Abnormal rhythms (or arrhythmias) may arise from abnormalities of impulse formation, or impulse conduction (or sometimes both). Arrhythmias can be classified in this way by their electrophysiologic mechanism, but it is sometimes simpler to recognize them based on their site of origin:

 Sinus rhythms (rhythms originating from the sinoatrial node)

 Key features: Normal P waves, normal QRS complexes

 Regular sinus rhythm

 Sinus arrhythmia

 Sinus tachycardia

 Sinus bradycardia

 Supraventricular rhythms (rhythms not originating from the sinoatrial node, but from above the ventricles)

 Key features: Abnormal or absent P waves, normal QRS complexes

 Atrial premature complexes

 Atrial tachycardia

 Junctional tachycardia

 (Supraventricular tachycardias)

 Atrial flutter

 Atrial fibrillation

 Ventricular rhythms (rhythms originating from the ventricles)

 Key features: wide, bizarre QRS complexes

 Ventricular premature complexes

 Ventricular tachycardia

 Ventricular fibrillation

 Ventricular escape complexes

 (nb: bundle branch block also results in wide bizarre complexes)

Principles of ECG Interpretation

It is often best to work from first principles when attempting to interpret ECGs. If one understands what the P-QRS deflections represent, it is often possible to unravel arrhythmias that appear unfathomable on initial inspection. By applying a set of basic questions it is possible to distinguish sinus rhythms from ectopic rhythms, supraventricular rhythms from ventricular rhythms / abnormal conduction, etc.

 Are there P waves visible?

 P waves indicate atrial depolarization

 Absence of P waves suggests either

 No normal atrial depolarization, e.g., atrial fibrillation, atrial standstill

 The P waves are hidden within the QRS complexes, e.g., ventricular tachycardia, junctional tachycardia

 Is there a P wave for every QRS complex, and a QRS complex for every P wave?

 P waves without associated QRS complexes

 Indicate atrial depolarization that has not been conducted through the atrioventricular node to the ventricles, i.e., atrioventricular block

 QRS complexes without P waves

 Either ectopic complexes (premature or escape), atrial fibrillation or sinoventricular complexes (atrial standstill)

 What is the relationship between the P waves and QRS complexes?

 Normal, constant PQ interval = sinus rhythm

 Long PQ interval = 1st degree

 Some P waves NOT followed by QRS = 2nd degree AV block

 No consistent relationship = 3rd degree AV block

 Are the QRS complexes narrow (normal) or wide (bizarre-looking)?

 Narrow complexes indicate rapid conduction through the ventricular myocardium, which can only occur when conduction spreads via the bundle branches and Purkinje system

 Narrow complexes therefore indicate complexes of supraventricular origin

 Wide complexes suggest that the ventricles have taken more time than normal to become depolarized. This means that spread of conduction has not taken place via the bundle branches and Purkinje system, but instead by passive cell-to-cell conduction, which is much slower.

 Wide complexes indicate

 Ventricular ectopic complexes (premature or escapes)

 Bundle branch block

 Is the rhythm regular or irregular?

 Some rhythms are usually very regular, e.g., supraventricular tachycardias, sustained ventricular tachycardia

 Some rhythms are always irregular, e.g., sinus arrhythmia, atrial fibrillation, sinus rhythm interrupted by ectopics

 Do all the complexes look the same?

 If there is more than one form of QRS complex, one should attempt to identify to classify the complexes as:

 Sinus--normal, narrow QRS complexes consistently associated with P waves and a normal PQ interval

 Supraventricular--normal, narrow complexes

 Ventricular--wide, and bizarre

 What is the heart rate?

 Slow heart rates (<65/min in dogs)

 Sinus arrhythmia

 Sinus bradycardia

 2nd/3rd degree AV block

 Atrial standstill

 Sinus arrest

 Fast heart rates (>160/min in dogs)

 Sinus tachycardia

 Atrial fibrillation

 Supraventricular tachycardias

 Ventricular tachycardia

After assessing the ECG by answering these questions, it should be possible to identify the rhythm or rhythms. The waveforms should be measured in detail, selecting sinus complexes from a lead II recording.