L-Wave

The L wave is a mid-diastolic forward transmitral flow wave seen between the E wave and A wave on pulsed-wave Doppler of mitral inflow. It represents continued forward flow from the left atrium into the LV during diastasis, after early rapid filling has already occurred and before atrial contraction

In a normal heart, after the E wave, LA and LV pressures almost equalise. There may still be a tiny amount of forward flow because pulmonary venous return continues to enter the LA, but this is usually very low velocity, typically <20 cm/s. The 2024 BSE diastolic function guideline states that a higher-velocity L wave, >20 cm/s, reflects impaired relaxation with elevated left atrial pressure / LV filling pressure

An L wave appears when LV relaxation is delayed enough to allow LV pressure to keep falling into mid-diastole, while LA pressure remains elevated because pulmonary venous return continues to fill a non-compliant or pressure-loaded LA; this recreates a mid-diastolic LA-to-LV pressure gradient and produces a second forward flow wave before atrial contraction

Why it happens in delayed relaxation

In impaired relaxation, the LV pressure fall is slow and prolonged. The BSE guideline explains that impaired relaxation is associated with a longer tau, slower LV pressure decay, prolonged IVRT, reduced early filling rate, and reduced early transmitral velocity

So, after the initial E wave, the LV has not completed relaxation. Instead of LV pressure simply rising and equalising with LA pressure, LV pressure may continue to fall or remain relatively low during mid-diastole. This creates an opportunity for additional forward flow across the mitral valve in the middle of diastole

Why the L wave becomes important when filling pressures are high

Delayed relaxation alone can create the timing conditions for an L wave, but a prominent/pathological L wave needs enough driving pressure from the LA

This is where elevated filling pressure comes in. If the LA is dilated, stiff, or chronically exposed to high pressure, then pulmonary venous return continues to raise LA pressure during diastasis. The BSE guideline specifically states that continued pulmonary venous flow into a dilated and incompliant LA can cause persistent LAP elevation after early LV filling, producing transmitral flow during diastasis; when this flow exceeds 20 cm/s, it reflects impaired relaxation and elevated LAP

E wave: early LA-to-LV pressure gradient due to LV relaxation/recoil

L wave: mid-diastolic gradient caused by ongoing pulmonary venous filling of a high-pressure LA plus delayed LV relaxation

A wave: late diastolic gradient caused by atrial contraction

So the L wave is not simply “extra filling.” It is a sign that the normally quiet phase of diastasis is no longer quiet. There is still enough pressure gradient across the mitral valve to push blood forward

What it means clinically

A clear L wave, especially >20 cm/s, should make you think:

'This patient has impaired relaxation, but not just simple Grade I delayed relaxation. There is also evidence that LA/LV filling pressure is elevated'

That distinction is crucial. In simple impaired relaxation with normal filling pressure, you expect a small E wave, dominant A wave, prolonged deceleration time, low e′, and normal LA pressure. But when a significant L wave appears, it suggests that the LA pressure has risen enough to drive mid-diastolic flow, moving the physiology toward impaired relaxation with raised filling pressures rather than isolated low-pressure delayed relaxation

Caution: tachycardia, long PR interval, E/A fusion, and eccentric aortic regurgitation can make the L wave difficult or unreliable to measure.