Canine Cardiology: A multimedia guide to cardiac auscultation
Content and Layout by Dr Jonathan M. Naylor, Diplomate ACVIM, Dr Anthony P Carr, Diplomate ACVIM, Mr Ryan E. Walker.
Funded in part by CONVINCE. This site is hosted by Vet Visions.
Other Learning Resources
Vet Visions also has complete audiovisual guides to auscultation in horses
and cattle. We produce a multimedia equine textbook, an
interactive instructional CD on passing a nasogastric tube in the horse which is proven to
improve learning and equine welfare, books on techniques in equine medicine
and handling horses, and a series of equine case simulations. Our
cattle books include Cattle Claw Care and a series of Calf Case Simulations.
Canine Cardiology
Contents:
The Components of a Stethoscope
How To Use a Stethoscope
Correctly Anatomy The Apex Beat
Cardiac Auscultation
Basic Cardiac Function
Audible Sounds Detected During
Aucultation
The First and Second Heart Sounds
The Third and Fourth
Heart Sounds Other Normal
Sounds
Normal Resting Heart Rate Values for Canine
Arrhythmias
Classification of
Arrhythmias
Normal Sinus Impulse Formation
Normal Sinus Impulse Formation
Altered
Supraventricular Impulse Formation
Disrupted Impulse Conduction
Murmurs
Problems and Strategies for Murmur
Localization
The Most Common Murmurs Afflicting Dogs and their Features
Tricuspid Regurgitation
Terminology
Diamond-Shaped Machinery Murmur Stenosis Information
The four basic principles required for successful auscultation are:
1. Correct use of the stethoscope.
2. Recognition of external landmarks that correspond to valve locations and
that due to acoustical effects within the heart, the locations where sounds are
heard loudest do not necessarily correspond to their anatomical source.
3. Correct differentiation, interpretation and understanding of normal versus
abnormal sounds. Studies have shown that both students and veterinary
practitioners can correctly describe the physical features of heart sounds,
however, their ability to correctly interpret these sounds is often lacking
(Naylor et al., In Press). This finding may be because traditional teaching
methods rely on a verbal description of an audible anomaly rather than actual
audio recordings. The authors of this website hope that with the multimedia
technologies used herein, specifically the pairing of actual cardiac audio
recordings with visual interpretation, individuals will improve their
diagnostic and interpretive skills. For further information on cardiology, you
may consult "Hearing Horse Hearts: An Interactive Guide to Equine Cardiac
Auscultation" (Naylor, 2000).
4. The ability to convey your understanding and interpretation of what you
heard to others using standard veterinary terminology.
The Components of a Stethoscope Top
Most stethoscopes are designed with a bell and a diaphragm. The diaphragm is
designed to pick up high frequency sounds and should be held firmly against the
skin. The bell is designed to amplify lower frequency sounds when applied with
light pressure. When using the bell, avoid applying firm pressure because the
skin beneath the bell will act as a diaphragm and negate the amplification of
low frequency sounds (Fox, 1988). The ear pieces should fit comfortably in your
ear canals. If the stethoscope comes with different sized ear pieces, select
the ear pieces that form the tightest seals in your ear canals. The tubing connecting
the binaurals to the bell-diaphragm may be single or double layered. Double
layered tubing eliminates more background noise.
How To Use a Stethoscope Correctly Top
For auscultation to be effective, it should be performed in a quiet room with
little extraneous noise. The stethoscope is most effective if:
1. When placed in the ears, the binaurals follow the same direction as the ear
canals (see diagram below).
2. The ear pieces fit snugly in the ears forming a tight seal and the hand
holding the stethoscope is still, relaxed and placed against the animal with a
constant amount of pressure.
Anatomy Top
The canine heart projects into both thoracic cavities, particularly the left,
from the third to the sixth intercostal space. The long axis of the heart is
rotated cranially so that it lies at an angle with the base more cranial than
the apex. The base of the heart is fixed by the great veins and arteries while
the apex can move freely within the pericardial sac. The so-called right and
left sides of the heart are more correctly understood to be the dextro-cranial
and levo-caudal sides because the left ventricle lies behind and slightly left
of the right ventricle. The left ventricle is more conical and massive than the
right ventricle which is more crescent shaped.
Landmarks
If the dog is standing square, much of the heart lies medial to the triceps
mass. A horizontal line drawn through the point of the shoulder lies slightly above
the level of the heart valves. As opposed to using features of the forelimbs
(e.g. the point of the shoulder and position of the olecranon) to locate heart
valves, palpation of the apex beat is more accurate because its position is
independent of the dogs forelimbs.
