A heart murmur doesn’t actually show up on an ECG. A murmur is a sound, detected by a stethoscope, caused by turbulent blood flow through or near the heart’s valves. An ECG, on the other hand, records only the heart’s electrical activity. These are two completely different types of information, so no ECG tracing will display a murmur itself. What an ECG can reveal are indirect clues about the underlying condition causing the murmur, such as thickened heart walls, enlarged chambers, or abnormal rhythms.
Why a Murmur Is Invisible on an ECG
Think of it this way: a murmur is the sound of blood whooshing through a narrowed or leaky valve, while an ECG is a recording of the tiny electrical signals that tell the heart muscle when to contract. An ECG shows the timing and strength of those signals as waves on a tracing. It has no way to detect sound or blood flow patterns. That’s why doctors use a stethoscope to hear a murmur and then order additional tests, like an echocardiogram (an ultrasound of the heart), to actually see what’s happening with the valves.
An ECG is still commonly ordered when a murmur is detected, though. It won’t confirm the murmur, but it can pick up electrical signatures of the structural problems that cause murmurs. Those signatures are what most people are really asking about.
What an ECG Can Show When a Murmur Is Present
When a valve problem forces the heart to work harder, parts of the heart muscle thicken or chambers stretch to compensate. These physical changes alter the electrical signals the heart produces, and those altered signals do show up on an ECG. The specific pattern depends on which valve is affected.
Aortic Valve Problems
Aortic stenosis, a narrowing of the valve that controls blood flow from the heart to the body, creates one of the most recognizable murmurs. Over time, the left ventricle (the heart’s main pumping chamber) thickens because it has to push blood through a tighter opening. On an ECG, this thickening shows up as taller-than-normal voltage spikes in the leads that look at the left side of the heart. In more advanced cases, the ECG also shows what’s called a “strain pattern”: the segment of the tracing just after the main spike slopes downward, and the following wave flips in the opposite direction from normal. This strain pattern is a sign that the thickened muscle is under significant stress.
Mitral Valve Problems
Mitral stenosis, a narrowing of the valve between the left atrium and left ventricle, forces the left atrium to work harder and eventually enlarge. This enlargement changes the shape of the P wave, the small initial blip on an ECG that represents the electrical signal traveling through the upper chambers. Instead of a smooth, single-peaked wave, it becomes broad and notched with two distinct peaks in lead II, a pattern sometimes called “P mitrale.” The total duration of the P wave stretches beyond 110 milliseconds, and in another lead (V1), the P wave develops a deep downward dip at its end. These are classic markers of left atrial enlargement, and mitral stenosis is the textbook cause.
Mitral regurgitation, where the valve leaks and allows blood to flow backward, often leads to a different ECG finding: atrial fibrillation. The leaking valve overloads the left atrium with extra blood volume, which can stretch it and disrupt its normal electrical signals. In one study of patients with severe mitral regurgitation, about 38% had atrial fibrillation or atrial flutter at the time of evaluation. On an ECG, atrial fibrillation replaces the normal P waves with a chaotic, irregular baseline, and the heart’s rhythm becomes unpredictable.
Pulmonic Valve and Right-Sided Issues
When the pulmonic valve (on the right side of the heart, directing blood to the lungs) is narrowed, the right ventricle thickens in response. The ECG clues here are essentially a mirror image of left-sided thickening. The heart’s electrical axis shifts to the right beyond 110 degrees, meaning the overall direction of electrical flow tilts away from its usual orientation. The R wave in lead V1 grows unusually tall (above 7mm), and deep S waves appear in the leads that monitor the left side of the heart. These findings together point strongly to right ventricular hypertrophy.
Congenital Heart Defects
An atrial septal defect, a hole between the heart’s upper chambers that’s present from birth, produces a murmur from the abnormal flow of blood between chambers. On an ECG, this defect commonly produces an incomplete right bundle branch block, where the electrical signal to the right ventricle is slightly delayed, widening the final portion of the QRS complex. A particularly distinctive finding is the “crochetage” sign: a small notch near the peak of the R wave in the leads that look at the bottom of the heart. When this notch appears alongside the bundle branch block pattern, the combination is highly specific for an atrial septal defect. Interestingly, about 35% of patients who had surgical repair of the defect lost the crochetage pattern afterward, even though the bundle branch block often persisted.
Innocent Murmurs Typically Show Nothing
Many murmurs, especially in children, are “innocent” or “functional.” They’re caused by normal blood flow that happens to be audible, not by any structural problem. Because there’s nothing wrong with the heart’s structure or electrical system, the ECG in these cases is usually completely normal. This is actually one reason the American Academy of Family Physicians cautions against reflexively ordering an ECG for every murmur. In children without symptoms or signs of heart disease, ECGs are not cost-effective and can even be misleading. One study found that ECG results were more likely to cause a murmur to be incorrectly classified as either innocent or pathological. In other words, the test sometimes added confusion rather than clarity.
The Test That Actually Shows a Murmur
If you’re looking for a test that directly visualizes what’s causing a murmur, that’s an echocardiogram, not an ECG. An echocardiogram uses ultrasound to create a real-time image of the heart’s valves and chambers. It can show a valve that doesn’t open fully, one that doesn’t close properly, or a hole between chambers. Doppler technology within the echocardiogram even lets doctors see the speed and direction of blood flow, effectively making the murmur visible as a color-coded jet of turbulent flow on the screen.
An ECG remains a useful first-line test because it’s fast, painless, and inexpensive. It can raise red flags that push a doctor toward ordering the echocardiogram. But on its own, it tells you about the electrical consequences of a valve problem, not the valve problem itself. If your doctor heard a murmur and ordered an ECG, the goal is to screen for those indirect signs of structural strain, not to see the murmur on the tracing.