Caisson Work

CAISSON WORK (occupational hazards and occupational diseases). Hygiene of labor in caissons. Caissons represent a device consisting of a working chamber, a shaft tube extending upward from it, ending at the top in an apparatus chamber, and a lock connected to the apparatus chamber. The working chamber is that part of the caisson in which caisson work proper is carried out, i.e., the excavation and removal of soil. It is usually made of reinforced concrete, but can be iron and even wooden. The shaft tube is intended for lowering people and materials into the chamber and for raising excavated soil from it. It consists of separate sections joined one on another as the caisson is lowered, and has a strictly vertical ladder for people. The apparatus chamber contains simple mechanisms used for raising soil from the working chamber and lowering materials into it, and is usually operated by two workers inside it. The lock has a special purpose both medical and production-related, representing a chamber (or chambers) in which any intermediate air pressure can be created between atmospheric pressure and the pressure in the caisson without changing the pressure in the caisson itself. The creation of such intermediate pressures is necessary to protect people from the danger of injuries and diseases associated with pressure changes, as well as to maintain the necessary pressure in the working chamber when people exit or when soil is removed and materials are supplied. Usually from 6 to 14 people work in the working chamber. When the caisson reaches the required stable soil, further work on earth removal ends, the working chamber is filled with concrete, as is the first section of the shaft tube; the entire remaining part of the caisson is then removed, and the resulting unfilled space in the masonry is also filled with concrete, after which the pier is ready. The purpose of C.W. is that when the soil lacks sufficient strength for a given structure (when there is an aquifer beneath it) or when it is necessary to work at the bottom of rivers, etc., piers and supports are placed under the structure being built (bridge, building, etc.), extending to stable soil, which necessitates passing through water. For this purpose, water in the corresponding layer is displaced by air forced under pressure into a special device called a caisson. The magnitude of the air pressure corresponds to the depth of the caisson's location; this is calculated based on the principle that for every 10 m of depth the caisson is lowered, the pressure of the air supplied to it should increase by 1 atmosphere. Air is forced into the caisson by compressors from a compressor station through air pipelines. Since air heats up considerably when compressed, if special cooling measures are not taken, it enters the caisson considerably heated, resulting in excessively high temperature in the caisson in such cases. This also occurs when the pipeline network is not insulated and is heated by the sun. In winter, uninsulating the pipeline network leads to the opposite results: air can enter the caisson cooled, and the temperature in the caisson may be excessively low. Since air for compressors can be taken from an unsuitable location (in terms of its dustiness) and since as it passes through compressors lubricated with oils, it can become contaminated with them, sometimes the air in the caisson can also be heavily contaminated. Humidity in the caisson is always unavoidably very high, exceeding 90% and often reaching complete saturation. It is particularly high in the locks during decompression, because as pressure continuously decreases in the lock, fog forms in it and vapors condense into water. The same phenomenon also occurs in the working chamber during periods of caisson lowering, carried out by reducing pressure in it. Ventilation in the caisson depends on the amount of air supplied to it, as well as the quality of the soil; with soils that are easily permeable to air (sandy), ventilation of the caisson and in particular the working chamber is satisfactory; in the case of soil poorly permeable to air (clayey, silty), ventilation of the caisson can suffer considerably if not ensured by special measures. To ensure the necessary air purity (removal of oil, condensate, and dust impurities), reservoirs are included in the pipeline network, which if necessary can also be equipped with filters. Current legislation in the USSR requires that the amount