Mitral valve

What it normally does

• Two leaflets — a tall anterior leaflet and a broader, scalloped posterior one — coapt to seal the atrioventricular opening throughout systole. The left ventricle generates around 120 mmHg while the left atrium stays near 10 mmHg, so any gap in that seal drives blood backwards down a gradient of roughly 100 mmHg. That gradient exists from the first moment of ventricular contraction to the last, which is why a leaking mitral valve gives a murmur that occupies the whole of systole.

  mitral regurgitation is pansystolic and starts with (and can bury) the first heart sound — and because loudness tracks turbulence rather than regurgitant volume, a quiet murmur does not mean a small leak.

• In diastole the orifice opens to roughly 4–6 cm² in an adult, and the ventricle fills largely passively at a mean left atrial pressure of only about 8–12 mmHg, with atrial contraction adding the last fifth or so of filling volume. Forward flow across the valve happens only in diastole — systole is spent with the valve shut.

  anything that narrows the orifice, or shortens diastole (tachycardia), or removes the atrial kick (atrial fibrillation) forces left atrial pressure up, and that pressure passes straight back into the pulmonary veins and capillaries.

• The leaflets are tethered by chordae tendineae to two papillary muscles that contract with the ventricle and stop the leaflets everting into the atrium. The posteromedial papillary muscle is usually supplied by a single artery (the posterior descending, most often from the right coronary); the anterolateral muscle has a dual supply.

  rupture a chorda or infarct the posteromedial papillary muscle and the seal fails instantly, while chronic elongation of chordae lets the leaflet balloon backwards (prolapse) — the tethering apparatus, not the leaflet alone, is what fails.

• The left atrium is a thin-walled, low-pressure, compliant chamber. Loaded slowly it stretches and accommodates large volumes for years at modest pressure; loaded suddenly, before it has remodelled, it sits on the steep part of its pressure–volume curve. A chronically stretched, fibrosed atrium also fibrillates and forms thrombus, classically in the appendage.

  the same regurgitant volume gives years of compensated breathlessness when it arrives gradually and flash pulmonary oedema when it arrives in one beat — and it explains why atrial fibrillation and systemic embolism are the shared complications of both mitral stenosis and chronic mitral regurgitation.

• The leaflets are thin, largely avascular folds of endocardium over a collagen core, kept competent by intact commissures, pliable leaflet tissue and chordae of the right length. This tissue heals by fibrosis and calcification, not by regeneration.

  immune-mediated valvulitis, myxomatous degeneration and infective destruction all end in fixed scarring — so no drug restores a mitral valve, and definitive treatment is mechanical (balloon commissurotomy, surgical repair or replacement).

What goes wrong

• Acute rheumatic fever and rheumatic heart disease← from “The leaflets are thin, largely avascular folds…”

  The immune response to a group A streptococcal infection — classically pharyngitis, with skin infection also implicated in high-burden Australian settings — cross-reacts with valve tissue by molecular mimicry, both antibody and T-cell mediated, typically a couple of weeks after the infection. Acute valvulitis makes the leaflets swell and coapt poorly, so the first attack causes regurgitation, not stenosis. Each episode heals by fibrosis, and recurrent episodes add more, until the thin pliable leaflets and free commissures of the normal valve become thickened, fused and immobile — which is where stenosis comes from, years later.

  First attack leaks, repeat attacks scar and fuse: acute rheumatic carditis gives mitral regurgitation, chronic rheumatic disease gives mitral stenosis. Diagnosis is clinical criteria plus echo plus streptococcal serology, with a lower threshold in high-risk populations. Acute rheumatic fever and rheumatic heart disease are notifiable in the Australian jurisdictions running register-based control programs, and the register is what delivers secondary prophylaxis — which is the only intervention that changes the valve's trajectory.

  You would find: A child or young adult with fever, migratory large-joint arthritis and a new murmur. Under the Australian (high-risk population) modification of the Jones criteria, monoarthritis and polyarthralgia also count, because the classic migratory picture is often blunted by early analgesia. In Australia this burden sits overwhelmingly with Aboriginal and Torres Strait Islander peoples, especially in remote parts of the NT, WA, Qld and SA, where rates are among the highest recorded anywhere; Māori and Pacific Islander peoples living in Australia are also at increased risk. Acute rheumatic fever remains a clinical diagnosis backed by evidence of preceding streptococcal infection, usually serological (ASOT, anti-DNase B), because the throat swab is often negative by the time symptoms appear. RHDAustralia guidance is that every suspected case gets an echocardiogram, since carditis is frequently subclinical — and echocardiography is also what diagnoses established rheumatic heart disease. Sore throats and skin sores in these communities are treated, not watched.

• Mitral stenosis← from “In diastole the orifice opens to roughly 4–6 c…”

  Rheumatic scarring fuses the commissures and thickens and retracts the leaflets and chordae, so the 4–6 cm² diastolic opening narrows towards 1 cm². The atrium can now only push blood through by generating a pressure gradient across the valve, so left atrial pressure rises and is transmitted back into the pulmonary veins and capillaries; sustained elevation produces pulmonary hypertension and eventually right heart failure. The dilated atrium fibrillates, blood stagnates in the appendage, and thrombus forms.

