4.1.4 Interpretation of an ECG
Evaluation of an ECG ('reading' an ECG) should be carried out in a thorough methodical manner. It is helpful if a consistent system is adopted so that errors are not made. A check list for assessing ECGs is shown in Table 4.2. The identification of specific arrhythmias is described in detail in Chapter 7. A typical normal ECG is shown in Figure 4.2. Identification of the waveforms and interval measurements are shown in Figure 4.3 and Table 4.3.
Heart rate is easily calculated by counting the number of complexes in a known period, for example counting the number within a 12 second interval and mulby 5. Alternatively, the rate per minute can be calculated by dividing 60 by the R-R duration in seconds. ECG paper is divided into small boxes (of 1 mm) and large boxes (of 5 mm). At a paper speed of 25 mm/sec, each small box represents an interval of 0.04 seconds, and each large box therefore represents 0.2 seconds. If accurate measurements of durations are required then a faster paper speed (50 mm/sec) aids measurement. Often the paper is marked with lines above the divisions on the trace which represent one second intervals when a paper speed of 25 mm/sec is used. Some ECG machines will mark one second intervals on the paper.
Mean electrical axis
Calculation of the mean electrical axis from the limb leads is of little clinical value in horses because the vector in the frontal plane is extremely variable and because the use of Einthoven's triangle is less appropriate in horses than in small animals (see section 1.5.5). From a practical standpoint, the amplitude of the complex is not a useful guide to heart size. However, the amplitude may be reduced if there is a pleural or pericardial effusion, and increased during exercise, or if the animal is excited, or if a complex has a ventricular origin.
It is important to be able to recognise normal complexes. P waves are small deflections of a few millivolts which are usually positive in a base-apex or Y lead recording. They are often notched and may have a positive and a negative component (biphasic). The QRS complex is usually a larger deflection (up to 3-4 mV in a base-apex lead). The first negative deflection is the Q wave, the first positive deflection is the R wave, and the second negative deflection is the S wave. The T wave follows and is very labile in size and orientation (see below).
The ECG should be examined to determine the duration of the intervals between complexes. The principal purpose of this process is to determine heart rate, although interval duration is also dependent on the conduction process and may be affected by autonomic tone, electrolyte levels and drugs. Unfortunately, the effects of electrolyte disturbances on wave morphology and interval duration are less predictable in the horse compared with small animals, therefore evaluation of the ECG is not a particularly useful method of assessing abnormalities in electrolyte levels.
It is important to be able to recognise artefacts so that they are not interpreted as abnormalities and, ideally, to be able to eliminate them. The most common artefacts are due to movement, either of the horse or of the leads or clips. Any such movement should be kept to a minimum. It is helpful to hold the leads close to the clips so that they do not swing. Movement artefact is particularly difficult to avoid when recording an ECG in an exercising animal. These artefacts are seen as sharp deflections which are haphazard but may occasionally resemble a QRS complex. Muscle tremor may produce a fine motion of the baseline of lower amplitude. Large undulations in the baseline may be caused by exaggerated respiratory motion. This may be less marked on a Y lead than a base-apex lead.
Interference from electrical mains hum is a common cause of artefact, espewith mains-powered ECG machines. This is apparent as regular waves at a frequency of 50 Hz. Mains interference can be reduced by ensuring that all the leads have good electrical coupling with the horse. A neutral electrode should be used. Standing the horse on a rubber insulating mat will help. Turning off other electrical appliances in the room, including fluorescent lights, also reduces the interference. The ECG machine and the mains supply should be properly earthed. Battery-powered machines pick up more interference when they are running low on power. If electrical artefact is still present, switch the ECG filter on. This is specifically designed to cut out high-frequency noise such as AC interference.
T-wave morphology is extremely variable and is particularly dependent on heart rate. Even beat to beat changes in the R-R interval often affect the size and polarity of the T wave. T-wave changes may also be seen in animals with cardiac disease, electrolyte disturbances and systemic disease. However, the changes are extremely variable and non-specific and they are therefore of little use in making a specific diagnosis. Some veterinarians have attached a lot of significance to T-wave changes. It has been suggested that these abnormalities are suggestive of myocarditis or heart strain, and that they are related to poor athletic performance. The pathophysiological basis of the condition described by the term heart strain is unclear, indeed it is difficult to establish what is meant by this term. T-wave changes may occur in myocarditis; however, since the T-wave changes are not specific and it is difficult to define a normal T-wave, in the author's opinion they are not helpful in the diagnosis of myocarditis. Recently, it has been shown that T-wave changes are a common finding in horses in training which have a normal performance record, although they are less commonly found in animals which are being rested.