Internal landmarks for the heart valves largely rely upon their positions
relative to intercostal spaces and costochondral junctions. The following
guidelines (Tilley and Goodwin, 2001) may be helpful for auscultation:
The Apex Beat Top
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.
Palpation of the Apex Beat
Cardiac Auscultation Top
Cardiac auscultation should be performed in a quiet room free of excessive
noise. Cardiac auscultation should also be performed as soon as the animal
enters the exam room or when the dog is stressed since this increases the
probability that a transient or subtle murmur will be detected. The probability
of detecting a murmur increases with stress because sympathetic activation
increases heart rate, cardiac contractility and cardiac output. Turbulent flow,
which gives rise to murmurs, is more likely at higher blood velocities.
Cardiac auscultation should proceed in a logical manner. The apex beat (mitral
valve area) should be palpated and the heart rate measured either by cardiac
auscultation or palpation of the femoral pulse. The femoral pulses should be
palpated in each hindlimb and compared for fullness, sharpness and regularity.
Next the femoral pulse should be palpated simultaneously with cardiac
auscultation in order to detect pulse deficits due to arrhythmias. Each valve
should be ausculted in the order Mitral, Aortic, Pulmonic (acronym MAP). Some
palpate the apex beat (mitral valve area) and move cranially from there.
However, if you wish to auscult in a particular intercostal space it is easier
if you start counting spaces from the last rib (13th) cranially.
Basic Cardiac Function Top
The cardiac cycle consists of two phases: Systole (ventricular contraction) and
diastole (ventricular relaxation). At rest, systole occupies one-third of the
cardiac cycle while diastole occupies two-thirds of the cardiac cycle. The
normal rhythm (Poodle example) originates
from the sino-atrial node. When the rhythm is completely regular (the timing of
the heart sounds remain in a uniform, repeating cycle) it is called a normal
rhythm. Sinus rhythm refers to the normal rhythmic contractions of the heart
initiated after the sino-atrial node discharges. It may be completely regular
or the interval between beats may wax and wane. This is referred to as sinus
arrhythmia and is normal in dogs. An ECG (electrocardiogram) is required to
confirm the presence of sinus rhythms or arrhythmias. In the following table
the width of the columns represent the duration of the audible characteristics
of the cardiac cycle. All of the normal characteristics of the cardiac cycle
are represented in this movie (Normal
Heart example)
Audible Sounds Detected During Aucultation Top
Research has revealed that there are four main sounds produced during the
cardiac cycle of which only the first and second are normally heard in canines.
The third and fourth heart sounds are pathological if ausculted.
Normal
S1 - The first heart sound (lub) is the result of the closure of the left and right
atrioventricular valves (mitral or bicuspid and tricuspid valves respectively).
S2 - The second heart sound (dup) is the result of the closure of the pulmonic
and aortic (semilunar) valves.
Pathological
S3 - The third heart sound is the result of the addition of more blood into a
partially filled ventricle thus creating turbulence and sound waves.
S4 - The fourth heart sound is the result of atrial contraction. Although S4 is
labelled the fourth heart sound; if present; it will be heard at the start of
the cardiac contraction cycle.
The First and Second Heart Sounds Top
Listen to the recordings of isolated heart sounds and see if you can detect the
differences in duration, intensity and pitch. S1 (S1)
is slightly longer in duration and of lower pitch than S2 (S2). More reliable clues are the timing of the
sounds and that S1 is louder than S2 when you listen at the apex beat (Left4).
A Comparison of S1 and S2
The Third and Fourth Heart Sounds Top
In canines, the third and fourth heart sounds (S3 and S4) are not heard in
normal animals and their presence is an indication of pathology. The presence
of S3 is often associated with dilated cardiomyopathy (DCM) or chronic volume
overloads due to acquired mitral insufficiency while S4 is associated with
hypertrophic cardiomyopathy, pressure overloads (semilunar valvular stenosis)
or chronic hypertension (Fox, 1988). Since S3 and S4 are both low frequency
sounds, they are best heard with the stethoscope bell.
Qualities of S3 and S4
Other Normal Sounds Top
Other normal sounds include those sounds produced by the gastrointestinal tract
and the respiratory tract. Both gut and respiratory sounds are clearly
distinguishable from cardiac sounds. The gut
sounds will be variable and irregular in timing while the respiratory
sounds are consistent and regular in timing.
