EARLY CHANGES IN LEFT-VENTRICULAR REGIONAL DIMENSIONS AND FUNCTION DURING CHRONIC VOLUME OVERLOADING IN THE CONSCIOUS DOG

As the left ventricle dilates during chronic volume overloading, it assumes a more spherical configuration, suggesting a nonuniform regional myocardial response. Early alterations in left ventricular geometry and regional function induced by chronic volume overloading were studied in the canine heart subjected to an aortocaval fistula. Mongrel dogs (9) were instrumented with ultrasonic crystal pairs placed perpendicularly to the long axis to measure instantaneous segment lengths in the apical, midventricular, and basal portions of the left ventricle; another pair monitored an apical internal diameter. Control measurements of end-diastolic dimensions and percent shortening in the conscious state were obtained. An aortocaval fistula was then constructed, resulting in an increase in resting end-diastolic pressure (EDP) from 11 .+-. 1 (SEM [standard error of the mean] to 22 .+-. 2 mm Hg and resting heart rate from 100 .+-. 4.9 to 131 .+-. 5.0 beats/min at 1 mo. (P < 0.01). At 10-20 days, mid-basal regional end-diastolic dimensions were significantly greater than control (8.8 .+-. 2.4% increase, P < 0.01), while there were no significant increases in apical dimensions when compared at the same EDP. By 20-40 days, all dimensions were significantly increased (11.4 .+-. 3.3% increase at the mid-base, P < 0.01), 7.8 .+-. 1.6% at the apex (P < 0.01), and 11.2 .+-. 4.5% at the diameter (P < 0.05). Regional function as measured by percent shortening at the same EDP exhibited early augmentation in all segments by 2 wk compared to control (3.8 .+-. 1.3% increase at the diameter, 3.8 .+-. 1.9% at the base, 7.2 .+-. 2.2% at the middle, and 12.2 .+-. 4.7% at the apex, P < 0.05). From 4-6 wk, function was not different from control, with a slight reduction in all regions from the peak values obtained at 2 wk. In the canine heart there are regional differences in the rate of left ventricular dilation to chronic volume overload, suggesting differential regional hypertrophy, with the earliest response at the mid-basal level. Despite dilation, regional function is preserved or even augmented, to varying degrees in each region.