The horse is very reactive to autonomic input to the heart, and some normal horses will have resting HRs less than 24 beats per/min due in part to parasympathetic nerve input (vagal stimulation).
Heart rhythm
The heart rhythm is typically regular with even timing between beats. Slight irregularity may normally occur spontaneously or in response to the respiratory cycle.
Auscultation
Heart sounds are generated by the turbulence in blood flow resulting from closure of the heart valves. The valves do not have sufficient mass to generate sounds.
Auscultation on the left side is performed between the third and fifth intercostal spaces (ICS) on the lower third of the chest. Sounds associated with the aortic, pulmonic, and mitral valve closure are heard at the base of the heart, the mitral sound is located in a more caudal position.
Auscultation on the right side is performed between the fourth and fifth ICS on the lower third of the chest. Sound associated with closure of the tricuspid valve is heard at the base of the heart.
Left side heart sounds:
Mitral valve at fifth ICS, caudal to the elbow.
Aortic valve at fourth ICS, just dorsal to the elbow.
Pulmonic valve at third ICS, under the triceps muscle, just below the level of the elbow.
Right side heart sounds:
Tricuspid valve at fourth ICS, just under the triceps muscle.
Normal heart sounds (see Figure 3.1)
Two to four heart sounds may be heard in the horse.
S1 and S2 are usually louder than S3 and S4.
S1 may be “split” (two sounds) due to slight variation in contraction in the right and left sides of the heart (uncommon).S1 is associated with closure of the left and right AV valves.S2 is associated with closure of the aortic and pulmonic valves.S3 is associated with rapid ventricular filling.S4 is associated with atrial contraction.
ECG (see Section III below)
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