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DMT: Diving Medicine: Pressure Physiology, Barotrauma and Decompression Illness

DMT · Work through your exam, one topic at a time.

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Diving Medicine: Pressure Physiology, Barotrauma and Decompression Illness

The Diving Medicine domain tests whether you can predict what pressure does to gas-containing spaces and to dissolved gas loads, then turn that prediction into a diagnosis and a first action. Stems are short vignettes: a diver surfaces with a symptom, and you must name the problem, start the right treatment, and decide whether recompression is urgent.

Gas laws as clinical tools

  • Boyle (P1V1 = P2V2): explains every squeeze on descent (ear, sinus, mask, suit, tooth) and every expansion injury on ascent (pulmonary barotrauma, pneumothorax, mediastinal and subcutaneous emphysema, arterial gas embolism).
  • Dalton: the partial pressure of a gas is its fraction times absolute pressure, so PO2 at depth sets both oxygen dosing and oxygen toxicity risk.
  • Henry: dissolved gas is proportional to partial pressure, the basis of nitrogen loading and DCS.
  • Charles/Gay-Lussac: warmed gas expands, which matters for sealed spaces and for patient temperature.

Barotrauma versus decompression illness

  • Squeeze injuries occur on descent; expansion injuries occur on ascent.
  • Type I DCS: joint and limb pain, cutaneous symptoms. Type II DCS: neurologic, vestibular (vertigo, hearing loss, vomiting) or cardiopulmonary (cough, chest pain, hypovolemic shock).
  • AGE presents within seconds to minutes of surfacing and can follow a short, shallow breath-hold ascent. DCS usually appears within 15 minutes to hours.
  • Both are recompression emergencies: 100% oxygen, supine position, no further diving, urgent transport to a chamber. Imaging and labs must never delay recompression.

Oxygen limits you will be asked to calculate

  • Working limit for in-water diving: PO2 of about 1.4 ATA, with 1.6 ATA the ceiling for resting, shallow exposures.
  • HBO treatment tables run higher (2.4-2.8 ATA) because the bubble-reduction benefit outweighs seizure risk when air breaks are used.

Worked example

A diver breathes 32% nitrox at 30 msw. Absolute pressure = 30/10 + 1 = 4 ATA. PO2 = 0.32 x 4 = 1.28 ATA, acceptable. Maximum operating depth for a 1.4 ATA limit: (1.4 / 0.32 - 1) x 10 = about 33 msw.

Traps that cost marks

Assuming AGE needs a deep or long dive; classifying vestibular DCS as Type I; forgetting that a convulsing patient must never be decompressed (breath-holding at depth causes pulmonary barotrauma and AGE); confusing OTUs with UPTDs.