04: ECG

What does an ECG tell us

About the electrical activity of the heart

What is absolutely required for another action potential to occur

Repolarization

If you suspect heart disease, what is the only way to definitively tell what type of disease it is

Echocardiogram

Where is most of the salt in the body

Outside the cell

Where is most of the potassium in the body

Inside the cell

Two types of heart cells

Pacemaker cells and cardiac myocytes

Which type of heart cell intrinsically depolarizes at a regular rate

Pacemaker cell

Region of the heart that takes the lead on electrical activity

SA node

Which lead do we interpret most often in vet med and why

Lead II; it is parallel to the direction that the AP spreads and most deflections are positive

Patient positioning for a diagnostic ECG

Right lateral recumbency

Standard ECG paper speed

25 mm/s

Electrical activity associated with the P wave

Atrial depolarization

Electrical activity associated with the QRS complex

Ventricular depolarization

Electrical activity associated with the T wave

Ventricular repolarization

Where do we see atrial repolarization on an ECG

We don’t, it is hidden by the QRS complex

Where is the SA node

R atrium

How do we calculate HR for an ECG paper speed of 25 mm/s

• 1 large box is 0.2 s → 5 large boxes is one second, count how many are between an R-R interval. 300/# boxes = bpm

• Or: count how many QRS complexes are in 3 seconds and multiply by 20

For this class: normal HR for dogs

50-150

For this class: normal HR for cats

120-220

What can we say about the signal if an ECG has a normal P and a normal QRS

Signal comes from the SA node

What can we say about the signal if an ECG has a no P and a normal QRS

Signal comes form the base of the heart (above the ventricles) but not the SA node

What can we say about the signal if an ECG has a no P and a wide and bizarre QRS

Signal comes from the apex/ventricles

Premature beat

Beat comes too fast

Escape beat

Beat comes too late

What do we call a normal sinus rhythm that is faster than 150 (220) bpm

Sinus tachycardia

What do we call a normal sinus rhythm that is slower than 50 (120) bpm

Sinus bradycardia

Sinus arrhythmia

HR speeds up during inspiration and slows during expiration

Species that may normally have a sinus arrhythmia

Dogs, NOT CATS

What causes a high sinus arrhythmia

High vagal tone

Sinus pause

Temporary pause in ECG <2 R-R intervals or <2 seconds

Sinus arrest

Pause in ECG >2 R-R intervals or >2 seconds

Speed of the ventricular pacemaker

• Dog: 20-40

• Cats: 60-80

Ventricular escape rhythm

No P wave, ventricular beats at a rate of 20-40 bpm (60-80 bpm in cats)

Accelerated idioventricular rhythm

No P wave, ventricular beats at a normal bpm

Ventricular tachyardia

No P wave, ventricular beats >180 bpm

Ventricular fibrillation

No P wave, ventricular beats >180 bpm with small, uncoordinated waves

Ventricular premature complex

No P wave, ventricular beat that is premature

Ventricular rhythms we do not treat

AIVR and single VPCs

Ventricular rhythm we don’t treat with lidocaine EVER

Ventricular escape rhythm

Supraventricular tachycardia AKA sinus tachycardia

Abnormally fast rhythm with normal R-R interval that originates from an ectopic foci above the AV node/ventricle (normal QRS), may not see P wave if it is buried in a T wave

Atrial fibrillation

Irregularly irregular abnormally fast rhythm with abnormal R-R interval that originates from an ectopic foci above the AV node/ventricle, and there is no P wave

Cause of atrial fibrillation

Enlarged L atrium

Lone atrial fibrillation

Idiopathic afib seen in large dogs and horses with no underlying heart disease

Types of AV dissociation

• 1st degree

• 2nd degree

• 3rd degree

1st degree AV block

Increased length between Q and R, but there is still a P for every QRS

2nd degree AV block

Some P waves are followed by QRS complexes, and some P waves don’t get through to the ventricles

3rd degree AV block

Zero P waves are getting through to the ventricles, and the only ventricular beats are a ventricular escape rhythm

Device that allows for 24/7 ECG monitoring

Holter monitor