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VETERINARY CLINICAL CARDIOLOGY
CARDIOLOGY CONCEPTS
Electrocardiology
Differentials for Abnormalities on the ECG
1. What if there is 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?
2. What if the P wave is totally absent throughout the rhythm?

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)
3. What if the P wave is 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.
4. What if the P wave is absent for only 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)
5. What if there is no QRS complex following the 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)
6. What if there is 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.
7. What if there is an 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?
8. What if there is just one isolated P wave that is of an 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).
9. What if the P wave is of variable shapes 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)
10. What if the P wave is of a consistent unusual shape?

Rule outs include:

  1. An ectopic atrial rhythm (a normal sinus-origin P wave should be positive in lead II)
11. What if the QRS is of an 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?
12. What if there is just one isolated QRS that is of an 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.
13. What if the QRS is of 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.
14. What if the 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
15. What if the 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)
16. What if the 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)
17. What if the R wave is tall in lead II?

Rule outs include:

  1. Left ventricular enlargement (if the rhythm is supraventricular in origin)
18. What if the ST segment is 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
19. What if the ST segment is 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
20. What is there is coving of the 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)
21. What if the P wave is wide?

Rule outs include:

  1. Left atrial enlargement
  2. Normal variant
22. What if the P wave is tall?

Rule outs include:

  1. Right atrial enlargement
  2. Pulmonary artery hypertension
  3. Normal variant
23. What if the 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)
24. What if the P wave is not associated with the 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)
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