DAN medics and re­searchers an­swer your ques­tions about dive medicine

Scuba Diver Australasia + Ocean Planet - - Buyer's Guide -

Q: My stu­dents asked what hap­pens to the ni­tro­gen bub­bles in un­treated decompression sick­ness (DCS). My guess was that they even­tu­ally get re­ab­sorbed or of­f­gassed via the lungs. What is the best an­swer?

A: Divers ac­cu­mu­late ni­tro­gen (and/or other in­ert gases in their breath­ing mix) while div­ing. The deeper and longer the dive, the more gas ac­cu­mu­lates. In cases of DCS, the in­ert gas load ex­ceeds the tis­sues’ ca­pac­ity, so bub­bles form.

Gas en­ters the body through the lungs, moves into the cir­cu­la­tory sys­tem and then into other tis­sues. Of­f­gassing oc­curs by the same mech­a­nism in re­verse: In­ert gas moves from the body’s tis­sues into the blood­stream and then into the lungs, where it is ex­haled.

A diver of­f­gasses when in shal­low wa­ter af­ter having been in deeper wa­ter (dur­ing as­cent and while per­form­ing a decompression stop or safety stop, for ex­am­ple). Of­f­gassing con­tin­ues af­ter the diver ex­its the wa­ter. The hu­man body has no means by which to in­def­i­nitely re­tain gas or bub­bles.

The vast ma­jor­ity of the in­ert gas is of­f­gassed within a few hours, and al­most all of it leaves recre­ational divers’ bod­ies within about 24 hours. This is why symp­toms of DCS usu­ally ap­pear in the first hours fol­low­ing a dive and why on­set of symp­toms more than 24 hours af­ter a dive gen­er­ally does not rep­re­sent DCS (ex­cept in spe­cial cir­cum­stances such as sat­u­ra­tion div­ing or sub­se­quent al­ti­tude ex­po­sure, for ex­am­ple).

Bub­bles cause in­flam­ma­tion and lo­cal tis­sue in­juries. The larger the bub­ble load, the more se­vere the in­jury and the faster the on­set. Hy­per­baric cham­ber treat­ment within the first 24 hours can elim­i­nate gas and bub­bles while the in­jury is still oc­cur­ring. Af­ter about 24 hours, the in­jury has al­ready oc­curred. Fewer, if any, bub­bles are present, but the in­jury per­sists. Hy­per­baric treat­ment af­ter the first day can still be very help­ful by pro­mot­ing heal­ing and re­cov­ery.

Al­though leav­ing DCS un­treated is not rec­om­mended, its gen­eral progression is im­prove­ment over time. Some divers who do not get treated re­cover com­pletely, but oth­ers have per­sis­tent prob­lems that range from mild to se­vere.

[Frances Smith, MS, EMT-P, DMT]

Q: What is “oxy­gen ear”?

A: Also known as mid­dle-ear oxy­gen ab­sorp­tion syn­drome, oxy­gen ear de­scribes a gas vol­ume im­bal­ance in the mid­dle ear af­ter div­ing with breath­ing gas that has a higher oxy­gen frac­tion than air. The phe­nom­e­non is com­monly as­so­ci­ated with open-cir­cuit div­ing us­ing nitrox and closed-cir­cuit re­breather div­ing. The high-oxy­gen-con­tent gas fills the mid­dle-ear space over the course of the dive. Post-dive, the tis­sues metabolise the oxy­gen, re­duc­ing the to­tal gas vol­ume be­low what it would be if the space were filled with air. If this loss in gas vol­ume is not equalised, rel­a­tive neg­a­tive pres­sure will de­velop. This is in ef­fect a squeeze, which can present as ear full­ness, mild dis­com­fort and/or im­paired hear­ing.

This prob­lem can be avoided eas­ily with oc­ca­sional equal­i­sa­tion for sev­eral hours af­ter div­ing. A per­son who is ac­tive, talk­ing and/or laugh­ing dur­ing this pe­riod may have no need to ac­tively equalise. On the other hand, a per­son who goes to bed im­me­di­ately af­ter div­ing might wake up sev­eral hours later with mild dis­com­fort. Full res­o­lu­tion is best achieved us­ing gentle equal­i­sa­tion tech­niques.

