Approach to ECGs Flashcards
Define what an ECG is
The ECG is a recording of changes in electrical potential difference occurring during the depolarisation and repolarisation of the myocardium plotted against time. i.e. It is a Voltage - Time Graph
The P wave is the sum of … ?
All the atrial action potentials (depolarisation)
What acts as the pacemaker within the heart?
Sinoatrial node
The slowed conduction through the AV node permits…?
Ventricular filling which allows the atria to completely contract before depolarisation
The QRS complex is shown on the ECG due to?
Rapid ventricular depolarisation
The T wave represents?
Ventricular repolarisation
The ECG electrodes are colour coded (UK scheme) and attached to:
- Red
- Yellow
- Green
- Black
Red: Right Fore
Yellow: Left Fore
Green: Left Hind
Black: Right Hind (the “earth lead”)
The most important ECG lead to examine is? Why?
LEAD II – this usually shows the largest complexes in the dog and cat. All measurements are made from this.
Lead I, II and III connect?
I = RF and LF II = RF and LH III = LF and LH
What questions need to be asked when looking at any ECG?
- What is the HR?
- Are the R-R intervals regular, regularly irregular or chaotically irregular ?
- Is there a P wave preceding every QRS complex and a QRS complex following every P wave?
- Do the QRS complexes look normal (i.e. using the His-Purkinje system normally?)
Sinus arrhythmia can be normal in which species?
Dog
When would periods of sinus arrest of up to 5 seconds be normal?
In sleeping dogs
In which species is sick sinus syndrome most commonly seen?
Westies
Which monitor can be used to investigate collapse and periods of sinus arrest?
Holter monitors
A prolonged P wave on the ECG represents?
Left atrial enlargement
A tall P wave on the ECG represents?
Right atrial enlargement
A tall R wave on the ECG represents?
Left ventricular enlargement
A prolonged QRS complex on the ECG represents?
Ventricular enlargement / hypertrophy
How would first degree AV block appear on an ECG?
The P:QRS ratio remains 1:1, but the P-R (PQ) interval is longer than normal for the species. This may be due to high vagal tone, the effect of certain drugs (e.g.
digoxin) or disease.
How would second degree AV block appear on an ECG?
There are some P waves not followed by QRS complexes, the QRS complex should look normal. The P-R interval varies,
and tends to lengthen beat to beat.
How would third degree AV block appear on an ECG?
The P waves bear no relationship to the QRS complexes. The P waves occur at their own, normal rate and the QRS complexes arise due to much slower automaticity of the Purkinje system or ventricular myocardium.
T waves are oppositely directly.
T waves which go in the opposite direction are also known as?
Ventricular escape complexes
How would atrial fibrillation appear on an ECG?
• QRS complexes and T waves look relatively normal, but the rate is fast and the interval between them (R-R interval) is highly variable (chaotic).
• There are no P waves (fibrillation waves may be seen along the baseline, but are not
always evident).
What are some possible systemic causes of ventricular premature complexes?
- Gastric dilatation volvulus
- Splenic lesions / splenectomy
- Sepsis
- Pancreatitis
- Pyometra
- Thoracic trauma (myocardial contusions)
- CNS lesions
How can a morbitz type I and II degree AV block be distinguished?
Type 1: Variable P-R interval before “dropped” P wave
Type II: Constant P-R interval (may be normal)
- Fixed ratio of P waves: QRS complexes
Interpret and diagnose the following ECG findings:
- HR 220 bpm
- Normal QRS (using His-Purkinje system)
- Chaotically irregular R-R intervals
- No discernible P waves – irregular baseline
Atrial fibrillation
How would supraventricular tachycardia present on an ECG?
May be very fast (>300 bpm), but regular PQRS complexes
Which ECG lead is used for standard measurements in dogs and cats?
Lead II
If the QRS duration is longer than 0.06s, what is indicated?
Ventricular hypertrophy
What are some possible causes of dampened QRS complexes?
- Conformation (massive breed)
- Fat
- Pleural effusion
- Pericardial effusion
- Thoracic mass
Left ventricular enlargement can be assessed by looking at which part of the ECG?
R wave height
A prolonged P wave duration is an indicator of…?
Left atrial enlargement
On an ECG, what can be seen which may indicate atrial stretch?
Supraventricular premature complex: Normal QRS complex, premature
Describe where the following ECG changes show enlargement at different points of the heart?
- Prolonged P wave
- Tall P wave
- Tall R wave
- Prolonged QRS
- Left atrial enlargement
- Right atrial enlargement
- Left ventricular enlargement
- Left ventricular enlargement
What is the ECG used for in large animals?
To document cardiac rhythm
How does a left bundle branch block appear on an ECG?
- Normal P:QRS ratio
- Tall, wide QRS complexes, T waves on opposite direction
- QRS duration > 0.08 seconds
How does a right bundle branch block appear on an ECG?
- Normal P:QRS ratio
- Deep, wide S waves, so –ve QRS complex, T waves on opposite direction
- QRS duration > 0.08 seconds
What does the QT interval represent?
Total time for ventricular depolarisation and repolarisation
The QT interval is dependant on?
Heart rate (increased QT interval with slower heart rate)
What other factors can affect the QT interval?
- Hypocalcaemia
- Vomiting and diarrhoea
- Hypothermia
- Dehydration
Changes in T waves are indicative of?
Metabolic changes
How will hyperkalaemia affect an ECG?
- Tall spikey T waves
- Atrial standstill: no P waves
- Variable R-R interval
On an ECG, the ST segment represents?
The period between depolarisation and repolarisation
ST segment coving (slurring) indicates …?
Left ventricular hypertrophy
and/or
Myocardial hypoxia
