Although the left atrial appendage has a quite unique structure, its f
unction remains unclear. To clarify the function of the left atrial ap
pendage, changes in its anteroposterior, transverse and longitudinal d
imensions, and in the anteroposterior dimension of the left atrial bod
y were measured by a sonomicrometer during volume loading in open ches
t dogs. In the control state, fractional shortening of the transverse
dimension of the appendage was greater than that of the atrial body. A
fter dextran infusion, each dimension of the appendage and body, measu
red just before atrial contraction, increased curvilinearly. The perce
nt increase in appendage dimensions was greater than that of the atria
l body dimension (p < 0.05). Systolic shortenings of the appendage inc
reased until mean left atrial pressure reached approximately 15 mmHg b
ut after further pressure elevation, it decreased. In postmortem isola
ted hearts, the appendage volume was 17.2 +/- 4.4% of the whole left a
trial volume. These findings indicate that the appendage has a conside
rable volume with a greater compliance and assists left ventricular fi
lling during atrial contraction by a Frank-Starling mechanism.