[Neal W. Pol­lock, Ph.D.]

Q: I’m 20 years old and will be having four wis­dom teeth (third mo­lars) re­moved soon. I un­der­stand I’ll have air pock­ets where my teeth were, and I as­sume those could cause prob­lems while div­ing. How long should I stay out of the wa­ter?

A: Fol­low­ing an un­com­pli­cated dental ex­trac­tion, four to six weeks is nor­mally suf­fi­cient time for the risk of in­fec­tion to re­solve. This as­sumes good heal­ing and that gum tis­sue has filled in the empty sock­ets, elim­i­nat­ing any air pock­ets. An un­healed socket can be a route for in­fec­tion as well as for air to en­ter sub­cu­ta­neous tis­sues and cause fur­ther in­jury. Pain med­i­ca­tion can im­pair your judge­ment un­der­wa­ter, so wait at least a cou­ple of days af­ter you fin­ish tak­ing it to re­sume div­ing.

Fol­low­ing the ex­trac­tion of up­per wis­dom teeth, the den­tist should ver­ify that there is no si­nus in­volve­ment.

The root tip of a mo­lar can breach the max­il­lary si­nus floor, re­sult­ing in a com­mu­ni­ca­tion (ab­nor­mal con­nec­tion) be­tween the mouth and the si­nus. This is not com­mon, but if it oc­curs it will fur­ther de­lay your re­turn to div­ing. If there is si­nus in­volve­ment, you should wait un­til the fis­tula (hole) is closed and healed be­fore you re­sume div­ing. Your den­tist or oral sur­geon will be able to tell you how long this should take.

Dis­com­fort, ten­der­ness or de­layed heal­ing can make it dif­fi­cult to hold a reg­u­la­tor mouth­piece, depend­ing on the tooth or teeth in­volved and the length of the mouth­piece’s flanges. Af­ter your den­tist or en­dodon­tist ap­proves a re­turn to div­ing, con­sider tak­ing your scuba gear to a lo­cal pool. Swim laps un­der­wa­ter to con­firm that breath­ing through your reg­u­la­tor does not cause any dis­com­fort.

[Frances Smith, MS, EMT-P, DMT]

Q: A few days ago on a dive boat, I heard the captain say to the group some­thing to the ef­fect of: “Stay hy­drated – de­hy­dra­tion is the num­ber one cause of DCS.” Is this true?

A: While de­hy­dra­tion can be a risk fac­tor for DCS, it is al­most cer­tainly not the most im­por­tant. The dive pro­file is the lead­ing risk fac­tor. The tim­ing and in­ten­sity of ex­er­cise and ther­mal sta­tus are likely to play the lead sec­ondary roles. State of hy­dra­tion is no more than a ter­tiary fac­tor – con­ve­nient to blame, but gen­er­ally less dra­matic in im­pact. This is not to say that hy­dra­tion should be ig­nored, just that it must be kept in per­spec­tive.

Drink­ing an ex­ces­sive amount of fluid will not elim­i­nate the risk of DCS and can even work against safety by in­creas­ing a diver’s sus­cep­ti­bil­ity to immersion pul­monary oedema. As with most things, ex­tremes in ei­ther di­rec­tion can be haz­ardous. Sound hy­dra­tion is im­por­tant for gen­eral health and div­ing health, but ig­nor­ing the ele­phant in the room (the depth/time pro­file) and fac­tors such as ill-timed ther­mal and ex­er­cise stress is def­i­nitely not good prac­tice.

[Neal W. Pol­lock, Ph.D.]

OP­PO­SITE PAGE Most in­ert gas is of­f­gassed within a few hours of div­ing, which is why DCS symp­toms ap­pear soon af­ter a dive BE­LOW DCS oc­curs when the in­ert gas load ex­ceeds the tis­sues’ ca­pac­ity, form­ing bub­bles that cause in­flam­ma­tion

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