Atrioventricular Block, Third Degree

Atrioventricular Block, Third Degree



Basic Information image



Definition


Total dissociation between the atrial (P waves) and ventricular (QRS complexes) rhythm because there is no conduction through the atrioventricular (AV) node. The ventricles depend on their own (slow) escape rhythm.



Synonym(s)




• Third-degree AV block


• Third-degree heart block


• Complete 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


Anesthetized foals with uroperitoneum



Clinical Presentation



History, Chief Complaint




• Marked exercise intolerance


• Weakness


• Syncope or collapse



Physical Exam Findings




• Progressive filling of the jugular vein during long diastolic pauses with visible pulsation secondary to right atrial contractions.


• Auscultation


• Bradycardia

• Regular or irregular rhythm

• Loud first heart sound

• Soft atrial sounds can often be heard, usually at an increased rate (>60/min)

• Signs of congestive heart failure may be found.


• Intermittent syncope or presyncope caused by long diastolic pauses. Immediately before syncope, the horse briefly shows an increased respiratory effort. Horses usually fall backward, sideways, or both.


• Lifting the head of a bradycardic horse may induce syncope.


• Syncopal episodes are usually short (seconds) and, after a few moments, the horse is able to stand again and looks relatively normal.



Etiology and Pathophysiology




• Etiology


• Structural lesions of the AV node (fibrosis, inflammation, degeneration) that may be associated with endocarditis, myocarditis, or infiltrative processes

• Severe hyperkalemia

• Intoxication (rattlesnake envenomation)

• Immediately after delivery of an intracardiac direct current electrical shock (eg, treatment of atrial fibrillation), a temporary high-degree AV block may occur

• Idiopathic

• Pathophysiology


• Clinical signs of weakness and intermittent syncope are related to the ability of the ventricles to generate their own (slow) escape rhythm.

• The escape rhythm may emanate from pacemaker cells from the distal AV node, His bundle, or ventricle.

• Bradycardia results in low blood pressure and a reflex increase in the atrial rate (commonly 60–120/min).

• Lifting the horse’s head seems to further decrease blood flow to the brain and may elicit syncope.


Diagnosis image



Differential Diagnosis




• Causes of fainting or collapse


• Other cardiovascular causes such as tachyarrhythmias, structural cardiovascular disease (congenital defects, aortocardiac fistula, cardiomyopathy), severe pulmonary hypertension, systemic hypertension, embolism, intracardiac neoplasia.

• Intracranial disease.

• Narcolepsy (cataplexy): A narcoleptic episode usually starts with lowering the head and buckling at the knees and may progress to collapse.

• Vasovagal syncope.

• Metabolic (eg, hypoglycemia).

• Causes of bradycardia


• Sinus bradycardia: Slow rate of P waves, normal AV conduction.

• Sinus arrest: Pause without P wave or QRS complex; AV conduction is normal.

• Advanced second-degree AV block: Three or more consecutive P waves are blocked at the AV node; AV conduction is still present.
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Jul 24, 2016 | Posted by in SMALL ANIMAL | Comments Off on Atrioventricular Block, Third Degree
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