Murmurs.   The Apex Beat

                                                  Murmurs

Murmurs are sounds produced by turbulent blood flow. Rapid flow, a wide vessel, low blood viscosity and an uneven or constricted vessel wall all predispose to cardiac murmurs. They can be physiological, for example high blood flow though the aortic outflow tract. Pathological murmurs reflect heart disease, for example degeneration and roughening of a valve surface. Veterinarians require a uniform method of describing murmurs to facilitate communication between each other via a common understanding. Five parameters have been developed that serve to describe all of the important aspects of a murmur. Of the five parameters, the most important ones are position in the cardiac cycle, intensity, duration and pattern of intensity. The point of maximal intensity (PMI) identifies the location where the murmur is heard loudest and is often described using the valve location nearest (e. g. Mitral valve area). On the following page is a table summarizing the parameters and their descriptions (Naylor, 2000). In dogs, systolic or continuous murmurs are more common than diastolic murmurs.

In describing the duration of murmurs, pan- refers to a murmur that obliterates both heart sounds either through systole or diastole. Holo- refers to a murmur that lasts throughout systole or diastole but does not obliterate any heart sounds. A continuous or machinery murmur lasts throughout most or all of systole and diastole and may or may not obliterate heart sounds. Early- and late- describe murmurs that are positioned closer to one heart sound than to another. Crescendo, decrescendo or diamond are terms that describe the intensity profiles of murmurs as increasing, decreasing or increasing and then decreasing in loudness. Musical and blowing are terms used to describe the frequency profile of a murmur. Grade refers to the absolute intensity of murmurs determined on a 6 point scale where the higher the grade the more severe the murmur (Example: Grade 2 versus a grade 5 regurgitant murmur).

Research shows that most clinicians correctly describe the grade of a murmur. Localization of the murmur to systole or diastole is less consistent. A clue is the timing of the heart sounds (systolic murmurs occur in the short pause), however loud murmurs can be perceived as being of longer duration than they really are (Naylor et al., In Press). Another useful method is to palpate the pulse during auscultation. Pan- or holo-systolic murmurs should be heard coincident with the pulse wave.

                   Problems and Strategies for Murmur Localization

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.