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Basics ⬇

Outline

BASICS

Overview!!navigator!!

  • Any cardiac arrhythmia associated with a slow heart rate
  • Physiologic—first- and second-degree AVB, sinus arrhythmia, and sinus blocks and pauses
  • Pathologic—atrial standstill and advanced second- and third-degree AVB

Signalment!!navigator!!

N/A

Signs!!navigator!!

  • A slow regularly irregular rhythm
  • Physiologic—no clinical signs, easily abolished by exercise or excitement
  • With second- and third-degree AVB, audible fourth (atrial) heart sounds heard during the pauses
  • Pathologic—weakness and syncope

Causes and Risk Factors!!navigator!!

  • Physiologic—a common normal mechanism to modify heart rate and blood pressure
  • Pathologic—uncommon but can occur with potassium disturbances and myocardial pathology
  • Profound hyperkalemia is associated with sinus bradycardia, atrial standstill, and third-degree AVB and can be seen with renal failure and in foals with uroperitoneum
  • Advanced second-degree AVB can also occur with α2-adrenergic sedative drugs and with halothane
  • Pathologic bradyarrhythmias occur as a terminal event

Diagnosis ⬆ ⬇

Outline

DIAGNOSIS

Differential Diagnosis!!navigator!!

  • Sinus bradycardia
  • AVB
  • AF with a slow heart rate

CBC/Biochemistry/Urinalysis!!navigator!!

  • Hyperkalemia may be present.
  • Cardiac troponin I or T may be increased with myocardial pathology

Imaging!!navigator!!

  • ECG to characterize the bradyarrhythmia and to identify AF
  • With AVB, there is prolongation of the PR interval (first degree), intermittent P waves without a following QRS complex (second degree), or complete dissociation of the P and QRS complexes (third degree)
  • With atrial standstill, P waves are absent
  • With sinus bradycardias, every P is followed by a QRS complex, but there is intermittent waxing and waning of the P–P interval and the R–R interval; with sinus pauses, there is a prolonged P–P interval; and with sinus blocks, the P–P interval is intermittently prolonged to more than two normal cardiac cycles
  • Echocardiography to identify underlying heart disease with pathologic bradyarrhythmias

Treatment ⬆ ⬇

TREATMENT

  • Physiologic—no treatment. An exercising ECG documents the disappearance of the arrhythmia
  • Pathologic—identify predisposing causes (drugs or hyperkalemia) and remove them if possible
  • With uroperitoneum, abdominal drainage is indicated, but the foal's electrolyte status must be stabilized before general anesthesia and surgery
  • With pathologic bradyarrhythmias secondary to myocardial pathology, anti-inflammatory medications may be appropriate
  • Some cases of third-degree AVB have successfully been treated by placement of transvenous pacemakers

Medications ⬆ ⬇

Outline

MEDICATIONS

Drug(s) of Choice!!navigator!!

For treatment of hyperkalemia consider the following drugs:

  • If symptomatic (bradycardia, muscle weakness) or serum potassium concentration >7.0 mmol/L
    • Calcium borogluconate 23% 0.2–0.4 mL/kg IV
    • Dextrose 0.5 g/kg with soluble insulin 0.1 unit/kg in 500 mL saline as IV infusion over 30–45 min
    • Sodium bicarbonate 1 mEq/kg IV over 15 min, can be repeated
  • If not symptomatic and <7.0 mmol/L
    • Diurese with at least 5 mL/kg/h lactated Ringer's solution
    • Furosemide 1 mg/kg IV if horse well perfused
  • For treatment of myocardial pathology, corticosteroids such as prednisolone 1 mg/kg PO every 48 h or dexamethasone 0.05–0.1 mg/kg IV or 0.1 mg/kg PO every 24 h for 3 or 4 days and then continued every 3–4 days in decreasing dosages are recommended
  • Where life-threatening bradyarrhythmias are observed during cardiopulmonary resuscitation, atropine or glycopyrrolate (glycopyrronium) can be administered at 0.005–0.01 mg/kg IV

Contraindications/Possible Interactions!!navigator!!

  • Care should be taken that discontinuation of dextrose infusions does not lead to hypoglycemia, particularly when insulin has been administered concurrently
  • High-dose corticosteroid therapy has been associated with laminitis, particularly when other laminitis risk factors are present

Follow-up ⬆ ⬇

Outline

FOLLOW-UP

Patient Monitoring!!navigator!!

Horses with pathologic bradyarrhythmias should have their ECG monitored frequently until the arrhythmia resolves.

Possible Complications!!navigator!!

Pathologic bradyarrhythmias can be fatal.

Expected Course and Prognosis!!navigator!!

The clinical course and prognosis are generally determined by the underlying cause.

Miscellaneous ⬆ ⬇

Outline

MISCELLANEOUS

Associated Conditions!!navigator!!

  • Uroperitoneum
  • Renal failure
  • Myocardial disease

Age-Related Factors!!navigator!!

More common in foals with uroperitoneum

Pregnancy/Fertility/Breeding!!navigator!!

Third-degree AVB will lead to a profound decrease in cardiac output and compromise blood supply to the fetus. A transvenous pacing device should be considered in pregnant mares.

Abbreviations!!navigator!!

Suggested Reading ⬆ ⬇

Reef VB, Bonagura J, Buhl R, et al. Recommendations for management of equine athletes with cardiovascular abnormalities. J Vet Intern Med 2014;28:749–761.

Author(s) ⬆

Author: Virginia B. Reef

Consulting Editor: Celia M. Marr and Virginia B. Reef

Acknowledgment: The author acknowledges the prior contribution of Celia M. Marr.