The heart-weight to body-weight ratios were calculated for 242 dogs in 11 groups ranging from one day to adult. The base-apex diameters, internal transverse diameters, external transverse diameters, and the anterior, posterior, lateral, and interventricular septal wall thicknesses were measured in left ventricles of hearts taken from 45 animals of nine different age groups. The ductus arteriosus was examined for patency in all puppies.
The heart-weight to body-weight ratios were found to be highly variable and failed to show significant changes with age. The average ratios increased during postnatal development from 7.17 gm/kg in the newborn to 8.87 in adults.
The left ventricular diameters and wall thicknesses increased progressively with age and were four times larger in adults than in the newborn. Diameter to wall-thickness ratios were calculated and did not change with age. Since the heart weight increased 10–16 times faster than the wall thickness, wall-thickness to heart-weight ratios were found to decrease with age indicating a faster lengthening than thickening of the myocardial fibers. The ductus arteriosus was functionally open in all puppies less than four days of age and did not close anatomically until 7–8 days of age.