ABPM Undersea and Hyperbaric Medicine cheat sheet

Absolute pressure in seawater (ATA) = (depth in metres ÷ 10) + 1; in freshwater use depth ÷ 10.3. Thus 30 m seawater = 4 ATA.

Boyle's law: at constant temperature, gas volume is inversely proportional to absolute pressure; a bubble at 10 m seawater (2 ATA) occupies half its surface volume and at 20 m (3 ATA) one third.

Dalton's law: total pressure of a mixture is the sum of partial pressures; nitrogen partial pressure in air at 30 m seawater = 0.79 × 4 ATA = 3.16 ATA.

Henry's law: dissolved inert gas is proportional to partial pressure and solubility, so deeper and longer dives increase tissue nitrogen loading and subsequent bubble risk.

Ascent rate should not exceed about 9–10 m/min (30 ft/min); a 3–5 minute safety stop at 5 m is standard after recreational no-stop dives.

Oxygen partial pressure limits: 1.4 ATA for recreational diving and 1.6 ATA for working dives; above 1.6 ATA the risk of CNS oxygen toxicity seizures rises steeply.

Nitrogen narcosis impairs judgement and dexterity from about 30 m, becomes marked beyond 60 m, and resolves on ascent; helium-based mixtures reduce it.

Decompression sickness: Type I involves joints, skin, or lymphatic symptoms; Type II involves neurological or pulmonary (chokes) symptoms. Give 100% oxygen, lie supine, and arrange urgent recompression.

Middle ear barotrauma is the most common diving injury; pulmonary barotrauma can force gas into the circulation and cause arterial gas embolism, often with collapse soon after ascent.

HBOT is breathing near-100% oxygen at pressures above sea level, most commonly 2.0–2.5 ATA for routine wound and radiation indications.

Core mechanisms include hyperoxygenation of hypoxic tissue, vasoconstriction with reduced oedema, enhanced neutrophil killing, fibroblast activity, collagen synthesis, and angiogenesis.

Untreated pneumothorax is the only absolute contraindication; relative contraindications include bleomycin, disulfiram, doxorubicin, uncontrolled seizures, and severe COPD with CO2 retention.

Carbon monoxide poisoning: COHb half-life is about 320 minutes on room air, 90 minutes on 1 ATA oxygen, and 20–30 minutes at 2.5–3 ATA.

US Navy Treatment Table 6 is the standard recompression table for decompression sickness: 2.8 ATA oxygen cycles with air breaks followed by gradual ascent, lasting about 4.5 hours.

Accepted indications include decompression sickness, arterial gas embolism, carbon monoxide poisoning, clostridial myonecrosis, necrotising soft tissue infection, crush injury, compromised flaps and grafts, chronic refractory osteomyelitis, diabetic foot ulcers (Wagner grade 3 or higher), osteoradionecrosis, radiation cystitis or proctitis, thermal burns, and idiopathic sudden sensorineural hearing loss.

Air breaks of about 5 minutes between oxygen cycles reduce pulmonary and CNS oxygen toxicity during prolonged treatment tables.

Hyperbaric oxygen supports compromised flaps and grafts by increasing oxygen diffusion, reducing oedema, and promoting neovascularisation.

Fire risk is the dominant chamber hazard because raised pressure and oxygen concentration greatly accelerate combustion; the fire triad of fuel, ignition source, and oxygen must be controlled.

Multiplace chambers are compressed with air and patients breathe oxygen by mask or hood; chamber oxygen concentration is kept below about 23.5% by volume.

Pre-dive checks include chamber pressure integrity, air and oxygen supply reserves, communications, fire suppression, medical equipment, and attendant readiness.

Compression is slowed or paused when a patient cannot equalise middle ear pressure; Valsalva, swallow, or Frenzel manoeuvres are performed before pressure is increased further.

A trained inside attendant is required in multiplace chambers to assist with masks, equalisation, medical problems, and emergencies.

A medical lock allows drugs, equipment, or specimens to be passed into a pressurised chamber without decompressing the main compartment.

In a chamber fire, oxygen delivery is stopped, the fire is suppressed, and decompression is carried out as rapidly as safety permits while occupants breathe air and protect their airways.

Relief valves and pressure-testing programmes protect against overpressurisation; backup gas supplies must cover the full treatment and decompression period.

US commercial diving is regulated under OSHA 29 CFR 1910 Subpart T, which requires dive plans, a designated supervisor, tenders, decompression tables, and medical fitness.

Flying after diving: wait at least 12–18 hours after no-stop recreational dives and 24 hours after decompression dives before ascending to altitude.

Saturation diving keeps workers at storage depth for days or weeks, with a single decompression at the end that reduces cumulative inert gas exposure.

Dysbaric osteonecrosis affects juxta-articular bone, especially femoral and humeral heads, in divers and compressed-air workers; it is often asymptomatic and detected by radiography or MRI.

Compressed gas cylinders require visual inspection and periodic hydrostatic testing, commonly every 5 years, plus clear markings and safe storage.

Altitude diving requires reduced depth limits and adjusted tables because surface pressure is lower and decompression stress is greater.

Cold exposure in diving increases heat loss and impairs manual dexterity and judgement, raising the risk of errors during safety-critical tasks.

Compressed-air workers such as tunnellers are at risk of caisson disease; staged decompression in air locks and adherence to decompression schedules are the main preventive measures.