Atrioventricular Block, Second Degree
The supraventricular impulse is intermittently blocked at the level of the atrioventricular (AV) node, resulting in a “missed” ventricular beat. At rest, this arrhythmia occurs in about 45% of healthy horses.
• Physiologic second-degree AV block (most common arrhythmia in healthy horses)
• Advanced (high-grade or high-degree) second-degree AV block (pathologic): Three or more consecutive P waves are blocked, resulting in a large difference between atrial and ventricular rate. AV conduction is still present.
History, Chief Complaint
Physical Exam Findings
• Heart rate normal or decreased
• Pulse deficit (missing pulse)
• In case of advanced second-degree AV block: Slow or normal heart rate. Weakness or exercise intolerance may be present. Multiple dropped beats in a row that may not disappear with exercise or vagolytic agents may occur.
Etiology and Pathophysiology
• Caused by high vagal tone: This is a regulatory mechanism at rest whereby a beat-to-beat increase in blood pressure leads to a blocked beat to maintain a stable blood pressure.
• Drug administration: α2 agonists (detomidine, xylazine) or calcium channel blockers (verapamil).
• Toxicity (digitalis, injection of iron preparations).
• Structural lesions of the AV node (degeneration, inflammation, fibrosis). In these cases, advanced second-degree AV block may progress to third-degree AV block.
• Immediately after delivery of an intracardiac direct current electrical shock (eg, treatment of atrial fibrillation [AF]), advanced second-degree AV block may occur temporarily.
• Second-degree AV block does not produce any clinical signs unless it results in a very slow heart rate (ie, advanced second-degree AV block).