A clinical appraisal of left atrial function.

The left atrium modulates left ventricular filling through three components: a phase of reservoir or expansion during systole, a conduit phase during diastole, and an active contractile component (when sinus rhythm is present) during late diastole. This active contractile component of the left atrium has an important role in patients with ventricular dysfunction as a ‘booster pump’ to augment ventricular volume. Augmented left atrial booster function is one of the mechanisms compensating for decreased early filling in patients with reduced left ventricular compliance, whereas a loss of atrial contraction, as a result of atrial fibrillation or ventricular pacing, reduces cardiac output by approximately 15–20%. During exercise left atrial reservoir and booster functions are augmented, whereas conduit function is not; increased reservoir function may play an important role in accelerating left ventricular filling by helping to maintain an enhanced atrioventricular pressure gradient during diastole and also by increasing left atrial booster function through an increase in preload. An isolated decrease in left atrial compliance is associated with relative increases in the conduit function of the left atrium. The ability to optimally redistribute left ventricular filling among reservoir, conduit and boosterpump functions is a potentially important adaptation that may occur in the left atrium in response to changing haemodynamics.