EFFECT OF ENDURANCE TRAINING ON CARDIAC MORPHOLOGY IN ALASKAN SLED DOGS

The cardiac morphology of 77 conscious Alaskan sled dogs before and af ter 5 mo of endurance training (20 km/day team pulling a sled and mush er) was studied using two-dimensional and M-mode echocardiography. Sub groups included dogs with at least one season of previous training ('' veterans'') and dogs undergoing their first season of training (''rook ies''). Training resulted in a significant (P < 0.05) decrease in rest ing heart rate(-15%) and significant increases in interventricular sep tal thickness (systole, 15%; diastole, 13%), left ventricular (LV) int ernal dimension in diastole (LVIDd, 4%), LV free wall thickness in sys tole (9%) and diastole (LVWd, 9%), and left atrial diameter (5%) in al l dogs, but the increase in LVWd was greater in rookies (16%) than in veterans (7%). Training increased end-diastolic volume index (8%), LV mass index (24%), and heart weight index (24%) and decreased the LVIDd -to-LVWd ratio (-6%) but did not alter cardiac index. We conclude that increased LV mass attributable to LV dilation and hypertrophy is asso ciated with endurance training in Alaskan sled dogs. Disproportionate LV wall thickening accompanying LV dilation suggests that cardiac morp hological changes are due to volume and pressure loading. These traini ng-induced changes are similar to those documented in human athletes u ndergoing combined isometric and isotonic training and differ from stu dies of dogs trained on treadmills.