of air supplied should ensure at least a threefold exchange per hour in the working and apparatus chambers. Under existing conditions, this standard provides about 50 m³ of air per person. Insufficient ventilation of the caisson is particularly important because due to the very high humidity in the caisson, conditions unfavorable for the body's thermoregulation are more easily created, and disturbances in thermoregulation occur more quickly, which directly affects the occurrence of caisson diseases. Furthermore, in the prevention of caisson diseases, the temperature regime in the caisson is of great importance. The most suitable temperature is within 17-22°, established by a resolution of the USSR People's Commissariat of Labor on February 5, 1930, and at the higher limits it would be very useful to provide a corresponding air movement speed in the working chamber (up to 0.6 m per sec.) to ensure necessary body heat dissipation. Caisson diseases, their pathogenesis, symptomatology, treatment and prevention. The transition of the body from normal pressure to increased pressure leads to changes in its tissues and organs, which gradually acquire the pressure of the surrounding environment. If this transition occurs gradually and over a period of time sufficient for the body to adapt to the changed pressure conditions, and if there are no pathological changes in the body that prevent this adaptation, then the body tolerates the transition to high pressure and stay in it without difficulty. If, however, one of these conditions is violated, then corresponding damage to the body is inevitable. Practically in C.W. this amounts to the fact that during the period of direct compression ('compression'), when pressure in the lock rises too quickly or when a person in the lock suffers from some pathological process in the auditory apparatus or nasopharyngeal space, perforation of the eardrum can easily occur due to unequal pressure in the tympanic cavity with external pressure. Therefore, legislation in all countries prescribes increasing pressure in the lock in accordance with a special time table. In particular, Soviet legislation requires gradual pressure increase from normal to one additional atmosphere in 5 min.; from one additional atmosphere to two atmospheres - 3 min., and in total from normal pressure - 8 min.; from two to three atmospheres - 2 min. (from normal 10 min.); from three to four atmospheres - 2 min. (from normal 12 min.). On the other hand, it is required that people descending into the caisson should not suffer from any pathological changes or processes in the auditory apparatus or nasopharyngeal space. Nevertheless, even when all precautions are observed by the person in the lock, especially a little trained person, unpleasant sensations and even acute pain in the ears may appear due to the pressure on the eardrum from inside and outside not yet being equalized. In this case, passing air through the Eustachian tube (by the Valsalva method or swallowing) opens it, whereby pressure in the tympanic cavity quickly equalizes and unpleasant sensations and pain quickly disappear. Thus, the lesions of the eardrum that may occur during the compression period are purely mechanical in nature and do not actually belong to caisson diseases. When the pressure in the lock reaches the level of working pressure in the caisson, the door to the middle, apparatus chamber is opened (until this moment this is impossible, as all doors in the caisson open toward greater pressure), and people who have undergone compression pass through this chamber into the shaft tube, down the ladder of which they descend into the working chamber, where they remain for the entire duration of their shift. This time depends on the magnitude of pressure in the caisson, and according to new rules of 1930, at pressures up to 1.75 additional atmospheres it should not exceed 7 hours per day, from 1.75 to 2.5 additional atm. - 6 hours, from 2.5 to 3 atm. - 5 hours, from 3 to 3.5 atm. - 4 hours and above 3.5 atm. - 2 hours, and in all these cases two mandatory shifts per person per day are established (except for pressures above 3.5 atmospheres, for which one shift is established), so that the indicated time is spent in the caisson in two periods. This time also includes the time for compression, decompression, descent into the working chamber and ascent from it. While in compressed air, a person usually experiences no noticeable disorders.

