Decompression sickness during the sinking of a caisson; a study of some factors in the pathogenesis of caisson disease.

Most of the studies of decompression sickness in industry have been made during the construction of tunnels. The sinking of a caisson, however, is a much briefer undertaking involving certain special features not encountered in tunnel work. Work of this type has received relatively little attention in recent years, although caisson sinking is performed all over the world. This paper describes an investigation into the incidence of decompression sickness during the sinking of reinforced concrete caissons in the Thames between February and May, 1950. The engineering problem was to build aqueducts to carry river water to a power station for cooling the turbines. The heated water had to be discharged by another outlet aqueduct down stream, well away from the inlet, so that the cooling efficiency would not be reduced by sucking in water already warmed by the turbines. The aqueducts were about 7 ft. in diameter. There were two stages in the construction of each aqueduct: first, caissons were sunk just below low water line in the river; tunnels were then driven out to join them. The aqueduct was completed so that its opening was well above the silt of the river bed. During the sinking the earth was removed by buckets and hoisted up through a " muck lock ". The air pressure in the caisson varied according to its depth below water-level, and was maintained to prevent water or mud rising into the caisson before it was sealed with concrete. The maximum pressure used was about 35 lb./sq.in., though it varied by 10 lb./sq.in. or more with the tide (Fig. 2). The working shift (usually four or five men, sometimes as many as eight) entered and left the caisson by a blister lock in which compression and decompression took place. They reached the caisson bottom by means of a vertical ladder. There were usually three eight-hour shifts. (See Fig. 1 for a diagram of the caisson.) Methods Data about cases of decompression sickness were obtained principally from the medical orderlies; they were always on the spot and were responsible for supervising the recompression in the medical lock. Records of all compressions were supplied by the engineer's office. Measurements were made of the carbon dioxide concentrations in the air in the blister lock of the caisson, during and at the end of a normal decompression. Since this lock is completely filled when it contains five men, there would have been no room for an observer and gas analysis apparatus in it. Accordingly arrangements were made to attach a length of hose to the outlet pipe, from which samples of the air escaping from the chamber during decompression were taken and sealed over mercury. It was thus possible to get estimates of the composition of the air in the decompression chamber while it was being used in the normal way, fully packed with men coming off shift. When a sample was taken another observer recorded the gauge pressure at the time. The oxygen and carbon dioxide were estimated by means of a Haldane gas analysis apparatus.