May 2007

Vol. 22, No. 5     What's this?

Tank Blackout Can Be Fatal

from the May, 2007 issue of Undercurrent  

A few years ago, a 46-year-old man donned his scuba gear, dived into a 12-foot-deep freshwater canal to recover a lost motor, and died underwater. His friend could not see him due to the murky water but estimated afterwards that the bubbles coming to the surface stopped after just a few minutes. The diver was reportedly a good swimmer, not using drugs or alcohol, and his medical history was clean. The autopsy’s only findings were those of drowning. So what killed him?

His tank was a 71-cubic-foot steel cylinder, approximately three decades old. An investigation revealed the diver had filled the tank at a dive shop three months before his fatal dive, but the last hydrostatic test had been performed four years prior. The tank had been partially used after the last fill and then stored at home. An analysis of the remaining 200 psi revealed an oxygen concentration of just 2.5 percent. When sawed open, the tank had a markedly corroded interior with black scale and some orange-red spots of rust.

This was the case study highlighted by three doctors as the epitome of “tank blackout” during their presentation at the Undersea and Hyperbaric Medicine Society’s annual conference last June. Tank blackout is one type of hypoxic blackout, a generic term for a group of loss-of-consciousness causes in divers due to an insufficient supply of oxygen to the brain. Oxygen can be depleted because of improperly filtered air, a hydrostatic test that accidentally keeps moisture inside the tank, draining the tank down to no air at all, and pushing the purge button inadvertently when underwater.

In steel tanks especially, rust forms even with a small amount of water and consumes the oxygen, reducing its concentration below that needed to keep conscious while breathing. Salt water speeds up the rusting. After months of storage, even with fully pressurized air, a corroded tank can lose the vast majority of its oxygen. A diver goinfg underwater with a corroded tank can quickly experience dizziness, then stupor and eventually blackout. Tank blackout is rare but usually fatal because hypoxia spreads quickly, says Stuart Miller, M.D., director of education at Long Beach Memorial Medical Center’s Department of Hyperbaric Medicine and lead author of the study. “Few determinations of gas analysis are made of the tank’s remaining air after a dive-related injury or death, so it’s difficult to judge how many were caused by tank blackouts. Probably more than on record.”

Smart divers have an annual visual inspection of their tanks (V.I.P.). Tank manufacturers and dive agencies recommend that dive shops give an annual check, but there’s no requirement on the books for recreational divers. “They’re the ones most susceptible because when they rent or borrow tanks, they have no idea if those tanks have been properly cared for,” says Miller. He recommends that divers keep at least 50 psi in used tanks during storage and always test the gas content with a portable analyzer before use.