The murmur of hypertrophic obstructive cardiomyopathy (HOCM) is a harsh, crescendo-decrescendo systolic sound typically heard best along the left lower sternal border and at the cardiac apex. What sets it apart from virtually every other heart murmur is its dynamic behavior: it gets louder or softer depending on what the person is doing at the moment, from standing up to bearing down to simply clenching a fist. That responsiveness to simple bedside maneuvers is what makes it one of the most clinically distinctive murmurs in cardiology, and understanding those changes is often the fastest route to suspecting the diagnosis before any imaging is ordered.
Where the Sound Comes From
In HOCM, part of the heart muscle, usually the interventricular septum, is abnormally thick. During each heartbeat, as the left ventricle contracts and pushes blood out through the outflow tract, the thickened septum narrows the path. At the same time, the front leaflet of the mitral valve can get pulled forward into the outflow tract, a phenomenon called systolic anterior motion (SAM). Together, these two things create a dynamic obstruction that generates turbulent blood flow, and that turbulence is the murmur you hear through a stethoscope.
The key word is “dynamic.” Unlike a calcified aortic valve that is stuck in one position all day long, the degree of obstruction in HOCM shifts from moment to moment depending on three variables: how full the left ventricle is, how hard it is squeezing, and how much resistance the blood faces when it leaves. Anything that shrinks the ventricle’s volume, increases its squeeze, or lowers the resistance downstream makes the septum and the mitral leaflet crowd closer together, the obstruction worsens, and the murmur gets louder. The reverse also holds. That is why body position and breathing patterns change the murmur so dramatically.
Bedside Maneuvers and What They Do to the Murmur
Clinicians have used simple physical maneuvers for decades to sort out what kind of murmur they are hearing. For HOCM, a handful of these maneuvers are especially telling because the murmur responds in the opposite direction from most other systolic murmurs. A landmark study of bedside diagnosis found that several of these maneuvers could distinguish the HOCM murmur from all other systolic murmurs with high accuracy.
Maneuvers That Make It Louder
The Valsalva maneuver, where you bear down as if straining on the toilet, reduces the amount of blood returning to the heart. With less blood filling the ventricle, the outflow tract gets narrower, and the murmur intensifies. In the bedside study, the Valsalva strain phase increased the murmur’s intensity with about 65 percent sensitivity and 96 percent specificity for HOCM, meaning almost no other murmur behaves this way.1PubMed. Bedside diagnosis of systolic murmurs That near-perfect specificity is what makes Valsalva such a useful tool at the bedside.
Standing up quickly from a squatting position has a similar effect. Squatting pools blood in the legs and increases resistance in the arteries, both of which fill the ventricle and reduce obstruction. When you suddenly stand, that extra filling drops away. The same study found that squatting-to-standing increased the HOCM murmur with 95 percent sensitivity and 84 percent specificity.2PubMed. Bedside diagnosis of systolic murmurs
Maneuvers That Make It Softer
Squatting down from a standing position does the opposite: it increases the blood returning to the heart and raises arterial resistance, expanding the ventricle and pulling the septum and mitral leaflet apart. The murmur fades. Sensitivity for detecting this decrease was 95 percent, with 85 percent specificity.3PubMed. Bedside diagnosis of systolic murmurs A case report documenting this in two patients with obstructive HCM confirmed that during a prompt squat, the murmur disappeared in one patient and dropped to a barely audible grade in the other.4PubMed Central. Squat in Obstructive Hypertrophic Cardiomyopathy
Passive leg elevation, where a patient lying down has their legs lifted by the examiner, also increases venous return and quiets the murmur, with about 85 percent sensitivity and 91 percent specificity.5PubMed. Bedside diagnosis of systolic murmurs And sustained handgrip, like squeezing a rolled-up towel hard for 30 seconds or more, raises the resistance the blood has to push against when it leaves the heart, which opens up the outflow tract. A study of nine HOCM patients found that during handgrip exercise the systolic murmur became inaudible or decreased in every single patient, confirmed both by ear and by phonocardiographic recordings.6PubMed. The effect of isometric exercise on the systolic murmur of patients with idiopathic hypertrophic subaortic stenosis In the broader bedside study, handgrip decreased the HOCM murmur with 85 percent sensitivity and 75 percent specificity.7PubMed. Bedside diagnosis of systolic murmurs
Telling HOCM Apart from Aortic Stenosis
Both HOCM and aortic stenosis produce systolic murmurs that can sound similar, and confusing the two is a genuine clinical trap. Both can cause a harsh, mid-systolic sound. But the maneuvers described above split them apart nicely: the murmur of aortic stenosis typically gets softer with Valsalva and louder with squatting, while HOCM does the opposite. That pattern alone resolves most diagnostic uncertainty at the bedside.
