Phosphodiesterase type 5 (PDE5) inhibitors — such as Viagra, Cialis, Levitra, Vivanza, Mvix and Lodenafil — are a class of popular drugs prescribed to treat erectile dysfunction and are often sold on the black market as sexual-function enhancers. It is reasonable to assume that many divers use PDE5 inhibitors while on a diving vacation, although the drugs’ possible effects on decompression safety have not been studied previously. In a recent paper, Blatteau et al.¹ presented the results of a study on rats treated with sildenafil (Viagra) and then exposed to a simulated dive.

PDE5 inhibitors increase blood flow by prolonging the life of cyclic guanosine monophosphate (cGMP), a molecule that relaxes the vascular smooth muscle. This effect is present overall, but it is most expressed in penile erectile tissues, the lungs and, in a lesser degree, the brain. Another molecule that affects cGMP is nitric oxide (NO); an increased level of NO in tissues increases blood flow, and the absence of NO reduces blood flow.

In diving, modifications of blood flow affect the on-gassing and off-gassing of inert gas. The summary effect of PDE5 inhibitors on the outcome of decompression is not known. NO reduces the risk of decompression sickness (DCS); in addition to its effects on blood flow, NO also reduces bubbles, protects inner vascular surfaces and diminishes inflammatory reactions, including platelet consumption. Blatteau et al. have shown that rats pretreated with the PDE5 inhibitor sildenafil (Viagra) suffered more DCS than untreated ones (34.3 percent vs. 6.25 percent), and more pretreated rats died than untreated ones (34.3 percent vs. 9.4 percent).

The authors believe that the increased occurrence of DCS in sildenafil-treated animals may be related to vasodilation in the central nervous system (CNS), with increased cerebral blood flow previously described by Demchenko et al.² This effect may increase the load of inert gas during the dive and thus create conditions for local generation of bubbles and severe neurological DCS. The previous study showed that PDE5 inhibitors reduce the onset time of convulsions caused by oxygen toxicity.

This study was done on an animal model, which is commonly used for DCS studies. Human data are not available, but it is prudent to assume that sildenafil may have the same effects in humans and thus should be avoided while diving. If PDE5 inhibitors are prescribed for pulmonary hypertension or prevention of immersion pulmonary edema, divers should discuss their options with a diving medicine specialist.

References

1. Blatteau J-E, Brubakk AO, Gempp E, Castagna O, Risso J-J, et al. (2013) Sidenafil Pre-Treatment Promotes Decompression Sickness in Rats. PLoS ONE 8(4): e60639. doi:10.1371/journal.pone.0060639

2. Demchenko IT, Ruehle A, Allen BW, Vann RD, Piantadosi CA (2009) Phosphodiesterase-5 inhibitors oppose hyperoxic vasoconstriction and accelerate seizure development in rats exposed to hyperbaric oxygen. J Appl Physiol 106(4): 1234–1242. doi: 10.1152/japplphysiol.91407.2008