Decompression Sickness and Dysbaric Osteonecrosis in a Compressed Air Tunnelling Project in Hong Kong

This dissertation, "Decompression Sickness and Dysbaric Osteonecrosis in a Compressed Air Tunnelling Project in Hong Kong" by Tai-hing, Lam, 林大慶, was obtained from The University of Hong Kong (Pokfulam, Hong Kong) and is being sold pursuant to Creative Commons: Attribution 3.0 Hong Kong License. The content of this dissertation has not been altered in any way. We have altered the formatting in order to facilitate the ease of printing and reading of the dissertation. All rights not granted by the above license are retained by the author. Abstract: Abstract of thesis entitled "Decompression sickness and dysbaric osteonecrosis in a compressed air tunnelling project in Hong Kong" Submitted by Dr. Tai Hing LAM for the degree of Doctor of Medicine at the University of Hong Kong in July 1988. This thesis is concerned with the application of epidemiology to the study of health and morbidity experience of compressed air workers. The study was both descriptive and analytical and the results should have practical implications for the provision of occupational health services and prevention of morbidity for this group of workers. In the largest compressed air tunnelling project of the Island Line of the Mass Transit Railway System in Hong Kong, 1916 men had an initial medical examination for fitness to work in compressed air. There were 367 men who were declared unfit for employment resulting in an unfit rate of 19.2%. The major disqualifying conditions were lung and cardiovascular diseases. Compressed air work started in October 1982 and ended in May 1984. The maximum working pressure (m.w.p.) used was 3.3 kg/cm2. The Blackpool Tables were used for decompressions. There were 142,140 man-decompressions of 1 kg/cm2 or above and 792 cases of Type I and one case of Type II decompression sickness. The bends rate was 0.56%. The bends rate wasfound to increase with m.w.p. and duration of exposure showing that the Blackpool Tables although satisfactory overall, became less physiologic as pressure and duration of exposure increased. The manifestations of the 793 cases of decompression sickness were generally similar to those reported elsewhere. Oxygen treatment was given to 9 'cases and all were successfully treated. Minimum Effective Pressure Treatment was given to the rest with 9.6% requiring two or more recompressions. Stepwise multiple regression showed that there were four predictors for the pressure of relief and the highest pressure used in recompression respectively, viz. m.w.p., interval before treatment, bends sequence and duration of exposure. The relative risk of new starters in having decompression sickness was 3.69. The individual risk factors for decompression sickness (presence or absence), as shown by logistic regression were m.w.p., number of exposures, past number of bends, job (being a miner) and Quetelet Index. Stepwise multiple regression revealed five predictor variables for the number of bends experienced, viz. ethnicity, m.w.p., Quetelet Index, number of exposures and past number of bends. A total of 912 men had radiological examination of the major joints and 12 definite and 30 suspected cases of dysbaric osteonecrosis were detected. The overall prevalence of definite cases was 1.3% and the prevalence of definite and suspected cases together was4.6%. The distribution of the lesions was symmetrical and 43.1% of the lesions were juxta-articular. All the men with the lesions were asymptomatic at the time of diagnosis. The risk factors for osteonecrosis were studied by including definite and suspected cases. Logistic regression analysis showed that the number of bends ever experienced, the number of hours of work in compressed air in the present contract and age were the three most important risk factors. The finding that those men with more bends had a higher risk of osteonecrosis suggested that measures to reduce bends would also have the effect of reducing osteonecrosis. The incidence of decompression