There is another, subtler difference involving premature ventricular contractions (PVCs), the occasional extra heartbeats that many people experience. After a PVC, the next normal heartbeat tends to be stronger because the ventricle had an extra moment to fill. In aortic stenosis, that stronger beat pushes more blood across the fixed narrowed valve, so the murmur gets louder after a PVC. In HOCM, the relationship is less predictable. A study comparing these responses found that 95 percent of patients with aortic stenosis or discrete subaortic stenosis had post-PVC murmur increases, versus only about 64 percent of patients with HOCM.8PubMed. The postextrasystolic murmur response to gradient in hypertrophic cardiomyopathy Among those HOCM patients who had substantial resting gradients, only two out of seven showed the expected increase. The study concluded that the systolic murmur in HOCM, unlike in aortic stenosis, does not track consistently with the outflow tract gradient.9PubMed. The postextrasystolic murmur response to gradient in hypertrophic cardiomyopathy This inconsistency makes the post-PVC response a less reliable sign for HOCM, but it also means a murmur that does not clearly get louder after a PVC should raise suspicion that you are not dealing with plain aortic stenosis.
What Happens When There Is No Resting Murmur
Not everyone with HOCM has a murmur at rest. Some patients have what is called latent or labile obstruction, where the outflow tract gradient only appears under certain conditions like exercise, dehydration, or a heavy meal. At a routine visit with the patient sitting quietly on the exam table, the stethoscope may reveal nothing unusual. This is one of the reasons HOCM can be missed on standard physical exams.
Provocative testing can unmask these hidden gradients. Amyl nitrite inhalation, which rapidly drops blood pressure and reduces the ventricle’s filling, has been used for this purpose. However, research has shown that the afterload reduction from amyl nitrite may not be comparable to a more natural stressor like exercise, and symptom-limited exercise testing tends to be more reliable for provoking latent gradients.10PubMed. Provocation of latent left ventricular outflow tract gradients with amyl nitrite and exercise in hypertrophic cardiomyopathy In current practice, exercise echocardiography has largely replaced chemical provocation for this purpose, since it reproduces the actual conditions under which symptoms occur.
What the Echocardiogram Adds Beyond the Stethoscope
Auscultation raises the suspicion, but echocardiography confirms the diagnosis and quantifies how severe the obstruction is. One of the distinctive findings on Doppler imaging is the so-called “dagger-shaped” signal. When continuous-wave Doppler is aimed across the left ventricular outflow tract, the tracing initially shows a convex contour reflecting early systolic flow before obstruction has kicked in. Then, as the mitral valve leaflet gets pulled forward and obstruction builds mid-systole, the contour transitions to a concave shape at an inflection point, producing a late-peaking, asymmetric envelope that looks like a curved dagger.11PubMed Central. Continuous-wave Doppler interrogation in valvular heart disease: pearls and pitfalls – Section: Shape of the aortic stenosis continuous-wave Doppler signal This is different from the more symmetric, rounded Doppler envelope you see in fixed aortic stenosis, and it is a strong visual clue to the dynamic nature of the obstruction.
Echocardiography also reveals the thickened septum, mitral valve SAM, and often accompanying mitral regurgitation, which can produce its own murmur directed toward the left atrium. In fact, many HOCM patients have two murmurs overlapping: the outflow tract obstruction murmur and a mitral regurgitation murmur. The mitral regurgitation typically produces a blowing sound radiating to the axilla, while the outflow obstruction murmur is harsher and heard closer to the sternum. Distinguishing the two by ear alone is difficult, but the maneuvers described earlier affect both in parallel since the same mechanism drives both.
