Page 235 - Cote clinical veterinary advisor dogs and cats 4th

Atrioventricular Block 101

• In cats, tachycardia (or normal heart rate)
does not rule out atrial standstill, but in
VetBooks.ir Technician Tips Diseases and Disorders
dogs, atrial standstill almost always causes
bradycardia.

• Mix potassium chloride thoroughly when
injecting into IV fluid bags to avoid settling
(and direct IV infusion of high KCl con-
centrations from the bottom of the bag).
• Supplemental potassium must never be given
ATRIAL STANDSTILL Electrocardiogram of a young Australian blue heeler dog with atrial standstill and faster than 0.5 mEq/kg/h.
right-sided congestive heart failure. Cause is atrial myopathy, based on a normal serum potassium level. Patient
responded well to permanent pacemaker implantation. Rhythm is regular (constant R-R interval), consistent SUGGESTED READING
with a junctional escape rhythm. No P waves are seen in this lead II tracing, nor were they present in any other
lead. 50 mm/sec, 1 cm/mV. Nakamura RK, et al: Adult-onset nemaline myopathy
in a dog presenting with persistent atrial standstill
and primary hypothyroidism. J Small Anim Pract
• Digitalis toxicosis–associated atrial standstill: Failure to note atrial standstill (loss of P 53:357-360, 2012.
prognosis depends on severity of intoxication waves on ECG) in patients with hyperkale-
and underlying cardiac disease mia may allow hyperkalemia to worsen, AUTHOR: Mandi Kleman, DVM, DACVIM
leading to cardiac arrest. EDITOR: Meg M. Sleeper, VMD, DACVIM
• Artifactual hyperkalemia is common in dogs
PEARLS & CONSIDERATIONS and cats: platelets release potassium in vitro
when activated (e.g., blood clotting in a
Comments red-top tube). Artifact is ruled out by measur-
• Atrial standstill in hyperkalemic patients is ing potassium on blood from a heparinized
a harbinger of life-threatening arrhythmias. (green-top) tube.

Atrioventricular Block

BASIC INFORMATION Epidemiology through the AV junction, the PQ interval
is variable (Mobitz type I) or is fixed (Mobitz
Definition SPECIES, AGE, SEX type II).
Subclassification: first-, second-, and third- • First-degree block: cats and dogs of any age ○ Classic Mobitz type I second-degree AV
degree atrioventricular (AV) block • Second-degree block: dogs > cats block includes progressively prolonged PQ
• First-degree block: delayed conduction • Third-degree block: cats and dogs of any interval preceding nonconducted (blocked)
from the sinoatrial (SA) node through age; more common in older animals P wave (Wenckebach phenomenon)
the AV junction (AV node and bundle • Mobitz type II second-degree AV block more
of His) to the ventricles; electrocardio- GENETICS, BREED PREDISPOSITION often pathologic and unrelated to vagal tone
graphically manifested as prolongation of • First- and second-degree AV block: common ○ Further described by the ratio of P waves
the PQ interval (PR interval if no Q wave in cocker spaniels, dachshunds, and brachy- to QRS complexes (e.g., 5 : 4)
present) cephalic dog breeds ○ High-grade/advanced second-degree AV
• Second-degree block: intermittent failure of • Second-, third-degree AV block: secondary block: two or more consecutive noncon-
transmission of SA nodal impulses through to His bundle stenosis in pugs ducted P waves (P/QRS ratio of 3 : 1 or
AV junction; some P waves not followed by higher)
a QRS ASSOCIATED DISORDERS
• Third-degree block: persistent blockage of • Reported association between Lyme disease HISTORY, CHIEF COMPLAINT
SA nodal impulses through the AV junction; and third-degree AV block in dogs and • First-degree AV block: incidental finding on
P waves occur regularly and at a normal or people electrocardiogram (ECG); clinical signs relate
elevated sinus rate; QRSs also occur regularly • Mobitz type I second-degree AV block usually to underlying condition, if any
but independently and at a slower escape associated with increased vagal tone; often • Second-degree AV block
rate. occurs concurrently with respiratory sinus ○ Mobitz type I: as for first-degree AV block
○ QRS morphology may be wide and bizarre arrhythmia and/or sinus bradycardia ○ Mobitz type II: if bradycardia is present,
(dogs > cats) or narrow and normal in • Third-degree AV block reported in association episodic or persistent signs of low cardiac
appearance (cats > dogs). with hypertrophic cardiomyopathy in cats output (e.g., lethargy, weakness, syncope)
and as possible complication for some dogs or congestive heart failure (CHF) (e.g.,
Synonyms with myxomatous mitral valve degeneration dyspnea, abdominal distention) are common
• For AV block: heart block Clinical Presentation ○ Third-degree AV block: episodic or per-
• For Mobitz type I second-degree AV block sistent signs of low cardiac output or CHF;
(see below): Wenckebach phenomenon DISEASE FORMS/SUBTYPES cats often show no clinical signs, likely
• For third-degree AV block: complete heart • Second-degree AV block subclassified into due to a more rapid ventricular escape
block Mobitz types I and II: for impulses conducted rhythm compared to dogs

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