Hyperbaric Medicine: Physics, Chambers, and Indications
Core concepts
Hyperbaric oxygen (HBO) therapy delivers 100% oxygen at 2.0–3.0 ATA in a monoplace or multiplace chamber. Understand the pressure–volume and gas density consequences of compression, the oxygen window concept (reduced inert-gas tension during oxygen breathing), and the mechanisms of HBO: hyperoxygenation, vasoconstriction with preserved oxygen delivery, enhanced neutrophil killing, fibroblast and collagen activity, and bubble-size reduction via nitrogen washout.
The candidate must know the accepted indications and their rationale: decompression sickness and arterial gas embolism (first-line), carbon monoxide poisoning (including smoke inhalation and pregnancy considerations), clostridial myonecrosis, necrotizing soft-tissue infections, chronic refractory osteomyelitis, osteoradionecrosis, compromised flaps and grafts, crush injury and compartment syndrome, thermal burns, sudden sensorineural hearing loss, central retinal artery occlusion, and idiopathic sudden hearing loss. Know which are "primary" (DCS, AGE, CO) versus adjunctive.
Traps
Questions test absolute versus relative contraindications - untreated pneumothorax is absolute; bleomycin, doxorubicin, and disulfiram are relative. Expect distractors that list HBO for conditions without evidence (e.g., routine wound care, autism). Also expect confusion about chamber types and the need for an inside attendant in a multiplace chamber.
Worked rule of thumb
For CO poisoning, the half-life of carboxyhemoglobin is ~4–5 h on room air, ~90 min on 100% oxygen at 1 ATA, and ~20–30 min at 3 ATA. Use that ladder to justify HBO in severe cases (syncope, neuro findings, ischemia, pregnancy).