  Severity is echocardiographic, not auscultatory: a valve area of about 1.5 cm² or less is severe, and the mean gradient is rate-dependent so a low gradient in a bradycardic patient does not exclude severe disease. A shorter interval between the second sound and the opening snap means higher left atrial pressure and tighter stenosis, and both the loud S1 and the snap disappear once the valve is rigidly calcified. Drugs buy diastole and dry the lungs; the valve is fixed by percutaneous balloon mitral commissurotomy in suitable pliable non-calcified valves without significant regurgitation or left atrial thrombus, otherwise by surgery.

  You would find: Exertional breathlessness progressing to breathlessness at rest, sometimes haemoptysis. A loud first heart sound, an opening snap after the second sound, and a low-pitched rumbling mid-diastolic murmur at the apex, heard with the bell, patient rolled into the left lateral position; a tapping apex, and with pulmonary hypertension a right ventricular heave and loud P2. An irregularly irregular pulse means atrial fibrillation — and the patient often decompensates on the day it starts, because the fast rate steals the diastole they depend on and they lose the atrial contraction that was supplying the last fifth of filling.

• Chronic mitral regurgitation← from “Two leaflets — a tall anterior leaflet and a b…”

  The systolic seal fails — prolapsing myxomatous leaflets or ruptured chordae (primary, degenerative disease), rheumatic scarring, or an annulus stretched and papillary muscles displaced by a dilated or ischaemic ventricle (secondary, functional disease). Every systole part of the stroke volume goes backwards into the atrium instead of the aorta. The atrium dilates, remodels and fibrillates; the ventricle volume-overloads and dilates eccentrically, and because it is ejecting partly into a low-pressure chamber it looks deceptively well for years before contractile function and forward output fall.

  In asymptomatic severe primary mitral regurgitation, surgery is indicated once the ejection fraction falls to 60% or below, or the left ventricular end-systolic diameter reaches about 40 mm — a 'normal-looking' 60% here is already a failing ventricle. Echocardiographic severity uses regurgitant volume, regurgitant fraction and effective regurgitant orifice area rather than murmur loudness. Repair beats replacement for degenerative disease; transcatheter edge-to-edge repair is an option for selected patients unfit for surgery. Distinguish primary (valve is the problem — fix the valve) from secondary (ventricle is the problem — treat heart failure first).

  You would find: A blowing pansystolic murmur at the apex radiating to the axilla, a soft first heart sound, a displaced hyperdynamic apex beat and often a third heart sound, with exertional fatigue and breathlessness. Beware the ejection fraction: part of each beat empties into a low-pressure atrium, so the ventricle can look like a good pump when it is not, and the thresholds that trigger action are set higher than in other diseases.

• Acute mitral regurgitation← from “The left atrium is a thin-walled, low-pressure…”

  A chorda ruptures, or a papillary muscle infarcts and ruptures — classically the posteromedial muscle after an inferior myocardial infarction, because its single blood supply gives it no collateral protection. The tether that held the leaflet shut is gone, and a normal-sized, unremodelled left atrium receives a large regurgitant volume while sitting on the steep part of its pressure–volume curve, so left atrial pressure climbs abruptly and is transmitted directly to the pulmonary capillaries. Forward stroke volume collapses at the same time. Other causes: infective endocarditis destroying a leaflet, and chordal rupture in myxomatous disease.

  Flash pulmonary oedema plus shock plus an unimpressive murmur, days after an inferior infarct: think papillary muscle rupture. The atrium's lack of time to remodel — not the size of the leak — is what makes it lethal. It is a surgical emergency; vasodilators, an intra-aortic balloon pump and ventilatory support are a bridge to theatre, not a treatment.

  You would find: Sudden severe breathlessness, pink frothy sputum and hypotension, typically a few days after a myocardial infarction. The murmur may be short, early-systolic or inaudible because atrial and ventricular pressures equalise so quickly — a quiet chest in a crashing patient does not exclude it. Urgent echocardiography (transoesophageal if the transthoracic study is inconclusive) and immediate cardiothoracic surgical referral.

• Mitral valve prolapse← from “The leaflets are tethered by chordae tendineae…”

  Myxomatous change makes the leaflets redundant and floppy and elongates the chordae, so as the ventricle empties in mid-systole a leaflet balloons back past the annular plane into the atrium. The chordae snap taut as it does — the click — and the seal breaks late in systole, giving a late systolic murmur.

  Click then late systolic murmur, moving earlier and longer on standing or Valsalva — the one murmur that gets louder and longer when the ventricle gets smaller. Mostly benign and followed by echocardiography; the concerns are progression to severe regurgitation, chordal rupture and, in a minority with bileaflet prolapse and inferolateral T-wave changes or ventricular ectopy, arrhythmia.

  You would find: A mid-systolic click followed by a late systolic murmur at the apex. Manoeuvres that reduce ventricular volume — standing from squatting, or the strain phase of Valsalva — let the leaflet prolapse earlier, so the click moves earlier and the murmur lengthens; squatting and handgrip do the opposite. It affects a few per cent of the population and is usually benign, but degenerative myxomatous disease is the commonest cause of primary mitral regurgitation in Australia, and some progress to severe regurgitation or chordal rupture.

What we give, and how it works

Open a drug to see what it binds, what that does to the cell, and what you then see in the patient.