After finishing work in the caisson, people return to the lock for decompression; the door from the lock to the chamber is closed, and slow decompression begins in the lock until the pressure is reduced to normal. Usually, decompression is carried out by the medical staff on duty. Decompression is carried out in accordance with special rules, and according to the legislation of the USSR, it must meet the following standards: when a person is transferred from a pressure of 1 additional atm to normal pressure, decompression should last 5 minutes, from 1½ additional atm to normal - 10 minutes, from 2 additional atm to normal - 30 minutes, from 3 additional atm to normal - 45 minutes, from 4 additional atm to normal - 1 hour. The most serious and life-threatening disorders in the body occur in the post-compression period. If decompression is carried out quickly, violating established norms, cases of eardrum perforation with bleeding from the ears may occur in this period as well, but due to the internal pressure exceeding the external pressure. However, such cases are rare, as violations of decompression norms would have to be very gross for this to happen. Caisson diseases depend on the fact that tissues and organs saturated with air (mainly nitrogen) during the body's stay under pressure do not have time to get rid of it during decompression, and the body transitions to normal pressure with excess gas in the tissues. Saturation of the body with air (saturation) occurs through the blood, which transfers it from lung tissue to all tissues and organs by diffusion. When transitioning to normal pressure, the reverse process occurs - desaturation of tissues and body fluids from excess gas. Its speed depends on the degree of saturation of the body with air (and this last depends on the magnitude of pressure, the duration of its action, and the saturation abilities of individual tissues), while the essence consists in the excess gas (nitrogen) striving to pass from the tissues saturated with it into the blood and through it into the exhaled air and leave the body with it. If the degree of saturation of the body with nitrogen is significant, then naturally large amounts of nitrogen enter the blood vessels, the emboli of which, by clogging various vessels, can cause corresponding disorders in the body. Thus, caisson diseases are a consequence of gas embolism of various localizations. Depending on these, all caisson diseases can be schematically divided into 3 groups. The first group includes local skin lesions in the form of subcutaneous emphysema, explained partly by gas embolism of skin vessels, partly by the release of gas directly into the subcutaneous tissue; the phenomena of emphysema explain skin itching, although some consider the cause of itching to be irritation of the posterior roots of the spinal cord by gas bubbles in the cerebrospinal fluid. The skin lesion is characterized by mottling or marbling, depending on the embolism of superficial skin veins. This group also includes the most common lesions of joints, bones, and muscles in caisson workers (caisson rheumatism, 'zhalomai' of Russian caisson workers). Cases of caisson joint rheumatism are most common, especially diseases of the knee joint. These cases are not uncommon even at relatively low pressures (up to 2 atm.). The mechanism of origin of these lesions is not entirely clear. It can be assumed that it comes down to the pressure on nerve endings by gas accumulations under the fascia, under the periosteum, in the yellow marrow of tubular bones, as well as in the joint cavities. Symptoms of these diseases: increased tendon reflexes, sensitivity of nerve trunks, swelling of the affected limb, friction noise, effusion, and crepitus in the joint. The second group includes lesions of the central nervous system from embolism of its vessels and from the accumulation of gas bubbles in it. These lesions can affect both the spinal cord and the brain. Cerebrospinal lesions manifest in the form of paraplegia (more often spastic), monoplegia, paralysis of the bladder and rectum, disorders of sensitivity and coordination, etc. These phenomena may be transient if gas accumulations and emboli are resorbed. However, if destruction of nervous tissue (mainly in the posterior columns and posterior parts of the lateral columns of the thoracic spinal cord) or hemorrhages into it (hematomyelia) occur, these phenomena become persistent and often, after several weeks, end fatally. Cerebral symptoms come down to dizziness, headaches, speech disorders, clouding of consciousness, stuporous state. As a result of gas embolism of cerebral vessels, collapse and death may occur. Hemiplegia and convulsions, which are a consequence of focal softening of the brain, often join the above-mentioned cerebral phenomena. Accumulations of gas in the labyrinth can cause deafness and Meniere's symptom complex. Finally, the third group includes phenomena depending either on the passage of large emboli into the right heart or on embolism of coronary vessels with cessation of cardiac activity and death per syncope, or on obstruction of pulmonary vessels with death per asphyxiam. The latter case is characterized by severe shortness of breath with intense asthma attacks on the basis of developing pulmonary edema. While the first group of caisson diseases occurs at relatively low pressures and is characterized by localized and transient lesions, the third group represents generalized lesions always with rapid and fatal outcome and occurs after transition from the highest pressures (4-3 