Physical examination of the cat or dog with suspected heart disease provides valuable information that allows "short-listing" of differential diagnoses to facilitate appropriate diagnostic and treatment recommendations.

Normal Cardiac Auscultation

In a normal dog or cat, two heart sounds are audible. The first heart sound (S1) is attributable to closure of the mitral and tricuspid valves at the onset of systole. The second heart sound (S2) is audible at the completion of ventricular systole and is attributable to closure of the aortic and pulmonic valves. Third and fourth heart sounds should not be appreciated in a normal dog or cat.

Heart Murmurs

A heart murmur is defined as a prolonged series of auditory vibrations emanating from the heart or blood vessels, most commonly attributable to turbulent blood flow.¹ In a normal heart, blood flow is characteristically laminar and hence silent. When heart disease produces a heart murmur, it is because dynamic or fixed pathology disturbs laminar blood flow and creates turbulence. In some cases, turbulent blood flow not only produces a heart murmur, but also a thrill that is palpable on the chest wall.

Heart murmurs are routinely described by 1) intensity (loudness); 2) timing in the cardiac cycle; and 3) location. Accurate description of a heart murmur facilitates determination of its possible genesis, whether it be physiologic or attributable to cardiac disease. When attributable to cardiac disease, murmur characteristics reflect which underlying cardiac disease may be present.

Murmur Description

1.  Intensity refers to the loudness of a murmur. Murmur intensity or "grade" is defined below.¹ If murmur grade varies, as it often does in cats, the term "dynamic" is used and the grade is given as a range. For example, a dynamic grade I–III/VI murmur describes a murmur that has an intensity that varies throughout the auscultation period, from difficult to hear (grade I) to immediately identifiable (grade III).

2.  Timing in the cardiac cycle.
A systolic murmur is one that is heard during ventricular systole; that is, between the first (S1) and second (S2) heart sounds. A diastolic murmur is heard during ventricular diastole; that is, immediately after S2. Murmurs may also be described as continuous; that is, a murmur is continuously present throughout the cardiac cycle. In dogs and cats, isolated diastolic murmurs are very rare.

3.  Location of the murmur.
The point of maximal intensity of the murmur is described as left or right, apical or basilar. The left apex is directly over the region of the mitral valve. The left base is the region of the pulmonic and aortic valves. The right apex is the region of the tricuspid valve. Describing a murmur more specifically as "pulmonic" or "aortic" based on identification of a left basilar murmur assumes a degree of accuracy that is unreasonable given the intimate proximity of these anatomical sites.
For cats, the majority of systolic murmurs are attributable to dynamic right ventricular outflow tract obstruction (DROVOTO) or systolic anterior motion (SAM) of the mitral valve with hypertrophic cardiomyopathy (HCM). The mechanism by which these murmurs are generated precludes their localization at the base or apex. Moreover, the small feline heart makes isolation of a murmur to the apex or base challenging even for those cats with fixed anatomical defects. As such, heart murmurs in cats are generally described as being located on the left or right parasternum or sternum.

Cardiac Auscultation Technique in Dogs and Cats

1.  Place palms over the left and right thorax to identify if a thrill is present.

2.  Use palms to locate the apex beat (the place where the heartbeat feels strongest, on the left thoracic wall).

3.  Place the diaphragm of the stethoscope over the apex beat and listen.

4.  Count the heart rate.

5.  Evaluate heart rhythm: regular, regularly irregular, irregularly irregular?

6.  Assess synchronicity of the right femoral pulse with the heartbeat.

7.  Determine if a murmur is present. If so, consider the intensity and timing. Then, gradually move the stethoscope craniodorsal (over about 2 rib spaces) and identify how murmur intensity changes. If murmur gets softer, the location of the murmur is apical; if murmur gets louder, the location is at the base.

Interpreting Heart Murmurs

The location of a murmur indicates the anatomical origin of the turbulent blood flow, while the timing of the murmur indicates the functional mechanism by which the turbulence is generated.

