Casus 1


A 12 year old female spayed Cocker Spaniel presented for investigation of syncope. On physical examination a bradycardia of 36 BPM was noted. The following ECG trace was then obtained:

The above ECG is most consistent with:

  1. Advanced second degree AV block with a junctional escape focus
  2. Third degree AV block with a ventricular escape focus
  3. Accelerated idioventricular rhythm
  4. Sinus bradycardia with left bundle branch block
  5. Ventricular premature complexes



There is a regular p wave rate (~166/minute) and a regular R-R interval (~36/minute) however there is complete atrioventricular dissociation. This is evidenced by the varying P-Q time and occasional P waves noted to occur between the QRS and T wave. The escape focus is regular but clearly too wide (>50ms) which is most consistent with an escape focus of ventricular origin (so no ventricular premature complexes). The positive polarity of the QRS in lead II suggests the escape focus is originating from within the right ventricle.

Complete AV dissociation excludes second degree AV block and sinus bradycardia.

The heart rate is far too low for it to be considered accelerated idioventricular rhythm.

Answer: Third degree AV block with a ventricular escape focus


Casus 2


An 8 year old MN Cavalier King Charles Spaniel presented to an afterhours emergency service following acute onset of dyspnoea. Left sided congestive heart failure was confirmed on thoracic radiographs. The rhythm was noted to be fast (> 200 bpm) and irregular on auscultation. The following ECG trace was obtained:



The above ECG is most consistent with:

  1. Atrial flutter
  2. Supraventricular tachycardia
  3. Atrial fibrillation
  4. Sinus arrhythmia
  5. Atrial standstill



The 4 hallmark ECG features of atrial fibrillation include: tachycardia, irregularly irregular R-R intervals, QRS of normal (i.e. narrow) configuration and absence of discernable P waves. There are also periods on the baseline where it appears to undulate. These are most consistent with ‘f’ waves or fibrillation waves. However these ‘f’ waves are not considered by most to be one of the hallmarks of atrial fibrillation as they are not always evident (particularly in small-breed dogs and most cats with atrial fibrillation).

Atrial flutter tends to have a characteristic ‘saw-tooth’ pattern to the baseline.

Supraventricular tachycardia tends to be fast but regular.

Sinus arrhythmia is a variation of normal sinus rhythm therefore P waves should be present. It should also have a relatively slow (yet normal) rate.

Atrial standstill is generally slower and more regular with a junctional or ventricular escape focus.


Answer: Atrial fibrillation


Casus 3


A 3-year old FS Labrador presented to the primary care veterinarian following intermittent episodes of sudden weakness and exercise intolerance occurring mainly at rest. The dog had not experienced recent exercise, exertion or excitement prior to presentation. On physical examination a very fast tachycardia was detected (~300 bpm). The following ECG trace was obtained during an episode of tachycardia:

The above ECG is most consistent with:

  1. Supraventricular tachycardia
  2. Ventricular tachycardia
  3. Torsades de pointes
  4. Atrial fibrillation
  5. Sinus tachycardia



The QRS complexes have a normal narrow (< 50ms) configuration which is consistent with a supraventricular focus (and excludes any ventricular arrhythmia including ventricular tachycardia and Torsades). The rhythm is also very fast and regular which makes atrial fibrillation less likely (atrial fibrillation typically has irregularly irregular R-R intervals however when very tachycardic this can at times be difficult to concern). There do also appear to be P waves visible at times in the trace (they are mainly noticeable as subtle spikes in the downward portion of the T wave in lead II) which definitively excludes atrial fibrillation. In the absence of any history, sinus tachycardia is a possible explanation for this ECG; however severe (and sustained) tachycardia is clearly abnormal in any dog that has not experienced recent exercise or exertion (as in this dog).

Answer: Supraventricular tachycardia


Question 4


A 4-year old Doberman presented for investigation of breathing difficulty and presumed syncopal episodes. The following ECG was recorded:

The above ECG is most consistent with:

  1. Sinus rhythm with tachycardia-dependent bundle branch block
  2. Supraventricular tachycardia with aberrant conduction
  3. Paroxysmal ventricular tachycardia
  4. Paroxysmal ventricular flutter
  5. Ventricular fibrillation



The ECG shows sudden onset of paroxysms of monomorphic ventricular tachycardia. “Monomorphic” refers to the fact the QRS complexes are fairly uniform in configuration which suggests the ventricular rhythm is originating from a single focus within the ventricle. The negative polarity (in lead II) of the ventricular ectopics suggests the focus is somewhere within the left ventricle. Notice also there are 4 normal sinus complexes in the middle of the tracing as well as a single ventricular ectopic followed by a compensatory pause (i.e. the underlying sinus rhythm is not disturbed).

Supraventricular rhythms (including sinus) with aberrant conduction cannot be completely excluded however the absence of discernable P waves clearly preceding each QRS makes these diagnoses less likely. Ventricular flutter essentially has a ‘sine’ wave configuration and no baseline is seen. Ventricular fibrillation reveals a disorganised, chaotic pattern of ventricular depolarisations.

Answer: Paroxysmal ventricular tachycardia