An electrocardiogram provides a graphic record of the voltage produced by cardiac muscle cells during atrial and ventricular depolarization and repolarization plotted against time. The electrical forces produced possess both direction and magnitude and can therefore be regarded as true cardiac vectors. The cardiac or mean electrical axis (MEA) represents the summation of many different forces. The MEA is the cardiac vector representing the average direction of spread of the depolarization wave through the ventricles as viewed from the frontal plane, that is the heart is viewed from the front with the animal standing up on its hind limbs.
Each lead on an ECG represents a hypothetical line drawn between two sites on the body surface to which electrodes are attached. Each lead has a positive and negative pole and views the heart and the cardiac vector from a slightly different position. In most cases a hexaxial lead system (Bailey’s hexaxial lead system) will provide an ECG of diagnostic quality. This combines three bipolar limb leads (leads I. II and III) with three unipolar limb leads (aVR, aVL and aVF).
The main uses of an ECG are as follows:
- Evaluation of chamber enlargement due to hypertrophy or dilation. ECG changes which suggest chamber enlargement should always be interpreted in association with radiographic abnormalities.
- Detection of cardiac arrhythmias. An ECG may be helpful in determining the type, origin and severity of an arrhythmia.
- Evaluation of cardiac therapy, for example with cardiac glycosides or anti-arrhythmic drugs.
- Detection of electrolyte or metabolic disturbances.
- Evaluation of the prognosis and progression of a disease process.
- Anaesthetic monitoring during surgery.
An ECG is normally performed with the animal placed in right lateral recumbency on a rubber topped table or a blanket to prevent 50 cycles interference. With cats or any critically ill animal, especially those which are severely dyspnoeic, the ECG is best performed with the animal placed in sternal recumbency or in any position which the animal finds comfortable. The position of the animal should be taken into consideration when interpreting the ECG; sternal recumbency increases the amplitude of the P and R waves and may shift the mean electrical axis by as much as I0° to the left.
Continuous 24 hour ambulatory electrocardiography
A routine ECG records only a brief period of heart rhythm yielding information relevant only to the time of assessment. Certain types of arrhythmia such as paroxysmal ventricular tachycardia may only be precipitated by exercise or excitement and, therefore, may not be detected on a routine ECG. As a result, episodes of collapse, even if truly cardiogenic in origin, may remain undiagnosed because many cases appear clinically normal when examined. Continuous 24 hour ambulatory ECG monitoring circumvents this problem and provides a reliable and simple means of correlating clinical signs with disturbances of cardiac rhythm in the animal’s home environment.
In one study, continuous ECGs performed on a series of healthy dogs showed marked sinus arrhythmia with associated long sinus pauses (5-7 s in some dogs) in both brachycephalic and non-brachycephalic breeds during sleep. Intermittent ventricular premature complexes were also noted. Twenty-four hour ECG monitoring has proved useful for monitoring the progress and response to therapy in German shepherds with an inherited form of ventricular ectopy which may result in sudden death between 4 and 8 months of age.
The ECG and the cardiac cycle
The phases of the cardiac action potential are the result of sequential changes in cell membrane ion permeability and movement of ions into and out of the myocardial cells. Each phase of the cardiac cycle corresponds to a specific segment on the ECG trace. The P wave represents atrial depolarization; the right atrium contracts fractionally before the left atrium. Enlargement of the atria, particularly the left atrium, accentuates this asynchrony and results in widening and notching of the P wave. The QRS complex represents ventricular depolarization and the T wave ventricular repolarization. The first negative deflection on lead II is the Q wave which is produced by discharge of the middle and apical portions of the ventricular system. Depolarization of the left and right ventricular free walls produces the R wave (under normal circumstances, the R wave represents depolarization of the left ventricle which is thicker; the deflection produced by the thinner right ventricle is essentially cancelled out). The R wave is the first positive deflection of the QRS complex on lead II. The S wave is the negative deflection which terminates the R wave; it is often difficult to define accurately and does not consistently appear on every ECG. The P-R interval represents the delay in conduction of the sinoatrial impulse through the atrioventricular node.
Basic principles of ECG interpretation
Normal ECG parameters, the criteria used for documenting chamber enlargement, and the electrocardiographic features of the more common arrhythmias are given in Tables. ECG abnormalities should always be interpreted in conjunction with the radiographic findings. An ECG docs not differentiate between dilatation and hypertrophy.
The following questions should be answered before the ECG is analysed in detail.
- Is there a P wave for every QRS complex?
- Is the relationship of the P waves to QRS complexes constant? Slight differences in the duration of the P-R interval due to variation in vagal tone are acceptable, but should not exceed 0.01-0.02 s.
- Are all the P waves and QRS complexes similar? Altered configuration may indicate an ectopic focus or an abnormal (blocked) conduction pathway.
Ectopic impulses caused by foci of abnormal electrical discharge may originate outside the sinoatrial (sinoatrial) node in the atria, atrioventricular junctional tissue (atrioventricular node) or ventricles. Supra ventricular ectopic impulses result in normal or near normal QRS complexes and altered or absent P waves. Ventricular ectopic impulses result in distorted, often biphasic QRS complexes which are wider than normal and are not associated with a P wave.