The E/A ratio is a marker of the function of the left ventricle of the heart.
The atria are separated from the ventricles beneath by valves that act like trap doors. They transfer blood to the ventricles in 2 steps: in the first step, the weight of the collected blood in each atrium causes it to fall into the ventricles below when the "trap door" valves open; focusing on the left side, the speed at which the blood moves during this initial action is called the "E" ( for early) filling velocity. But some blood always remains, so toward the end of the atrial emptying cycle (diastole), the second step occurs in which the atria contract to squeeze out that last bit ("atrial kick"). The speed of the blood filling the ventricle in this step is the "A" (for atrial) filling velocity.
The E/A ratio is the ratio of the early (E) to late (A) ventricular filling velocities.
In a healthy heart, the E velocity is greater than the A velocity. In certain pathologies and with aging, the left ventricular wall can become stiff, increasing the back pressure as it fills, which slows the early (E) filling velocity.
When the 'A' velocity becomes greater than 'E' velocity, this is often accepted as a clinical marker of diastolic dysfunction, in which the left ventricular wall becomes so stiff as to impair proper filling, which can lead to diastolic heart failure. This can occur, for instance, with longstanding untreated hypertension.
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