Artifacts
During cardiac auscultation you can hear additional sounds produced by movement
or the environment. In order to eliminate these sounds the location where
auscultation is being performed should be free of excessive noise, the dog
should be properly restrained and the vet should take care in handling the
stethoscope. To reduce the sound of hair
rubbing against the stethoscope, the dogs coat may be moistened with
alcohol over the target area.
Normal Resting Heart Rate Values for Canines Top
The "normal" heart rate for canines varies with the age, physical
size, breed, level of arousal and physical condition of the animal (Tilley and
Goodwin, 2001). Smaller dogs have faster heart rates than larger ones. Compare
the recordings from an adult Poodle with
that from a Greyhound, and use your watch
to practice taking the heart rate. As a general rule, clinicians will take the
heart rate over a period of 10 or 15 seconds depending on how tachycardic the
animal is. If you have difficulty counting the faster rate try counting in tens
and remembering every set of ten by extending a finger. The answers are here.
Arrhythmias Top
An arrhythmia or dysrhythmia is a deviation from the regular rhythm. In dogs
this may be normal or abnormal and may result from abnormal cardiac impulse
formation, conduction, rate or regularity.
Regularity
Regularity refers to the predictability of an arrhythmia. Some arrhythmias
occur in a predictable fashion and are said to be regularly irregular. These
rhythms may be normal (e.g. sinus arrhythmia) or pathological. In others the
onset of the next beat is completely unpredictable and the rhythm is said to be
irregularly irregular (e.g. atrial fibrillation). Irregularly irregular rhythms
are pathological in origin.
Classification of Arrhythmias Top
Tilley and Goodwin (2001) classify arrhythmias according to:
Origin
Supraventricular arrhythmias arise from the atria or AV node whereas
ventricular arrhythmias arise from the ventricles.
Rate
Arrhythmias with slow rates are bradyarrhythmias while those with fast rates
are tachyarrhythmias.
Regularity
Fibrillation is a rapid, irregular, chaotic rhythm while tachycardia is a rapid
but regular rhythm.
Normal Sinus Impulse Formation Top
Sinus Arrhythmia
Sinus arrhythmia is a regularly irregular sinus rhythm which is a normal
finding in most dogs (especially brachycephalic breeds). SinusArrhythmia is
characterized by slight variations in the S1-S1 interval. These variations are
related to changes in vagal tone to the heart and are often associated with
inspiration (negative pressure created in the thorax) or use of sedative or
anesthetic drugs. You can demonstrate sinus arrhythmia by palpating the radial
artery on your wrist. Once you feel your pulse take a big deep breath and you
should feel your pulse quicken and then slow down as you exhale.
Altered Sinus Impulse Formation Top
Sinus Bradycardia (Slow Heart Rate)
Sinus bradycardia has a regular rhythm and may result from systemic disease
(renal failure), toxicities, increased vagal tone, elevated intracranial
pressure or compression of the eyeball, hypothermia, hypothyroidism or drugs
(tranquilizers, propranolol, morphine, various anesthetics) (Fox, 1988). Sinus
bradycardia is diagnosed when the heart rate is less than 65 beats / minute and
an ECG shows sinus rhythm.
Sinus Tachycardia (Fast Heart Rate)
Sinus tachycardia; often caused by stress; is the most common arrhythmia
observed in dogs and has a regular rhythm. Sinus tachycardia may result if
there is increased metabolism and oxygen demand or increased requirement for
cardiac output (pain, fright, excitement), pathology (fever, shock, anemia,
hypoxia, hyperthyroidism) or pharmacological agents (atropine, epinephrine,
ketamine) (Fox, 1988). Sinus tachycardia is diagnosed when the heart rate is
less than 160 beats / minute for most dogs (>180 bpm for small / toy breeds
or >220 bpm in puppies) and an ECG shows sinus rhythm (Fox, 1988) .
Altered Supraventricular Impulse Formation Top
Atrial Fibrillation
Atrial fibrillation is a common
pathological arrhythmia in dogs (atrial
fibrillation example). Auscultable characteristics of atrial fibrillation
include a completely unpredictable rhythm, sometimes called a
"jungle-drums" rhythm. Listen for long diastolic pauses between some
beats and very short intervals between others. Sometimes the beats are so close
together that S2 is not generated and two S1 sounds follow each other. The
other hallmark of atrial fibrillation is a pulse deficit. Sometimes this can be
detected because there is a large disparity between the heart rate and the
pulse rate. If the heart beat is slow it is more reliably detected by
simultaneous auscultation and palpation of the pulse. Normally every S1 heart
sound is followed by a pulse wave. Abscence of a wave is called a pulse
deficit.
