Equine cardiology: Arrhythmias

Question 1

How do you set up an equine ECG?

Answer 1

• Base apex lead
  – Left arm (+): heart apex
  – Right arm (-): mid right jugular furrow
  – Neutral: remote from heart

Question 2

What is the P wave?

Answer 2

• Atrial depolarisation - starting SA node

Question 3

What is the P-R segment?

Answer 3

• Delay at AV node

Question 4

What is the QRS complex?

Answer 4

• Ventricular depolarisation - starts at AV node

Question 5

What is the T wave?

Answer 5

• Ventricular repolarisation

Question 6

What happens if depolarisation starts away from the node ( at an ectopic focus)?

Answer 6

• Different wave of depolarisation = different shaped ECG

Question 7

What are common arrhythmias in horses?

Answer 7

Generally Physiologic:
* 1st and 2nd degree AV block*
* Sinus block
* Sinus arrhythmia/bradycardia/tachycardia

Generally Pathologic:
* Atrial Premature complexes (APC)**
* Atrial tachycardia
* Atrial fibrillation (AF)**
* Junctional escape complexes/premature contractions
* Ventricular Premature complexes (VPC)**
* Ventricular tachycardia (VT)
* Ventricular fibrillation
* 3rd degree AV block

Question 8

What is 2nd degree AV block?

Answer 8

• Regular SA node depolarisation (P-waves)
• AV node stops spread of depolarisation to ventricles every 3-4beats
• Resolves with increased sympathetic (Atropine/buscopan)

Question 9

What would 2nd degree AV block look like on an ECG?

Answer 9

Regular R-R intervals
* Normal QRS morphology
* Normal P waves (S4)
* No QRS every now and again (S1+S2)

Question 10

What is a 1st degree AV block?

Answer 10

• Prolonged P-R = doesn’t actually block

Question 11

What is a sinus arrhythmia?

Answer 11

• Periodic waxing + waning of R-R interval

Question 12

What is sinus block / arrest?

Answer 12

• Occasional pauses of 2+ R-R intervals
• NO P or QRS waves on ECG

Question 13

What would you hear with atrial fibrillation?

Answer 13

• Common in bigger horses
• Biggest cause of poor performance
• No 4th heart sound (B) - P waves
• Prominent S3 (D)
• Irregularly irregular
• Long pauses

Question 14

What should be done if atrial fibrillation?

Answer 14

• Check if continuous / paroxysmal
• Check for underlying pathology -
• echo
• serum electrolytes
• complete blood count
• cardiac troponin
• If no problems = primary atrial fibrillation - big heart

Question 15

What are presenting complaints for horses with atrial fibrillation?

Answer 15

• Poor performance - at high intensity exercise
• Fading during race
• Epistaxis

Question 16

What is secondary AF?

Answer 16

AF caused by other cardiac disease
* Mitral valve regurgitation + distention of atria

Question 17

How can you treat AF?

Answer 17

1. Pharmacologic conversion – Quinidine sulphate
   * Main effect = increases refractory period for atrial cells
   * STOP if horse does not convert or shows severe side effects
2. Transvenous electrical cardioversion

Question 18

What are atrial premature complexes?

Answer 18

• seen between QRS + T waves
• Can lead to AF

Question 19

What does different shaped P wave indicate?

Answer 19

• atrial contraction originating somewhere other than SA node

Question 20

What does different shaped QRS/T wave indicate?

Answer 20

• ventricular contraction originates somewhere other than AV node