Virginia Luis Fuentes, MA, VetMB, PhD, CertVR, DVC, MRCVS, DACVIM, DECVIM-CA (Cardiology)
Royal Veterinary College, Dept of Veterinary Clinical Sciences, Hatfield, United Kingdom
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
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
Ventricular rhythms (rhythms originating from the ventricles)
Key features: wide, bizarre QRS complexes
Ventricular premature complexes
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)
2nd/3rd degree AV block
Fast heart rates (>160/min in dogs)
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.