The most common causes of atrial fibrillation are chronic atrioventricular valvular
insufficiency in small breeds, dilated cardiomyopathy in large breeds, and
congenital heart defects. Less common causes include heartworm disease, cardiac
trauma, digitalis toxicity and severe metabolic disorders (Fox, 1988).
Auscultable or palpable characteristics of atrial fibrillation include
inconsistently filled femoral pulses, detection of an S1 without an S2 and a
pulse deficit.
Disrupted Impulse Conduction Top
Second Degree Atrioventricular (AV) Block
Second degree AV block may be of two types: Mobitz I, usually type A or Mobitz
II, usually type B. The two types of second degree AV block are best
distinguished by ECG. Mobitz I is a normal finding in dogs, especially in young
animals and disappears with exercise. Mobitz II is pathological in origin and
will not disappear with exercise. Both types of second degree AV block are
manifested by a dropped beat detectable during auscultation. By exercising and
immediately ausculting the dog, you can determine if the AV block is a Mobitz I
(the dropped beats have disappeared) or Mobitz II (the dropped beats are still
auscultable). Second degree AV blocks can be associated with sinus arrhythmia,
increased vagal tone, supraventricular tachycardia, electrolyte imbalances or
drugs (digitalis, intravenous atropine, xylazine) (Fox, 1988).
Murmurs Top
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 (diastolic murmur example).
In describing the duration of murmurs, pan
refers to a murmur that obliterates both heart sounds either through systole or
diastole but does not obliterate any heart sounds. Holo refers to a murmur that lasts throughout stystole or
diastole but does not obliterate any heart sounds. A continuous or machinery murmur (example) 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 and 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 Top
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 Top
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 (example)
(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
Patent ductus arteriosus (listen to a PDA) 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 Top
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.
Terminology Top
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
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 Top
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 when related to any one of the four heart valves refers to the
situation in which the valve fails to close properly. Blood flows against its
normal course producing turbulence and a murmur.
Machinery Murmur Top
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 manifested by a continuous murmur.
Plateau
Plateau describes an intensity pattern of a murmur that remains constant
through to completion.
Stenosis Top
Stenosis when related to any one of the heart valves, chambers or great vessels
refers to the situation in which the valve fails to open properly or the
chamber or vessel is abnormally narrow and the normal flow of blood is
hindered.
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.
Information Top
Other Learning Resources
Vet Visions also has complete audiovisual guides to auscultation in horses
and cattle. We produce a multimedia equine textbook, an
interactive instructional CD on passing a nasogastric tube in the horse which is proven to
improve learning and equine welfare, books on techniques in equine medicine
and handling horses, and a series of equine case simulations. Our
cattle books include Cattle Claw Care and a series of Calf Case Simulations.
References
Fox, P.R. 1988. Canine and Feline Cardiology. Churchill Livingstone Inc.
Naylor, J.M., Yadernuk, L.M.,
Naylor, J.M. 2000. Hearing Horse Hearts: An Interactive Guide to Equine Cardiac
Auscultation. Vet Visions Inc.
Tilley, L. P. and Goodwin, J. K. 2001. Manual of Canine and Feline Cardiology
3rd Ed. W.B. Saunders Company.
Copyright
This web site is intended as a resource for veterinarians and students. You may
link to the title page and we would be grateful if you would let us know by
E-mail when you do this. There is no charge. If you link to a page other than
the title page please acknowledge in the link that the page is part of Canine
Cardiology by J.M. Naylor, A.P. Carr and R.R.E. Wolker.
The content of this website, that is all text, graphics, tables, charts, audio
and video are copyright of Jonathan M Naylor and may not be downloaded in full
or part without the author's written permission.
Comments and suggestions:
Jon.Naylor@usask.ca
© J. M. Naylor 2001, 2009
A complete range of Vet Visions multimedia books and case simulations is available below:
Equine:
The Art of Equine Medicine
Passing a Nasogastric Tube in the Horse
Techniques in Equine Medicine
Handling Horses
Equine Case Simulations
The Art of Equine Auscultation
Bovine:
Cattle Claw Care
The Art of Bovine Auscultation
Calf Case Simulations