Occasionally, the mitral and tricuspid valves will close asynchronously, enabling you to detect a split S1. This finding may occur in normal large-breed dogs, may be associated with marked intraventricular conduction delays (i.e.
bundle branch blocks, artificial pacemakers, or ventricular arrhythmias), or may be heard in patients with mitral or tricuspid
valve stenosis.3 More commonly, S2 may be split into two audible components. Normally during inspiration, the aortic component of S2 precedes the pulmonic component because of the variations in the pulmonary vascular bed capacitance and an inspiratory increase
in right ventricular volume.2 So splitting of S2 may rarely be heard in normal dogs with large fluctuations of intrapleural pressure.4 More often, pathologic splitting of S2 is encountered when the pulmonic component is delayed because of prolonged right ventricular ejection in cases of pulmonary
hypertension, atrial or ventricular septal defects, pulmonic stenosis, or right bundle branch block.
Abnormal, transient heart sounds. While S1 and S2 represent the normal transient sounds audible in dogs and cats, we may also identify abnormal transient heart sounds such
as S3 and S4. Because of their low frequency, these sounds are usually best heard with the bell of the stethoscope and often impart a
galloping sound to the heart. Although this triple cadence is often referred to as a gallop rhythm, it does not represent an abnormality within the heart's conduction system or origin of the cardiac impulse. Instead, S3 and S4 gallops are most often associated with myocardial hypertrophy or elevated filling pressures.
An S3 gallop, also known as a ventricular gallop or protodiastolic gallop, is associated with the termination of the early, rapid ventricular filling phase of diastole.4 While this sound may be recognized in normal horses or cows, an audible S3 in dogs and cats is generally associated with the ventricular dilatation or eccentric hypertrophy recognized in dilated cardiomyopathy,
atrioventricular valve insufficiency, or substantial left-to-right shunting lesions. An S3 gallop is heard best over the left apex and should be listened for carefully with the bell of the stethoscope as it may be
the only auscultatory abnormality in patients with dilated cardiomyopathy.
S4 gallop sounds, also known as atrial gallops or presystolic gallops, are often heard when an augmented atrial contraction is required to fill a stiff, concentrically hypertrophied ventricle.
Similar to the low-frequency S3 gallop, this presystolic gallop is detected most readily with the bell of the stethoscope. S4 gallops may be the only auscultatory abnormality in patients with hypertrophic cardiomyopathy, systemic hypertension, and
hyperthyroidism, or they may be recognized in cases of semilunar valve stenosis. In cats with rapid heart rates, it may be
difficult to determine whether the gallop sound is an S3, an S4, or a summation of an S3 and S4. Nonetheless, the most important aspect is detection of the gallop sound, followed by an appropriate work-up.
While normal and abnormal heart sounds represent brief, transient auscultatory events, heart murmurs are prolonged auditory
vibrations produced when blood flows turbulently through abnormal communications between the cardiac chambers or through stenotic
or insufficient heart valves. They also occur subsequent to alterations in blood viscosity (e.g. anemia) or vessel diameter (the larger the vessel, the more likely blood flow is turbulent).
Figure 2. Characterizing Heart Murmurs
Heart murmurs are characterized by their timing and duration within the cardiac cycle, intensity (loudness), frequency (pitch),
configuration (shape), and location and direction of radiation.2 An overview of their characterization is presented in Figure 2.
Table 1. Heart Murmur Classification