RADIATION-INDUCED LIFE-SHORTENING 



315 



Similar investigations have been carried out with the mice exposed while 

 anaesthetized. The anaesthetic used was Nembutal, 60 mg/kg, administered 

 intraperitoneally. Table II shows the results for both the LDgg and life- 

 shortening. This time there seems to be no variation with dose-rate either for 

 the acute or for the long-term effects of radiation. 



o 

 o 



7r 



6 



5 



4 



3 



/ 



10 100 1,000 10,000 



Dose-rate (rad/min) 



100,000 



Fig. 2. Variation of life-shortening with dose-rate. 



A comparison of the values in Tables I and II shows that the protective 

 action of the anaesthetic against both acute and long-term effects of radiation 

 appears to diminish with increasing dose-rate, becoming undetectable at very 

 high dose-rates. 



Table II. Effect of anaesthesia 



The interpretation of these results is not easy. The obvious explanation 

 would be an oxygen effect. Thus it is possible that reduced senstitivity to 

 radiation in the anaesthetized animal is the result of the lowering of the 

 oxygen tension in tissues by the anaesthetic (Mack and Figge, 1952). A 

 similar explanation might be offered for the dose-rate effect. If the dose is 

 delivered in a very short time, the oxygen in the ceUs of vital tissues consumed 

 by the first fraction of the dose is not replenished quickly enough, and the 

 latter part of the irradiation is thus given under hypoxic conditions. Such an 



