Decompression sickness: a review.
Work in compressed air is a dangerous activity, whether it is carried on under water in a conventional diving suit, in self-contained breathing apparatus or in a diving bell, or in relatively dry conditions in a caisson or an underwater tunnel. Surprisingly large numbers of men have worked at one time or another in compressed air in tunnels or caissons and it is the dangers which arise to the health of these men that are considered here. When a tunnel is driven through water-logged strata or through porous ground under a river, the open end of the tunnel must be sealed off and air pumped in to balance the hydrostatic pressure, should there be any danger of water flooding in and bringing with it unmanageable quantities of silt or sand. A pier of a bridge may be constructed in deep water by means of a caisson (Fr. caisse, a box), which is a compressed air chamber in which a gang of men excavate foundations. In both tunnels and caissons men come and go through an air lock and must be compressed on entering and decompressed at the end of the work period. Although working conditions in these circumstances have much improved over the last 6o years, our understanding of the reactions of the human body to atmospheric pressures greater than the normal and of the pathogenesis of decompression sickness is still insufficient to prevent illness completely. In engineering practice it is customary for working pressure to be expressed as gauge pressure in pounds per square inch (p.s.i.g.) which is the pressure over and above the normal atmospheric pressure of I4-7 p.S.i. (I 03 kg./cm.2). For certain calculations absolute pressure (gauge pressure plus atmospheric pressure) is used, and for clarity it must always be stated whether gauge or absolute pressure is referred to. In naval practice it is usual to think in terms of feet of sea water, each 33 feet of depth corresponding to about one atmosphere or approximately I5 p.s.i. Sometimes the unit of pressure used is the atmosphere (atm.). On the
Decompression sickness in caisson workers
El Ghawabi, S. H., Mansour, M. B., Youssef, F. L., El Ghawabi, M. H., and Abd El Latif, M. M. (1971).Brit. J. industr. Med.,28, 323-329. Decompression sickness in caisson workers. An investigation of 55 bridge construction workers is reported. The overall bends rate was 0·97%. (The term `bends' as used in this study is defined in the paper.) Chokes were encountered in 67·27% of workers. A clinical, haematological, and radiological study was performed. Definite bony changes were found in 43·6% of all workers; 91·6% of these had lesions around the elbow. The presence of dense areas in the neck of the scapula is reported in two cases for the first time. The relatively high haematocrit value is thought to play a part in the pathogenesis of bone infarction through its relation with blood viscosity.
The arterial supply of the adult humerus.
It is worth repeating that in all of these bones the periosteum with its rich blood supply had been removed. A true picture of the blood supply can only be obtained by keeping this abundant source of arterial blood in mind. The constancy of the main nutrient artery of the humeral shaft is remarkable. It would appear to be prudent to guard against injuring this vessel in operations on the humeral shaft. The danger of damaging this artery will probably be greatest in open reductions of fractures of the mid-shaft of the bone. Fractures through the shaft at the junction of the middle and lower thirds will probably destroy the main nutrient at the time of injury. The upper end of the lower fragment will then depend for arterial blood supply on vessels entering from the periosteum and those ascending from the epicondyles. Extensive stripping of the periosteum of the lower fragment in open reductions in this region would probably be best avoided. The upper half of the shaft has an excellent blood supply from the ascending branch of the main nutrient artery and from the accessory nutrient arteries. This may account for the predilection of secondary malignant deposits for this part of the bone.

The main blood supply of the humeral head enters it anterolaterally above the common site of fractures of the surgical neck. Both bone ends thus have a good blood supply. This may account for the rapidity of union of these fractures. However, in operating on this region, whether for repair of rotator-cuff injuries or for open reduction of fracture-dislocations of the surgical neck, it is perhaps worth while bearing in mind that the main blood supply of the head enters it via the upper end of the bicipital groove or via the adjacent parts of the greater and lesser tuberosities. If damage to these vessels is avoided the viability of the humeral head may be preserved.

The importance of preserving muscular and ligamentous attachments when operating on the lower end of the humerus is well known. The size of the posterior arteries entering the epicondyles and condyles is, however, worth bearing in mind. If damage to these posterior vessels is avoided in operations on fractures in this region, the viability of the bone fragments may well be preserved and subsequent degenerative changes in the elbow joint may be avoided.

The intramedullary course of the nutrient arteries (Fig. 16) must mean that they will be destroyed by the insertion of intramedullary nails. If at the same time an open reduction of the fractured shaft is performed, periosteal stripping of the bone ends will be inevitable. The blood supply of the shaft would then be unduly jeopardized. Both the branches of the main and accessory nutrient arteries in the medulla and the only possible local collateral circulation, the periosteal vessels, would be destroyed.