Equine Arrhythmias Flashcards
Arrythmias are the same in the horse except?
- Very large heart, with large myocardium
- Huge excess capacity at rest, therefore very high vagal tone
The vagus nerve controls which parts of the heart? What is the effect of this?
SAN and AVN
Increased vagal tone affects these nodes
The cardiac action potential conduction system pacemaker and threshold potentials can be influenced by?
Autonomic Nervous system
K+, Na+ and Ca2+ concentrations
Drugs
What is the function of the AV node?
Slow down depolarisation between the atria and ventricles so they don’t contract at the same time
What is the resting potential?
The difference between positive and negative potential difference in the myocytes
Resting potential is negative
What is the refractory period?
The time when a cardiac myocyte cant carry another wave of depolarisation
How is the pacemaker action potential different to the cardiomyocyte action potential?
Pacemaker action potential has a creeping resting potential that slowly increases until threshold is reached and it has a dramatic increase
Regularly sends out waves of depolarisation
Describe how to carry out an equine ECG
Base apex lead:
- Left arm (+): heart apex
- Right arm (-): mid right jugular furrow
- Neutral: remote from heart
The P wave represents?
Atrial depolarisation starting at the SA node
The P-R segment on an ECG represents?
Delay at the AV node
Prevents synchronous atrial / ventricular contraction
The QRS complex represents?
Depolarisation that starts at the AV node
The T wave on an ECG represents?
Ventricular repolarisation
- can change morphology / polarity at different heart rates = normal
Describe the heart sounds in the horse
Arial depolarisation and contraction = S4 ‘B’
Ventricular depolarisation and contraction = S1 ‘LUB’
Ventricular repolarisation and relaxation = S2 ‘DUP’
What are the 2 consequences of depolarisation starting at an ectopic focus (spreads over the atria in a different direction)?
Different wave of depolarisation
Different shaped ECG (p-wave)
Describe how an ectopic ventricular pacemaker alters the ECG
- Normally starts at the AV node and spreads down the conduction fibres.
- If the ectopic focus starts at another location i.e. the ventricular wall, it spreads in a different direction and takes longer. So the ECG is wider and appears bizarre
- Ectopic ventricular pacemaker close to conduction system: run of ventricular tachycardia
Name the 2 common physiological arrythmias in horses
1st and 2nd degree AV block
Sinus arrythmias/bradycardia/tachycardia
Name the 4 common pathological arrythmias in horses
Atrial Premature complexes (APC)
Atrial fibrillation
Ventricular Premature complexes
Ventricular tachycardia
Name the 4 less common pathological arrythmias in horses
- Atrial tachycardia
- Junctional escape complexes/premature contractions
- Ventricular fibrillation
- 3rd degree AV block
Describe the features of second degree AV block in horses
- Common physiological arrhythmia at rest
- Regular SA node depol.
- Conduction pathway normal
- AV node stops spread of depolarisation to ventricles every 3 - 4 beats
Describe the ECG of a horse with 2nd degree AV block
- Regular R-R intervals
- Pause is approx double R-R interval
- Normal QRS morphology
- Normal P waves (and therefore S4)
- Dropped beat
What change is seen on the ECG in a horse with 1st degree AV block?
Prolonged P-R interval
What change is seen on the ECG in a horse with sinus arrythmia?
Periodic waxing and waning of the R-R interval
What change is seen on the ECG in a horse with sinus block or arrest?
- Occasional pauses of 2+ R-R intervals
- No P or QRS T on ECG
Describe how an ECG would appear if a horse has atrial fibrilation
- Irregular R-R intervals
- Absence of P waves
- ‘F’ (fibrillation) waves
- QRS normal morphology
