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Fig. 3.9. Lead II ECG (25 mm/s, 10 mm/mV) showing ventricular tachycardia. The heart rate is approximately
180 bpm. The rhythm is regular. QRS complexes are wide and bizarre, negatively deflected in lead II, with large
positive T-waves. Occasional P-waves are seen (see arrows) but are not temporally related to QRS complexes.

Fig. 3.10. Lead II ECG (25 mm/s, 10 mm/mV) showing polymorphic ventricular tachycardia. The heart rate is
approximately 160 bpm. All QRS complexes are wide and bizarre, but are of different morphology, with some
positively and some negatively deflected in lead II. All QRS complexes have large T waves of opposite polarity to
the QRS complex. There is a large amount of baseline artifact due to patient motion and panting (may be mistaken
for flutter/fibrillation waves). Because ectopic beats originate from multiple foci within the ventricles, the rhythm is not
perfectly regular.

term management of ventricular arrhythmias most tracts’; see Fig. 3.11). Occasionally, supraventricu-
commonly involves oral sotalol (combined potas- lar ectopy can be caused by drug toxicity, trauma,
sium-channel blocker and weak nonselective and severe systemic diseases. From a practical
β-blocker) and/or mexiletine (sodium-channel perspective, supraventricular and atrial ectopy are
blocker; see Table 3.6). synonymous terms; technically ‘supraventricular’
is more anatomically correct since it includes
ectopic beats originating from the region of the AV
Supraventricular tachyarrhythmias
node itself.
Supraventricular ectopy is caused either by abnor- Supraventricular ectopic QRS complexes appear
mal automaticity (ectopic activity) in atrial cells or the same as sinus complexes for that patient, since
re-entrant loops of electrical activity involving the the abnormal impulse or circuit still conducts
atria. Supraventricular ectopy most commonly through the AV node, and ventricular depolarization
occurs in patients with structural heart disease and past the AV node proceeds as normal. Normal P
severe left atrial enlargement. These arrhythmias waves are not present for these beats since the
can also be seen as an isolated phenomenon in impulse for atrial depolarization does not originate
dogs (particularly Labrador Retrievers) with in the SA node. Abnormal atrial depolarization
abnormal ‘accessory pathways’ of conduction tis- waveforms (P′ waves) can occur before, during, or
sue linking the atria and ventricles, allowing car- after the QRS complex and may be positive, nega-
diac depolarization to return to the atria via a tive, or isoelectric depending on where the ectopic
re-entrant loop that bypasses the AV node (‘bypass activity or re-entrant circuit are located within the

Electrocardiography 59