Copyright © 2004 by the European Society of Cardiology.
Letter to the Editor
Risk of decompression illness among 230 divers in relation to the presence and size of patent foramen ovale
Military Hospital Brussels, Centre for Hyperbaric Oxygen Therapy, Bruynstraat 200, 1120 Brussels, Belgium
* Corresponding author. Tel.: +32 2 264 48 68; fax: +32 2 264 48 61 (E-mail: peter.germonpre{at}mil.be).
Dear Sirs,
Sandra Rea Torti et al., in the June issue of the Journal,1 report on a group of divers, examined for patency of the foramen ovale (PFO), and claim to be able to calculate the relative risk for decompression illness (DCI) when a diver has a PFO.
Where it is true that in large samples with a low occurrence of a disease in proportion to a risk factor, the Odds Ratio may approach the value of the Relative Risk, in this series that claim is unjustified.2
First of all, the authors undertook a retrospective study. Relative Risks can only be reliably calculated from prospective analyses. This is the reason why we ourselves refrained from calculation of the RR in our publication3 we did however (in spite of what Torti et al., pretend) calculate the Odds Ratio, which we found to be 5.6 for 'undeserved cerebral decompression sickness' in divers with Grade 2 PFO.
Secondly, the subject selection is not detailed: was it a random sample out of the sports divers federation, or were divers selected upon their response to a call for participation? If so, what measures were taken to ensure that divers would not present on the basis of previously experienced symptoms that might have been attributable to DCI? Such a selection bias is most likely, as is the fact that these divers would not 'admit' they had had DCI.4 In the 'no-PFO' group there were 10 out of 167 divers (5.9%) who had experienced serious DCI. It is easy to calculate how, if only 10 more divers would have 'self-selected' on the basis of previous DCI symptoms, the Odds Ratio would be halved, with 95% confidence intervals of 1.4–6.0.
The article by Torti, and most likely the way the abstract was written, has caused quite some concern in sports diver's lay literature. In these articles, it is concluded that PFO is a major risk factor and that closure of PFO should be considered in all sports divers. Torti et al., mention only in brief terms near the end of their report, that diving with a PFO can be as safe as diving without a PFO, if the dive profile is conservative enough not to produce venous nitrogen bubbles upon ascent and after surfacing. It is the inappropriateness of the currently used dive profiles, not the PFO, which is the cause of decompression illness. The Divers Alert Network Europe, an international diving safety organization, is conducting studies aimed at developing adapted dive profiles towards 'low-bubble' decompression.5
Finally, we were delighted to find a bibliography reference pointing towards a possible "opening" of the PFO in saxophone players.6 Although we have proposed this mechanism to be possible in sports divers already in 2002,7 we have not been aware of medical literature backing up this hypothesis in other groups of people. Imagine our disappointment when the cited reference did not contain any mention of PFO!
References
- Torti SR, Billinger M, Schwerzmann R, et al. Risk of decompression illness among 230 divers in relation to the presence and size of patent foramen ovale Eur Heart J 2004;25:1014-1020.
[Abstract/Free Full Text] - Motulsky H. Intuitive biostatisticsNew York: Oxford University Press; 1995.
- Germonpre P, Dendale P, Unger P, et al. Patent foramen ovale and decompression sickness in sports divers J Appl Physiol 1998;84(5):1622-1626.
[Abstract/Free Full Text] - Hagberg M, Ornhagen H. Incidence and risk factors for symptoms of decompression sickness among male and female dive masters and instructors – a retrospective cohort study Undersea Hyperb Med 2003;30(2):93-102.[ISI][Medline]
- Marroni A, Bennett P, Cronjé F, Cali Corleo R, Germonpre P, Pieri M, et al. Comparison of precordial doppler detected bubbles and computer mesured gas tensions in dives to 25 m with linear ascents, deep and shallow stops Undersea Hyperb Med 2004;31:233-243.[ISI][Medline]
- Kinra S, Okasha M. Unsafe sax: cohort study on the impact of too much sax on the mortality of famous jazz musicians Brit Med J 1999;319:1612-1613.
[Free Full Text] - N'Guyen A-F, Germonpré P, Balestra C, et al. Time related opening of the foramen ovale in divers Eur J Underwater Hyperb Med 2002;3(3):73.
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