DAN Medics Answer Your Questions about Dive Medicine

In entry-level classes, students are told never to hold their breath on ascent for fear of a lung-expansion injury. What is the actual mechanism by which lungs are injured by expanding gas? And why exactly is the last bit of an ascent the most dangerous?

Lung-expansion injuries can be the most dramatic and life-threatening emergencies in scuba diving. They are generally a result of lung overinflation due to pathological air trapping (lung disease) or breathholding during ascent. A good understanding of lung anatomy is essential to comprehend the associated risks.
The main bronchi divide into smaller airways called bronchioles and continue to branch and reduce in size until they form the respiratory bronchioles, which terminate in the alveolar sacs. The alveoli are the key functional unit of the respiratory system where gas exchange takes place. These fragile air sacs are surrounded by a delicate membrane only one to two cell layers thick and are encompassed by a network of capillaries.

While exposed to atmospheric pressures at sea level, our lungs are in a state of equilibrium as we inhale and exhale. Slight pressure changes occur as we change elevation, yet equalization of the pressures inside and outside the lung is a passive and inconspicuous event with each breath. During descent into water, all gas-containing spaces in the body tend to shrink as the pressure surrounding the body increases; for example, the lung volume of a breathhold diver becomes smaller with the descent in the water column. Because scuba regulators deliver breathing gas at the ambient pressure of the diver, a higher concentration of the breathing gas enters the lungs, preventing the reduction in volume that would otherwise occur.

If the diver does not exhale during ascent, the lungs will progressively increase in volume until the elastic limit of the alveoli is exceeded and lung injury occurs. This forces gas into one of three locations: 1) the space within the chest cavity (pleural space), a condition known as pneumothorax; 2) the tissue planes within the lung itself (interstitial space), from where it may travel into the space around the heart, the tissues of the neck and the larynx (mediastinal emphysema); or 3) the blood. In this latter condition (arterial gas embolism, or AGE), gas bubbles can then pass from the pulmonary capillaries via the pulmonary veins to the left side of the heart and then to the carotid or basilar arteries (causing cerebral arterial gas embolism, or CAGE).

It is important to note that a breathhold ascent after inhaling from a scuba tank from a depth as shallow as 4 fsw may be sufficient to tear alveolar sacs, causing lung injury and one of these three disorders.

Boyle's law explains why changes in depth while in shallow water can be more hazardous than equivalent changes of depth in deep water. In essence, British physicist/chemist Robert Boyle discovered that at a constant temperature and mass, the volume of a gas is inversely proportional to the pressure exerted on that gas. When the pressure is doubled, the volume is reduced to one-half of the original volume. Conversely, when the pressure is reduced by one-half, the volume doubles.

For a diver at 15 fsw, the total pressure acting on the body is 1.5 atmospheres absolute (ata). A sudden ascent to the surface would therefore result in a 30 percent pressure reduction and assuming a compliant chest wall, a volume increase of 50 percent — which may result in lung injury.

When compared to a vertical change of 15 fsw occurring from a depth of 66 fsw, where the gas was compressed to 3 ata, the 0.5 atmosphere of depth change would result in only a 16 percent reduction in pressure and a 20 percent increase in lung volume — and thus, would be less likely to cause lung injury.
—Daniel A. Nord, BFA, EMT-P, CHT, director of DAN Medical Services

Q: I've heard varying opinions about the use of heat, ice and vinegar in treating cases of jellyfish stings. What do you actually recommend?

A: Given the distressing and painful nature of these injuries, it is no surprise that people will try just about anything to relieve symptoms. According to marine envenomation expert Lisa-ann Gershwin, Ph.D., of the Queen Victoria Museum in Tasmania, Australia, the best approach depends on whether one is in a tropical or nontropical environment. Since stings in nontropical environments are rarely life threatening, the primary goal is pain relief. In tropical environments, where a higher proportion of stings are life threatening, the primary goal is preserving life.

With this approach in mind, rescuers in a tropical environment should first stabilize the victim of a jellyfish sting and provide basic life support as necessary. Next, the sting site should be saturated with vinegar. Vinegar does not relieve pain, but it is the only substance known that will instantly denature 100 percent of box jellyfish (Cubozoa) nematocysts (stinging cells) and is considered safe for use on human skin. After vinegar has been applied, tentacles will be neutralized and can be removed. Ice can then be used for pain relief.

In nontropical environments, rescuers should endeavor to remove any visible nematocysts from the skin before they fire. Use seawater to rinse the sting site; freshwater (or ice) is known to cause marine nematocysts to discharge. Saltwater rinses can be very effective in removing the layer of nematocyst-rich slime that jellyfish leave on the skin, but tweezers or gloved hands will likely be required to remove tentacles. Ice can then be used for pain relief, as with tropical stings.

Although heat may relieve pain as well as (or perhaps even better than) ice, it is not generally recommended since heat dilates capillaries in the skin, increasing the circulation of venom throughout the body. There is one notable exception to the general rule of using ice for pain relief: confirmed Portuguese Man o' War (Physalia) stings in nontropical environments. Portuguese Man o' War stings should be immersed in 113ºF (45ºC) water for 20 minutes. This is about as hot as the patient will be able to tolerate. Because heat has not been shown to relieve symptoms of stings by other species as readily, ice is preferred for pain relief in stings by all other types of jellyfish.
—Brian Harper, EMT, DMT, DAN medical information specialist

Q: I recently had surgery; when can I dive again?

A: The type or complexity of the surgery directly affects recovery time. The act of diving does not necessitate specific consideration or a protracted recovery time following most uncomplicated surgeries. Exceptions to this general rule involve major organ surgery, i.e., heart, lung, brain, spinal cord and orthopedic repairs requiring hardware, joint and bone reconstruction. Your surgeon is the best resource to discuss specifics such as the procedure, recovery time and potential complications. If the surgery prompts the need for structured rehabilitation, it can be considered more complex and will most likely indicate a longer recovery period.

When can one safely return to diving after general surgeries? The essential advice is to refrain from diving activities until you are completely healed. The incision(s) should be well healed, and you should be free of pain and recovered from any post-operative complications. When your surgeon releases you for activity without restriction, you can consider a return to diving. Remember that despite complete healing, you may still need time to adequately recover strength, stamina and exercise capacity. Consider a cautious approach to your initial dives. Start in a controlled environment prior to remote travel or dives in rough conditions. The healing process often takes longer than we would like, so please be patient and follow these recommendations to increase your chances for a successful recovery.
—Marty McCafferty, EMT-P, DMT-A, DAN medical information specialist

Ask Us!
The DAN Medical Information Line is here to answer all your dive-related medical questions. You can reach the medical staff during regular business hours (Monday through Friday, 9 a.m. to 5 p.m. ET) by calling +1-919-684-2948, ext. 222. You can also submit an email at http://www.DiversAlertNetwork.org

© Alert Diver — Summer 2010