Comparison of left atrial and left ventricular performance in conscious dogs

Summary: The velocity of contraction has been reported to be more rapid in left atrium (LA) than left ventricle (LV), but the mechanics of contraction in these two chambers have not been compared under physiological conditions. Accordingly, sonomicrometer crystals and solid state pressure transducers were used to measure LV and LA systolic function simultaneously in seven conscious dogs. Two sets of crystals (LA-1, LA-2) were placed perpendicularly across the LA. Phenylephrine was infused at increasing doses from 0.4 to 1.2 μg·kg^-1·min^-1 to increase LV and LA afterloads. Constant heart rates were maintained by atrial pacing. The results indicated that LA motion was virtually identical in both diameters. LA fractional shortening (0.07±0.01) and ejection time (0.057±0.002 s) were much smaller than the ventricular values (0.21±0.01 and 0.18±0.01 s, respectively). The velocity of circumferential fibre shortening (V_cf) for the LV (1.16±0.05 circ·s^-1) was not significantly different from LA-1 V_cf (1.19±0.09 circ·s^-1) or LA-2 V_cf (1.22±0.08 circ·s^-1). During phenylephrine infusions, LV systolic pressure increased from 120.0±3.4 to 190.8± 15.5 mmHg and LV end-diastolic pressure increased from 3.6±0.3to27.7±2.7 mmHg. When V_cf for each chamber was plotted against the peak systolic pressure in that chamber, the curve of the LA was located upward and to the right of the LV curve. Thus in the heart of the same conscious animal, LA and LV have very similar wall velocities at rest, but the velocity of LV shortening was more sensitive to increases in afterload.