Effects of Aspirin on Diving

Divers who take aspirin should dive conservatively to minimize their risk of DCI and be sure they are free from congestion (i.e., have no difficulty equalizing).

My doctor recommended that I take an 81 mg aspirin (a "baby aspirin") every day. Has there been any research into how this might affect recreational divers' risk of decompression sickness (DCS)?

Though there is insufficient space here to address every aspect of this question, my colleague and I will address a few of the major issues. First, whatever research paper we read will likely have participants that differ in some way from recreational divers (they might be Navy divers, for example) or environmental conditions that differ (they might involve hyperbaric chamber dives rather than submersions). Therefore, whenever we look to research for definitive guidance we look for a meta-analysis that summarizes everything that has been shown in gold-standard human experiments. Unfortunately, there has not yet been a meta-analysis of predive aspirin use in divers, so we must look at individual studies. This is a bit like looking at photographs in that they show only snapshots of particular moments and may not convey the totality of what was going on at the time.

My colleagues and I found that giving rats a daily dose of aspirin for two days before hyperbaric diving reduced the incidence of DCS after a very specific compression profile. The limitations of this single study, however, outweigh the practical lessons we can take from it. In particular, the profile was well beyond what recreational divers might dive, the experiment was in a chamber instead of in the water, the dose was very high, and the signs of DCS in rats are different from those commonly seen in recreational divers. Although the results of this small study were not statistically significant, the experiment did suggest that high doses of aspirin might have a protective benefit (in rats).1

This was a potentially important finding because in 2014 the U.S. Food and Drug Administration (FDA) denied an application to recommend a daily dose of aspirin for primary prevention of heart attack or stroke.2 The FDA has approved recommending aspirin for secondary prevention (in patients who had already had a stroke or heart attack, for example), but it requires more evidence before it will approve aspirin for primary prevention (prescribed as a preventive measure to people who are at elevated risk of a stroke or heart attack). The U.S. Preventive Services Task Force recently reviewed the available evidence and recommended aspirin for use by certain "at-risk" individuals over age 50 following consultation with their primary care provider.3 Given the aging population of divers in the U.S., this could affect many divers.

In some hyperbaric treatment facilities, aspirin is prescribed as an adjunct to hyperbaric therapy based on the hypothesis that it will "thin" the blood and increase offgassing, thus increasing the efficacy of hyperbaric oxygen therapy (HBOT).4 Although this sounds prudent, the evidence supporting its prescription is controversial; what are needed next are randomized clinical trials in which some patients receive aspirin and others do not. Until then we will not know for sure if aspirin is beneficial in HBOT. Taking it before diving, however, is another matter.

In terms of regular predive administration, such as in divers who take an aspirin a day, this is a vexing question, so I turn to my colleague in medicine to describe some potential considerations.

— Peter Buzzacott, MPH, Ph.D.,
DAN Director of Injury Monitoring and Prevention

As mentioned previously, even the decision to take aspirin on a daily basis is not clear-cut. You must discuss with your physician the risks and benefits of taking this remarkable and inexpensive medication. Even with a large body of evidence supporting aspirin use in the prevention of cardiovascular disease and some cancers, the risk of gastrointestinal and intracranial bleeding moderates its universal prescription for those it might benefit.

Introducing your desire to dive into the discussion may have an impact on the decision for you to take low-dose aspirin on a daily basis. I will primarily discuss the effects of its anticlotting function since this has the main impact on divers. In the future, research may prove its anticancer effects, and this may also have a direct impact on the diver.

Aspirin is a potent medication that has antithrombotic effects by affecting platelet function. Platelets play an important role in the clotting function. This effect can be seen with low doses of aspirin (baby aspirin) and is supported by multiple studies. The analgesic effects of aspirin appear to require higher doses to achieve analgesia by yet a different cellular pathway with an associated increase in bleeding risk.

The same bleeding risks (gastrointestinal, intracranial, etc.) for the general population affect divers in even more locations in the body. The problem for divers is that this antithrombotic or anticlotting effect can have negative effects when impacted by a diving injury from barotrauma and possibly severe neurologic decompression illness (DCI).

Bleeding from trauma that includes injuries from barotrauma may experience increased bleeding. This may occur with middle-ear or sinus barotrauma (squeeze). These areas do not allow easy access to control such bleeding.

A concern by many diving medicine experts is that with serious neurologic DCI, hemorrhage or bleeding has been seen in sensitive neurologic tissue. With aspirin-induced anticoagulation, this bleeding may take on increased significance for both treatment response and long-term outcome.

Nonsteroidal medications, which share some of the same mechanisms of action as aspirin, have been used in conjunction with the hyperbaric treatment of DCI. Those investigators reported some benefit in the number of hyperbaric treatments required. Some benefit of its analgesic properties may account for this. Unfortunately, this effect will probably not be seen with low-dose aspirin since the cellular pathway producing this analgesia is minimally affected.

In addition to the discussion of studies that outline the risks and benefits of low-dose aspirin for the general population, divers must also consider with their physician concerns over increased bleeding in the event of barotrauma or decompression injury. If taking low-dose aspirin, divers should pay strict attention to the ability to equalize, have no evidence of congestion or upper-respiratory illness and employ conservative measures to prevent DCI.

— Jim Chimiak, M.D.,
DAN Medical Director

1. Lambrechts K, Pontier JM, Mazur A, Theron M, Buzzacott P, Wang Q, et al. Mechanism of action of antiplatelet drugs on decompression sickness in rats: a protective effect of anti-GPIIbIIIa therapy. J Appl Physiol. 2015; 118(10):1234-9.

2. Kux L. Citizen petition denial response from FDA to Bayer Healthcare LLC. Rockville, Md.: FDA; May 2, 2014. Available at: http://www.regulations.gov/document?D=FDA-1977-N-0018-0101. Accessed Sept. 21, 2016.

3. Whitlock EP, Burda BU, Williams SB, Guirguis-Blake JM, Evans CV. Bleeding risks with aspirin use for primary prevention in adults: a systematic review for the U.S. Preventive Services Task Force. Ann Intern Med. 2016; 164(12):826-35.

4. Bessereau J, Coulange M, Genotelle N, Barthélémy A, Michelet P, Bruguerolle B, et al. [Aspirin in decompression sickness] (French). Therapie. 2008; 63(6):419-23.

© Alert Diver — Q1 Winter 2017