Simplified “Diving medicine”

1. Simplified “Diving medicine” HP Shum, Oct 2010 Ref.: Introductory course in dive medicine, Royal NZ Navy Slark Hyperbaric Unit

2. History • Breath holding diving started since 4500BC • Dived for food, pearls, sponges and shell • Reclaim sunken valuables, and sometime for military purpose

3. History • Diving with equipment started since 300BC • Snorkel • Leather breathing bladders • Diving bells • Diving helmet • SCUBA diving +/- close breathing system

8. Physiological response during diving • Respiratory system • Increase airway resistance due to change of gas density • Decrease vital capacity by 10-15% • Decrease lung compliance • Increase work of breathing • V/Q mismatch (increase in perfusion of lung units due to capillary engorgement, decrease ventilatioin due to reduce lung compliance coupled with increased airway resistance)

9. Physiological response during diving • Cardiovascular system • Redistribution of peripheral blood into central circulation • Increase stroke volume • Bradycardia • Increase CO • BP unchanged

10. Physics of diving • Diver is subjected to atmospheric pressure plus pressure exerted by the water – absolute pressure

11. Pascal’s principle • Pressure exerted anywhere in a confined incompressible fluid is transmitted equally in all directions throughout the fluid such that the pressure ratio (initial difference) remains same

12. Pascal’s principle • Underwater blast – blast wave transmitted through the water and body until they reach a liquid-gas interface where extensive damage may occur

13. Ideal gas law • The product of absolute gas pressure (P) and volume (V) must equal the product of the number of moles of gas (N), the gas constant (R) and the absolute temp (T) • PV=NRT

14. Boyle’s law • Describes the inversely proportional relationship between the absolute pressure and volume of a gas, if the temperature is kept constant • P1V1=P2V2 • Most damage done by expanding gases during decompression of divers occurs close to the surface

15. Charles’s law • At constant pressure, the volume of a given mass of an ideal gas increases or decreases by the same factor as its temperature on the absolute temperature scale • V1/T1=V2/T2

16. Gas solubility • Henry’s law -> at a constant temperature, the amount of a given gas dissolved in a given type and volume of liquid is directly proportional to the partial pressure of that gas in equilibrium with that liquid • Increase temp decrease gas solubility -> more gas dissolve at cold water during diving and release during re-warming • Nitrogen has greater solubility in lipid than He, which will increase risk of narcosis

17. Complications of diving • Decompression illness • Inert gas nacrosis • Other complications

18. Decompression illness • Occur when bubbles form in blood or tissue during or after a decrease in environmental pressure • Can occur in compressed gas divers, aviators and astronauts

19. Bubbles formation • Partial pressure of inert gas dissolved within the tissue during a dive may exceed ambient pressure during ascent • “Supersaturation” occur and provoke bubble formation • Bubbles form first in tissue • Microbubbles developed over the endothelium and subsequently form large bubbles which detach from vessel wall and causing gas embolism

21. Bubbles formation • Most arterial bubbles arise from tissue and veins but can also from pulmonary barotrauma • Lung providing good bubble filtering effect but can be overwhelmed by excessive volume of bubbles, pulmonary vasodilation (from oxygen toxicity / bronchodilator use) and presence of pulmonary AV communication

22. Bubble – mechanical effect • Inside non-compliance tissue causing arterial, venous, nerves, lymphatic and sensory cells compression

23. Bubble – biochemical effect • Activate inflammatory and coagulation cascade • Extravasation of fluids resulting in hemoconcentration

24. Decompression illness • Previously divided into decompression sickness, arterial gas embolism and other forms of pulmonary barotrauma • New system based on clinical descriptive classification • Acute vs. chronic • Evolution of symptom and signs (static, progressive, relapsing, resolving) • Organ involvement • DCI • Evidence of barotrauma • eg acute progressive cerebral DCI with mediastinal emphysema

