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Veterinary Heart Murmurs

Why a Loud Murmur Does Not Tell You How Sick the Heart Is

Wait, What? A Tiny Defect Can Make a Loud Noise

A very loud heart murmur sounds alarming. It is tempting to assume that louder always means more severe heart disease.

That is not a safe universal rule. A small ventricular septal defect can generate a loud murmur because blood is accelerated through a narrow opening. Some animals with important myocardial disease can have only a soft murmur—or no murmur at all.

sound intensity ≠ disease severity by itself.

The Scientific Job

This manual owns one Veterinary World job:

How do veterinarians interpret murmur timing, location, intensity and quality together with the animal’s age, clinical state and cardiac measurements without treating loudness as a universal severity meter?

The RFE is: characterise the sound, infer the likely flow disturbance, measure whether the heart and circulation are actually affected, and choose the next test that most reduces uncertainty.

This page does not re-own normal cardiac electrophysiology or human cardiology. It owns veterinary murmur interpretation and its handoff to cardiac imaging and haemodynamic assessment.

Quick Answer

A useful murmur description includes:

timing — systolic, diastolic or continuous;
point of maximal intensity — where it is loudest;
radiation — where else it can be heard;
intensity — how loud it is under defined auscultation conditions;
quality and pitch — harsh, musical, blowing, high or low frequency;
configuration — how intensity changes during the cardiac cycle;
patient context — species, age, breed, heart rate, anaemia, fever, pregnancy, stress and exercise state.

Those features help generate hypotheses. They do not replace echocardiography, radiography, ECG, biomarkers or blood-pressure assessment when those are needed to measure cardiac structure and consequence.

Primary Entry — A Murmur Is the Sound of Disturbed Flow

Normal blood flow through the heart is usually quiet enough that valves closing dominate what the stethoscope detects. A murmur appears when blood flow or vibrating structures produce audible energy.

Merck describes most veterinary murmurs as vibrations generated by turbulent high-velocity blood flow, with less common musical murmurs arising from vibrating cardiac structures.

Explore Merck Veterinary Manual — Diagnosis of Heart Disease in Animals →

Part 1 — Timing Narrows the Valve and Vessel Question

A murmur’s position within the cardiac cycle is one of its highest-value coordinates.

Timing	What it means scientifically	Examples that may enter the differential
Systolic	Disturbed flow occurs while ventricles contract	Regurgitant valve flow, outflow obstruction, septal defect, innocent/physiologic flow
Diastolic	Disturbed flow occurs while ventricles fill	Aortic insufficiency and selected other lesions
Continuous	Pressure gradient persists across systole and diastole	Patent ductus arteriosus and other vascular communications

Timing does not name the disease by itself, but it removes many impossible explanations.

Part 2 — Location Is a Map, Not a Final Diagnosis

Veterinarians listen over standard areas of the thorax and note where the murmur is strongest. A left apical systolic murmur in a dog raises different possibilities from a right-sided murmur or a left basilar ejection murmur.

But sound travels through tissue. Thoracic shape, body size, lung inflation and the direction of a turbulent jet can move the audible maximum away from the exact anatomical lesion.

point of maximal intensity → localisation clue, not anatomical proof.

Part 3 — Why Murmur Grades Exist

Veterinary murmurs are traditionally graded from I to VI. The scale describes ausculted intensity and whether a palpable precordial thrill is present.

That makes grading useful for communication and longitudinal comparison—but not a universal biological severity scale.

Merck specifically notes that grades III and IV are generally not predictive of disease severity and gives examples where a small ventricular septal defect can be loud. In cats, murmur intensity cannot be assumed to track severity of cardiomyopathy.

Secondary Deepening — Velocity, Gradient and Orifice Size Interact

Why can a small opening be loud? A narrowed pathway can create a high-velocity jet. Higher velocity increases shear and turbulence, which can increase acoustic energy.

In simplified haemodynamics, pressure gradients across narrow jets are related to velocity. Echocardiography can estimate these gradients from Doppler measurements. The important conceptual point is:

small opening can create high velocity → high velocity can create loud turbulence → loud sound does not automatically mean large volume of disease.

