Equine Cardiology 2

Question 1

1. What are many physiological equine cardiac arrhythmias related to?
2. When should they disappear?
3. Physiological equine arrhythmias.
4. Pathological equine arrhythmias.

Answer 1

1. High vagal tone at rest.
2. During exercise or excitement.
3. 1st and 2nd AV black, sinus arrhythmia.
4. Atrial premature depolarisations, atrial tachycardia, atrial fibrillation, ventricular premature depolarisations, ventricular tachycardia, 3rd degree AV block.

Question 2

1. Most common regular rhythm in horses.
2. Most common regularly irregular rhythm in horses.
3. Most common irregularly irregular rhythm in horses.

Answer 2

1. Normal sinus rhythm.
2. 2nd degree AV block.
3. AF.

Question 3

Appearance of sinus arrhythmia on ECG.

Answer 3

R-R intervals vary (longer, shorter, longer, shorter…). Seen occasionally after exercise but not common.

Question 4

2nd degree AV block.

Answer 4

At low HRs.
Disappears w/ exercise.
Hear dropped beat in which S4 heard and not followed by S1 or S2.
ECG - dropped beats in which P wave not followed by QRS complex.

Question 5

3rd degree AV block.
Common?
Cause?
Seen on ECG?
Patient presentation?
Px?
Tx?

Answer 5

Rare.
Complete conduction block between atria and ventricles.
No relationship between P and QRS complexes.
Recumbent, weak, profound exercise intolerance, v low HR.
Poor.
None, euthanise. Pacemakers?

Question 6

Atrial fibrillation.
Presentation?
Cause?
Auscultation rhythm?
Seen in ECG?

Answer 6

Incidental finding in non-athletic horses, or exercise intolerance / wobbly / epistaxis in athletic horses.
High vagal tone, large atrial size in horses, large breed horses, cardiac disease leading to atrial dilatation (MVR). Circus movement and re-entry of waves of depolarisation.
Irregularly irregular rhythm, no S4.
ECG – baseline irregularity = f waves.
QRS complex normal. R-R interval irregularly irregular.

Question 7

Why is AF more clinically significant at exercise compared to at rest?

Answer 7

Ventricles fill passively at rest so atrial contraction is less important to CO.
But during exercise, w/ high HR, atrial contraction more important for ventricular filling so CO decreased at exercise which results in exercise intolerance.

Question 8

Actions following Dx of AF.

Answer 8

Echocardiographic exam to identify underlying cardiac disease (e.g. MVR) leading to atrial dilatation. If disease detected, Px for conversion to sinus rhythm poor and recurrence high so Tx not usually attempted.

Question 9

Tx options for horses w/ cardiac arrhythmias.

Answer 9

Horses w/o underlying cardiac disease can be treated to restore sinus rhythm.
Oral quinidine sulphate.
Transvenous electrical cardioversion (TVEC) proving successful.
85% conversion w/ case selection.
Recurrence of AF ~30%.

Horses not converted should only be used for low level recreation and only after exercising ECGs analysed.

Question 10

Quinidine Sulphate.
What does it do?
Dose, route, frequency, until?
Actions while administering.
Issues?

Answer 10

Prolongs refractory period.
20mg/kg, NG tube, every 2hrs, until conversion, unmanageable side effects or 6 doses given.
Hospitalisation and continual ECG.
Many unpleasant side effect incl. death (<5%).

Question 11

TVEC.
Aim.
Risks.

Answer 11

Electrical shock to the heart to cancel out all f waves at same time and restore sinus rhythm.
Must be done under GA - risks associated.

Question 12

Atrial premature depolarisations.

Answer 12

Electrical impulses that originate too early somewhere in atrial myocardium (not node).
P wave occasionally abnormal in configuration, QRS morphology typically same as normal sinus beats.
4 or more in a row = atrial tachycardia.
Can be found in otherwise healthy horses.
Also associated w/ disease e.g. myocarditis and electrolyte imbalances.
Might increase likelihood of AF.

Question 13

Ventricular premature depolarisations and ventricular tachycardia.

Answer 13

Depolarisation too early from ventricular myocardium.
Different morphology and usually longer duration than normal.
4 or more in a row = ventricular tachycardia.
May suggest underlying pathology.
May initiate VT or VF - danger of syncope or sudden death.

Question 14

Clinical decision making.

Answer 14

Treat arrhythmias, not murmurs or VSD.
Dx by ECG and/or echocardiography.
Decision making based on:
Safe to ride? - compare risk to that of an unaffected horse.
Likely to progress?
Regular monitoring required?
Frequency of re-examination?