980 RADIATION BIOLOGY 



tion of hyaluronate. While the decline in platelets and appearance of 

 heparinoidlike substances are probably largely responsible for capillary 

 bleeding, it is likely that other factors also play a role in its development. 



The typical acute reaction to radiation injury may embarrass the 

 circulation initially and terminally; the intermediate period, however, 

 appears to be relatively free of any gross circulatory disturbance. This 

 applies to penetrating radiation; unfortunately there are no data that 

 relate to circulation after superficial irradiation other than local blood 

 vessel effects. It is, of course, well known that these responses are 

 separated in time by periods of relative recovery. A shocklike state has 

 been seen in dogs following massive X irradiation of the whole body or of 

 the abdomen (Moon et al., 1940, 1941). This is characterized by progres- 

 sive hemoconcentration, dehydration, and hypotension, with death occur- 

 ring several days after exposure. The severity of this type of early 

 reaction to large radiation dosages often parallels the recognizable tissue 

 damage. Post-mortem visceral changes are described as those character- 

 istic of circulatory failure of the shock type, and the physiological dis- 

 turbances have been thought to be a consequence of acute toxicity result- 

 ing from absorption of the products of tissue necrosis, although, as in the 

 case of irreversible shock from other causes, this has been difficult to 

 confirm experimentally. 



The shocklike syndrome is generally not observed after median lethal 

 irradiation. Early circulatory changes may appear in some species, how- 

 ever. A fall in blood pressure has been detected in rats and rabbits but 

 not in dogs during the first 24 hours after X irradiation (Prosser, Painter, 

 Lisco, et al., 1947; Painter et al., 1947; Weber and Steggerda, 1949; 

 Strauss and Rother, 1924). Hypotension is reported in the rabbit after a 

 total-body exposure as low as 50 r (Painter et al., 1947). A similar 

 early decrease in blood pressure has been noted in patients receiving 

 X-ray therapy (Bediirftig and Griissner, 1949). There is reason to 

 believe that part of the early hypotension is reflex, since vagotomy and 

 atropinization can reduce the blood pressure response (Strauss and 

 Rother, 1924; Painter et al., 1947). That the immediate hypotension is 

 neural in origin is suggested further by its absence in the spinal animal 

 (Montgomery and Warren, 1951). Repeated injections of epinephrine 

 are also effective in antagonizing initial hypotension in rabbits (Painter 

 et al., 1947). Blood pressure changes have been described after irradia- 

 tion of the hind legs, but the effects are not consistent. The direction of 

 early blood pressure change may depend upon the area that is exposed 

 (Toyoma, 1933b). A small increase in blood pressure, which is not 

 affected by atropine or nicotine, has been observed during irradiation of 

 the cardiac area. A rise also occurs during exposure of midbrain; when 

 irradiation includes the entire head of the animal, there is a decrease that 

 can be prevented by vagotomy. 