Part 4 — A Quiet Murmur Can Also Mislead

Severe disease can reduce the conditions needed to generate a loud murmur. If ventricular pumping becomes weak, the velocity or pressure gradient driving turbulent flow may fall. Some cardiomyopathies affect muscle more than valves and may not generate conspicuous turbulence.

This reverses the novice assumption: less noise can coexist with more pump failure.

Part 5 — Innocent and Physiologic Murmurs Are Real

Young puppies and kittens can have soft innocent murmurs without structural heart disease. High-flow states such as anaemia, fever or pregnancy can also produce murmurs because blood velocity rises even when valves are anatomically normal.

Merck also notes that some healthy horses and cats can have murmurs without clinically important structural disease.

Explore Merck Veterinary Manual — Cardiovascular Abnormalities in Animals →

Part 6 — Auscultation and Echocardiography Answer Different Questions

Method	Main information
Auscultation	Timing, intensity, location, quality and rhythm clues
ECG	Electrical rhythm and conduction
Thoracic radiography	Heart silhouette, pulmonary vessels and evidence of pulmonary congestion
Echocardiography	Chamber size, valve motion, myocardial structure, Doppler flow and pressure-gradient estimates
Blood pressure	Systemic pressure load and perfusion context
Cardiac biomarkers	Evidence related to myocardial stretch or injury in appropriate contexts

A stethoscope hears the flow consequence. Echocardiography can inspect the structure and measure flow more directly.

Part 7 — A Murmur Is Not the Same as Heart Failure

Heart failure is a clinical syndrome in which the cardiovascular system cannot maintain appropriate circulation without abnormal filling pressures, leading to consequences such as pulmonary oedema, pleural effusion, ascites or poor perfusion depending on the mechanism.

An animal can have a murmur for years without heart failure. Another animal can have serious heart disease with little or no murmur.

murmur = acoustic finding; heart failure = physiological syndrome.

Part 8 — The Same Murmur Can Mean Different Things at Different Ages

A murmur in a young puppy raises congenital and innocent-flow possibilities. A newly detected left apical systolic murmur in an older small-breed dog raises acquired mitral valve disease more strongly. A murmur in a cat may require careful separation of dynamic flow, stress-related changes and cardiomyopathy.

Age, species and breed therefore change the prior probability before any advanced test is ordered.

JC Deepening — Murmur Interpretation Is an Inverse Fluid-Dynamics Problem

The veterinarian hears an acoustic output and works backward toward the hidden flow field that produced it.

sound waveform → timing/location/quality → candidate pressure gradient or regurgitant jet → structural hypothesis → Doppler measurement → cardiac consequence.

Several different lesions can generate similar sound. The inverse problem becomes better constrained when Doppler echocardiography, chamber size and clinical signs are added.

Part 9 — Why Disease-Specific Correlations Still Matter

Rejecting “louder always means worse” does not mean intensity is useless. In some specific diseases, murmur intensity correlates with the severity of regurgitation or obstruction.

The correct rule is narrower:

severity relationship must be validated for that lesion, species and physiological state.

Part 10 — Serial Examination Creates a Time Axis

A single murmur grade is a snapshot. Repeated examinations can reveal whether the sound, resting respiratory pattern, pulse quality, exercise tolerance, chamber size or other measurements are changing.

The useful clinical receipt is not merely “murmur still present.” It is whether measurable cardiac structure and function remain stable or progress over time.

How Do We Know?

Veterinary cardiology compares auscultatory findings with echocardiography, Doppler velocities, chamber dimensions, radiographs, ECGs, biomarkers and clinical outcomes. This allows researchers and clinicians to test when murmur features correlate with specific lesions and when they do not.

Observation vs Inference

Observation: a grade V systolic murmur with a palpable thrill is heard.
Inference: vigorous turbulent flow is present; exact lesion and physiological severity still need localisation.
Observation: a soft murmur is heard in a cat.
Inference: a cardiac-flow abnormality is plausible, but cardiomyopathy severity cannot be inferred from loudness alone.
Observation: Doppler shows a high-velocity jet across a narrowed outflow tract.
Inference: an obstruction-generated pressure gradient becomes much more strongly supported.

