Atrioventricular Block, Second Degree

Atrioventricular Block, Second Degree



Basic Information image



Definition


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.



Synonym(s)




• Second-degree AV block


• Second-degree heart block


• The term high-grade or high-degree AV block is used for both advanced second- and third-degree AV block.



Epidemiology



Risk Factors




• High vagal tone


• Drug administration (eg, α2 agonists)



Associated Conditions and Disorders


Other vagally induced arrhythmias such as sinus arrhythmia, sinus block, first-degree AV block



Clinical Presentation



Disease Forms/Subtypes




• Physiologic second-degree AV block (most common arrhythmia in healthy horses)


• Mobitz type I (Wenckebach): The PQ interval progressively prolongs until a P wave is blocked. Variations in pp intervals are often present.

• Mobitz type II: Constant PQ interval with an intermittently blocked P wave.

• 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




• Usually no complaints


• Drug administration


• For advanced second-degree AV block: Exercise intolerance, weakness, and occasionally syncope



Physical Exam Findings




• Heart rate normal or decreased


• Pulse deficit (missing pulse)


• Auscultation


• A dropped beat: Beat-to-beat interval is double the preceding intervals.

• An atrial sound (S4) is often heard during the pause.

• The irregularity often appears at regular intervals (eg, every 4 or 5 beats).

• The irregularity abolishes with stress, exercise, or vagolytic agents.

• 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).


• Electrolyte imbalance.


• 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.


• Idiopathic.


• 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).

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Jul 24, 2016 | Posted by in SMALL ANIMAL | Comments Off on Atrioventricular Block, Second Degree
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