Example #1. A left apical systolic murmur. Here the location of the murmur at the left apex indicates that turbulent blood flow is associated with the mitral valve. The murmur is audible during systole, a time in the cardiac cycle when the mitral valve should be closed. Hence, a murmur that is audible at this location during systole is indicative of mitral valve insufficiency. This mitral insufficiency may be attributable to a limited number of diseases: myxomatous mitral valve degeneration, dilated cardiomyopathy (where the mitral valve apparatus is stretched and unable to coapt normally), mitral valve endocarditis, and congenital mitral valve dysplasia.

Example #2. A grade V/VI left basilar systolic murmur. Here the location of the murmur at the left base indicates that turbulent blood flow is associated with either the pulmonic or aortic valves. The murmur is audible during systole, a time in the cardiac cycle when the aortic and pulmonic valves should be open. Hence, a loud murmur that is audible at this location during systole indicates that either the aortic or pulmonic valve is stenotic and obstructing blood flow. Tetralogy of Fallot, of which pulmonic stenosis is a component, is also a possibility.

These two examples highlight the value of accurate murmur characterization. A clinically well dog with a left apical systolic murmur attributable to myxomatous mitral valve disease may have a long preclinical phase and may or may not develop pulmonary oedema. However, a clinically well dog with a left basilar systolic murmur attributable to severe pulmonic stenosis is at risk for syncope/sudden death, and intervention (i.e., balloon valvuloplasty) may prevent development of clinical signs and allow normal longevity.

A list of differential diagnoses based on murmur characterization is presented in tabulated form at the end of these notes.

Mucous Membranes

Capillary refill time (CRT) reflects the time taken for the capillary bed to be reperfused once it has been blanched by digital pressure, and it is a crude means of evaluating tissue blood flow. Dogs and cats with cardiac disease that severely and acutely compromises cardiac output (e.g., pericardial effusion, rarely dilated cardiomyopathy) can have prolonged CRT, but for most animals it is maintained even in the presence of advanced disease.

Cyanosis is a very insensitive means of detecting hypoxaemia. It develops only when arterial oxygen saturation falls below 80–85%, so identification of normal pink mucous membranes does not exclude significant hypoxaemia with pulmonary oedema.

Femoral Pulses

Femoral pulses should be evaluated by applying digital pressure sufficient to occlude arterial flow and then gently releasing pressure until maximal pulsation is felt. This pulse pressure is the difference between systolic and diastolic pressure. A "weak" pulse due to reduced pulse pressure can be attributable to reduced systolic pressure (e.g., low cardiac output with heart disease, hypovolaemia, haemorrhage). A "bounding" pulse may be attributable to increased systolic pressure (e.g., sympathetic stimulation) or reduced diastolic pressure (e.g., patent ductus arteriosus).

Thoracic Auscultation

In a normal cat or dog, respiratory rate is typically < 30 breaths per minute (bpm) at rest, and soft bronchovesicular sounds can be appreciated on thoracic auscultation.²

For cats and dogs with cardiogenic pulmonary oedema, respiratory rate is typically elevated (> 50 bpm). Crackles, which are the sounds created when collapsed airways open during inspiration, may be appreciated with moderate to severe oedema but can be difficult to auscultate because tachypnoea can preclude tidal volumes sufficient to open collapsed terminal airways.

For cats with pleural effusion, respiratory rate may or may not be elevated. It is not uncommon for cats with pleural effusion to have respiratory rates of < 40 bpm; however, cats with severe pleural effusion will have increased respiratory effort with a prolonged inspiratory phase. Breath sounds are absent ventrally, and for compliant cats, percussion of the ventral thorax is dull.