Evidence Boundaries

loud murmur ≠ severe heart failure.
soft murmur ≠ mild disease universally.
no murmur ≠ normal heart.
murmur grade ≠ valve-lesion size.
one auscultation ≠ stable disease over time.
heart enlargement ≠ congestive failure automatically.
educational cardiology ≠ diagnosis of an individual animal.

Common Misconceptions

Misconception	Better model
Grade VI means the worst possible heart disease.	Grade describes sound intensity, not universal physiological severity.
A murmur proves valve disease.	Outflow obstruction, septal defects and high-flow states can also create murmurs.
No murmur means no heart disease.	Myocardial and rhythm disorders can exist without obvious turbulent flow.
An echocardiogram is just a clearer stethoscope.	It measures structure and Doppler flow rather than sound alone.

Unfamiliar Transfer

Dog A has a very loud murmur from a small congenital defect but normal chamber size and no exercise intolerance. Dog B has only a faint murmur but marked ventricular dysfunction and pulmonary oedema.

Which dog has the more consequential cardiac problem? A strong learner refuses to rank them from murmur grade alone and asks for haemodynamic consequence, chamber response and clinical state.

Checkpoint Questions

What physical process produces most heart murmurs?
Why is murmur timing useful?
Why is point of maximal intensity only a localisation clue?
How can a small defect create a loud sound?
Why can serious myocardial disease be quiet?
How is a murmur different from heart failure?
What information does Doppler echocardiography add?
Why do species and age alter interpretation?
When can murmur intensity still be useful?
What is the RFE after a murmur is detected?

Answer key

Turbulent high-velocity blood flow or, less commonly, vibration of cardiac structures.
It locates disturbed flow within systole, diastole or both, narrowing mechanisms.
Sound radiates through the thorax and may not peak exactly over the lesion.
A narrow opening can accelerate blood and create high-energy turbulence.
Weak pumping can reduce turbulent velocity, and some myocardial diseases do not produce major valve jets.
A murmur is an acoustic finding; heart failure is a physiological syndrome.
It visualises anatomy and measures flow velocity/direction and pressure-gradient surrogates.
Different lesions and physiologic murmurs have different prevalence across species and life stages.
When a disease-specific relationship has been validated for that lesion and species.
Characterise the sound, localise the likely flow problem, then measure structure and physiological consequence.

Edge Science — Can a Digital Stethoscope Diagnose the Lesion?

Digital stethoscopes can record phonocardiograms, preserve frequency information and feed machine-learning classifiers. Algorithms may become useful for screening or for recognising recurring acoustic patterns.

But the RFE remains unchanged: an acoustic model only sees the signal captured at the chest wall. It cannot directly measure chamber enlargement, pulmonary oedema, blood pressure or microscopic myocardial disease unless those states have been separately validated against the sound pattern.

better sound classification ≠ complete cardiac state reconstruction.

Veterinary World Direction Graph

Veterinary heart murmurs → blood-flow physics → valve disease → congenital shunts/stenosis → ECG → echocardiography → cardiac imaging → blood pressure → heart failure physiology → exercise tolerance → veterinary critical care.

Normal sinoatrial-node physiology remains Living World. Human cardiology remains Medicine. This page owns interpretation of the animal-patient murmur as evidence.

Research Sources and Further Reading

Educational boundary: Collapse, breathing difficulty, marked exercise intolerance or suspected heart failure can require urgent veterinary assessment. This manual explains murmur science only and does not diagnose or treat an individual animal.

Teaching Guide for Parents, Tutors and Teachers

For the people who teach because somebody depends on them.

Begin with the contradiction: “Which is worse—a whispering murmur or a roaring one?” Do not accept “roaring” until the learner asks what lesion produced the sound and what the heart is actually doing.

hear → characterise timing/location → propose flow mechanism → measure cardiac structure → measure physiological consequence → revise severity judgement.

The mastery test is an unfamiliar case in which loudness and clinical severity disagree. A strong learner should recognise that the contradiction is not an error; it is evidence that sound intensity and system failure are different variables.

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eduKate Learning Manual: Veterinary Heart Murmurs | Why a Loud Murmur Does Not Tell You How Sick the Heart Is