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Basics ⬇

Basics

Definition

  • All atrial impulses are blocked at the AV junction; atria and ventricles beat independently. A secondary “escape” pacemaker site (junctional or ventricular) stimulates the ventricles.
  • Atrial rate normal.
  • Idioventricular escape rhythm slow.

ECG Features

  • Ventricular rate slower than the atrial rate (more P waves than QRS complexes)-ventricular escape rhythm (idioventricular) usually <40 bpm; junctional escape rhythm (idiojunctional) 40–60 bpm in dogs and 60–100 bpm in cats.
  • P waves-usually normal configuration (Figure 6a).
  • QRS complex-wide and bizarre when pacemaker located in the ventricle, or in the lower AV junction in a patient with bundle branch block; normal when escape pacemaker in the lower AV junction (above the bifurcation of the bundle of His) in a patient without bundle branch block.
  • No conduction between the atria and the ventricles; P waves have no constant relationship with QRS complexes; P–P and R–R intervals relatively constant (except for a sinus arrhythmia).

Pathophysiology

Slow ventricular escape rhythms (<40 bpm) result in low cardiac output and eventual heart failure, often when animal is excited or exercised, since demand for greater cardiac output is not satisfied. As the heart fails, signs increase with mild activity.

Systems Affected

Cardiovascular

Genetics

Can be an isolated congenital defect

Incidence/Prevalence

Not documented

Geographic Distribution

N/A

Signalment

Species

Dog and cat

Breed Predilections

  • Cocker spaniel-can have idiopathic fibrosis.
  • Pug and Doberman pinscher-can have associated sudden death, AV conduction defects, and bundle of His lesions.

Mean Age and Range

Geriatric animals, except congenital heart disease patients. Median age for cats-14 years.

Predominant Sex

Intact female dogs

Signs

Historical Findings

  • Exercise intolerance
  • Weakness or syncope
  • Occasionally, CHF

Physical Examination Findings

  • Bradycardia
  • Variable third and fourth heart sounds
  • Variation in intensity of the first heart sounds
  • Signs of CHF
  • Intermittent “cannon” A waves in jugular venous pulses

Causes & Risk Factors

Isolated congenital defect

  • Idiopathic fibrosis
  • Infiltrative cardiomyopathy (amyloidosis or neoplasia)
  • Hypertrophic cardiomyopathy in cats
  • Digitalis toxicity
  • Hyperthyroidism in cats
  • Myocarditis
  • Endocarditis
  • Electrolyte disorder
  • Myocardial infarction
  • Other congenital heart defects
  • Lyme disease
  • Chagas disease

Diagnosis ⬆ ⬇

Diagnosis

Differential Diagnosis

  • Advanced second-degree AV block
  • Atrial standstill
  • Accelerated idioventricular rhythm

CBC/Biochemistry/Urinalysis

  • Abnormal serum electrolytes (e.g., hyperkalemia, hypokalemia) possible.
  • High WBC with left shift in animals with bacterial endocarditis.

Other Laboratory Tests

  • High serum digoxin concentration if AV block is due to digoxin toxicity.
  • Lyme titer and accompanying clinical signs if AV block due to Lyme disease.

Imaging

Echocardiography and Doppler ultrasound to assess cardiac structure and function.

Diagnostic Procedures

  • Electrocardiography
  • His bundle electrogram to determine the site of the AV block.
  • Long-term (Holter) ambulatory recording if AV block is intermittent.

Pathologic Findings

Degeneration or fibrosis of the AV node and its bundle branches, associated with endocardial and myocardial fibrosis and organized endomyocarditis.

Treatment ⬆ ⬇

Treatment

Appropriate Health Care

  • Temporary or permanent cardiac pacemaker-only effective treatment in symptomatic patients.
  • Carefully monitor asymptomatic patients without a pacemaker for development of clinical signs.

Nursing Care

Cage rest prior to pacemaker implantation; when the pulse generator is put into a subcutaneous pocket, a non-constricting bandage is required around the ventral neck or abdomen for 3–5 days to prevent seroma formation or pacemaker movement.

Activity

Restrict if symptomatic

Diet

No modifications unless required to manage underlying condition (e.g., low-salt diet).

Client Education

  • Temporary or permanent cardiac pacemaker-only effective treatment in symptomatic patients.
  • Asymptomatic patients without a pacemaker-must be carefully monitored for development of clinical signs.

Surgical Considerations

  • Most patients-at high anesthetic cardiopulmonary risk; usually paced preoperatively with a temporary external pacemaker system.
  • The small size of cats makes pacemaker implantation more difficult than in dogs.

Medications ⬆ ⬇

Medications

Drug(s) Of Choice

  • Treatment with drugs-usually of no value. Traditionally used to treat complete AV block: atropine, isoproterenol, corticosteroids, and dobutamine.
  • Intravenous isoproterenol infusion may help increase the rate of the ventricular escape rhythm to stabilize hemodynamics.
  • If CHF-diuretic and vasodilator therapy may be needed before pacemaker implantation.

Contraindications

Avoid digoxin, xylazine, acepromazine, beta blockers (e.g., propranolol and atenolol), and calcium channel blockers (e.g., verapamil and diltiazem); ventricular antiarrhythmic agents are dangerous because they suppress lower escape foci.

Precautions

Vasodilators-may cause hypotension in animals with complete AV block; monitor closely if used, especially prior to pacemaker implantation.

Follow-Up ⬆ ⬇

Follow-Up

Patient Monitoring

  • Monitor-pacemaker function with serial ECGs.
  • Radiographs-following pacemaker implantation, to confirm the position of the lead and generator.

Prevention/Avoidance

N/A

Possible Complications

Pulse generators-broad range of clinical life; pacemaker replacement necessary when battery is depleted, pulse generator malfunction occurs, or exit block develops; pacemaker leads can become dislodged and infected.

Expected Course and Prognosis

Poor long-term prognosis if no cardiac pacemaker implanted, especially when the animal has clinical signs. Cats can sometimes survive >1 year.

Miscellaneous ⬆

Miscellaneous

Associated Conditions

None

Abbreviations

  • AV = atrioventricular
  • CHF = congestive heart failure
  • ECG = electrocardiogram
  • WBC = white blood cell

Internet Resources

www.vetgo.com/cardio

Suggested Reading

Bright JM. Pacemaker therapy. 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).

Kellum HB, Stepien RL. Third-degree atrioventricular block in 21 cats (1997–2004). J Vet Intern Med 2006, 20:97–103.

Schrope DP, Kelch WJ. Signalment, clinical signs, and prognostic indicators associated with high-grade second or third-degree atrioventricular block in dogs: 124 cases (January 1, 1997–December 31, 1997). J Am Vet Med Assoc 2006 , 228:1710–1717.

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

Author Larry P. Tilley

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

Acknowledgment The author and editors acknowledge the prior contribution of Naomi L. Burtnick.

Client Education Handout Available Online