Notebook: Electrocardiography 3

These notes were taken during a lecture by Dr. Meg Sleeper, veterinary cardiologist.

- Ectopic beats come from outside the sinus node; they can be either early or late.
- Junctional beat is the same as an AV nodal beat
- Junctional escape vs. ventricular escape

BASIC ECG CLASSIFICATION
- aberrant conduction (the conduction path is abnormal)
- bundle branch block (RBBB) occurs commonly.
- right bundle branch block occurs most commonly.
- left bundle branch block is more often associated with structural heart dz.
- arrythmia (either of supraventricular or ventricular origin)

2 differentials for a wide QRS complex:
- ventricular aberrant beat (a ventricular complex)
- block in conduction (blockade through conduction through the ventricles)
- with a RBBB, you see negative deflection

SUPRAVENTRICULAR CARDIAC RHYTHMS

Classification of conduction defects
1st degree: prolonged PR interval (slower than normal conduction through the AV node
- can be caused by fibrosis of the AV node, or certain drugs (i.e. digoxin)

2nd degree type 1 or Wenchebach: some of the P waves don't get through. Unconducted p waves typically occur as the R to R interval is lenghtening. May see subtle lengthening of the PR interval.
- in a dog where this is physiologic, increasing sympathetic tone will cause this to go away (i.e., running the dog in the hallway for a bit)

2nd degree type 2: P wave that is not followed by a QRS complex. Most typically it is progression of AV disease or fibrosis. (If this animal is exercised, the rhythm won't go away.)

3rd degree: complete AV block. Nothing is conducting down. It may seem that the P waves are very regular on their own and the QRS complexes are very regular on their own; however, the regularity of the two may not seem to be related. T waves are wider than normal with discordant QRS complexes.

Asides:
Remember, the first heart sound is caused by closure of the mitral valve. Premature beats are usually very loud b/c they occur when the valve is wide open.

Summation occurs when two cardiac events appear on the ECG paper at the same time.

If a QRS is not very wide, it may be coming from the AV node.

Case example: Dog has been fainting; ECG shows a sinus bradycardia with a ventricular escape beat. Atropine an be given as a diagnostic test (Atropine Response Test).

If the pattern goes away in response to atropine, the pattern is a sinus tachycardia. (A normal dog would also respond this way.) This means that the heart can respond normally when vagal tone goes away. For some reason this dog has abnormally high vagal tone. Note: If the dog had not presented for fainting, and the atropine response test caused the pattern to go away, this dog probably would not need to be treated.

Sinus rhythm is common in cats but pretty rare in dogs. Sinus arrhythmia is common in dogs. Remember, an arrhythmia is not necessarily abnormal. Sinus tachycardia is a normal response to pain, fever, stress, fear, etc.; this is a way the sinus node responds to elevated sympathetic tone.

Supraventricular tachycardia is also known as paroxysmal atrial tachycardia. If vagal tone is increased, does this slow down gradually or abruptly? -- To answer this question, the carotid sinuses or eyeballs are pressed as a (tentative) diagnostic test (this doesn't always work). If the pressure is released and the pattern resumes abruptly, the pattern is a supraventricular tachycardia. If the pattern resumes gradually, it is a sinus tachycardia.