Information acquired from an equine ECG is limited to the HR, rhythm and presence or abnormalities of electrical complexes. This is due to the rapid depolarization of the horse heart (ventricles) because of the almost complete penetration of nerve Purkinje fibers across the ventricular myocardium (Type II Purkinje system).
Lead placement – A base‐apex lead is commonly used in the horse. One method of applying the ECG leads for this is as follows:Negative lead (RA) White on Manubrium.Positive lead (LA) Black on Xyphoid.Ground lead (LL) Red on loose skin of neck cranial to the scapula.Note: Europe, RA red, LA yellow, LL green.
II Heart murmurs
Murmurs are a result of turbulent blood flow within the heart.
Murmurs may result from normal or abnormal blood flow.
A Location
The point‐of‐maximal intensity (PMI) and distribution over which the murmur is heard should be defined.
First defined as left‐ and/or right‐sided.
Left‐sided murmurs are further defined as to the location on the chest wall.
The murmur is further defined on how widely the sound is heard over the cardiac silhouette.Examples of this are very focal, focal, radiating, widely radiating.Very focal, valve‐associated murmurs may be detected at expected anatomic locations.
B Timing in cardiac cycle
The occurrence of the murmur in the cardiac cycle is identified as systolic, diastolic or continuous.
Many clinicians assess systole as the short portion of the cardiac cycle when the horse is at rest and diastole as the longer duration.Caution should be used because the systolic and diastolic periods may be equal in length or the diastole period shorter with excitement, exercise or disease.
Systole is readily identified if a cardiac impulse is palpated while auscultating the heart (common in foals or horses in thin body condition). If the cardiac impulse cannot be felt, simultaneous palpation of a peripheral pulse while auscultating the heart should be performed.An example of this is palpation of the transverse facial artery while listening to the heart. This may be difficult in larger framed horses whereby palpation of the radial artery at the medial aspect of the carpus while listening to the heart may be done.
Systolic murmurs should be further defined as to when the murmur occurs, i.e. throughout, early, middle or late.Holosystolic murmurs occur throughout systole and S1 and S2 are distinct from the murmur.Pansystolic murmurs overlie one or both S1 and S2.
C Intensity
Murmur intensity is categorized on a scale of 1 (very quiet) to 6 (very loud).
In general, soft murmurs (grade 1 and 2) are non‐pathologic while the louder murmurs are associated with pathology in the heart. However, there are exceptions to this rule‐of‐thumb.
Grade 1
Very soft, small area; the heart must be auscultated for several minutes to detect.
Grade 2
A faint murmur. The heart must be auscultated for a short period to detect.
Grade 3
The murmur is readily heard when auscultation begins. It is localized.
Grade 4
A loud murmur which is widespread (radiates), but there is no palpable thrill (vibration felt on the chest wall).
Grade 5
A loud, widespread murmur with a palpable thrill.
Grade 6
A very loud widespread murmur with a palpable thrill. The murmur can still be heard when the stethoscope is lifted slightly off of the chest wall.
Sounds
The murmur sound quality may be categorized as soft, moderate or coarse; other terms may include blowing, rumbling, musical or other such descriptor.
Sound contour may be described as band‐shaped (equal sound intensity over time), crescendo (gets louder), decrescendo (gets softer) or a combination of these.
D Physiologic (non‐pathologic) murmurs
Occur due to normal blood flow from the ventricles to the aorta and pulmonary artery.
This type of murmur is usually holo‐ or early‐systolic, soft blowing, band‐shaped murmur.
The PMI is at the left heart base (forth ICS).
Diagnosis is based on ruling out abnormal flow through the heart valves and heart defects with the use of ultrasonography.
E Pathologic murmurs
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