Cardiac Auscultation of the Cat

Heart murmurs in cats are very common with several studies identifying that roughly one-third of overtly healthy adult cats have heart murmurs.^3-5 These murmurs are most commonly dynamic in nature, whereby the intensity of the murmur varies with sympathetic tone. Typically the murmur gets louder with provocation such that the higher the heart rate, the louder the murmur, and the murmur can disappear entirely when sympathetic stimulation is attenuated and heart rate slows. As such, a murmur may be audible when the clinician first auscultates the patient, but the murmur may soften or disappear as the cat relaxes during the examination.

Murmurs will be attributable to heart disease for some cats (anywhere from 30–85%);^3-6 however, for a large proportion of cats, a heart murmur is "innocent." For those cats with heart disease as the cause for the murmur, hypertrophic cardiomyopathy (HCM) and systolic anterior motion (SAM) of the mitral valve is the most common diagnosis. Dynamic right ventricular outflow tract obstruction (DRVOTO)⁷ is the mechanism by which most "innocent" murmurs (i.e., not associated with underlying cardiac pathology) are generated in cats. It is important to not dismiss a murmur that is intermittent, since murmurs attributable to DRVOTO and SAM are both dynamic and are audibly indistinguishable.

Normal cardiac auscultation of a cat does not exclude the possibility of heart disease. HCM is identified by several studies in 16% of the overtly healthy adult cat population.^3,4,8 However, up to 70% of these cats do not have a murmur at rest.³ Furthermore, 30–40% of cats presenting with clinical signs attributable to heart disease do not have a heart murmur.^9,10

Gallop rhythms are identified in a small population of cats. Here, a fourth heart sound (S4) is heard in association with each sinus beat producing a regular "lub-dub-dub." The additional heart sound is typically generated by atrial contraction, which is necessary to augment ventricular filling in cats with diastolic dysfunction associated with HCM and other cardiomyopathies. This gallop rhythm is best identified with the bell of the stethoscope, and its presence is very suggestive of heart disease.

Heart murmur characterization and differential diagnoses

Heart murmur characterization and differential diagnoses (cont'd)

References

1.  Kittleson MD, Kienle RD. Small Animal Cardiovascular Medicine. St. Louis, MO: Mosby Inc.; 1998.

2.  Johnson LR. Clinical Canine and Feline Respiratory Medicine. Iowa: Blackwell Publishing; 2010.

3.  Paige CF, Abbott JA, Elvinger F, Pyle RL. Prevalence of cardiomyopathy in apparently healthy cats. J Am Vet Med Assoc. 2009;234:1398–1403.

4.  Wagner T, Fuentes VL, Payne JR, McDermott N, Brodbelt D. Comparison of auscultatory and echocardiographic findings in healthy adult cats. J Vet Cardiol. 2010;12:171–182.

5.  Cote E, Manning AM, Emerson D, Laste NJ, Malakoff RL, Harpster NK. Assessment of the prevalence of heart murmurs in overtly healthy cats. J Am Vet Med Assoc. 2004;225:384–388.

6.  Nakamura RK, Rishniw M, King MK, Sammarco CD. Prevalence of echocardiographic evidence of cardiac disease in apparently healthy cats with murmurs. J Feline Med Surg. 2011;13:266–271.

7.  Rishniw M, Thomas WP. Dynamic right ventricular outflow obstruction: a new cause of systolic murmurs in cats. J Vet Intern Med. 2002;16:547–552.

8.  Riesen SC, Kovacevic A, Lombard CW, Amberger C. Echocardiographic screening of purebred cats: an overview from 2002 to 2005. Schweiz Arch Tierheilkd. 2007;149:73–76.

9.  Fox PR, Liu SK, Maron BJ. Echocardiographic assessment of spontaneously occurring feline hypertrophic cardiomyopathy. An animal model of human disease. Circulation. 1995;92:2645–2651.

10. Rush JE, Freeman LM, Fenollosa NK, Brown DJ. Population and survival characteristics of cats with hypertrophic cardiomyopathy: 260 cases (1990–1999). J Am Vet Med Assoc. 2002;220:202–207.