Left atrial diameter
LA AP diameter · PLAX, end-systole (frame just before mitral valve opening), perpendicular to the aortic root long axis
Normal range & thresholds
Men 3.0–4.0 cm · Women 2.7–3.8 cm; indexed 1.5–2.3 cm/m².[1]
Health warning built into the guideline: the anteroposterior diameter alone is an inadequate measure of left atrial size. The atrium is constrained anteroposteriorly by the sternum and spine and enlarges first superoinferiorly and mediolaterally, so a normal AP diameter frequently coexists with a clearly dilated atrium. Report volume; keep the diameter only because specific risk scores still use it.[1,17]
Health warning built into the guideline: the anteroposterior diameter alone is an inadequate measure of left atrial size. The atrium is constrained anteroposteriorly by the sternum and spine and enlarges first superoinferiorly and mediolaterally, so a normal AP diameter frequently coexists with a clearly dilated atrium. Report volume; keep the diameter only because specific risk scores still use it.[1,17]
Pathophysiology
The left atrium is a low-pressure, thin-walled reservoir in direct hydraulic continuity with the pulmonary veins and, in diastole, with the left ventricle. Chronic elevation of left atrial pressure stretches it; the stretch triggers fibrosis and electrical remodelling, which begets atrial fibrillation, which begets further dilatation. Size is therefore a cumulative, time-integrated record of filling pressure — the ‘HbA1c of diastolic function’ — not a snapshot of it.[2,23]
Raised by
- Any chronic elevation of LA pressure: HFpEF, HFrEF, hypertensive heart disease, HCM, restrictive and infiltrative disease
- Mitral valve disease — regurgitation (volume) and stenosis (pressure); the largest atria in clinical practice are rheumatic
- Atrial fibrillation — both cause and consequence; also atrial flutter and chronic tachyarrhythmia
- Obesity, obstructive sleep apnoea, chronic kidney disease, diabetes
- High-output states, chronic anaemia, endurance athlete’s heart (up to ≈40 mL/m² without disease)
- Left-to-right shunts at atrial or ventricular level; constrictive pericarditis (mild)
Lowered by
- Small body habitus — index before interpreting
- Hypovolaemia, dehydration, aggressive diuresis, post-dialysis
- Cardiac tamponade
- Extrinsic compression — large pleural effusion, hiatus hernia, mediastinal mass, descending aortic aneurysm
Technique & pitfalls
- End-systole, i.e. the frame immediately before the mitral valve opens, when the atrium is largest.
- Measure perpendicular to the long axis of the aorta, from the posterior aortic wall to the posterior LA wall, leading edge to leading edge.
- Do not use the M-mode trace alone if it is not perpendicular.
- Report the indexed value; and add volume, because the two frequently disagree.
- In an atrium distorted by a large left ventricle or a kyphoscoliotic chest, the AP diameter is not interpretable at all.
Pseudo-change & artefact
- An AP-only measurement in an asymmetrically dilated atrium — the dominant false-negative in routine practice.
- Oblique PLAX planes overestimate; foreshortening underestimates.
- Volume status: 2–4 mm swing with diuresis or a dialysis session.
- Measuring in diastole rather than end-systole underestimates.
- In severe kyphosis or a narrow chest, sternal compression makes a genuinely large atrium measure normally.
Treatment thresholds
- Hypertrophic cardiomyopathy: LA diameter (in mm, from PLAX) is a continuous input to the ESC HCM Risk-SCD model, which drives the ICD decision — one of the few places the anteroposterior diameter, not volume, is the validated variable.[14,38]
- Rheumatic mitral valve surgery: a giant left atrium and long-standing AF support a concomitant Cox-maze/left atrial appendage procedure at the time of valve surgery.[6,7]
- AF rhythm strategy: LA diameter >5.0–5.5 cm predicts ablation failure and is used in shared decision-making about rhythm versus rate control — an expectation-setting variable, not a guideline contraindication.[40]
- Where a guideline sets an atrial trigger for valve surgery it uses volume (LAVI ≥60 mL/m²) — see card 12.[6]
Next step
- Next: measure biplane LA volume index — always, whenever the diameter is abnormal or the clinical question is diastolic.
- Then: establish the mechanism: mitral valve disease, filling pressures (E/e′, TR velocity, LA reservoir strain), rhythm history.
- Then: ambulatory rhythm monitoring — an unexplained dilated atrium in sinus rhythm is an indication to look for paroxysmal AF, and it changes stroke prevention.
- Then: LA strain and, where AF ablation is contemplated, CMR or CT for atrial fibrosis and pulmonary vein anatomy.
Drugs
- Reduced by anything that lowers filling pressure: diuretics (acutely and reversibly), RAAS blockade, ARNI, SGLT2 inhibitors, and effective rate/rhythm control.[68,69]
- RAAS blockade reduces new-onset AF, plausibly by limiting atrial remodelling.
- Increased by any agent that raises filling pressure or promotes tachyarrhythmia; also by chronic alcohol.
- Weight loss and OSA treatment produce measurable atrial reverse remodelling.
Reversibility
Partly reversible, over months. After successful mitral valve repair for primary MR the atrium shrinks by roughly 15–25% within 6–12 months. After AF ablation with maintained sinus rhythm, reductions of 10–20% are typical. Treated hypertension and effective decongestion give smaller reductions. The very large fibrotic atrium of long-standing rheumatic disease or permanent AF does not return to normal — and it is that irreversible remodelling which justifies intervening in valve disease before the atrium is destroyed.[6,40]