How Treatment Changes the Murmur
Because the murmur is a direct reflection of the degree of outflow tract obstruction, any therapy that reduces the obstruction will also reduce the murmur. For decades, the first-line approach has been beta-blockers or the calcium channel blocker verapamil, both of which slow the heart rate, reduce contractility, and allow the ventricle more time to fill. By decreasing the force of each contraction, these drugs widen the outflow tract and quiet the murmur.12PubMed Central. The diagnosis and treatment of hypertrophic cardiomyopathy – Section: Results
A newer class of drugs, cardiac myosin inhibitors, has changed the treatment landscape. Mavacamten, approved in several countries starting in 2022-2023, works by directly dialing down the molecular motor that makes heart muscle fibers contract.13PubMed Central. The Diagnosis and Treatment of Hypertrophic Cardiomyopathy A meta-analysis of myosin inhibitor trials found that compared with placebo, these drugs reduced the resting outflow tract gradient by roughly 62 percent on average and the Valsalva-provoked gradient by about 54 percent, with patients being over three times as likely to improve by at least one functional class.14PubMed. The Efficacy of Cardiac Myosin Inhibitors Versus Placebo in Patients With Symptomatic Hypertrophic Cardiomyopathy: A Meta-Analysis and Systematic Review
The effect on the murmur can be dramatic. In a case series of patients with mid-ventricular obstruction treated with mavacamten, one patient’s grade 3 out of 6 systolic murmur dropped to a grade 2 after 16 weeks of treatment, and her outflow obstruction gradient was completely abolished.15PubMed Central. Efficacy of mavacamten in patients with hypertrophic cardiomyopathy and mid-ventricular obstruction: case series – Section: Case summary For clinicians tracking patients over time, a quieter murmur after starting therapy is one of the simplest bedside signals that the drug is working.
When Medication Is Not Enough
If drugs fail to control symptoms and the gradient persists, two invasive options exist. Surgical myectomy involves cutting away a strip of the thickened septum to physically widen the outflow tract. Alcohol septal ablation achieves a similar result by injecting alcohol into a small coronary artery that feeds the obstructing muscle, causing a controlled, localized injury that thins it over time. Both procedures require a high level of expertise at specialized centers. When performed successfully, they lead to sustained gradient reduction and clinical improvement, though septal ablation carries a 10 to 20 percent chance of needing a permanent pacemaker afterward due to conduction system damage.16PubMed Central. Hypertrophic Obstructive Cardiomyopathy After either procedure, the murmur typically diminishes substantially or disappears altogether, mirroring the reduction in obstruction.
The Murmur in Athlete Screening
HOCM remains the most common cause of sudden cardiac death among young athletes in the United States, and a systolic murmur detected during a pre-participation physical exam is sometimes the first and only clue.17Current Sports Medicine Reports. Cardiac Auscultation in Sports Medicine: Strategies to Improve Clinical Care The problem is that HOCM may or may not present with a typical murmur at rest, especially in young athletes whose hearts are well-conditioned and whose resting heart rates are slow. An athlete sitting calmly in a clinic may have minimal obstruction and no audible murmur.
This is why the dynamic maneuvers matter so much during screening exams. Having an athlete stand up quickly from a squat, or perform a Valsalva maneuver, can unmask a murmur that was silent at baseline. Any new or changing systolic murmur that gets louder with these provocations should prompt urgent echocardiography. Still, many athletes with HOCM slip through screening because their murmur is absent at rest and provocative maneuvers are not always performed during routine sports physicals. The debate over whether to add echocardiography or electrocardiography to standard screening continues in sports medicine, in part because of this limitation.
How the Murmur Was First Recognized
The clinical entity of HOCM came into focus in the late 1950s and early 1960s, though descriptions of unexplained cardiac hypertrophy in autopsy specimens date back further. A pivotal moment occurred at the National Institutes of Health, where the hemodynamicist Eugene Braunwald and cardiac surgeon Andrew Glenn Morrow encountered a patient who had a systolic murmur and a pressure gradient below the aortic valve, suggesting an obstruction that did not involve the valve itself.18PubMed Central. Storytelling of Hypertrophic Cardiomyopathy Discovery This was puzzling because, at the time, outflow obstruction was understood to be caused by valve disease. The discovery that the obstruction was muscular and dynamic, changing from beat to beat and with different physiological conditions, was genuinely novel. Morrow eventually developed the surgical myectomy procedure that still bears his name, and the bedside maneuvers that modulate the murmur became central to clinical teaching because they reflect the same hemodynamic principles Braunwald first mapped out in the catheterization lab.
The condition went through a confusing thicket of names over the following decades: idiopathic hypertrophic subaortic stenosis (IHSS), hypertrophic obstructive cardiomyopathy (HOCM), muscular subaortic stenosis, and others. The current convention uses “hypertrophic cardiomyopathy” (HCM) as the umbrella term, with “obstructive” added when a significant outflow gradient is present. But many clinicians and patients still use HOCM colloquially, especially when referring to the subset of cases where the murmur and gradient are the defining clinical features.