Electrocardiography

I. Potential Information to Be Gained from the ECG Includes:

1. Cardiac rate and rhythm - high sensitivity/specificity.
2. Cardiac chamber enlargement - low sensitivity/ specificity.
3. Ischemia, hypoxemia, electrolyte disorders and other extracardiac disorders - low sensitivity/specificity.

II. ECG of the Normal Neonate

1. Normal ECG values reported in thoroughbred and pony foals are shown in Table 1 and Table 2 [1,2]. Like adult horses, there is considerable variability in the QRS morphology and frontal plane mean QRS electrical axis of foals. Although there is some individual tendency for the electrical axis to evolve from a cranial rightward to caudal leftward orientation over the first few months, this is extremely variable and of little clinical value in distinguishing foals with significant heart disease.
2. Heart rate of neonatal horses varies from a relative bradycardia immediately after birth (65-70/min), to a relative tachycardia during the period of initial activity (120-130/min), to a resting heart rate of about 90-100/min in the first few days up to 2 weeks. Thereafter, heart rate gradually declines toward adult levels over several months.

III. Cardiac Enlargement

1. The P wave is usually positive or biphasic in leads I, II, III, and aVF. P waves greater than 0.4 mV or wider than 60 msec may indicate left and/or right atrial enlargement.
2. The QRS complex is quite variable and may be positive or negative in each limb lead, but is usually less than 1.5 mV in amplitude. QRS duration greater than 60 msec or amplitudes greater than 2 mV are probably abnormal and may indicate ventricular enlargement. With LV enlargement the QRS complexes tend to become mainly positive in leads I, II, aVF, and left precordial leads. With concentric RV hypertrophy the QRS complexes may be mainly negative in these leads.

IV. Cardiac Arrhythmias and Conduction Disorders [3].

1. The normal cardiac rhythm of foals is a sinus rhythm with minimal sinus arrhythmia.
2. Second degree AV block, which is commonly found in adult horses, is usually not present in foals. Escape beats due to atrioventricular dissociation are also uncommon in foals.
3. Cardiac arrhythmia recognition requires an ECG, usually using lead II or a base-apex lead to facilitate identification of P waves and QRS complexes.
4. >Premature contractions: supraventricular (atrial and junctional) premature beats are marked by a premature, but normal or near normal QRS complex, often but not always proceeded by an altered P wave. Ventricular premature beats are marked by a premature, wide and aberrant QRS complex not preceded by a P wave, but followed by a compensatory pause, with the next P wave occurring at the expected time.
5. Tachycardia: Sinus tachycardia is marked by normal complexes, including P waves, and often by periods of acceleration or deceleration. Atrial tachycardia is a type of supraventricular tachycardia characterized by a rapid ectopic atrial rhythm (sustained premature atrial complexes, PAC) and a normal QRS complex, with regular R-R intervals indicating a lack of direct involvement of the AV node. Atrial flutter is uncommon in foals and is characterized by abnormal atrial re-entry activity with distinct and continuous flutter waves (up to 300-400/min), normal QRS complexes (size, shape, frequency), but irregular R-R intervals. It may progress to atrial fibrillation. Atrial fibrillation is marked by absence of P-waves, variable baseline fibrillation waves, and normal QRS complexes with irregular R-R intervals (See Resuscitation Part 2 - Evaluation of Cardiovascular Status). Supraventricular tachycardia (atrial or junctional in origin) is marked by normal or near normal QRS complexes, P waves (when visible) differently shaped from P waves associated with initial pacemaker, and little variation in rate unless it is intermittent. AV nodal and junctional tachycardias have not been extensively studied in horses although they usually are characterized by a normal P-wave followed by a narrow or wide QRS. Ventricular tachycardia (VT) can be uniform or multiform depending on the shape of QRS complexes. VT is marked by wide, aberrant QRS complexes, capture or fusion beats, and AV dissociation. Torsades de pointes (twist of point) is a unique form of multiform ventricular tachyarrhythmia, where the electric axis of the heart and the shape of waves are rapidly changing. Ventricular fibrillation is the end stage of ventricular tachyarrhythmias and is characterized by the absence of P and T-waves and wide, low amplitude fibrillation waves as well as significant decrease in cardiac output followed by fainting, syncope, seizure and death.
6. Bradycardia: Sinus bradycardia is marked by normal complexes occurring at a slow rate. Causes include hypothermia, hyperkalemia, and other cardiac and extracardiac disorders. Other types of consistent or intermittent bradycardia include sinoatrial arrest, marked by a pause in the rhythm without P waves, and second or third degree AV block, marked by one or more non-conducted P waves between QRS complexes. Evaluation and management of these rhythm disorders should follow guidelines discussed in other texts.