Differentials for ECG Abnormalities

No P wave

Several situations must be differentiated if “absence of P waves” is the abnormality.

• Is the P wave totally absent throughout the rhythm?
• Is the P wave obscured or intermittently present?
• Is the P wave absent for only one beat?

Totally Absent P wave

Rule outs include:

1. Atrial fibrillation (note the QRS to QRS interval must be irregular; the heart rate is usually high normal or elevated). Baseline undulations or f waves can be misinterpreted as P waves. If there is no consistent P morphology (usually there are too many possible P morphologies to select from due to the undulating baseline) and these are at variable distances before the QRS, then likely there are no P waves.
2. Hyperkalemia (note the QRS to QRS interval tends to be regular; the heart rate tends to be low normal or slow)
3. Silent atrium (a disorder wherein the atrial myocardium is replaced by fibrous tissue, also called permanent atrial standstill) (note the QRS to QRS interval tends to be regular; the heart rate tends to be slow, the rhythm is called an escape rhythm)
4. Supraventricular tachycardia (in fact there is always a P wave associated with the QRS here but in many cases the P wave is completely buried in the previous QRS or T wave such that we can’t visualize it)

P wave obscured or intermittently present

Rule outs include:

1. A very small P wave. The P wave can be difficult to see or find in some dogs and many cats because it is very small. The chest leads tend to give the biggest P waves, so always check here carefully.
2. A buried P wave. When the heart rate is fast the P wave can be buried in the preceding T wave. Look for pauses in the rhythm (such as after a VPC) to check for P waves, or try to slow the heart rate and look for P waves.
3. Ventricular tachycardia. Several P waves are usually observed intermittently in most cases; the other P waves are buried in the QRS or T wave of the VPC.
4. Sick sinus syndrome. Intermittent periods of pause with no P or QRS, or just no P waves with an escape rhythm.

P wave absent for one beat

Rule outs include:

1. A premature ventricular contraction if the QRS occurs early (the QRS must meet the criteria for VPC)
2. A supraventricular premature contraction if the QRS occurs early (the QRS must meet the criteria for a supraventricular premature contraction)
3. A ventricular escape beat if the QRS occurs late (>1 sec in dogs, > 0.5 sec in cats) (the QRS must be wide and bizarre in morphology)
4. A supraventricular escape beat if the QRS occurs late (>1 sec in dogs, > 0.5 sec in cats) (the QRS must be narrow and similar to the sinus beats)

No QRS following a P wave

Rule outs include:

1. 2nd degree heart block (Intermittently, P waves are not followed by QRS complexes, but for the QRS complexes that are present, they are the result of the associated P wave in front i.e. conducted)
2. 3rd degree heart block (No P waves are conducted and there is complete A-V dissociation and the ventricular rhythm is a slow escape rhythm)

No T wave

Rule outs include:

1. Artifact. All depolarizations must have a repolarization wave. If a deflection looks like it might be a QRS (although perhaps somewhat bizarre) and there is no identifiable T wave, then that deflection is not a QRS, it is an artifact.

Unusual shaped P wave

Several situations must be differentiated if the P wave is of an unusual shape:

• Is there just one isolated P wave that is of an unusual shape?
• Is the P wave of variable shapes and occurring in a pattern?
• Is the P wave of a consistent unusual shape?

One isolated P wave of unusual shape

Rule outs include:

1. A premature (ectopic) P wave (usually this occurs as an early P wave). This P wave may or may not be followed by a QRS complex (the latter is an APC with block).

P wave variable and occurs in a pattern

Rule outs include:

1. Wandering atrial pacemaker (there is usually a pattern, for example: the P wave might be tall on the short cycles and absent, isoelectric or negative on the long cycles; usually occurs in the setting or sinus arrhythmia)

P wave consistent unusual shape

Rule outs include:

1. An ectopic atrial rhythm (a normal sinus-origin P wave should be positive in lead II)

QRS unusual shape

Several situations must be differentiated if the QRS is of an unusual shape:

• Is there just one isolated QRS that is of an unusual shape?
• Is the QRS is of a consistent unusual shape?

