The basics of electrocardiography (Proceedings)


The basics of electrocardiography (Proceedings)

Aug 01, 2010

Proper electrocardiography (ECG) starts with proper positioning. For a diagnostic ECG, the patient must be restrained in right lateral recumbency with the legs perpendicular to the body and parallel to each other. The patient should be placed on a pad or thick blanket to minimize noise that can interfere with the ECG signal. Electrodes can be attached by a multitude of methods. The most common and most economical electrode attachment available is the alligator clip. Because these can be painful to your patient, some will file the teeth. Another less painful use of the alligator clip is to wet the hair with alcohol and ultrasound gel, and clip the electrode on the hair next to the skin. This requires that the patient have sufficient hair to hold the clip, and it is sometimes noisier than desired for a proper ECG, but it is useful with sensitive patients. Another, more costly option is Grass® platinum electrodes. These are small needles placed subcutaneously. Typically, most patients do not react if they are new and sharp. We use these several times before discarding. Adhesive electrodes are also available, but not as useful in our hairy patients, and they must be discarded with each use. These can also be placed on the paw pads if your patient's feet are not hairy. For a diagnostic ECG, the electrodes should be placed distal to the elbow and knee, with four electrodes attached, one on each leg. Monitoring ECGs can be performed with the electrodes on the body wall, but they do not represent true vectors.

For a standard electrocardiogram, the limb leads—I, II, and III—are the most often utilized to evaluate rhythm and mean electrical axis. Lead I is a dipole from the right to the left arm, lead II is from the right arm to the left leg, and lead III is from the left arm to the left leg. Augmented limb leads aVL, aVR, and aVF are also useful in calculating the mean electrical axis (MEA). Most machines will only print a single lead at one time. If this is the case in your facility, print several seconds worth of each lead—I, II, III, aVR, aVL, and aVF—and then return to lead II to print a longer "rhythm" strip that is representative of your patient's arrhythmia. We are fortunate enough to have a delay feature on our machine that when activated, stores the previous six seconds of the ECG, so we can choose to print only the arrhythmia. This is especially useful with intermittent arrhythmias.

There are two types of ECGs—diagnostic and monitoring. The difference in the two is the type of filter used. Diagnostic ECGs incorporate low frequency filters that reduce wandering baseline but do not affect the waveforms. Monitoring ECGs filter high-frequency noise, especially associated with electrical interference. The second can also distort waveforms, and therefore, for a diagnostic ECG, the machine must be set to the low frequency filter. The patient must be positioned properly, the leads must be attached correctly, and the machine must be calibrated. If these criteria cannot be met, the ECG can only be used to assess rate and rhythm.

To start assessing your printed ECG, you must start by knowing how normal appears and what each part of the ECG represents. A normal ECG consists of a P wave, representing the initiation of the electrical impulse in the sino-atrial node. The electrical impulse travels through the atria to the atrio-ventricular node, enters the Bundle of His and spreads through the Purkinje fibers, causing the ventricular myofibers to contract. This is represented by the QRS complex. Finally, with the T wave, the ventricles repolarize, or regain the potential to be affected by an electrical charge. Next, measure the direction of the impulse that travels through the heart by calculating the mean electrical axis (MEA). The easiest way to do this is to find the isoelectric lead—the lead in which the positive and negative deflections are similar. Your MEA is the lead perpendicular to that. In dogs with normal hearts, the MEA is +40° to +100°, or caudal and to the left. In a normal ECG, lead II will be upright. In cats, the MEA can be anywhere from 0° to +160°. Changes in this vector can be caused by different chamber enlargements, as well as electrical disturbances in the heart.

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