Abstract: The sternum precordial zone of dogs was impacted at velocities of 6. 7m/s, 8. 0m/s, 9. 1m/s and 10.0m/s with mechanical elastic-cord propelled impactor. Morphological changes were studied by histochemistry and electron microscopy. The results showed that: (1) No gross anatomic lesions but myocardial fiber waviness under microscope was detected. In ultrastructure myofibril contraction hand, slightly widened I band or even torn myofibrils were observed when impact was delivered at velocity of 6. 7m/s and 8. 0m/s. (2) At impact velocity of 9.1m/s and 10. 0m/s macroscopic cardiac contusions in associated with evident microscopic lesions were demonstrated. Platelet aggregation or thrombus formation occurred at the site of injured vascular endothelia. More severe ultrastructural damage was found. (3) There might be multiple lesions that revealed in various degrees of injuries in a traumatized heart. It is further demonstrated that lesions in microstructure, especially in ul trastructure could be identified without gross anatomic find ings in blunt cardiac trauma.

Key words: blunt cardiac trauma, histology, ultrastruc ture