Page 37 - Rapid Review of ECG Interpretation in Small Animal Practice, 2nd Edition

Approach to Evaluating Arrhythmias

Focal (ectopic) atrial tachycardia • One-to-one atrial to ventricular association is
Focal atrial tachycardia (FAT) occurs when localized
usually present during AVRT because the atria
VetBooks.ir regions in the atria (other than the sinus or AV node) and ventricles are both integral parts of the
arrhythmia circuit.
develop the ability to fire rapidly on their own.
Depending on the site of the focus, the morphology
of the P wave observed on the surface ECG varies. On BRADYARRHYTHMIAS AND
occasion, multifocal atrial tachycardia can develop, in CONDUCTION DISTURBANCES
which case the P wave morphology varies depending Sinus bradycardia
on the shifting atrial focus. The electrophysiological Sinus bradycardia (SB) is a type of sinus rhythm
mechanisms that initiate and maintain FAT include where the sinus node discharge rate is low (<50 bpm
abnormal automaticity, re-entry, and triggered activity. in an awake dog). SB of 45–60 bpm during sleep is
normal. However, if SB persists during excitement
ECG criteria (Fig. 3.6): or exercise, it can be a sign of sinus node pathology.
• Normal P waves or abnormal P waves that are
different in morphology from sinus P waves. ECG criteria (Fig. 3.7):
• There may be a “warm-up” period observed • Sinus rhythm with positive P waves in the
where the heart rate gradually accelerates after caudal leads (II, III, aVF) and normal QRS
initiation of the tachycardia. complexes, unless concurrent conduction disease
• Atrial rate generally >180 bpm. is present.
• The ventricular rhythm may be irregular when • Slow sinus node discharge rate.
the atrial rate is so rapid that not every atrial • Escape beats (junctional or ventricular) may be
beat is conducted to the ventricles (physiological present.
second-degree AV block).
SICK SINUS SYNDROME
Atrioventricular re-entrant tachycardia Sick sinus syndrome (SSS) is a disease process in
Atrioventricular re-entrant tachycardia (AVRT) is a which the spontaneous sinus node discharge is either
macro-re-entry arrhythmia whose circuit comprises slower than normal (primary SB) or intermittently
the AV node as well as an accessory pathway (AP) absent (sinus arrest or sinus block). It is characterized
which can conduct impulses between the atrial by an atrial rate that is inappropriate for a patient’s
and ventricles, often time in either direction, thus activity. The subsidiary pacemaker tissue (AV node
bypassing the AV node and His–Purkinje system. and Purkinje fibers) is often also abnormal, resulting
During typical AVRT, the depolarization proceeds in inadequate escape rhythms, such that complete
from the atria to the ventricles through the AV asystole (pauses) can occur, often lasting many
node, and back up to the atria from the ventricles seconds. Clinical signs associated with SSS may
in the retrograde direction via the AP. An AP can include fainting or intermittent weakness. Some
behave like a two-way street for electric conduction. patients experience supraventricular tachycardia
If during normal sinus rhythm, early activation (SVT) as well as bradycardia or sinus arrest.
(pre-excitation) of the ventricle occurs via atrial to
ventricular conduction along the AP, a delta wave ECG criteria (Fig. 3.8):
(slurring of the upstroke of the QRS complex) on • The ECG demonstrates a collection of
the surface ECG can occur. Some APs can only conditions that indicate sinus node and escape
conduct in the retrograde direction and therefore rhythm dysfunction.
do not show a pre-excitation pattern. • Periods of bradycardia or sinus arrest followed
by paroxysms of SVT.
ECG criteria: • AV block may also be present.
• Short PR interval (if the impulse is transmitted • Due to the intermittent nature of the
forward from atria to ventricle over the AP). arrhythmia, 24-hour ambulatory ECG (Holter)
• Delta wave (if the impulse is transmitted monitoring is often necessary to diagnose this
forward from atria to ventricle over the AP). condition definitively.
• During typical AVRT, QRS complexes are
usually narrow and the retrograde P wave
(negative polarity in leads II and III) may be seen
embedded into the early portion of the T wave.
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