A heart murmur grade, ranging from 1 to 6, describes how loudly turbulence can be heard with a stethoscope—not whether your cat has a real heart problem. The confusion arises because murmur loudness does not reliably predict disease severity; a faint murmur can signal serious disease, and a loud one may be harmless. If your veterinarian detects a murmur during a routine exam, the grade is just the first checkpoint. Tests like heart ultrasound and blood biomarkers are what actually determine whether the murmur reflects a structural heart condition or is benign and requires no treatment.
Table of Contents
- The 1-6 Murmur Grading Scale
- The Loudness-Severity Trap
- Tests That Actually Diagnose Heart Disease
- Interpreting Your Cat's Results
- What Should Happen Next
- Frequently Asked Questions
The 1-6 Murmur Grading Scale
A murmur's intensity is graded on a 1–6 scale based on loudness heard with a stethoscope. Grade 1 is barely audible in a quiet room, while Grade 6 can be heard without placing the stethoscope on the chest. Grades 2 through 5 represent intermediate loudness levels.
Grade 5 carries a physical sign called a "thrill"—a vibration you or your vet can feel with a hand placed on the chest. This sensation indicates significant turbulence but still does not confirm heart disease. The scale is simple: it measures what the ear detects, nothing more.
The Loudness-Severity Trap
Here is where murmur grades mislead. A Grade 1 or 2 murmur can hide serious heart disease, while a Grade 4 murmur may belong to a cat with a structurally normal heart. Hypertrophic cardiomyopathy (HCM), the most common feline heart disease, does not always produce an audible murmur at all; 31–62% of HCM-affected cats lack detectable murmurs. On the flip side, 25–33% of cats with low-grade systolic murmurs show no cardiac disease on echocardiography.
Many of these are innocent murmurs—benign turbulence heard in structurally normal hearts. The takeaway: grade is a signal to investigate, not a diagnosis. Louder murmurs (Grade 3 or higher) are significantly more likely to indicate cardiac disease than soft murmurs, but even higher grades require confirmation through imaging.
Tests That Actually Diagnose Heart Disease
Echocardiography (heart ultrasound) is the gold standard for determining if a murmur stems from actual heart disease. This painless, non-invasive procedure requires no anesthesia. With Doppler imaging, it reveals murmur location, identifies structural abnormalities, and assesses heart pump function and chamber size. If the echo shows a normal heart structure and function, the murmur is innocent—even if it was graded 5 or 6. A second tool, less definitive but useful as a supplement, is a blood test called NT-proBNP.
NT-proBNP is a biomarker released by stressed cardiac muscle; it shows 71% sensitivity and 92% specificity for detecting cardiomyopathy in asymptomatic murmuring cats. A normal NT-proBNP result makes undiagnosed heart disease less likely. However, this test cannot replace echocardiography—it supports, but does not replace, heart ultrasound.
Interpreting Your Cat's Results
If your kitten has a soft murmur and no other signs of illness, innocent murmurs in kittens often resolve by age 4–5 months and require no treatment; prognosis is excellent. Your vet may recommend a recheck exam in 2–4 weeks to confirm the murmur has faded. If your adult cat has a Grade 2 murmur and an echocardiogram shows a structurally normal heart, that murmur is innocent.
Your cat needs no medication and no activity restriction. If, however, the echo reveals thickened heart walls or other signs of HCM or valve disease, your vet will discuss monitoring and, if symptoms develop, treatment options. A Grade 4 or 5 murmur combined with an abnormal echo is more urgent and typically warrants closer monitoring, periodic follow-up ultrasounds, and medication to support heart function.
What Should Happen Next
A single heart murmur detection warrants full cardiac investigation in asymptomatic cats, including murmur re-examination, echocardiography, and possibly biomarker testing. Do not assume a soft murmur is harmless or a loud murmur is serious—ask your vet for an echocardiogram. If echocardiography is delayed or unavailable, NT-proBNP can provide interim reassurance, but it is not a substitute for ultrasound. Discuss timing with your vet, especially if your cat is showing any signs like lethargy, coughing, or difficulty breathing—these warrant urgent evaluation.
Once testing is complete, follow your vet's recommendation for follow-up. Innocent murmurs need no further action. Cats diagnosed with early HCM or other cardiomyopathies benefit from periodic monitoring before symptoms arise, which can significantly extend quality of life.
Frequently Asked Questions
Can a Grade 1 murmur turn into something serious?
A Grade 1 murmur itself does not progress or worsen. However, an underlying heart condition that produced a quiet murmur could progress. This is why echocardiography is essential—it clarifies whether the murmur reflects a condition that needs monitoring.
My kitten's murmur was detected at 8 weeks. Should I be worried?
Not necessarily. Many kittens have innocent murmurs that disappear by 4–5 months. Your vet will likely recommend a recheck in a few weeks to confirm it has faded. If it persists, echocardiography can clarify its cause.
Is a normal NT-proBNP enough to rule out heart disease?
NT-proBNP is a helpful screening tool, but echocardiography remains the definitive test. A normal NT-proBNP makes undiagnosed heart disease less likely, but it does not replace ultrasound imaging of the heart structure and function.
What if the echo is normal but the murmur is loud?
A loud murmur with a normal echocardiogram is innocent. No treatment or restrictions are needed. Your vet may recommend periodic rechecks if the murmur is new or changing, but most innocent murmurs remain stable.