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:
- 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.
- Hyperkalemia (note the QRS to QRS interval tends to be regular; the heart rate tends to be low normal or slow)
- 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)
- 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:
- 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.
- 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.
- 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.
- 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:
- A premature ventricular contraction if the QRS occurs early (the QRS must meet the criteria for VPC)
- A supraventricular premature contraction if the QRS occurs early (the QRS must meet the criteria for a supraventricular premature contraction)
- 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)
- 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:
- 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)
- 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:
- 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:
- 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:
- 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:
- 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:
- A premature ventricular contraction if the QRS is early (the QRS must meet the criteria for VPC)
- A supraventricular premature contraction if the QRS occurs early (the QRS must meet the criteria for a supraventricular premature contraction), conducting with aberrancy
- A fusion beat if there is a typical P wave with a shorter PR interval (usually occurs in the company of other VPCs)
- 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
- 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
- A sinus beat that conducts with aberrancy (as bundle branch block). A normal P wave must be present and associated with this QRS complex.
- 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:
- Bundle branch block if the rhythm is sinus or of supraventricular origin (the QRS meets the criteria for right or left bundle branch block)
- Ventricular tachycardia (the QRS meets the criteria for ventricular tachycardia)
- Right ventricular enlargement (a supraventricular rhythm and the QRS is negative in lead two; and meets the criteria for right ventricular enlargement)
- An escape rhythm. It could be supraventricular conducting with aberrancy or ventricular in origin.
- An idioventricular rhythm. A form of slow ventricular tachycardia.
QRS has a notch in it
Rule outs include:
- MIMI (refers to microscopic intramural myocardial infarct) (the beats must be sinus in origin or supraventricular origin)
- Not significant if present on a VPC or a ventricular escape beat
- Not significant if bundle branch block is present
QRS is wide
Rule outs for a persistently wide QRS include:
- Left ventricular enlargement (the QRS is only slightly widened and the R wave is tall)
- A sick myocardium (dilated cardiomyopathy or heart failure can be associated with a wide QRS associated with slow myocyte to myocyte conduction)
- Bundle branch block (the rhythm meets the criteria for bundle branch block and the rhythm is sinus or supraventricular) (the QRS is markedly widened)
- Right ventricular enlargement (the QRS is only slightly widened)
- Ventricular tachycardia (meets the criteria for VPC)
- A ventricular escape rhythm
- A pre-excited beat (via an accessory pathway. Has a short PR interval)
QRS is small
Rule outs include:
- Thoracic effusions (pleural or pericardial)
- A sick myocardium (dilated cardiomyopathy or heart failure can be associated with a low amplitude QRS)
- Obesity
- Hypothyroid (probably related to obesity with hypothyroid)
- Pneumothorax
- A normal variant
- A fusion beat; occurs as a single beat (meets the criteria for a fusion beat)
R wave tall in lead II
Rule outs include:
- Left ventricular enlargement (if the rhythm is supraventricular in origin)
ST segment depressed
Rule outs include:
- Left ventricular enlargement (if the left ventricular enlargement criteria are met; all by itself this is probably not enough to suggest left ventricular enlargement)
- Myocardial ischemia
- Not of significance if not associated with a supraventricular beat (such as in a VPC)
- Not of significance if associated with bundle branch block
ST segment Elevated
Rule outs include:
- Pericardial disease
- Myocardial ischemia
- Not of significance if not associated with a supraventricular beat (such as in a VPC)
- Not of significance if associated with bundle branch block
Coving of ST segment
Rule outs include:
- A sick myocardium (dilated cardiomyopathy or heart failure can be associated with this)
- This may be normal (many ECG units tend to produce some degree of coving)
P wave is wide
Rule outs include:
- Left atrial enlargement
- Normal variant
P wave is tall
Rule outs include:
- Right atrial enlargement
- Pulmonary artery hypertension
- Normal variant
P wave is notched
Rule outs include:
- Left atrial enlargement (only if the P wave is also too wide)
- Normal variant (if the P wave is not too wide)
P wave not associated with a QRS
Rule outs include:
- 3rd degree heart block (the ventricular rate is slower than the atrial rate)
- Ventricular tachycardia (the P waves are often difficult to observe as they are buried in the QRS or T wave)
- Non-paroxysmal junctional tachycardia
- Idioventricular rhythm (a slow VT)