Rapid ascent from depth, most commonly associated with SCUBA diving, exposes the human body to a dramatic change in ambient pressure. This pressure reduction forces dissolved gases to come out of solution too quickly, leading to Decompression Illness (DCI). DCI is a serious medical emergency that can range from mild joint pain to catastrophic neurological injury and death. Understanding the underlying physics is essential, as the adverse effects are a direct consequence of how gases behave under varying pressures.
The Physics of Pressure Change
Understanding DCI relies on two fundamental gas laws governing how gases interact with the body underwater. Henry’s Law explains why inert gases, primarily nitrogen, dissolve into the body’s tissues under pressure. This law states that the amount of gas dissolved in a liquid is directly proportional to the partial pressure of that gas above the liquid. As a diver descends, increasing ambient pressure causes more nitrogen to dissolve into the blood and tissues, a process called “on-gassing.”
The problem arises during ascent when ambient pressure rapidly decreases. This pressure drop causes dissolved nitrogen to come out of solution, similar to opening a carbonated soda bottle. Boyle’s Law states that pressure and volume are inversely proportional for a fixed amount of gas. As a diver surfaces, decreasing pressure causes gas volumes within the body to expand. This expansion is responsible for both the formation of nitrogen bubbles in the tissues and mechanical over-expansion injuries to the lungs.
Decompression Sickness (The Bends)
The most recognized consequence of surfacing too quickly is Decompression Sickness (DCS), often called “the bends.” DCS occurs when dissolved nitrogen leaves the tissues too rapidly, forming bubbles within the blood vessels and tissues. These bubbles cause damage through mechanical blockage and inflammatory responses. Symptoms typically manifest within an hour of surfacing, though they can be delayed.
DCS is categorized into two types based on severity. Type I DCS is the less severe form, primarily affecting the musculoskeletal system and skin. The most common symptom is deep, dull, or aching pain in the joints and surrounding muscles, often in the shoulders and elbows. Other symptoms include skin mottling, itching, a rash known as cutis marmorata, and unusual fatigue.
Type II DCS is severe, involving neurological, spinal cord, or cardiopulmonary systems. The spinal cord is particularly vulnerable, with symptoms ranging from numbness, tingling, and weakness to partial paralysis or loss of bladder control. Inner ear involvement can cause vertigo, dizziness, and hearing loss. Neurological DCS can present with stroke-like symptoms, and a rare form called “the chokes” causes shortness of breath and chest pain due to bubbles in the pulmonary circulation.
Pulmonary Barotrauma and Arterial Gas Embolism
While DCS results from dissolved gas, Pulmonary Barotrauma and Arterial Gas Embolism (AGE) are immediate mechanical injuries caused by gas expansion, as predicted by Boyle’s Law. Pulmonary Barotrauma, or lung over-inflation injury, occurs if a diver holds their breath during ascent or if pre-existing lung issues trap air. As ambient pressure drops, the expanding gas cannot escape the lungs, leading to the rupture of lung tissue.
This rupture allows gas bubbles to escape the lungs and enter the pulmonary circulation. The most serious outcome is an Arterial Gas Embolism (AGE), where these bubbles travel through the heart into the systemic arterial circulation. They frequently lodge in the small arteries supplying the brain, causing immediate, stroke-like symptoms.
AGE is the most immediate and dangerous acute risk of rapid ascent, with symptoms often appearing within minutes of reaching the surface. A diver may experience sudden unconsciousness, visual disturbances, seizures, paralysis, or confusion. The sudden onset and severity of AGE differentiate it from the slower progression of DCS, making it a life-threatening emergency requiring immediate medical intervention.
Emergency Response and Treatment
Decompression Illness (DCI) covers both DCS and AGE, and the emergency response is similar for both. The most immediate first aid measure is administering 100% oxygen using a tight-fitting mask. This helps shrink nitrogen bubbles and accelerate the elimination of inert gas from the body. The affected person should be laid down, kept warm, and monitored for signs of shock or unconsciousness.
It is imperative to contact emergency medical services immediately and inform them that DCI is suspected, as specialized care is needed. The definitive treatment for all forms of DCI is Hyperbaric Oxygen Therapy (HBOT), or recompression therapy. This involves placing the patient in a sealed chamber where atmospheric pressure is increased to simulate depth.
The increased pressure physically shrinks the gas bubbles, reducing their damaging effect, and forces nitrogen back into solution. The patient then breathes pure oxygen, which creates a steep gradient to help the body rapidly eliminate excess nitrogen. Treatment is often lengthy and requires immediate transport to a facility with a hyperbaric chamber. The sooner HBOT begins, the better the outcome and the lower the risk of permanent neurological damage.