On the left side, the pulmonic and aortic roots lie next to each other and it is difficult to separate their respective valvular sounds. Both produce sounds that are best heard cranio-dorsally on the left side of the thorax at the second or third intercostal spaces. Since the aortic valve is more centrally located and produces louder sounds some aortic murmurs are also heard on the right side. Mitral valve problems produce sounds that are heard more caudally centered on the fourth or fifth intercostal space. On the right side, tricuspid and ventricular septal defects produce murmurs that are heard ventrally around the fourth or fifth intercostal space. A problem with localizing the origin of murmurs is that loud murmurs can radiate over a wide area and on both sides of the thorax. Despite this, the point at which they are loudest is often close to the lesion.
Sometimes it may prove challenging to correctly identify the likely origin of a murmur. Generally by following a logical process like the one outlined here, insight may be gained into the type of murmur being dealt with. First of all the stethoscope should be moved around to all the valve areas on each side of the thorax in order to ascertain where the PMI is located and which; if any; valve is involved. With the location of the PMI known the murmur's intensity may be accurately graded and the character and quality judged. Finally, by simultaneously ausculting the PMI and palpating the femoral pulse an accurate indication of the position and duration of the murmur within the cardiac cycle may be obtained. Additionally, note that by examining the animal as soon as it enters the exam room or when it is stressed, the probability of detecting a transient or subtle murmur increases because the intensity increases in accordance with the sympathetic effects of stress.
The Most Common Murmurs Afflicting Dogs and their Features
In order of prevalence:
Mitral Regurgitation
Mitral Reguritation; the result of mitral insufficiency; allows backflow of blood into the left atrium. Typical features of mitral regurgitation include a normal to increased arterial pulse, a PMI located at the left apex, a plateau or decrescendo quality and systolic position in the cardiac cycle (Fox, 1988). Mitral regurgitation is most often the result of acquired valvular disease (e.g. mitral valve endocardiosis) and is usually observed in older dogs.
Patent Ductus Arteriosus (PDA)
Patent ductus arteriosus results when the ductus arteriosus fails to close properly (functional closure normally occurs by 72 hours after birth while anatomic closure is complete within the first few weeks). PDA is therefore most commonly seen in young dogs with a higher prevalence in purebreds and females (Fox, 1988). This murmur will feature an increased arterial pulse, a normal to increased venous pulse, a PMI located at the left base and a machinery or continuous quality as it is present throughout most or all of systole and diastole (Fox, 1988).
Tricuspid Regurgitation
Tricuspid regurgitation; the result of tricuspid insufficiency; allows backflow of blood into the right atrium. Like mitral regurgitation, tricuspid regurgitation is most often caused by acquired valvular disease and is usually observed in older animals. Features of a tricuspid regurgitant murmur include an increased venous pulse, a PMI located at the right apex, a plateau or decrescendo quality and a systolic position in the cardiac cycle (Fox, 1988).
The following two diagrams represent the locations where specific cardiac pathologies will be auscultated best.
The Apex Beat
The apex beat is an impact vibration produced at the start of ventricular contraction as the heart hits the chest wall. In the normal dog it is palpated on the left side, ventrally in about the fifth intercostal space. The apex beat should be identified by palpation before the heart is listened to. It is important in lesion localization because the mitral valve lies close by and S1 is loudest at this point.
Terminology
Some of the common terms used in cardiology are:
Blowing
Blowing describes the frequency profile of a murmur in which there is no single predominant frequency.
Continuous Murmurs
Continuous murmurs occur throughout both systole and diastole and are often associated with patent ductus arteriosus (PDA).
Crescendo
Crescendo describes an intensity pattern of a murmur that increases as it progresses towards completion.
Crescendo-Decrescendo - See Diamond-Shaped
Decrescendo
Decrescendo describes an intensity pattern of a murmur that decreases as it progresses towards completion.
Diamond-Shaped
Diamond-Shaped refers to an intensity pattern of a murmur that first increases and then decreases towards completion.
Fibrillation
Fibrillation refers to the situation in which muscle tissue spontaneously enters a state of rapid, irregular and completely random contractions.
Flutter
Flutter describes a state of rapid, regular and uniform muscular pulsations or contractions in the range of 200-320 per minute.
Holo-
Holo- describes the duration of murmurs which last from the end of S1 to the beginning of S2 or the end of S2 to the beginning of S1 and these murmurs do not obliterate S1 or S2.
Insufficiency
Insufficiency refers to the situation in which a heart valve fails to close properly. Blood flows against its normal course producing turbulence and a murmur.
Machinery Murmur
Machinery describes a murmur that occupies most of systole and diastole and may be used interchangeably with continuous.
Musical
Musical describes the frequency profile of a murmur when there is a single prominent primary or fundamental frequency with secondary harmonics.
Pan-
Pan- describes the duration of murmurs which last from the beginning of S1 to the end of S2 or the beginning of S2 to the end of S1 and thus the murmur obliterates both of these heart sounds.
PDA (Patent Ductus Arteriosus)
PDA is a congenital condition characterized by the post-natal persistence of a lumen in the ductus arteriosus between the aorta and the pulmonary artery. PDA is often manifested by a continuous murmur.
Plateau
Plateau describes an intensity pattern of a murmur that remains constant through to completion.
Stenosis
Stenosis describes the situation in which a heart valve fails to open properly or the chamber or vessel is abnormally narrow and the normal flow of blood is impeded.
Thrill
Thrill is a vibration caused by turbulent fluid movement through an incompetent valve which is palpable on the thoracic wall. Thrill is typically observed with grade 5 or 6 murmurs.
Regurgitation
Regurgitation results with valvular insufficiency and is characterized by blood flow against its normal course. In canines mitral valve regurgitation is the most common regurgitant-type murmur that will be encountered.
VSD (Ventricular Septal Defect)
VSD is a congenital condition characterized by the persistent patency of the ventricular septum post-natally thus allowing blood to flow directly between the ventricles. Since the blood can bypass the pulmonary circulation cyanosis may be present in addition to a grade 5/6 systolic murmur.