A rapid ascent from a dive subjects the human body to a sudden and extreme change in surrounding pressure. Tissues and air-filled spaces become dangerously compromised when they cannot adjust quickly enough to this rapid shift. This transition from high pressure underwater to lower pressure at the surface can lead to two distinct, life-threatening medical emergencies: decompression sickness and barotrauma. Understanding the physics governing these changes explains why a quick trip to the surface is an emergency situation.
The Physics of Pressure Change
The fundamental danger of a rapid ascent is rooted in two principles of physics that govern gas behavior. Henry’s Law describes how the amount of gas dissolved in a liquid is directly proportional to the pressure of that gas above the liquid. Underwater, elevated pressure forces gases, primarily nitrogen, to dissolve into the diver’s bloodstream and tissues at higher concentrations.
During a safe, slow ascent, the pressure reduction allows dissolved nitrogen to leave the tissues slowly and be exhaled. A rapid ascent causes the pressure to drop too quickly, resulting in nitrogen gas coming out of solution immediately, similar to opening a carbonated drink. This rapid off-gassing forms bubbles within the body, directly causing decompression sickness. Boyle’s Law explains the inverse relationship between the volume and pressure of a gas. As a diver rises, the external pressure decreases, causing the volume of any trapped gas within the body’s air spaces to expand dramatically.
Decompression Sickness: Symptoms and Types
The formation of nitrogen bubbles in the body, known as Decompression Sickness (DCS), is the most common consequence of insufficient decompression. These bubbles can obstruct blood flow, trigger inflammation, and mechanically damage tissues. The severity and location of symptoms determine the classification of the injury.
Type I DCS
Type I DCS is the milder form, primarily involving the musculoskeletal system, skin, and lymphatic system. The most recognized symptom is joint pain, often called “the bends,” typically affecting the elbows, shoulders, knees, and hips. Other manifestations include skin rashes, itching, and swelling. Although not immediately life-threatening, Type I DCS requires medical treatment to prevent long-term complications.
Type II DCS
Type II DCS is a far more severe injury, resulting from bubbles affecting the central nervous system (CNS), brain, or pulmonary system. Neurological symptoms are common, ranging from dizziness, visual disturbances, and confusion to significant motor weakness, paralysis, or loss of bladder control. Spinal cord involvement is particularly vulnerable to damage. Pulmonary DCS, sometimes called “the chokes,” is a rare but potentially fatal form where bubbles lodge in the lung blood vessels, causing chest pain and a persistent cough.
Barotrauma: Injuries from Expanding Air
Distinct from DCS, barotrauma is a mechanical injury caused by the expansion of air trapped within the body’s cavities, as dictated by Boyle’s Law. This injury is often linked to holding one’s breath during ascent or inadequate equalization. The most dangerous form is Pulmonary Barotrauma (lung overexpansion injury), which occurs when rapidly expanding air cannot escape the lungs.
The trapped air can rupture delicate lung tissue, forcing gas into surrounding areas, such as the chest cavity or under the skin. The most severe outcome is an Arterial Gas Embolism (AGE), where air forces its way into the pulmonary blood vessels and travels directly to the arterial circulation. These bubbles can block blood flow to the brain, producing stroke-like symptoms like loss of consciousness, seizures, or partial paralysis, often within minutes of surfacing.
Less severe, but more common, are barotraumas affecting the middle ear and sinuses. As the diver ascends, expanding air in these confined spaces can cause pain and tissue damage if the pressure cannot be equalized through the Eustachian tubes or sinus openings. While painful, these injuries are generally not life-threatening, unlike AGE or severe DCS.
Immediate Action and Prevention
If a diver exhibits any symptoms of decompression illness following a rapid ascent, immediate action is necessary.
- The injured person should be placed on 100% oxygen delivered by a non-rebreather mask to help reduce the size of nitrogen bubbles.
- Emergency medical services (EMS) must be contacted immediately.
- The emergency dispatcher should be informed of the suspected diving injury to facilitate transport to a facility with a hyperbaric oxygen chamber.
- Specialized resources, such as the Divers Alert Network (DAN), should also be contacted for expert consultation on emergency management.
The best defense against these injuries is prevention through adherence to safe diving practices. Divers should strictly monitor their ascent rate, never exceeding 30 feet per minute, to allow time for nitrogen off-gassing. Performing a safety stop, typically for three to five minutes at 15 to 20 feet, is a standard preventative measure to further reduce bubble formation. Divers must never hold their breath while ascending, as this is the primary cause of dangerous lung overexpansion injuries.