Dysbaric osteonecrosis in a compressed air tunnelling project in Hong Kong.

In the largest compressed air tunnelling project using the Blackpool Decompression Tables in Hong Kong, 912 men had radiological examination of major joints and 12 definite and 30 suspected cases of dysbaric osteonecrosis were detected by the compressed air physician. Distribution of the lesions was symmetrical and 43.1 per cent were juxta-articular. All the cases were asymptomatic. The infrequent occurrence of osteonecrosis in Hong Kong in general and in this project in particular suggests that the Blackpool Tables were relatively effective in preventing the disease. Definite and suspected cases were pooled together for the study of risk factors for osteonecrosis. Logistic regression analysis showed that the number of bends ever experienced, number of hours worked in the present contract, age and obesity were important independent risk factors. Because of the impossibility of a suitably long follow-up for all workers, the prevalence of osteonecrosis may be under-reported in Hong Kong.

Analysis of some individual risk factors for decompression sickness in Hong Kong.

Individual risk factors for decompression sickness (DCS) were studied in 932 men who had worked for 12 shifts or more at maximum working pressure (MWP) of 1 bar or above in a compressed air tunneling project in Hong Kong. Two dependent variables were used: presence or absence of bends and number of bends experienced by a man. Three hundred and fifty-six men (38.2%) had one or more bends. Univariate analysis showed that many variables were associated with presence or absence of bends. Logistic regression showed that the best equation included five independent variables: MWP, number of exposures, past number of bends, job (being a miner), and Quetelet Index (or Body Mass Index). The number of bends was also associated with many variables. Stepwise multiple regression revealed five important independent variables: ethnicity, MWP, Quetelet Index, number of exposures, and past number of bends. Obesity and past number of bends were therefore important risk factors for DCS after taking into account MWP and number of exposures. The age effect observed in univariate analysis could be due to obesity. Miners and Japanese had higher risks of DCS, probably due to their strenuous labor.

Manifestations and treatment of 793 cases of decompression sickness in a compressed air tunneling project in Hong Kong.

In the largest compressed air tunneling contract for the construction of the Island Line of the Mass Transit Railway system in Hong Kong, 154,390 man-decompressions occurred, of which 142,140 were after exposures to 1 bar (1.97 ATA, 14.7 psig) or above. The maximum working pressure (MWP) was 3.30 bar (4.26 ATA, 47.9 psig). There were 792 cases of type I and 1 case of type II decompression sickness. The manifestations of the cases were generally similar to those reported elsewhere. Oxygen treatment was given to 9 cases and all were successfully treated with no recurrence of symptoms. Minimum effective pressure treatment on 783 type I cases was successful, with 9.6% requiring two or more recompressions. The pressure required to relieve symptoms was more closely related to the interval between completion of decompression after work and commencement of treatment than to the delay between onset of symptoms and treatment. For every 1-h interval or every 1-h delay, an additional pressure of 0.04 bar (0.04 ATA, 0.58 psi) above MWP was required for pain relief. Step-wise multiple regression analysis showed that the four predictors for pressure of relief and the highest pressure used in recompression, respectively, were, in order of descending importance, maximum working pressure, interval before treatment, bends sequence (the nth attack of bends experienced in the present contract, i.e., the sum of previous attacks and the present attack), and duration of exposure.

Preemployment medical examinations in a compressed air tunneling project in Hong Kong.

One thousand workers intended to be employed in a compressed air tunneling project in Hong Kong had preemployment medical examinations for fitness to work in compressed air. Only 69.3% were declared fit and the overall unfit rate was 22.1%. The major disqualifying medical conditions were lung and heart abnormalities and chronic otitis media. Chest x-ray was found to be the most useful procedure in detecting the disqualifying conditions. The type I bends rate of the contract during the same period of examination was low: 1.39% at maximum working pressure of 2.45 kg/cm2, and there was no case of type II decompression sickness. Although many factors may affect the bends rate, it is suggested that the strict criteria adopted in the selection of workers might have contributed to the satisfactory outcome in the prevention of decompression sickness.

Two unusual cases of severe dysbarism after compressed air work in Hong Kong.

Two cases of unusual severe dysbarism are reported. One case occurred after working in compressed air at a pressure of 2.05 kg/cm2 (29 psi) for a duration of 27 min; the second case, after working for a duration of 6 h at a pressure of only 1.0 kg/cm2 (14.2 psi). The first case presented a combination of symptoms resembling "the chokes" and "the staggers," progressing into shock probably due to micro-air embolism. The second suffered chest pain and "the chokes," complicated by an underlying chest infection. Both were treated successfully by recompression.