25. Non specific symptom • Fever • General malaise • Headache • Lethargy

26. CNS • Rapid onset • Beware of associated pulmonary barotrauma • Sensory loss • Confusion • Psychosis • Seizure

27. Spinal cord • Low flow venous system predispose to bubbles formation • Sensory loss • Paralysis • Urinary retention

28. Inner ear • Either by barotrauma or by bubble formation • Vertigo • Nausea and vomiting • Hearing loss • Tinnitus

29. Others • Musculoskeletal • Polyarthralgia • Myalgia • Skin • Non specific erythema • Purpura • Pruritus • Hematological • Hemoconcentration • coagulopathy

30. PFO and DCI • High proportion of divers with neurological DCI had a PFO • However …. PFO is very common but neurological DCI is very rare … Why???

31. PFO and DCI • Inner ear DCI strongly linked with presence of large Rt to Lt shunt (eg PFO) • If arterial bubbles reach the labyrinthine artery (internal auditory artery), they must also be distributing widely in the brain ….. • This discrepancy could be explained by slower inert gas washout from the inner ear after diving and the consequent tendency for arterial bubbles entering this supersaturated territory to grow because of inward diffusion of gas

32. The models predict half-times for nitrogen washout of 8.8 min and 1.2 min for the inner ear and brain, respectively. The inner ear remains supersaturated with nitrogen for longer after diving than the brain. The prolonged inner ear inert gas supersaturation contributes to the selective vulnerability to short latency decompression sickness in divers with right-to-left shunt. Mitchell et al.: J Appl Physiol. 2009 Jan;106(1):298-301

33. Risk factors of DCI • Exceed diving table limits • Rapid ascent • Omitted decompression • Repetitive diving • Flying after diving (0.74 bar in commercial flight cabin) • Heavy work during diving • Age • Underlying illness

35. Inert gas narcosis • Nitrogen is the most important inert gas • Compressed air is the commonest breathing mixture for hyperbaric environment • Becomes significant if partial pressure of nitrogen >3.2 bar (ambient pressure >4 bar or >30MSW) • High variation in individual susceptibility

37. Management of injured diver • ABC + fluid resuscitation • High flow O2 • Supine position • Mx of hypo or hyperthermia • History taking • Duration and depth of dive • No. of ascent and nature • Symptom onset associated with dive process • PE (based on system involvement) • No just focus on diving specific Cx, should beware of diving non-specific conditions like fracture/ hemorrhage • If DCI suspected, transportation should not exceed 300m above sea level. Helicopter / aircraft pressured to sea level during flight should be considered

38. Recompression and HBOT • Reduce bubble size • Increase gradient for diffuse of inert gas out of bubbles

39. Decrease bubble size • Increase surface tension pressure and causing resolution of bubbles • Increase surface area of bubbles which enhance gas diffusion out of the bubbles • Decrease length of gas column and allow perfusion pressure to push it into the venous circulation • Reduce compression of adjacent structures • Decrease bubble tissue and bubble blood interface and decrease the inflammatory process

41. Oxygen use • Increase diffusion gradient of gas (nitrogen) from bubbles • Avoid further inert gas exposure • May improve hypoxia • Bubbles will grow for several min and then get progressively smaller • Rarely cause decompression illlness • But … risk of O2 toxicity causing CNS/ pulmonary damage and potential fire risk

42. Pressure during treatment • 2.8 bar is based on limiting O2 toxicity and not on predictions of bubble behavior • Risk of O2 induced convulsion <1%

43. Other treatments • Steroid -> not useful • Anti-PLT agent -> no useful • Anticoagulant -> potential bleeding complications • Lignocaine -> neuro-protective, induce deceleration of ischemic ion fluxes across the neuronal cell membrane and prevention of the consequent neurotoxic events, lowers neuronal metabolism, potent anti-inflammatory effect, can be considered in cerebral arterial gas embolism (CAGE) Undersea Hyperb Med. 2001 Fall;28(3):165-74.

44. Questions and comments