The Electrocardiogram (ECG)

The ECG is a graphic record of the heart's electrical activity plotted over time. It is a very useful test which is easy to perform and readily available. This lecture is a revision of the many uses and the practical application of electrocardiography in small animals.

Uses of Electrocardiography

 For the diagnosis of arrhythmias noted on clinical examination

 To rule out arrhythmias in animals with a history of collapse

 To provide information regarding chamber enlargement

 To provide information on certain electrolyte disturbances

 To provide supportive evidence of a diagnosis of pericardial effusion

 To monitor effectiveness of anti-arrhythmic therapy

 To monitor the heart rhythm under anaesthesia

Lead Systems

ECG electrodes are attached at multiple sites on the body to allow the electrical activity of the heart to be viewed from multiple angles. The standard leads are the bipolar limb leads I, II and III and the augmented limb leads aVR, aVL and aVF. Chest leads are sometimes recorded for further evaluation of cardiac chamber size, however these will not be covered in this lecture.

Leads I, II, and III compare electrical impulses between electrodes placed on two limbs. Leads aVR, aVL and aVF compare the signal at one limb with the average of the two other limbs. These are mainly used to calculate mean electrical axis.

Lead II is used to make the standard measurements of the P-QRS-T waveform.

ECG P-QRS -T Wave Form

The typical sinus electrical impulse originates in the sinus node in the right atrium and then passes through the right and left atrium before reaching the atrioventricular (AV) node. It slowly passes across the AV node and then down the bundle branches into the Purkinje network in the ventricular myocardium. Depolarisation is followed by repolarisation. The phases of the electrical activity are depicted on the ECG as follows:

 P Wave: Represents atrial depolarisation

 P-Q interval: Represents conduction across the atrioventricular node

 Q wave: Represents depolarisation of the interventricular septum (defined as the first negative deflection following the P wave in lead II)

 R wave: Represents depolarisation of the ventricles (defined as the first positive deflection in the QRS complex in lead II)

 S wave: Depolarisation of the basal portion of the ventricular free wall and septum (defined as the first negative deflection following the R wave in the QRS complex in lead II)

 T wave: Represents repolarisation of the ventricles

 Q-T interval: Represents electrical systole

What Type of Machine?

There are a wide variety of ECG machines available. Some useful traits include:

 A screen for prolonged monitoring

 A paper trace for a permanent record and to make measurements

 The ability to record all six leads

 A display of simultaneous leads can be very useful to pick up artefacts related to limb movement

 A good range of paper speeds

 A good range of sensitivities

 Simple controls that everyone in the practice can learn to use with ease

How to Record a Good Quality ECG

1.  Place animal in right lateral recumbency on an insulated surface.

2.  Attach electrodes to all four legs: Electrodes can be attached using filed down crocodile clips. Alternative systems include the use of sticky pads or plates. It is important to attach the electrodes at the standard positions high up on the legs, as positioning lower on the legs can alter the size of the complexes. If you are attaching an ECG just to monitor the rhythm then position of the electrodes on the legs is not as important and electrodes can be attached to the feet. It is important to have good contact to obtain a good quality trace with minimal artefacts. Use plenty of gel and/or spirit to improve contact, sometimes shaving may be necessary. Attach the electrodes to loose skin to avoid muscle interference. Standard positions of electrodes:

a.  Red: Right forelimb

b.  Yellow: Left forelimb

c.  Green: Left hindlimb

d.  Black: Right hindlimb

3.  Check paper speed: Paper may be run at 10, 25 or 50 mm/sec on most ECG machines. The standard speed is 25 mm/sec. However, if you need to measure intervals or complex width with accuracy, it is important to run the paper at the faster speed of 50 mm/sec for part of the trace.

4.  Check calibration: The majority of machines have three settings for calibration 5, 10 and 20 mm/mV. The standard setting is 10 mm/mV, however if the complexes are small it may be necessary to change the calibration to 20 mm/mV and likewise if the complexes are tall the calibration may need to be changed to 5 mm/mV to ensure that the complexes fit on the trace. Complexes must fit on the trace to be measured accurately.

5.  Record calibration mark: It is important to check that a calibration mark is present otherwise complex height cannot be measured accurately.

6.  Record a few complexes of each lead (1, 2, 3, aVR, aVL, aVF) at 50 mm/sec.

7.  Record a longer lead 11 rhythm strip at 25 mm/sec.

8.  Label with patient details/date/lead number/paper speed, etc!.

9.  Do not sedate if you are looking for rhythm disturbances. Sedation or anaesthesia will affect the rhythm.

10.  Avoid the filter function if possible as it can alter height of complexes.

Common ECG Artefacts and How to Avoid Them

Animal Movement

Frightened animals may shiver and move. Shivering will result in a poor quality ECG recording. It may be reduced by spending extra time calming the animal and ensuring the animal is warm. Sudden limb movement can result in artifacts that look like ectopic beats. Simultaneous lead recording are very useful to identify individual limb movement.

Respiratory Movement

Breathing may result in movement of the baseline. This problem can be decreased by ensuring the electrodes are not too close to the chest wall, placing a foam pad between the front legs may also help. Panting is best stopped by blowing on the animals face or holding the mouth shut briefly. Purring in the cat can cause baseline movement. It is difficult to stop, but is easily recognised by its intermittent nature.

