Amount of radiation which causes injury to CVS or CNS:

Exposures greater than 50 Gy usually cause death in 1 to 2 days.  The few humans who have been exposed at this level showed collapse of the circulatory system with a precipitous fall in blood pressure in the hours preceding death.  Autopsy revealed necrosis of cardiac muscle. Victims also may present with intermittent stupor, incoordination, disorientation and convulsions, suggestive of extensive damage to the nervous system.    White and Pharoah's Oral Radiology Principles and Interpretation 7th edition