atm.); the second group occupies an intermediate position, develops after staying under pressure of significant degrees (2.5-3.5 additional atm.) and can either be limited to localized transient or persistent lesions or be characterized by general lesions with fatal outcome. All diseases from compressed air are united under the common synonyms: aeropathy, aemia, pneumathemia, etc. In experiments on animals and in autopsies of people who died quickly in the decompression period, the right heart was found to be dilated by a large amount of gas, and the venous system was filled with gas bubbles. As a result, the blood foams at autopsy. Multiple gas emboli were found in the coronary vessels. In the spinal cord in cases of paralysis at autopsy, hemorrhages and focal softening were found, localized mainly in the lower thoracic and upper lumbar parts, which is explained by their weaker vascularization. From the side of the lungs at autopsy, edema and interstitial emphysema were detected. The liver, spleen, and kidneys also turn out to be affected at autopsy, although during life they did not give any symptoms. Cases of finding huge gas accumulations under the mucosa of the jejunum were noted. The rate of tissue saturation with gas, resp. nitrogen, depends on their properties. Thus, blood saturation occurs within 55 seconds, while fatty tissue saturates slowly and at the same time absorbs nitrogen 5 times more than blood and other tissues. Thus, this tissue, constituting up to 20% of body weight and poorly vascularized, will also slowly get rid of gas in the decompression stage, serving as a reservoir for nitrogen absorbed under pressure during this period. Therefore, nervous tissue, subcutaneous tissue, bone marrow, joints (especially the knee) are most often affected. To combat caisson, resp. decompression diseases, the most important thing is the appropriate professional selection of workers. These should be people with a good cardiovascular system, capable of handling the transport of gas portions from tissues to the lungs, with little developed fatty tissue, with a stable nervous system, etc. Working conditions (temperature, etc.) should not create obstacles to the normal functioning of the body, especially important in the decompression period; everything that can reduce the body's resistance during this period (cooling, colds, etc.) can be a direct cause of caisson disease and must be carefully eliminated. It is extremely important to observe the norms of stay under pressure, and especially the norms of decompression. The latter plays a cardinal role in the prevention of caisson diseases. In addition to strictly observing the rules of decompression and appropriate conditions in the lock (proper temperature, clean air, sufficient ventilation), it is important after a person leaves the caisson to stimulate their cardiac activity, for which it is advisable to give workers hot tea or coffee immediately, provide them with a short rest in a special room to bring the thermoregulatory apparatus into a stable state, to change clothes and dry off to avoid colds. Caisson diseases can occur not immediately upon leaving the caisson, but also after several (up to 24) hours. Therefore, it is necessary to observe appropriate precautions and for some time after leaving the caisson.

Special and particular medical supervision during Caisson Work is inevitable and necessary. The conditions of labor during Caisson Work in the USSR are regulated by the rules of the NKT USSR No. 38 of 5/XI 1930, which standardize the issues of design, equipment, and maintenance of caissons, auxiliary premises and devices adjacent to them, prescribe all necessary safety and hygiene measures during work in caissons, measures for the prevention of caisson diseases, establish the rules for organizing medical services, state the contraindications for admission to Caisson Work, and prescribe the mandatory methods for treating caisson diseases. By resolution of the NKT USSR No. 156 of April 30, 1929 (Section XI, item 5), an additional two-week vacation is established for caisson workers due to the special harmfulness of the work. The most effective method of treating caisson disease is recompression, returning to the pressure at which the person was working. For this purpose, caisson work must always be provided with a therapeutic lock with appropriate equipment, into which the sick can be admitted. Even in cases of paralysis, if the lesions are not persistent, the person easily and quickly fully recovers their health in the therapeutic lock. The therapeutic lock must be equipped with beds, electric lighting, heating devices, provided with a special window for external observation of the patient's condition, and have a chamber for administering medications to the patient, etc., without changing the pressure in the lock. The recompression method is based on the fact that under pressure, gas emboli decrease in size and dissolve, passing back into a dissolved state in the tissues. After recompression, when the person feels completely healthy, the pressure is slowly and carefully reduced. The admission of the sick person to the therapeutic lock must occur as quickly as possible and in any case no later than 12 hours after the onset of disease symptoms. Among palliative measures, it is necessary to mention pain-relieving medications, high temperature (dilation of blood vessels and acceleration of blood circulation), soothing ointments, massage, and baths. These measures can only be applied in mild cases (occurring from pressures up to 2 atmospheres).