The basics of electrocardiography (Proceedings)

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.

Assessing Rate

Begin by counting the heart rate on ECG. Start by determining the paper speed, generally 25 or 50 mm/sec. You can determine the instantaneous heart rate by counting individual boxes between similar points in the ECG, but the easiest way to assess heart rate is with a Bic® pen. They are always 10 cm long (with the cover on). Place the pen on the ECG, and count the number of complexes from one end of the pen to the other. If the rate is 25 mm/sec, multiply by 10. For 50 mm/sec, multiply by 20.

Abnormal Rhythms

Atrioventricular blocks, while not necessarily considered slow, are the most common bradyarrhythmias. AV blocks refer to delayed, intermittent, or total dissociation of the impulse from the SA node through the AV node. First degree AV block is a delay of conduction, and it is represented on the ECG as a prolonged PR interval. There are two types of intermittent, or second degree, AV block. The first is Mobitz I, or Wenckebach. In this type of second degree AV block, the P to R interval gradually lengthens until the impulse fails to conduct to the ventricles, and all that is seen is a P wave without a corresponding QRS. Mobitz type II second degree AV block is represented on an ECG by simply a P wave without a QRS. Third degree AV block is a complete dissociation between the SA and AV nodes. The QRS complexes occur independently from the P waves.

Sick sinus syndrome (SSS) is a group of arrhythmias. Characteristically, the beats appear to be normal sinus beats, but the SA node occasionally fails to fire completely. On the ECG, this is represented as a flat line. Patients with SSS can have long pauses that cause fainting. They can also have periods where the heart beats very fast and alternates between a fast rhythm and a slow one. This is called brady-tachy syndrome.

Another bradyarrhythmia is persistent atrial standstill. This condition is much rarer than any of the other bradycardias. The most striking feature is a lack of P waves along with a slow heart rate. Occasionally, the patient can have residual P waves, but atrial activity is not detectable on echocardiogram.

Supraventricular arrhythmias arise above the AV node. They appear similar to a normal sinus beat, in that they are upright in lead II and narrow. A single supraventricular beat is an extrasystole, and sounds like the heart is tripping. The most common of these is an atrial premature contraction. Several beats in a row are termed a paroxysm, and more than 30 seconds is sustained, and can cause symptoms.

One of the most common supraventricular arrhythmias is atrial fibrillation (AF). Typically seen with severe heart disease, it results from progressive enlargement of the atria. The impulse from the SA node is replaced by disorganized activity in the atria, and this leads to irregular conduction of impulses to the ventricles. With AF, the R to R interval varies and the baseline is noisy. The most common feature of AF, however, is the lack of a P wave.

Another, much more rare supraventricular arrhythmia is atrial flutter. The most typical feature of atrial flutter is a saw-tooth baseline. Each tooth in the saw is a P wave. Not every P wave conducts into a ventricular beat, so we say that the rhythm is a 3 or 4 to one block.

Ventricular arrhythmias propagate at or below the AV node. A single beat is also heard as a trip in the rhythm, an extrasystole. Ventricular premature contractions are typically wide, bizarre, and biphasic. Again, several ventricular beats fast and in a row are considered paroxysmal, and greater than 30 seconds of these beats is considered sustained ventricular tachycardia. The biggest danger of v-tach is the possibility that it will deteriorate into ventricular fibrillation. Like AF, the normal rhythm is lost, but unlike AF, if the ventricles lose their rhythm, the heart does not beat. On the ECG, this appears just as an irregular jagged line. This arrhythmia can respond to electrical defibrillation, but does so rarely in our patients.

Another rhythm that likes to mimic ventricular tachycardia is accelerated idioventricular rhythm, or AIVR. This rhythm is very common with systemic abdominal disease, and it is often seen after a splenectomy or GDV. It does not classify as a tachycardia because it is not fast. The heart rate is slightly faster than the patient's sinus rhythm, at about 180-200.

Veterinary technicians can be instrumental in diagnosing arrhythmias. We can be the first to listen to our patients, and notice if there is an abnormal beat. We are also important when it comes to diagnosing the arrhythmia. We can restrain our patients properly, and recognize when leads are not correctly connected. It is not necessary that we know what each arrhythmia is, but it is important to recognize when it varies from normal and when to call attention to it.