ABEM UHM common mistakes
ABEM UHM · Understand the traps and how to avoid them.
Treating a normal PaO2 or SpO2 on room air as evidence against significant right-to-left shunting in a diver with suspected patent foramen ovale and unexplained arterial gas embolism or cutaneous decompression sickness.
Candidates conflate oxygenation with shunt physiology; a modest shunt can be masked by compensatory hyperventilation and the question stem offers a normal room-air gas as a distractor.
What to do instead
Recall that shunt fraction is estimated from the A-a gradient on 100% oxygen (or echocardiography with bubble study), not from a room-air PaO2; when a stem gives a normal room-air gas plus a plausible shunt scenario, treat the gas as non-exclusionary and select the shunt-evaluating step.
Applying a single ascent-rate rule (for example 30 ft/min or 60 ft/min) to every scenario without distinguishing the recommended rate from the maximum rate and without adjusting for the depth range.
Multiple figures circulate in training and operational guidance, and candidates memorize one number rather than the concept that ascent rate limits are depth-dependent and that the maximum permissible rate exceeds the recommended rate.
What to do instead
Anchor on the principle: the recommended rate is slower than the maximum, and the maximum is depth-banded (slower at shallow depths). When a stem specifies a rate, compare it to the band for that depth range rather than to a single memorized figure.
Ordering recompression for a diver with a history of seizure, untreated tension pneumothorax, or an unstable airway before addressing the contraindication.
Recompression is the reflex answer for any diving injury, and candidates skip the safety screen because the vignette emphasizes the diving diagnosis.
What to do instead
Build a pre-recompression checklist into every treatment question: airway patency, pneumothorax excluded or drained, seizure risk and any implanted device, and hemodynamic stability. If a stem plants one of these, the correct next step is stabilization, not the chamber.
Calculating a decompression obligation or repetitive-group designation from a table without first confirming the actual bottom time, the correct depth (average versus maximum), and the surface interval.
Stems bury the relevant depth or time among distractors such as descent time, travel time, or a second dive, and candidates grab the first number that fits a familiar table.
What to do instead
Before any table lookup, write down the four inputs explicitly: maximum depth, bottom time measured from leaving the surface to starting ascent, surface interval, and repetitive group. If any input is ambiguous in the stem, the question is testing input selection, not arithmetic.
Missing the diagnosis of immersion pulmonary edema in a healthy diver who develops dyspnea, cough, and hypoxemia during a cold or exertional dive, and instead anchoring on decompression sickness or drowning.
The presentation overlaps with both, and candidates do not consider a non-barotraumatic cause of dyspnea in a diver who never went deep.
What to do instead
When a diver develops respiratory symptoms during or immediately after a dive with no significant depth exposure and no missed decompression, consider immersion pulmonary edema; the management is removal from water, oxygen, and supportive care, not recompression.
Confusing the mechanism and first-line management of arterial gas embolism from pulmonary barotrauma with that of decompression sickness, and delaying recompression for transport or imaging.
Both present with neurologic symptoms and both are treated with hyperbaric oxygen, so candidates treat them as interchangeable and forget that gas embolism is the more time-critical emergency.
What to do instead
Separate the two: gas embolism arises from alveolar rupture with gas entering the arterial circulation, typically on ascent or immediately at the surface, and warrants the most urgent recompression. When a stem describes immediate post-ascent neurologic symptoms, prioritize immediate recompression over imaging.
Failing to distinguish oxygen toxicity of the central nervous system from pulmonary oxygen toxicity and applying the wrong prevention strategy (for example, using pulmonary dose limits to prevent a seizure).
Both are oxygen toxicity and both involve oxygen exposure, so candidates merge them and pick whichever prevention measure they remember.
What to do instead
Map each syndrome to its exposure pattern: CNS toxicity is acute and high-partial-pressure, prevented by limiting inspired oxygen partial pressure and using air breaks; pulmonary toxicity is cumulative and lower-partial-pressure, tracked by total oxygen dose. Match the prevention to the syndrome named in the stem.
Assuming a diver who is asymptomatic after a dive has no decompression illness and withholding evaluation, or conversely treating every minor musculoskeletal ache as requiring recompression.
Candidates polarize on the 'no symptoms, no disease' heuristic or on over-inclusive treatment, and the stem usually contains a subtle finding that should tip the balance.
What to do instead
Use the exam's operational rule: unexplained symptoms after a dive warrant evaluation and a low threshold for recompression, while isolated mild joint pain without other findings may be observed with a defined reassessment plan. Identify whether the stem supplies a red flag (neurologic, pulmonary, cutaneous, or constitutional) before choosing.
Misapplying gas law relationships: using Boyle's law where Dalton's applies, or assuming a rigid versus flexible container behaves the same under pressure change.
The laws are taught together and the stems describe real equipment (a mask, a lung, a drysuit, a cylinder) without naming the law, so candidates pick the law by keyword rather than by which quantity is held constant.
What to do instead
Ask what is held constant before choosing a law: constant temperature means Boyle, constant volume with gas mixture means Dalton, constant pressure means Charles. Then ask whether the container is rigid (volume fixed) or compliant (pressure equilibrates), because that determines which variable changes.
Neglecting the dive profile and gas history when evaluating a breath-hold or mixed-gas diver, and applying scuba-based assumptions about inert gas loading, oxygen exposure, or carbon dioxide retention.
Most training emphasizes scuba, so candidates default to scuba physiology even when the stem describes breath-hold diving, rebreather use, or a gas mix that changes the expected oxygen and inert gas partial pressures.
What to do instead
Re-read the stem for the breathing mode and gas before reasoning. For breath-hold diving, consider carbon dioxide and hypoxia rather than inert gas loading; for mixed gas, compute the oxygen and inert gas partial pressures at the stated depth before choosing management.