On the role of separated gas in decompression procedures.

(i) E xperim en ts have been m ade on th e incidence and tim e of onset of decom pression sickness in m ice exposed in single or repeated exposures to raised pressures of n itrogen or helium . I f a first (conditioning) exposure to pressure is followed 5 m in la te r by a second (test) exposure, th e incidence of sickness is considerably higher th a n th a t for e ither exposure alone, or for a single exposure equal in leng th to th e sum of the o ther tw o exposures. The sam e resu lt is ob ta ined if the conditioning exposure is to sa tu ra tion . Sickness produced b y re ­ p ea ted exposures has a shorter la tency of onset th a n a fte r single exposures. These resu lts are explicable on th e basis of asym ptom atic bubble fo rm ation a fte r decom pression. (ii) This la te n t suscep tib ility to decom pression sickness, as revealed by th e te s t exposure, in itia lly increases w ith tim e a fte r decom pression, reaches a m axim um , an d th en declines. The ra te of decline is fas te r th a n can be accounted for on the basis of th e decay and local reabsorp tion of bubbles. I t is suggested th a t gas bubbles are also rem oved by passage from th e tissues th rough th e venous system to the lungs. (iii) The degree to w hich very sho rt second exposures to pressure (lasting only a few seconds) give rise to th e sym ptom s of decom pression sickness canno t be explained on the basis of the k inetics of bubble grow th. These sym ptom s could also arise from com pression by the second exposure of bubbles in th e tissues, allowing th em to en te r th e venous system and pass to th e lungs, w here, if expanded by decom pression before elim ination, th ey give rise to severe decom pression sickness. (iv) These observations are in d irect conflict w ith th e principles on w hich cu rren t decom ­ pression tab les are based. The relative success of th e la t te r m u st be a ttr ib u te d to th e em pirical m anner in w hich th e tab les have been constructed an d m odified in th e ligh t of experience. The resu lts suppo rt the theo ry th a t separa ted gas m ay be p resen t in sym ptom less d e­ com pressions an d raise the question of gas tran sp o rt to the lungs as a fac to r w hich should be tak en in to account in the design of decom pression tables.