K Robinson, M Byers. Diving Medicine
Total Page：16
File Type：pdf, Size：1020Kb
J R Army Med Corps 2005; 151: 256-263 J R Army Med Corps: first published as 10.1136/jramc-151-04-07 on 1 December 2005. Downloaded from Diving Medicine K Robinson, M Byers with diving. This article aims to review the physics and physiology of diving, discuss the common medical conditions that occur whilst diving, and consider the current thinking in relation to aetiol- ogy and treatment. A Brief History of Diving Breath-hold diving has probably been around from before written history. Divers were reportedly used in the Trojan Wars (1194-1184 BC) to disrupt the enemy navies. Aristotle described ruptured tympan- ic membranes and ear infections in sponge divers, and Marco Polo wrote of pearl divers reaching depths of 27 metres. Alexander the Great is supposed to have made a dive in a primitive diving bell, Roger Bacon described men walking on the seabed in 1240, and in 1535 Guglielmo de Lorena developed the first true diving bell (3). In 1774 Freminet used a bellows to deliv- er air from the surface to a diver and Siebe Introduction developed the diving suit in 1819 (3). This It could be argued that diving is the most consisted of a copper helmet and jacket, extreme sport of all. Survival in such a which was modified in 1839 to resemble the hostile underwater environment is waterproof suit recognised and in regular use http://militaryhealth.bmj.com/ dependent on sophisticated breathing to the present day. apparatus and an understanding of the Self-contained underwater breathing inherent risks. Diving incidents are well apparatus (SCUBA) was also developed in publicised, not least because of the the 19th century and in 1943 Jacques severe injuries that ensue, but remain an Cousteau and Emile Gagnan demonstrated uncommon occurrence. Yet despite, or the aqualung (3). perhaps because of its dangerous and challenging profile, recreational diving Physics and Physiology Air is composed of approximately 21% oxy- is on the increase with more people gen and 79% nitrogen. Each of these gases learning to dive each year. There are exerts a partial pressure in proportion to on September 30, 2021 by guest. Protected copyright. currently an estimated 700,000 recre- their concentration. At sea level man is sub- ational divers in the UK (1). ject to 1 atmosphere (101.32 kPa). Water, Diving within the military may be being denser than air, exerts a greater pres- occupational or recreational. sure per unit volume such that for each 10 Occupational diving involves a long and metres of sea water (msw) of descent adds rigorous training package designed to another atmosphere of pressure. A diving provide the diver with the necessary Miss K Robinson, cylinder filled with compressed air will con- skills to complete their task, be it in Specialist Registrar in tain the same partial pressures of nitrogen Emergency Medicine, mine clearance, underwater engineering and oxygen as environmental air but contain Derriford Hospital, or as a combat swimmer. It is Ministry a considerably larger volume, thus allowing a Plymouth of Defence policy that military diving diver to breath air from the tank for extend- Email: complies with Health and Safety at Work ed periods. For example a 12 litre cylinder of [email protected] guidance and with the Diving at Work air compressed to 232 bar will hold 2784 Regulations as far as reasonably practi- Lt Col M Byers litres of air (12 x 232). Normal air pressure cable (2). MRCGP Dip IMC DA (1 atm) = 1.01 bar, 760mmHg or 101 kPa. Diving injuries can be encountered in Dip SEM RAMC The pressure increases relating to diving barracks, on exercise and on operations 3 CS Med Regt, Gaza affect the inspired gas and also the gas filled and Medical Officers need to have an Barracks, Catterick spaces within the diver such as the lungs, Garrison understanding of the presentation and sinuses, ears and gastrointestinal tract. The Email: [email protected] management of conditions associated K Robinson, M Byers 257 J R Army Med Corps: first published as 10.1136/jramc-151-04-07 on 1 December 2005. Downloaded from Gas Laws make these changes clear. Vision underwater is altered due to the Boyle’s Law states that if temperature refraction of light making objects seem larger remains constant, the volume of a fixed mass and closer. Light has a differential absorption of gas is inversely proportional to the in water causing loss of colour (especially absolute pressure. As pressure increases by 1 red) at depth and many diving environments atmosphere for every 10m of descent in have poor visibility due to turbid water. water, a given volume of gas will halve in Sound travels more than four times faster in volume at 2 atmospheres. At 20m below the water than in