Introduction

• Cause: impulse of supraventricular origin (from sinus node or an atrial ectopic beat) is blocked at the AV node and does not progress to activate the ventricles.
• Significance: usually non-pathologic.
• Signs: dropped beat.
• Diagnosis: ECG and/or auscultation.
• Treatment: usually condition is physiological and no treatment is required, very rarely an artificial pacemaker, or sympathomimetic agents may be indicated in advanced (pathological) 2^nd  degree atrioventricular block (AVB).
• Prognosis: generally excellent due to physiological nature of the condition, but conversely prognosis is guarded to poor if arrhythmia is not abolished by exercise and horse is symptomatic.

Pathogenesis

Etiology

• Usually a normal homeostatic mechanism to regulate blood pressure. Mediated by the vagus.
• During the conducted beats, blood pressure gradually rises. This stimulates baroreceptors which act via the vagus to slow or block conduction at the atrioventricular and sinoatrial node.
• In advanced (pathological) second degree AVB:
  • Occasionally structural conduction system disease, eg congenital, degenerative, inflammation or neoplasia.
  • Drug induced, eg α2 agonists (detomidine Detomidine hydrochloride, romifidine Romifidine and xylazine Xylazine).
  • Electrolyte imbalances.
  • Digitalis toxicity.

Pathophysiology

• Usually a normal homeostatic mechanism to regulate blood pressure. Mediated by the vagus.
• Rarely pathological.

Diagnosis

Treatment

Outcomes