Third degree AV block
From Felipedia
Third degree AV block (complete) is a rare heart disease of cats characterised by the cardiac impulse completely blocked in the region of the atrioventricular (AV) junction and/or all bundle branches. The atrial rate (P to P interval) is normal. The idioventricular escape rhythm is slow.
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ECG Features
The ventricular rate is slower than the atrial rate (more P waves than QRS complexes). The ventricular escape rhythm (idioventricular) usually has a rate < 40 beats per minute, whereas a junctional escape rhythm (idiojunctional) has a rate of 40-60 in dogs and 60-100 in cats. The P waves are usually normal in configuration (figure). The QRS complex is wide and bizarre when the pacemaker is located in the ventricle, or in the lower AV junction in a patient with bundle branch block. The QRS complex is normal when the escape pacemaker is located in the lower AV junction (above the bifurcation of the bundle of His) in a patient without bundle branch block. There is no conduction between the atria and the ventricles. The P waves have no constant relationship with the QRS complexes. The P to P and R to R intervals are relatively constant (except for a sinus arrhythmia).
There is usually variable third and fourth heart sounds, variation in intensity of the first heart sounds, signs of congestive heart failure and intermittent "cannon" waves in jugular venous pulses.
Causes
- Isolated congenital defect
- Idiopathic fibrosis
- Infiltrative cardiomyopathy (amyloidosis or neoplasia)
- Hypertrophic cardiomyopathy
- Digitalis toxicity
- Myocarditis
- Endocarditis
- Electrolyte disorder
- Myocardial infarction
- Other congenital heart defects
- Lyme disease
Treatment
A temporary or permanent cardiac pacemaker is the only effective treatment in symptomatic patients. Asymptomatic patients without a pacemaker implanted must be carefully monitored for the development of clinical signs.
Treatment with drugs is usually of no value. Traditional drugs used to treat complete AV block include atropine, isoproterenol, theophylline, and corticosteroids. An intravenous isoproterenol infusion may help increase the rate of the ventricular escape rhythm to stabilize hemodynamics. Avoid digoxin, xylazine, acepromazine, beta-blockers ( e.g., propranolol and atenolol) and calcium channel blockers (e.g., verapamil and diltiazem).
References
1. Miller MS, Tilley LP, eds. Manual of canine and feline cardiology. 2nd ed. Philadelphia: WB Saunders, 1995.
