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579.e2 Left Bundle Branch Block

Left Bundle Branch Block
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states of heart disease, such as congestive heart
BASIC INFORMATION
failure (dyspnea, lethargy, abdominal distention) • Prolonged QRS complex duration (canine:
> 0.07 second; feline; > 0.06 second)
Definition or syncope. • Wide and positive QRS complexes in leads
An intracardiac conduction disturbance I, II, III, and aVF and in left precordial leads
sometimes associated with left ventricular PHYSICAL EXAM FINDINGS (CV 6 LL [V 2 ] and CV 6 LU [V 4 ])
enlargement. There is failure of normal (rapid) • Infrequently, split heart sounds may be due to • Negative QRS complexes in leads aVR and
conduction from the bundle of His through the prolonged left ventricular ejection time and CV 5 RL (rV)
left bundle branch to the Purkinje fibers in the delayed closure of the mitral valve (causing
left ventricle. Conduction to the left ventricle a split first heart sound). Advanced or Confirmatory Testing
occurs but is very slow because it must travel • LBBB does not alter the rhythm of the • Thoracic radiographs to evaluate for left
from muscle cell to muscle cell. This results heartbeat or pulse. ventricular enlargement
in a marked delay in conduction to the left • Findings related to underlying structural • Echocardiogram to assess heart structure and
ventricle, and the QRS complex becomes wider heart disease (heart murmur, arrhythmia, function
on the electrocardiogram (ECG). gallop sound, dyspnea) are common.
Epidemiology Etiology and Pathophysiology TREATMENT
SPECIES, AGE, SEX LBBB usually indicates underlying cardiac Treatment Overview
Dogs and cats of either sex and any age disease: No treatment is necessary for LBBB. LBBB
• Cardiomyopathy does not result in any hemodynamic sequelae
RISK FACTORS • Congenital heart disease, in particular by itself and therefore does not require specific
Left ventricular enlargement, particularly left subaortic stenosis treatment. However, LBBB is often associated
ventricular dilation • Cardiac ischemia with underlying structural heart disease, and
• Certain drug toxicoses (doxorubicin) treatment of the underlying condition may be
Clinical Presentation indicated.
DISEASE FORMS/SUBTYPES DIAGNOSIS
• Intermittent: in some cases, left bundle Possible Complications
branch block (LBBB) may occur only when Diagnostic Overview Complete block of the right and left bundle
the heart rate surpasses a certain individual The diagnosis is based entirely on the ECG. branches produces complete (third-degree)
threshold (heart rate dependent). LBBB is suspected when QRS duration is longer atrioventricular block.
• Left anterior fascicular block than normal (canine: > 0.07 second; feline:
○ Historically, it was thought the left bundle > 0.06 second), wide and positive R waves are PEARLS & CONSIDERATIONS
branched into anterior and posterior fas- seen in leads I, II, III, and aVF and the left
cicles as it does in humans and left anterior precordial leads, and the rhythm appears to be Comments
fascicular block was implicated in causing supraventricular in origin. • The bizarre QRS morphology seen with
a marked left axis deviation on the surface bundle branch block can be confused
ECG, with only slight prolongation of left Differential Diagnosis with premature ventricular complexes or
ventricular depolarization. Wide QRS complex(es) on ECG: ventricular tachycardia (ventricular ectopy).
○ The left bundle does not branch into two • Left ventricular hypertrophy ○ If P waves are present and the PR interval is
distinct fascicles in dogs and cats, making • Ventricular ectopy (premature ventricular consistent for every heartbeat, the complex
the extrapolation from human anatomy complexes, ventricular tachycardia) is likely coming from a supraventricular
inaccurate. • Ventricular escape rhythm site, and the bizarre QRS morphology is
○ A marked left axis deviation in cats, which • ± Motion artifact due to bundle branch block rather than
has been called left anterior fascicular block ventricular ectopy. However, P waves
by some, has often been observed in cats Initial Database are sometimes buried and not visible,
with hypertrophic cardiac disease and in Electrocardiography is the definitive diagnostic although the rhythm is supraventricular
normal cats. test; ECG shows characteristics of LBBB: in origin (particularly with tachycardias).
○ In 2012, a retrospective study including
153 cats demonstrated that the presence
of the left anterior fascicular block pattern
was not predictive of heart disease in that
population of cats.
○ Although the term left anterior fascicular
block is inappropriate, it is often used, and
the ECG pattern, which reflects the left
axis deviation, should be recognized: tall
R wave in leads I and aVL and S wave
in leads II, III, and aVF.
HISTORY, CHIEF COMPLAINT
Reflective of the underlying structural heart
disease. Chief complaints range in severity LEFT BUNDLE BRANCH BLOCK Lead II ECG from a dog shows normal sinus rhythm with a left bundle
from none (uncommon; LBBB is an infrequent branch block. The QRS duration is 0.1 second (normal ≤ 0.07 second). The heart rate is 90 beats/min. 50 mm/
incidental finding on ECG) to decompensated sec, 1 cm = 1 mV.

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