (e.g., cardiomyopathy, gastrointestinal disease, airway disease).

Nursing Care

N/A

Activity

Unrestricted; unless restriction required for an underlying condition.

Diet

No modifications or restrictions unless required to manage an underlying condition.

Client Education

Generally unnecessary

Surgical Considerations

None unless required to manage an underlying condition.

Medications ⬆ ⬇

Medications

Drug(s) Of Choice

Medications used only if needed to manage an underlying condition.

Contraindications

  • Avoid hypokalemia-increases sensitivity to vagal tone; may potentiate AV conduction delay.
  • Avoid drugs likely to impair impulse conduction further (calcium channel blocking agents, -adrenergic antagonists, 2-adrenergic agonists, amiodarone, propafenone).

Precautions

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

Possible Interactions

N/A

Alternative Drug(s)

N/A

Follow-Up ⬆ ⬇

Follow-Up

Patient Monitoring

Except in healthy young animals, monitor ECG to detect any progression in conduction disturbance.

Prevention/Avoidance

N/A

Possible Complications

N/A

Expected Course and Prognosis

  • Depends on underlying cause.
  • Prognosis usually excellent if no significant underlying disease is present.

Miscellaneous ⬆

Miscellaneous

Associated Conditions

None

Age-Related Factors

PR interval-tends to lengthen with advancing age

Zoonotic Potential

None

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.

Miller MS, Tilley LP, Smith FWK, Fox PR. Electrocardiography. In: Fox PR, Sisson D, Moise NS, eds., Textbook of Canine and Feline Cardiology. Philadelphia: Saunders, 1999,pp. 67–106.

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.

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