Initially assessed by auscultation.

       Further definition with echocardiography is indicated.

       Murmurs are generated by turbulent blood flow often due to back flow through a heart valve or flow through an abnormal or persistent anatomic opening (e.g. a ventricular septal defect (VSD), patent foramen ovale).

       Left‐sided systolic murmursLeft AV (mitral) valve regurgitationUsually occurs as a holosystolic, soft blowing murmur.May be focal or radiate mildly to widely. PMI at the left heart base.Ventricular septal defect (VSD)Murmurs are pansystolic, sound quality may be anywhere from soft‐blowing to a coarse, rumbling murmur.May be focal or they may radiate widely.Usually loudest on right side but may be heard on the left; may be louder on the left if there is right to left shunting.Less commonly, a VSD may be located such that there is shunting from the left ventricle to the right ventricular outflow tract. The PMI with this type of defect is at the left‐third ICS, under the triceps muscle.Valvular endocarditisCan occur at any or a combination of heart valves.Left‐sided lesions are more common in the horse than right‐sided lesions.Murmurs are due to both altered blood flow passing by the lesion (anterograde) as well as regurgitant flow through the valve.The quality generally depends on the size and location of the valvular lesion(s).

       Left‐sided diastolic murmursAortic valve regurgitation (AVR)A very common condition in older horses, usually over 15‐years‐of‐age.This is classically a decrescendo murmur which may be soft to coarse in quality.The sound may be quite profound and has been termed a “dive bomber” murmur.Aortic valve endocarditis – similar to AVR.

       Right‐sided systolic murmursRight AV (tricuspid) valve regurgitationThe PMI is usually at the fourth ICS due to flow from the right ventricle to the right atrium.Typically, a holosystolic, soft, blowing murmur.Aortic valve regurgitationThe PMI is usually left‐sided, but the murmur is often heard to a lesser degree on the right side (see above).Ventricular septal defectThe murmur is usually loudest on the right side of the chest as there is shunting from the left‐to‐right side of the heart.The common defects are located high in the interventricular septum.Occurs most commonly in Arabian or Arabian cross‐breed horses, indicating that this breed is genetically predisposed to the condition.

       Continuous (systolic and diastolic) murmursPatent (persistent) ductus arteriosus (PDA)This type of murmur may be heard in the newborn foal but usually resolves in the first days‐to‐weeks of life.It may persist beyond this period; however, this condition is very uncommon to rare in horses.

       P wave – depicts atrial depolarization.Due to the horse's large atria, the P wave may be biphasic, or bifid (notched).

       The QRS complex follows the P wave, and it depicts ventricular depolarization.Figure 3.3 Normal sinus rhythm.Figure 3.4 Second‐degree atrioventricular blockade.The Q, R, and S wave are not always present.The T wave follows the QRS, and it depicts ventricular repolarization.The T wave may be positive or negative at rest.During exercise or stress, the T‐wave polarity is opposite to that of the QRS complex.Tall T waves may be mistaken for QRS complexes.

       Evaluation of the ECG should be performed in a systematic manner.

       Absence of a P wave indicates sinoatrial block.

       P waves may be absent or “hidden” in the following conditions.Hyperkalemia: No P wave, tall T waves and wide QRS complexes.Atrial fibrillation: No P wave and the rate is irregular, and f waves are present.Ventricular tachycardia: P waves hidden in QRS complexes, and presence of wide and bizarre QRS complexes.P wave may be “hidden” in the previous QRS complex during atrial or sinus tachycardia. The morphology of the QRS complex is normal.

       Second‐degree heart block (see Figure 3.4).Occasional absence of AV conduction.P wave is not followed by a QRS complex (see below Section IV, C).

       Third‐degree heart block.The condition is rare in horses.P waves not followed by a QRS complex.This is a “complete heart block” and atrial impulses are not conducted through the AV node.The condition is most likely related to pathology of the AV node.The HR is slow because the ventricles are contracting at their intrinsic rate (escape rhythm).P waves have normal morphology but have no relationship with QRS complex.The QRS complex can be normal or have a bizarre shape depending on its location of origin.

       Horses have a high incidence of arrhythmias compared to other domestic animals and this is due to their high degree of vagal tone.

        Physiologic arrhythmias usually resolve during exercise or excitement when there is an increase in sympathetic tone and/or a decrease in vagal stimulation.

       Conversely, some arrhythmias may be exacerbated by exercise.

       Sinus bradycardia is considered to be present when the HR is <24 beats/min, but the RR interval is regular.The normal HR is between 24 and 50 beats/min.

       Usually, bradycardia is due to increased vagal stimulation.Generally, disappears with exercise.

       HRs lower than 24 beats/minute may occur in a fit horse, but may also reflect cardiac disease, such as sinus abnormalities and/or infiltrative processes.

       Characterized by a resting HR > 50 beats/min.The RR intervals are normal (variations >20% are considered to be abnormal).The relationship between the P waves and QRS complexes is normal.

       May be due to excitement or exercise in the normal horse.

       The maximal HR is between 220 and 240 beats/min.

       Abnormal increases at rest occur with many extra‐cardiac as well as cardiac disease processes.Extra‐cardiac causes include external stimuli to the heart, and are often due to one or a combination of the following: pain, hypovolemia, toxemia, septicemia. The “drive” may be a need for increase blood flow.Tachyarrhythmias have many causes and may be multifactorial. The origin may be atrial (SA node‐related or extra‐nodal, such as atrial fibrillation), supraventricular tachycardia (SVT) or ventricular tachycardia (VT).

       Type 1 (Mobitz type1, Wenckebach phenomenon)This is the most common