Electrical Interference

This is a very common problem. Try different rooms, ensure the machine is well earthed, use a well insulated table, ensure the electrodes are clean, and make sure the electrodes are not touching. If all else fails use the filter function however remember filtering can reduce the height of the complexes.

Lead Reversal

It is very easy to put the leads on the wrong legs which may result in an unusual trace.

ECG Interpretation

It is important to have a logical approach when interpreting an ECG. It helps to have a set of questions to work through. The following twelve questions can be used as a guideline. It is also worth having a standard form made up for recording the measurements for each ECG trace recorded.

1. What is the Heart Rate?

 Slow heart rates: Sinus arrhythmia, sinus bradycardia, 2nd/3rd degree AV block, atrial standstill, sinus arrest.

 Fast heart rates: Sinus tachycardia, atrial fibrillation, supraventricular tachycardia, ventricular tachycardia.

2. What is the Rhythm? Is it Regular or Irregular?

 Regular rhythms: Sinus rhythm, sinus tachycardia, supraventricular tachycardia, sustained ventricular tachycardia.

 Irregular rhythms: Sinus arrhythmia, atrial fibrillation, sinus rhythm interrupted by ectopic beats either supraventricular or ventricular.

3. Is There a P Wave for Every QRS Complex, and a QRS Complex For Every P wave?

 P waves without a QRS complex: Indicates atrial depolarisation which has not been conducted through the atrioventricular node to the ventricles, i.e., atrioventricular block.

 QRS complex without a P wave: These are either ectopic complexes (premature or escape), atrial fibrillation or sinoventricular complexes (atrial standstill).

4. What is the Relationship Between the P Waves and QRS Complexes?

 Normal PQ interval which remains constant: Sinus rhythm.

 Long PQ interval: First degree atrioventricular (AV) block.

 PQ interval is long and varies: Vagal influence on SA node--Wenckebach phenomenon.

 Short PQ interval: Seen with accessory pathways.

 No consistent relationship of any sort: Third degree (complete) atrioventricular block, AV dissociation.

5. Do All the P Waves Look the Same?

 Variable P waves: May represent a wandering pacemaker due to high vagal tone or could be due to an intermittent supraventricular rhythm disturbance.

 Negative P waves: May be due to a supraventricular ectopic focus or due to incorrect lead placement.

6. Do All the QRS Complexes Look the Same?

If there is more than one form of QRS complex, one should attempt to classify the complexes as:

 Sinus: Normal, narrow QRS complexes consistently associated with P waves.

 Supraventricular: Normal, narrow QRS complexes.

 Ventricular: Wide and bizarre QRS complexes.

7. Do the P Waves Measure Within Normal Limits?

The waveforms should be measured in detail, selecting sinus complexes from a lead II recording ideally recorded at 50 mm/sec. It is important to remember that only sinus complexes can be used to obtain information about chamber size.

 Tall P waves in lead II: Indicate right atrial enlargement.

 Wide P waves in lead II: Indicate left atrial enlargement.

8. Do the QRS Complexes Measure Within Normal Limits?

The waveforms should be measured in detail, selecting sinus complexes from a lead II recording ideally recorded at 50mm/sec.

 Tall R waves in lead II indicate left ventricular enlargement.

 Deep S waves in leads II indicate right ventricular hypertrophy.

 Small complexes are seen in pleural effusions, pericardial effusions, hypothyroidism, obesity and broad chested dogs.

 Variable height complexes are seen with pericardial effusions, this is known as electrical alternans.

 Left bundle branch block will result in tall R waves and wide QRS complexes, this must be differentiated from left ventricular enlargement.

 Right bundle branch block will result in deep S waves and wide QRS complexes, this must be differentiated form right ventricular enlargement.

9. Is the QT Interval Normal?

 Prolonged Q-T intervals: Hypocalcemia, hypokalemia, hypothermia, quinidine toxicity and CNS disorders.

 Shortened Q-T intervals: Hypercalcemia, digoxin toxicity and hyperkalemia.

10. Is the ST Segment Normal?

 Depression of the ST segment: Myocardial ischemia, myocardial infarction and digoxin toxicity.

 Elevation of the ST segment: Myocardial infarction, pericarditis and myocardial hypoxia.

 Coving of the ST segment: Is seen with left ventricular enlargement.

11. Are the T Waves Normal?

 T waves can be positive, negative or biphasic in dogs and cats.

 Tall spiked T waves are seen with hyperkalemia.

 Tall T waves may be seen with ventricular enlargement, conduction disturbances.

 Small T waves are seen with hypokalemia.

 T wave abnormalities may also result from myocardial hypoxia, myocardial infarction and bradycardia.

12. What is the Mean Electrical Axis?

 The mean electrical axis gives information on the main vector of depolarization of the heart and can be calculated by several different methods.

Conclusion

A consistent thorough approach to recording a good quality ECG and analyzing the trace can provide very useful information in the investigation of cardiac cases.

References

1.  Tilley LP. (1992) Essentials of canine and feline electrocardiography. Third edition. Philadelphia: Lea & Febiger.

2.  Martin MW. (2007) Small animal ECGS--an introductory guide. Second edition. Oxford: Blackwell publishing.