DIVE LOG AUSTRALASIA FEB 2025 ISSUE 410

activity is not within everyone’s level of capability. Moreover, care needs to be taken not to become dehydrated if the exercise is conducted very proximal to diving. Finally, because exercise after diving is not a good idea (see below) it is a difficult strategy to implement in multi-day diving over sequential days. Exercise at other stages of dives has mixed implications. Heavy exercise during bottom time is a definite disadvantage because exercise increases blood flow through tissues and thereby enhances the uptake of inert gas breathed at higher pressures during the bottom phase of the dive. A sensible perspective should be maintained on this issue. Obviously, some exercise is required to swim around and enjoy a dive, but what should be avoided, if possible, is prolonged heavy exercise at depth. Diver propulsion vehicles (DPVs) or “scooters” are a great help in this regard, and the group arguably most likely from their use is technical divers performing decompression dives. Exercise during decompression is potentially beneficial for the same reason. That is, continued mild exercise whilst decompressing enhance blood flow through tissues and thereby helps carry inert gas which is now leaving tissues back to the lungs. Rather than simply hanging at a safety stop or decompression stop, divers could usefully maintain gentle finning which will help outgassing. Note however, that more is not necessarily better in this setting because heavy exercise can promote the transfer of venous bubbles into the arterial circulation (see below). Exercise after a dive, particularly deep or decompression dives, is generally discouraged. There is a period (lasting hours) after surfacing when tiny bubbles often form in the veins. They are carried back to the lungs where they get trapped in the lung capillaries and the gas inside them is absorbed. In the vast majority of situations these bubbles are harmless. However, if a diver indulges in exercise while these bubbles are arriving in the lung circulation then there is a greater chance of them failing to be filtered out by the lungs. If these bubbles pass through the lungs into the arterial circulation, there is more potential for them cause harm. In addition, if the diver has a patent foramen ovale communication between the two upper heart chambers, exercise may make it more likely that the bubbles will cross the PFO from the veins into the arterial circulation. Divers should generally avoid significant exercise, including heavy lifting and straining for at least 4 hours after diving. In summary, if we want a simple characterization of the relationship between exercise and diving, it might

go like this: before = good, bottom = bad, decompression = good, surface = bad. No need to be obsessive about it, but there are options for risk reduction for those interested in taking them.

Professor Simon Mitchell MB ChB, PhD, DipOccMed, DipAdvDHM (ANZCA), FUHM, FANZA Simon works as an anaesthesiologist at Auckland City Hospital and is a Professor of Anaesthesiology at the University of Auckland. He provides on-call cover for the diving emergency service in New Zealand. He is widely published with two books and over 160 scientific journal papers or book chapters. He co authored the hyperbaric and diving medicine chapter for the last four evrsions of Harrison’s principles of Internal Medicine. He has been Editor-in-Chief of Diving and Hyperbaric Medicine Journal since 2019. He has twice been Vice President of the UHMS and in 2010 received the society’s Behnke Award for contributions to the science of diving. Simon has a long career in sport, scientific, commercial, and military diving. He was first to dive and identify three historically significant deep shipwrecks in Australia and New Zealand, including one in 2002 which was the deepest wreck dive undertaken at the time. He was conferred Fellowship of the Explorers’ Club of New York in 2006, and was the Rolex Diver of the Year in 2015. His most recent expeditions were the Pearse Resurgence cave exploration (New Zealand) in February 2020, a project to take arterial blood gas specimens from an elite freediver at 60m in January 2021, and hunting a shipwreck in sub-Antarctic in February 2022.

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Dive Log Australasia #410 February 25