air making it much more diffi- surface (3 atmospheres) the gas will be a cult to localise sound (4). If the diver is using third of its original volume and so on. The a gas mixture containing helium, the pitch of converse is true on ascent. A volume of gas the voice will increase making underwater will double in volume from ascending from voice communication, with wired or wireless 10m deep to the surface. This helps to systems, almost impossible without the use explain barotrauma. of expensive electronic equipment (5). Dalton’s Law states that in a mixture of Hearing acuity is reduced by around 60dB gases, the partial pressure of each gas present especially in the higher frequencies, localisa- is equal to the pressure that gas would exert tion is poorer and there are the thermal and if it alone occupied the original volume pressure effects due to being under water. (Table 1). From this it can be seen that with There is decreased sensation affecting dex- every 1 atmosphere increase in pressure, terity, 2-point discrimination, vibration and although the proportions of the gases stay other functions caused by the cold, the gases the same the partial pressure increases. The used for respiration, pressure and the protec- effects of this will be seen when decompres- tive equipment worn to survive in the envi- sion sickness is considered. ronment. Henry’s Law states that at a constant The increased density of water leads to an temperature the amount of gas that will dis- increased heat loss. Water temperatures vary o solve in a liquid is proportional to the partial between –2 to above 30 C but are usually pressure of a gas over the liquid.The amount below the body’s thermo neutral point o of gas that dissolves will also be affected by (31 C). Greater depth leads to lower temper- Bunsen’s coefficient of solubility, time and atures and water conducts heat 25 times temperature. This is important in the theory faster than air. Peripheral vasoconstriction of decompression sickness. occurs to maintain the core temperature, but this can cause loss of manual dexterity, and Physiological Demands of the alteration in blood flow may contribute Diving to decompression sickness. Protective ther- Being underwater affects many body systems mal equipment worn to survive the cold envi- and underwater work creates increased phys- ronment causes an additional loss of dexteri- http://militaryhealth.bmj.com/ iological demands on the body.The diver has ty. to move in a medium that is more viscous than air, breathe gases at increased density, The Gases cope with an increased hydrostatic pressure, and carry heavy and bulky equipment. Nitrogen As the diver descends the hydrostatic pres- Nitrogen is an inert gas that constitutes 79% sure around his legs may increase the of air and at 1atm has a partial pressure of intrathoracic blood volume by up to 700mls. 0.79atm. It is a weak anaesthetic agent. This alters the ventilation perfusion balance Diving below 30 metres of sea water (msw) and may cause gas trapping. The increased equivalent to 4 atmos and a piN2 of 320kPa pressures in the right atrium and pulmonary will lead to a narcotic effect.This is similar to on September 30, 2021 by guest. Protected copyright. artery stimulates a diuresis, which, com- the effects of alcohol, gets worse with depth pounded by cutaneous vasoconstriction in and causes cognitive and manual impair- cold temperatures, causes dehydration. ment. Similar effects will occur with other inert gases including helium and hydrogen Table 1.Effects of Depth on Lung Volume and Partial Pressures. (4). While the gas itself is not poisonous, it leads to a condition called nitrogen narcosis Effects of Depth on Partial Pressures (“being narked”), which can be fatal due to the effect of this condition on decision mak- Depth Lung Volume Pressure pO2 pN2 ing. Divers can be taught to acclimatise to Surface 100% 101kPa 21.2kPa 79.8kPa the milder effects of narcosis by building up 1 atmos to deeper dives gradually and regularly. It is not thought to be a physiological adaptation 10m 50% 202kPa 42.4kPa 159.6kPa but a learned response to repeated drills. As 2 atmos nitrogen is inert it is not metabolised by the body. It does, however, enter the body tis- 20m 33% 303kPa 63.6kPa 239.4kPa sues. During a dive the amount of nitrogen 3 atmos that enters the tissues is proportional to its 30m 25% 404kPa 84.8kPa 319.2kPa partial pressure in alveolar air. Therefore, as 4 atmos the dive becomes deeper, the amount of 258 Diving Medicine J R Army Med Corps: first published as 10.1136/jramc-151-04-07 on 1 December 2005. Downloaded from nitrogen entering the tissues will be greater. exposure to lower partial pressures causes Different tissues ‘on-gas’ at different rates progressive pulmonary fibrosis.