Electrocardiography
Introduction
Electrocardiography is one of the most commonly found piece of monitoring equipment in modern veterinary practices.
The ECG can determine the rate, rhythm and nature of cardiac depolarisation and repolarisation. It can also indicate changes in myocardial mass, conductivity between the heart and skin and the presence of metabolic abnormalities affect the heart.
Recording an ECG
The ECG detects the electrical activity of the heart through 3 electrodes. These electrodes are most commonly placed on the 2 forelimbs and the left hindlimb. An additional 'earth' electrode may be placed on the right hindlimb.
The electrodes are attached to the patient via ECG pads (most commonly), crocodile clips (more common in horses) and transcutaneous needles (rare). Any hair should be clipped to improve contact between the ECG pad and the skin. Pads should be secured with tape and additional electrode gel or alcohol should be used to improve contact between the patient and electrodes. The electrodes are connected to the ECG machine by colour-coded cables.
The colour coding system:
Yellow: Left forelimb
Red: Right forelimb
Green: Left hindlimb
Black: Right hindlimb
The ECG should be recorded in a calm and quiet environment. If the animal is stressed it may help to dim the lights. The patient should be kept as still and relaxed as possible as muscle tremors and movement can cause artifacts on the trace. Dogs should be placed in right lateral recumbency but positioning in a cat is less important. The cables should be positioned so that they do not drape over the animals chest as they can cause respiratory movement artifact. Electrical activity can cause interference on the ECG trace (known as 50Hz interference). To prevent this it should be taken on an insulated surface away from computers and fluorescent lighting and other electrical equipment.
Chemical restraint should be avoided if at all possible as this changes the ECG.
Reading an ECG Trace
An ECG supplies information about the electrical activity of the heart only. It indicates the heart rate and rhythm and can be used to detect any arrhythmias. It does not supply information about cardiac function. It is important to remember to treat the patient not the ECG!
To interpret an ECG it is necessary to understand the path of depolarisation through the heart: The sino-atrial node depolarises spontaneously and this wave of depolarisation spreads through the atria. The impulse is then conducted through AV node slowly. Then it rapidly passes through the bundle of His and bundle branches, spreading through the ventricular myocardium. The myocardium then remains depolarised for a period before repolarising. This depolarisation and repolarisation are detected as potential differences on the skin surface - producing the classic shape of the ECG trace.
| Stage | Represents |
|---|---|
| P | Atrial Depolarisation |
| QRS | Ventricular Depolarisation |
| T | Ventricular Repolarisation |
Six limb leads (ECG traces derived from a pair of electrodes) are produced from the reading of the four electrodes. These are named leads I, II, III, aVR, aLV, aVF. The trace should always be read from left to right. Ideally it should be read from the start but if this is too difficult then the most recognisable portion should be identified and the trace read from this point. and if it is hard to read then you should identify the
A Normal ECG Trace
Before you interpret abnormal ECG's you must know what a normal ECG looks like:
- A P-wave precedes every QRS complex
- A QRS complex follows every P-wave
- P and QRS are consistently and reasonably related
- P and QRS will all appear the same
- They will occur at a normal rate
An Abnormal ECG Trace
The following questions should be asked when interpreting every ECG:
Is E.C.G. of diagnostic quality?
It is important that no artefact is present on the trace. Interference from electrical equipment and fluorescent lighting (50Hz interference) and movement should be prevented. The ECG should be calibrated for both paper speed and vertical sensitivity, and the trace should remain within the paper edges. All leads should be demonstrated.
What is the heart rate?
Heart rate can be calculated using one of the following two methods:
(1) Instantaneous heart rate
1500/R-R interval (25 mm/sec)
3000/R-R interval (50 mm/sec)
(2) Number of R-R intervals in 6 seconds x 10
Where multiple rhythms exist, the rate of all rhythms present should be calculated.
An high heart rate may be due to sinus tachycardia, supraventricular tachycardia, ventricular tachycardia and atrial fibrillation. A slow heart rate may be caused by sinus bradycardia, sinus arrhythmia, second or third degree AV block, atrial standstill and sinus arrest.
What is the heart rhythm?
It is important to interpret whether the heart rate is regular or irregular. If it is irregular you should record whether it is regularly irregular or irregularly irregular.
What is the mean electrical axis?
This figure is of limited value in small animals. It can give some indication of ventricular enlargement and the presence of intraventricular conduction defects.
What are the individual complex measurements?
Changes in the morphology of the complexes are classed as a change in the shape, size or duration of the P wave, QRS complex or T wave. The magnitude and duration of deflections can be caused by hypertrophy of the myocardium, electrolyte abnormalities or an alterations in autonomic tone within the heart.
You should record whether all the deflections are the same, as some variations may be normal.
Variation in P-wave can be caused by a wandering pacemaker or atrial ectopy. Variation in QRS complexes may be caused by either variable conduction or electrical alternans.
No P to every QRS - this occurs when ventricular depolarisation follows an abnormal atrial depolarisation. This can be caused by premature ventricular or junctional complexes, sinus arrest with ventricular or junctional escape complexes, atrial standstill and atrial fibrillation.
No QRS to every P - this occurs the AV node fails to conduct impulses normally. This can be caused by second and third degree AV block.
The shape of a complex can be used to identify the location of the origin of a rhythm disturbance. Complexes that originate in the ventricles are produce wide and bizarre QRS complexes, whereas complexes of atrioventricular origin are narrow and upright.
Abnormal P-R interval: A variable P-R interval is caused by atrioventricular dissociation. This occurs secondary to junctional and ventricular rhythm disturbances and third degree AV block.
To calculate this the P-P, R-R and P-R intervals should all be measured. If the P-R interval is the only variable factor this is very suggestive of AV dissociation.
Common problems:
Differentiation of artefact from rhythm disturbance
Determining the origin of non-sinus depolarisations
Determining the significance of E.C.G. abnormalities which may be present.
Summary
ECG is the most useful tool for assessment of cardiac rhythm.