Undersea Medicine: Barotrauma, Gas Toxicity, and DCI Recognition
Core concepts
Undersea medicine is the clinical physiology of the diver in a water column. The candidate must be fluent in Boyle's law (pressure–volume relationships), Henry's law (gas solubility and inert-gas loading), Dalton's law (partial pressures of breathing gases), and Archimedes' principle (buoyancy and the hazards of rapid ascent). Know the pressure gradient: 1 atmosphere (atm) per 10 m of seawater (33 ft), so 30 m ≈ 4 atm absolute.
Barotrauma is the dominant acute injury. Middle-ear squeeze is the most common diving injury; pulmonary barotrauma with alveolar rupture is the most immediately lethal and produces pneumothorax, pneumomediastinum, and arterial gas embolism (AGE). Decompression sickness (DCS) is inert-gas bubble disease, distinguished from AGE by onset (DCS usually minutes to hours after surfacing; AGE typically within seconds to minutes of ascent) and by focal neurologic findings in AGE.
Gas toxicity: oxygen toxicity presents as CNS seizures (Paul Bert effect, deep, short latency) or pulmonary irritation ( Lorraine Smith effect, chronic exposure); nitrogen narcosis mimics alcohol intoxication at depth and resolves with ascent; CO2 retention causes headache, dyspnea, and narcosis. Hypoxia from a hypoxic mix at the surface is a classic trap.
Traps
Questions love the diver who surfaces asymptomatic and deteriorates en route, the "no symptoms so no treatment" distractor, and the assumption that shallow dives cannot cause DCS. Also expect confusion between AGE and DCS, and questions that omit the dive profile entirely.
Worked rule of thumb
Depth in meters ÷ 10 + 1 = atmospheres absolute. A diver at 20 m is at 3 ATA; a 10-L lung volume at depth holds 30 L at surface - the volume ratio is the injury potential on ascent.