Aortic Stenosis vs Mitral Regurgitation: Telling Two Systolic Murmurs Apart
Aortic stenosis (AS) and mitral regurgitation (MR) are both left-sided valvular lesions that produce a systolic murmur and, over time, drive the ventricle toward failure. The core axis that separates them is the type of hemodynamic burden: AS is an outflow obstruction that pressure-overloads the LV, whereas MR is backward flow that volume-overloads both the LA and LV. This difference dictates the murmur, the physical findings, and the pattern of ventricular remodeling.
How to tell them apart
| Feature | Aortic stenosis | Mitral regurgitation |
|---|---|---|
| Valve mechanics | The aortic valve does not open fully, creating obstruction to LV outflow. | The mitral valve does not close properly, allowing blood to leak backward from LV into LA during systole. |
| Hemodynamic burden | Pressure overload of the LV, which must generate higher pressures to overcome the obstruction. | Volume overload of both the LA and LV from the regurgitant volume. |
| Ventricular remodeling | Concentric hypertrophy—sarcomeres laid down in parallel produce thick walls with a normal-size cavity, raising the relative wall thickness (h/r ratio) to normalize wall stress per the Laplace law. | Eccentric hypertrophy—sarcomeres laid down in series enlarge the chamber radius disproportionately; absolute wall thickness stays normal or is only modestly increased, so the relative wall thickness (h/r ratio) falls (genuinely thin walls appear only with late decompensation). |
| Chambers affected | Primarily the LV, which hypertrophies and later fails. | Both the LA and LV progressively dilate. |
| Murmur quality and timing | Crescendo-decrescendo (ejection) systolic murmur, harsh in quality; in severe AS it peaks later in systole. | Holosystolic murmur. |
| Murmur location and radiation | Best heard at the right upper sternal border and radiates to the carotids. | Best heard at the apex and radiates to the axilla. |
| Characteristic peripheral/exam signs | Pulsus parvus et tardus (delayed, diminished carotid upstroke), narrow pulse pressure, S4, sustained (heaving) LV apical impulse, and an absent, single, or paradoxically split S2. | Laterally displaced, hyperdynamic apical impulse and an S3 gallop reflecting the volume-loaded, dilated LV, often with a soft S1. |
| Course of decompensation | Hypertrophied LV becomes stiff (diastolic dysfunction) then weak (systolic dysfunction); symptoms emerge when compensation fails. | Progressive LA and LV dilation, atrial fibrillation, pulmonary hypertension, and eventual LV dysfunction. |
The reasoning
Anchor on the murmur. A harsh crescendo-decrescendo ejection murmur at the right upper sternal border radiating to the carotids, accompanied by pulsus parvus et tardus, narrow pulse pressure, S4, a sustained LV heave, and a soft or single S2, defines aortic stenosis. A holosystolic murmur at the apex radiating to the axilla, with a laterally displaced hyperdynamic apical impulse and an S3, defines mitral regurgitation. The remodeling pattern arbitrates any doubt: AS pressure-overloads the LV into concentric hypertrophy (raised relative wall thickness), while MR volume-overloads the LA and LV into eccentric hypertrophy (dilated cavity with a fallen h/r ratio). Echocardiography then quantifies severity—valve area and gradient in AS, regurgitant flow and chamber dilation in MR—and guides the timing of valve replacement.
Key tests
- Echocardiography: in AS, shows a reduced valve area and elevated mean transvalvular gradient with concentric LVH; in MR, shows the regurgitant jet from LV to LA with LA and LV dilation. Echo is urgent when a new systolic murmur appears after MI (papillary muscle rupture causing MR).
- Cardiac auscultation/physical exam: an ejection murmur at the right upper sternal border radiating to the carotids with pulsus parvus et tardus and narrow pulse pressure points to AS, while a holosystolic apical murmur radiating to the axilla with a displaced, hyperdynamic apex and S3 points to MR.
- Exercise testing in asymptomatic severe AS: objectively unmasks 'symptom-free' status, exertional symptoms, or an inadequate blood-pressure response that would indicate the need for intervention.
What they share
- Both are left-sided valvular lesions that generate a systolic murmur.
- Both impose chronic hemodynamic stress that remodels the ventricle and can culminate in LV systolic dysfunction and heart failure.
- Both may ultimately require valve replacement, using either a mechanical valve (lifelong anticoagulation) or a bioprosthetic valve.
- Echocardiography is the pivotal study to define severity in each.
Pitfalls
- A new systolic murmur after MI is an emergency: MR from papillary muscle rupture is heard at the apex radiating to the axilla and demands urgent echocardiography and immediate surgical consultation—do not dismiss it.
- Vasodilators such as nitroglycerin and nitroprusside are contraindicated in aortic stenosis; reflexively vasodilating a decompensated AS patient is dangerous.
- Patients with severe AS often unconsciously limit their activity and deny symptoms—do not equate 'no complaints' with mild disease; use exercise testing to objectively confirm true asymptomatic status.
- Do not mistake the eccentric hypertrophy of MR for 'thin-walled' myopathy—absolute wall thickness is normal or modestly increased; genuinely thinned walls signal late decompensation, not the compensated volume-overload phase.
- Both lesions are systolic, so timing alone is misleading—use murmur location and radiation (carotids vs axilla) plus the pulse, apical impulse, and remodeling findings to separate them.
Practice this the way the exam tests it — on branching cases where your decisions shape the patient.