ABEM UHM cheat sheet

Barotrauma of descent (squeeze) affects the middle ear, sinuses, mask, and suit; equalization failure causes tympanic membrane and middle-ear hemorrhage.

Middle-ear barotrauma is graded by otoscopy (TEED scale 0-5); grade 0 is normal, grade 5 is tympanic membrane perforation.

Inner-ear barotrauma presents with vertigo, tinnitus, and sensorineural hearing loss after a forceful Valsalva; it is a contraindication to further diving until evaluated.

Alternobaric vertigo results from unequal middle-ear pressures during ascent and is usually transient; it can cause loss of orientation underwater.

Pulmonary barotrauma of ascent causes alveolar rupture with pneumothorax, pneumomediastinum, subcutaneous emphysema, or arterial gas embolism.

Arterial gas embolism symptoms typically occur within 10 minutes of surfacing; onset after 10 minutes suggests decompression sickness instead.

Saltwater near-drowning carries higher mortality from surfactant washout and pulmonary edema; freshwater aspiration can cause hemodilution and ventricular fibrillation.

Marine envenomations: stonefish and scorpionfish stings are treated with hot water immersion (45 degrees C) for pain; box jellyfish stings use vinegar to inactivate nematocysts.

Divers should not fly for at least 12-24 hours after repetitive dives without symptoms; longer intervals apply after decompression sickness treatment.

Hyperbaric oxygen is defined as breathing 100% oxygen at greater than 1 atmosphere absolute; treatment pressures are typically 2.0-3.0 ATA.

Mechanisms of HBO include hyperoxygenation, vasoconstriction, enhanced neutrophil killing, angiogenesis, and reduction of gas bubble size via Boyle's law.

Absolute contraindication: untreated pneumothorax. Relative contraindications include uncontrolled asthma, COPD with CO2 retention, and certain chemotherapeutic agents.

Central nervous system oxygen toxicity presents with tinnitus, visual changes, nausea, twitching, and seizures; incidence rises above 2.8-3.0 ATA.

Pulmonary oxygen toxicity (Lorrain Smith effect) causes tracheobronchial irritation and decreased vital capacity with prolonged exposure.

Oxygen toxicity risk is managed with air breaks; a common schedule is 20 minutes of air every 90-120 minutes of oxygen.

Carbon monoxide poisoning is treated at 2.5-3.0 ATA; indications for HBO include syncope, neurologic deficits, pregnancy, and severe metabolic acidosis.

Decompression sickness and arterial gas embolism are treated with US Navy Treatment Table 6 at 2.8 ATA with air breaks.

Pressure increases by 1 atmosphere absolute for every 10 meters (33 feet) of seawater; absolute pressure = 1 + depth/10 ATA in meters.

Boyle's law: at constant temperature, gas volume is inversely proportional to pressure; a 6-liter breath at 10 m holds 12 liters at the surface.

Dalton's law: total pressure equals the sum of partial pressures; oxygen partial pressure = fraction of oxygen times absolute pressure.

Henry's law: gas dissolved in a liquid is proportional to its partial pressure; this underlies nitrogen loading and decompression sickness.

Nitrogen narcosis typically appears below 30 meters and impairs judgment and reaction time; it resolves with ascent.

Oxygen partial pressure above 1.4 ATA increases CNS oxygen toxicity risk; 1.6 ATA is the usual maximum for working dives.

Hypothermia risk is high because water conducts heat about 25 times faster than air; cold increases nitrogen solubility and gas uptake.

Gas density rises with depth, increasing work of breathing; dense gas at depth contributes to CO2 retention and hypercapnia.

Descent causes gas spaces to shrink and tissue gas uptake; ascent causes gas expansion and off-gassing, which is the basis for decompression stops.

Initial management of decompression illness: supine position, 100% oxygen by nonrebreather mask, IV fluids, and urgent transport to a hyperbaric facility.

Do not delay recompression for imaging; decompression sickness is a clinical diagnosis, and treatment should not await imaging.

Decompression sickness types: Type I involves joints, skin, and lymphatics; Type II involves neurologic, pulmonary, or cardiac systems.

Arterial gas embolism and decompression sickness are often treated identically (US Navy Treatment Table 6) because the distinction is unreliable.

The dive history should record depth, bottom time, ascent rate, repetitive dives, surface interval, and time of symptom onset.

Rapid ascent with breath-holding is the classic mechanism for pulmonary barotrauma and arterial gas embolism in divers.

Neurologic decompression sickness can present as paresthesias, weakness, incontinence, or altered mental status; treat as an emergency.

Adjunctive therapy for severe decompression illness may include lidocaine for neurologic symptoms and hydration; avoid nitrous oxide and air travel until cleared.