Page 237 - Cote clinical veterinary advisor dogs and cats 4th

102 Atrioventricular Block

PHYSICAL EXAM FINDINGS by a QRS complex at a species-specific normal ○ Complete response: total resolution
• First-degree AV block: unremarkable interval. The PQ interval (or PR interval) of AV block and heart rate increases
VetBooks.ir normal or decreased; cardiac rhythm regularly duration) and conduction through the AV node tachycardia), suggesting physiologic, not
encompasses atrial depolarization (P-wave
by ≥ 50% (typically resulting in sinus
• Second-degree AV block: heart rate typically
pathologic
irregular with skipped beats (no ventricular
(PR segment), normally ≤ 0.13 second in dogs
contraction when block occurs); femoral
pulse strength typically normal; may be or ≤ 0.09 second in cats. ○ Incomplete response: some AV block
persists, and/or heart rate increases < 50%
supranormal for beats following pauses due Differential Diagnosis ○ Incomplete response may indicate indi-
to increased ventricular filling; no pulse • First-degree AV block vidual variation (normal) or structural AV
deficits (i.e., every ventricular contraction ○ Artifact (e.g., changing paper speed) nodal disease (e.g., fibrosis); syncope
generates a pulse); for Mobitz type II only, • Second-degree AV block decreases in a small minority of these
signs of low cardiac output and/or CHF ○ Auscultatory: pronounced respiratory sinus patients when treated with oral medica-
may be present arrhythmia (common), sinoatrial arrest or tions (see below), but most require
• Third-degree AV block: bradycardia (heart block (uncommon) pacemaker implantation.
rate typically < 50/min in dogs, < 140/min ○ ECG: third-degree AV block, rhythmic • Limited accuracy; Holter or cardiac event
in cats); intermittent prominent jugular pulse motion artifact (e.g., purring, shivering) monitoring (p. 1120) obtained at the time
(cannon a wave) caused by right atrial • Third-degree AV block of clinical event/collapse is superior for
contraction against closed tricuspid valve; ○ Historical: other causes of syncope (e.g., confirming a pathologic arrhythmia
intermittent prominent first heart sound tachyarrhythmias, structural heart disease,
(bruit de cannon) due to dissociation pulmonary hypertension, intracranial TREATMENT
between atrial and ventricular contraction; disease, metabolic disease) (p. 953)
other findings referable to low cardiac output ○ Auscultatory: Other bradyarrhythmias Treatment Overview
or CHF if present (e.g., sinus bradycardia, persistent atrial For second- and third-degree AV block, goals
standstill, high-grade second-degree AV are to restore normal cardiac output and resolve
Etiology and Pathophysiology block) CHF if present. When clinical signs are present
Etiology: (typically syncope), pacemaker implantation
• First-degree AV block Initial Database is almost always necessary.
○ May occur as normal variation (manifesta- • First-degree AV block
tion of prevailing vagal tone) ○ ECG: PQ or PR interval > 0.13 second Acute General Treatment
○ Iatrogenic: medications that slow AV nodal in dogs, > 0.09 second in cats • For first-degree and Mobitz type I second-
conduction (e.g., digoxin, beta-adrenergic ○ Additional testing as pertains to degree AV block: no specific therapy required
antagonists, calcium channel antagonists, underlying/concurrent condition, if any • For all cases of high-grade second-degree AV
opioids) • Second-degree AV block block causing clinical signs and all cases of
○ Diseases that increase vagal tone (e.g., ○ ECG: some P waves not followed by QRS third-degree AV block:
respiratory, gastrointestinal, and intra- complexes, resulting in a faster atrial rate ○ Standard therapy for CHF if applicable
cranial central nervous system disorders) than ventricular rate; also to rule out other (p. 408)
○ Less commonly: primary cardiomyopathies, arrhythmias ○ IV positive chronotropes (e.g., isoproter-
idiopathic AV nodal fibrosis, infiltrative ○ Echocardiogram: rule out structural enol 0.04-0.08 mcg/kg/min IV infusion):
myocardial disease, myocardial infarction intracardiac causes; incidental finding of temporary support (e.g., before pacemaker
• Second-degree AV block other abnormalities is common (e.g., implantation) but often ineffective, par-
○ As for first-degree AV block myxomatous AV valve disease) and not ticularly with third-degree AV block
○ May occur seconds to minutes after always related ○ Temporary and/or permanent artificial
intravenous (IV) atropine or glycopyrrolate ○ Thoracic radiographs: rule out CHF pacemaker implantation
(transient) ○ CBC, serum biochemistry profile, and
• Third-degree AV block urinalysis: unremarkable unless concurrent Chronic Treatment
○ Pathologic conditions as listed for first- conditions • Indicated for high-grade Mobitz type II
and second-degree AV block • Third-degree AV block second-degree AV block and all cases of
Pathophysiology: ○ ECG: unrelated faster atrial rate (P waves) third-degree AV block
• Above causes may produce slowing (first- and slower ventricular rate (QRS complexes) • Oral positive chronotropes (e.g., propanthe-
degree AV block) or blockage (second- and ■ Escape rhythm (QRS complexes) may line 0.5-1 mg/kg PO q 8h, terbutaline
third-degree AV block) of electrical impulse appear wide and bizarre or normal 0.2 mg/kg PO q 8-12h, or theophylline
conduction by the specialized cardiac (presumably arising from the ventricles 5-10 mg/kg PO q 8-12h) may be used but
myocytes of the AV junction. or lower AV junction, respectively) generally inadequate
• In third-degree AV block, ventricular ■ Rate: typically 25-50 beats/min in dogs, • Permanent pacemaker implantation
depolarization (and contraction) results from 70-140 beats/min in cats
an escape rhythm generated by pacemaker ■ Also rule out other arrhythmias Possible Complications
cells in the more distal part of the AV junc- ○ Echocardiogram, thoracic radiographs, • Mobitz type II second-degree AV block may
tion (His bundle) or in the ventricles. and minimum laboratory database as for progress to third-degree AV block.
• Result is faster atrial rate (P waves per second-degree AV block • Patients requiring pacemaker implantation
minute) and slower ventricular rate (QRS are at risk for sudden death before
complexes per minute) occurring indepen- Advanced or Confirmatory Testing implantation.
dently of one another. Atropine response test (0.04 mg/kg IV, SQ • Patients with high-grade Mobitz type II
[longer response time]) second-degree AV block or third-degree AV
DIAGNOSIS • For second-degree AV block, Mobitz type block are at risk for developing CHF
II (or Mobitz type I if accompanied by sinus
Diagnostic Overview bradycardia and vague clinical signs) Recommended Monitoring
All types of AV block require ECG for diagnosis. • To differentiate between physiologic and • First-degree AV block: no specific monitoring
On a normal ECG, each P wave is followed pathologic causes necessary

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