Dr. Jan Bright
The beating heart produces four distinct sounds that can be heard with a stethoscope. In small animal species and in people only two of these sounds can be heard with a stethoscope. The other two sounds are inaudible in these species but can be recognized and recorded with suitable electronic equipment. In large animal species all four heart sounds are often heard when listening with a stethoscope. The four normal heart sounds are designated S1 (first heart sound), S2 (second heart sound), S3 (third heart sound), and S4 (fourth heart sound). S1 and S2 are the two sounds that can be heard with a stethoscope in both small and large animal species.
The first two heart sounds are louder than S3 and S4 and are often expressed by the monosyllables “Lup-dup.” When the heart rate is sufficiently slow, the cardiac cycle is characterized as Lub-dup, pause, Lub-dup, pause, etc. Lub-dup is the systolic portion of the cycle, and the pause represents diastole. However, in animals with rapid heart rates there is very little pause.
The first heart sound, Lub, results from several factors: 1) closure of the atrioventricular valves (mitral and tricuspid valves), and 2) oscillations of the heart and large vessels due to changes in blood flow as a result of leaflet closure. Vibrations are transmitted through surrounding tissue to the chest wall where they can be heard and recorded. The first heart sound is low-pitched, dull, and relatively long. It’s intensity is affected by the force of ventricular contraction and the position (degree of opening) of the AV valve leaflets at the onset of systole.
The second heart sound, dup, defines the end of systole and is associated with closure of the semilunar valves (aortic and pulmonary valves). S2 is usually clear, sharp, and higher in pitch than S1. There may be slight asynchrony between closure of the aortic and pulmonary valves resulting in a splitting of S2 into two component sounds. However, because of the relatively high heart rates of small animals, it is not usually possible to distinguish a splitting of S2 in small mammals or birds.
The third heart sound (S3) is not normally heard with a stethoscope in dogs, cats, or rodents. This sound occurs during rapid filling of the ventricles (early diastole). If audible, therefore, it is heard immediately after S2. When the heart rate is sufficiently slow and S3 is audible, the cardiac cycle is characterized as Lub-dup-d, pause, Lub-dup-d, pause, etc. The third heart sound is thought to be due to vibrations of the ventricular walls as blood rushes into the ventricles.
S4 is also known as an “atrial contraction” sound, but it is important to point out that the contracting atrial myocardium is NOT actually producing vibration. It is the sudden acceleration of blood into the atrium as a result of atrial systole that results in vibration and, hence, sound. Because atrial contraction occurs in late diastole, the fourth heart sound, if audible, is heard immediately before S1. When the heart rate is sufficiently slow and S4 is audible, the cardiac cycle is characterized as b-Lub-dup, pause, b-Lub-dup, pause, etc. If all four heart sounds are audible and the heart rate is sufficiently slow, as is often the case in healthy horses, the cardiac cycle is characterized as b-Lub-dup-d, pause, b-Lub-dup-d, pause, etc. S4 will not be present in animals lacking an active atrial contraction. Therefore, S4 will not be present in patients with atrial fibrillation or atrial standstill. S4 may be heart without the other heart sounds in animals with second or third degree atrioventricular conduction block.
Heart murmurs are longer duration sounds than the normal heart sounds. Heart murmurs occur when blood flow deviates from laminar (streamlined) flow and becomes turbulent. The turbulence results in vibrations that are audible with a stethoscope as a murmur. Blood flowing at high velocity and large blood vessels promote turbulent flow. Decreased viscosity of the blood also promotes turbulence.
A murmur will be heard during that portion of the cardiac cycle during which the turbulent flow occurs. Therefore, determining the timing of the murmur provides important information regarding the possible cause of the murmur. Murmurs can be characterized according to whether the intensity of the murmur (loudness) changes or not (Figure 3). Murmurs that do not change in intensity are described as plateau or band-shaped murmurs. Band-shaped murmurs are typical of murmurs caused by AV valvular regurgitation or turbulent flow through a ventricular septal defect. Murmurs that increase in intensity are described as crescendo or ejection-type murmurs and are typical of semilunar valve obstruction and of some innocent murmurs. Murmurs that begin loud and fade are described as decrescendo murmurs. This type of murmur is typical for aortic valvular regurgitation. Murmurs are also characterized by their pitch. Most murmurs heard in veterinary patients are medium to high pitched murmurs, but occasionally a low pitched murmur (usually diastolic) is noted.