One isolated QRS of unusual shape

Rule outs include:

1. A premature ventricular contraction if the QRS is early (the QRS must meet the criteria for VPC)
2. A supraventricular premature contraction if the QRS occurs early (the QRS must meet the criteria for a supraventricular premature contraction), conducting with aberrancy
3. A fusion beat if there is a typical P wave with a shorter PR interval (usually occurs in the company of other VPCs)
4. A ventricular escape beat if the QRS occurs late (>1 sec in dogs, >0.5 sec in cats) (the QRS must be wide and bizarre in morphology) and it is not preceded by a P wave
5. A supraventricular escape beat if the QRS occurs late (>1 sec in dogs, >0.5 sec in cats) (the QRS must be meet the criteria for a supraventricular beat), conducting with aberrancy
6. A sinus beat that conducts with aberrancy (as bundle branch block). A normal P wave must be present and associated with this QRS complex.
7. Artifact. Artifact may be present as any of the following: if it occurs between the end of the preceding QRS and the first half of the T wave of that QRS (no real beat [normal or ectopic] can occur at this time because the myocardium is refractory); if it fails to alter the underlying rhythm (however interpolated VPCs can do this as well); or if it fails to be followed by a repolarization wave.

QRS a consistent unusual shape

Rule outs include:

1. Bundle branch block if the rhythm is sinus or of supraventricular origin (the QRS meets the criteria for right or left bundle branch block)
2. Ventricular tachycardia (the QRS meets the criteria for ventricular tachycardia)
3. Right ventricular enlargement (a supraventricular rhythm and the QRS is negative in lead two; and meets the criteria for right ventricular enlargement)
4. An escape rhythm. It could be supraventricular conducting with aberrancy or ventricular in origin.
5. An idioventricular rhythm. A form of slow ventricular tachycardia.

QRS has a notch in it

Rule outs include:

1. MIMI (refers to microscopic intramural myocardial infarct) (the beats must be sinus in origin or supraventricular origin)
2. Not significant if present on a VPC or a ventricular escape beat
3. Not significant if bundle branch block is present

QRS is wide

Rule outs for a persistently wide QRS include:

1. Left ventricular enlargement (the QRS is only slightly widened and the R wave is tall)
2. A sick myocardium (dilated cardiomyopathy or heart failure can be associated with a wide QRS associated with slow myocyte to myocyte conduction)
3. Bundle branch block (the rhythm meets the criteria for bundle branch block and the rhythm is sinus or supraventricular) (the QRS is markedly widened)
4. Right ventricular enlargement (the QRS is only slightly widened)
5. Ventricular tachycardia (meets the criteria for VPC)
6. A ventricular escape rhythm
7. A pre-excited beat (via an accessory pathway. Has a short PR interval)

QRS is small

Rule outs include:

1. Thoracic effusions (pleural or pericardial)
2. A sick myocardium (dilated cardiomyopathy or heart failure can be associated with a low amplitude QRS)
3. Obesity
4. Hypothyroid (probably related to obesity with hypothyroid)
5. Pneumothorax
6. A normal variant
7. A fusion beat; occurs as a single beat (meets the criteria for a fusion beat)

R wave tall in lead II

Rule outs include:

1. Left ventricular enlargement (if the rhythm is supraventricular in origin)

ST segment depressed

Rule outs include:

1. Left ventricular enlargement (if the left ventricular enlargement criteria are met; all by itself this is probably not enough to suggest left ventricular enlargement)
2. Myocardial ischemia
3. Not of significance if not associated with a supraventricular beat (such as in a VPC)
4. Not of significance if associated with bundle branch block

ST segment Elevated

Rule outs include:

1. Pericardial disease
2. Myocardial ischemia
3. Not of significance if not associated with a supraventricular beat (such as in a VPC)
4. Not of significance if associated with bundle branch block

Coving of ST segment

Rule outs include:

1. A sick myocardium (dilated cardiomyopathy or heart failure can be associated with this)
2. This may be normal (many ECG units tend to produce some degree of coving)

P wave is wide

Rule outs include:

1. Left atrial enlargement
2. Normal variant

P wave is tall

Rule outs include:

1. Right atrial enlargement
2. Pulmonary artery hypertension
3. Normal variant

P wave is notched

Rule outs include:

1. Left atrial enlargement (only if the P wave is also too wide)
2. Normal variant (if the P wave is not too wide)

P wave not associated with a QRS

Rule outs include:

1. 3rd degree heart block (the ventricular rate is slower than the atrial rate)
2. Ventricular tachycardia (the P waves are often difficult to observe as they are buried in the QRS or T wave)
3. Non-paroxysmal junctional tachycardia
4. Idioventricular rhythm (a slow VT)