Electrocardiogram Interpretation – Back to Basics

Rebecca Malakoff, DVM, DACVIM (Cardiology)
MSPCA-Angell West

Making an Electrocardiogram (ECG) diagnosis may feel like a daunting task when a veterinarian is infrequently faced with the challenge, but confidence can be gained by practice and a systematic approach.  It is often helpful to break down the interpretation into steps, rather than trying to jump to a diagnosis right away and to pay attention to each deflection as well as the overall pattern.

A quick review of the normal conduction pathway is very helpful when faced with a potentially abnormal ECG recording.  With normal conduction, the impulse starts in the sinus node, depolarizes the atria (represented by the P-wave on the ECG), and travels to the atrioventricular (AV) node.  The pause in transmission at the AV node leads to the PR interval on the ECG.  The electrical impulse then rapidly travels down the bundle of His and the left and right bundle branches to depolarize the ventricular myocardium (represented by the QRS wave on the ECG).  Finally, ventricular repolarization occurs (represented by the T wave on the ECG).  It may sound basic, but keeping in mind what each wave reflects is very helpful when you need to determine which electrical disruption is creating an arrhythmia.  For example, a prolonged PR interval represents a delay in conduction at the AV node (first degree AV block), and a widened QRS complex represents prolonged conduction through the ventricles, either due to a ventricular ectopic beat or abnormal intraventricular conduction such as bundle branch block.

One method of a systematic approach to ECG interpretation includes:

Step 1:  Determine heart rate

Step 2:  Determine whether heart rate is normal, fast, or slow

Step 3:  Are QRS complexes uniform in appearance or variable?  Are they normal (narrow) or wide?

Step 4:  Determine whether underlying rhythm (of the R waves) is regular or irregular.  If the rhythm is irregular, is it regularly irregular (follows a repeating pattern) or irregularly irregular?

Step 5:  Determine association between P waves and R waves (is there a P for every QRS, a QRS for every P?)  Are P waves uniform in appearance or variable?

If you have completed step one for a patient (counting heart rate) and determined that a tachycardia is present, the next step is to determine the type of tachycardia.  Before trying to jump to an exact diagnosis, it is helpful to determine which of the two general types of tachycardia is present by examining the QRS complexes and determining whether they are narrow or wide.  A narrow complex QRS appears narrow because the electrical impulse is using the rapid conduction system (His bundle and left and right bundles) to depolarize the ventricular myocardium.  This means the electrical impulse originates above, or at, the AV node.  Examples of a narrow complex tachycardia include sinus tachycardia, atrial tachycardia, AV junctional tachycardia, atrial flutter and atrial fibrillation.

Lead II, 25 mm/sec ECG showing a rapid, narrow-complex monomorphic (QRS complexes all the same shape) tachycardia. P’ waves are not visible, as they are superimposed on preceding QRS/T complexes. The heart rate is 330 bpm. The diagnosis is supraventricular tachycardia.


Lead II, 25 mm/sec. This is another example of a supraventricular tachycardia, showing a brief break in the tachycardia revealing two normal P waves (arrows). Again, P’ waves are superimposed on the preceding complexes during the tachycardia.


Lead II, 25 mm/sec. Although similar to the preceding examples, this ECG recording of a narrow complex tachycardia has a more irregular pattern of R-R intervals. During the longer R-R intervals, there is no evidence of a definitive P wave or repeating P-R interval (although the baseline undulations can at times mimic a P wave, and it is important not to mistake the preceding complex T wave as a P wave). The diagnosis is atrial fibrillation.

If a QRS complex is wide, this indicates depolarization of the ventricular myocardium is delayed, often because all or part of the electrical impulse is travelling by slower cell to cell conduction rather than using the rapid conduction system.  The majority of wide-complex tachycardia seen on ECG recordings represent ventricular tachycardia, but you can also have a wide-complex tachycardia when there is aberrant conduction of a sinus or supraventricular tachyarrhythmia, such as with bundle branch block.  If in doubt, it is safest to assume a wide-complex tachycardia is ventricular tachycardia and treat accordingly.

Lead II, 25 mm/sec ECG showing a wide complex tachycardia representing ventricular tachycardia.


Lead II, 25 mm/sec. Another example of ventricular tachycardia, with negatively deflected QRS complexes.

Looking at the association between P waves and QRS complexes is also very helpful for determining whether a rapid rhythm is a supraventricular vs. ventricular tachycardia, but a few key points must be kept in mind.  Most practitioners remember that a supraventricular premature complex is preceded by a P’ wave, which represents aberrant atrial depolarization, and has a regular P’R interval (association).  But it is easy to forget that this P’ wave may not be visible, as it may be buried in the preceding T wave or QRS complex.  Another common misconception is the expectation that there will be no P waves with a ventricular premature complex (VPC) or ventricular tachycardia.  A normal P wave may be present (normal atrial depolarization may occur), but it is has nothing to do with the subsequent VPC which was initiated by spontaneous ventricular depolarization (no PR association).  Often these P waves may be obscured or buried within the VPC, but if the VPC is only slightly premature, you may still see a P wave preceding it.  If P waves are regularly seen before wide QRS complexes, with a repeatable PR interval, this indicates bundle branch block (the electrical impulse was initiated normally in the sinus node, relayed through the AV node, but then disruption in conduction through the bundle branches led to the wide QRS appearance).

An example of VPCs where preceding P waves are visible (arrows), but not associated (no repeating PR interval) with the subsequent QRS complex.


A – Right bundle branch block


B – . Left bundle branch block. QRS complexes are wide, indicative of slowed intraventricular conduction, but there is a repeating PR interval preceding each complex.

When faced with an abnormally slow rhythm, it can be similarly helpful to then determine whether the QRS complexes are narrow or wide, and whether they are regularly preceded by P waves.  If a regular PR interval occurs, one should determine whether it is normal length or prolonged (first degree AV block).  P waves which sometimes are followed by a QRS (with repeatable PR interval) but sometimes not (“dropped P waves”) indicate second degree AV block.  This occurs when the AV node only intermittently transmits the signal to the ventricles to depolarize.  When there is no association between the P waves and QRS complexes, this indicates AV dissociation, most commonly due to third degree AV block (where the AV node fails to transmit any of the electrical impulses to the ventricles).  Wide QRS complexes which occur later than the expected next complex, with no preceding associated P wave, are ventricular escape beats.  These are fairly easy to recognize when occurring as part of third degree AV block, but may also occur with other pauses in rhythm- during sinus bradycardia or sick sinus syndrome.  It is important to keep in mind that these represent ventricular ectopy, but are not “VPCs” because they are not premature (and should not be treated the same).

Lead II, 25 mm/sec. Heart rate (QRS rate) is 60 bpm. There is repeating PR interval, but each QRS complex is followed by a nonconducted P wave (arrow) (The P waves are easy to overlook due to relative low amplitude). Diagnosis is second degree AV block with 2:1 conduction.


Lead II, 25 mm/sec. Heart rate (QRS rate) is 50 bpm. P waves are present, but there is no association with the QRS complexes (no repeating PR interval). Diagnosis is third degree AV block.

Thorough discussion of all considerations in ECG diagnosis and arrhythmia treatment is beyond the scope of a short article, but hopefully this review will help you in reaching an ECG diagnosis with increased ease and confidence.  Fortunately, in those cases where the diagnosis is still uncertain, Angell’s team of cardiologists are available to help.

For more information on this and all cardiology-related questions, please contact Angell’s Cardiology service at 617-541-5038 or Dr. Malakoff works full time at our Waltham location.

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