ABPM Undersea and Hyperbaric Medicine key terms

At constant temperature, the volume of a fixed mass of gas is inversely proportional to its pressure. Divers use it to predict how gas-filled spaces shrink on descent and expand on ascent.

The total pressure of a gas mixture equals the sum of the partial pressures of its component gases. This lets divers calculate the pressure each gas exerts at any depth.

The amount of gas dissolved in a liquid is proportional to the partial pressure of that gas above the liquid. This explains inert gas uptake in tissues during descent and bubble formation on ascent.

The share of total pressure contributed by one gas in a mixture, calculated as the gas fraction times ambient pressure. It determines physiological effect, such as oxygen dose or nitrogen narcosis.

A gas that does not take part in metabolism, chiefly nitrogen in air diving and helium in mixed-gas diving. Its uptake and elimination by tissues drive the decompression obligation.

A theoretical tissue in decompression models with a defined half-time for inert gas uptake and elimination. Fast compartments govern short dives; slow compartments govern saturation and long exposures.

The maximum inert gas partial pressure a tissue is thought to tolerate before decompression sickness becomes unacceptable. Dive tables are built by keeping tissue pressures below these limits.

Illness caused by inert gas bubbles forming in tissues or blood after a reduction in ambient pressure. Symptoms range from joint pain and rash to paralysis and cardiopulmonary collapse.

Gas entering the arterial circulation, usually from pulmonary barotrauma when a diver holds their breath on ascent, obstructing blood flow. It causes sudden neurological or cardiac symptoms and needs urgent recompression.

Tissue injury caused by a pressure difference between a gas-filled space and its surroundings when that space cannot equalise. Common sites are the middle ear, sinuses, lung, and face mask.

Reversible impairment of judgement, coordination, and mood caused by breathing nitrogen at raised partial pressures, typically noticed below about 30 metres. It resolves on ascent and is managed by depth limits.

Harmful effects of excessive oxygen partial pressure, including acute central nervous system seizures and chronic pulmonary irritation. Risk rises with dose and exposure time, so limits are set for diving and therapy.

Raised carbon dioxide in the blood, in diving often from inadequate ventilation, skip breathing, or contaminated gas. It causes headache, breathlessness, and confusion, and increases susceptibility to oxygen toxicity and narcosis.

A persistent opening between the atria of the heart present in roughly a quarter of adults. It can let venous bubbles reach the arterial circulation, raising the risk of unexplained decompression illness.

Fluid accumulation in the lungs during immersion, especially in cold water or with heavy exertion, causing breathlessness, cough, and low blood oxygen. It can affect fit, experienced divers and requires removal from the water.

A planned pause at a specified depth during ascent so inert gas can leave tissues before pressure is reduced further. Safety stops are short, shallow stops used to reduce bubble formation.

A technique in which tissues become fully saturated with inert gas, so decompression time no longer grows with exposure length. Divers live at depth in a habitat or chamber for days or weeks.

Intermittent breathing of 100% oxygen at pressures above sea level, usually 2.0 to 3.0 atmospheres absolute, inside a pressure chamber. It treats conditions such as decompression illness, carbon monoxide poisoning, and problem wounds.

The transient deficit in total dissolved gas pressure in venous blood, created because oxygen is consumed and carbon dioxide is highly soluble. It drives inert gas elimination and underpins hyperbaric treatment of bubbles.

A standard recompression protocol using 100% oxygen at 2.8 atmospheres absolute with scheduled air breaks, lasting about five hours. It is widely used for decompression sickness and arterial gas embolism.

Tissue hypoxia caused by carbon monoxide binding haemoglobin and cytochrome enzymes. Hyperbaric oxygen speeds elimination, reduces lipid peroxidation, and is considered for severe poisoning, neurological signs, or pregnancy.

Death of bone, usually the mandible, after radiotherapy, with poor healing and risk of pathological fracture. Hyperbaric oxygen is used adjunctively to improve tissue oxygenation and support surgical repair.

Chronic bone infection that persists despite appropriate antibiotics and surgery. Adjunctive hyperbaric oxygen raises oxygen tension and supports white cell killing, and is used when standard treatment has failed.

Trauma from prolonged compression of limb muscle, causing swelling, ischaemia, and release of toxic products. Hyperbaric oxygen is used adjunctively to reduce oedema and improve oxygenation in crush injury and compartment syndrome.

A non-invasive method that measures skin oxygen partial pressure using heated electrodes. It helps select patients for hyperbaric oxygen and predict wound healing in ischaemic tissue.

A pressure vessel that holds two or more people, allowing an inside attendant to accompany patients and give direct care. Oxygen is delivered by mask or hood, and air breaks are simple to perform.

A single-occupant pressure chamber, usually pressurised with oxygen so the patient breathes the chamber gas. It needs less space and staffing but limits direct patient access and attendant support.

A scheduled period of breathing normal air during hyperbaric oxygen exposure. It reduces pulmonary and central nervous system oxygen toxicity and is standard in treatment tables.

A device containing absorbent such as soda lime that removes carbon dioxide from a closed chamber atmosphere or breathing loop. Exhaustion of the absorbent causes rising carbon dioxide, headache, breathlessness, and impaired performance.

The condition of equipment and surfaces free of combustible contaminants such as oil and grease, achieved by specific cleaning and verification. It is required for oxygen service because contaminants can ignite and burn violently.

A small pressure-locking compartment on a chamber used to pass drugs, equipment, or food in or out without depressurising the main chamber. It preserves treatment pressure and session continuity.

A trained person who accompanies patients inside a multiplace chamber, monitoring them, operating equipment, and helping in emergencies. Also called an inside tender or chamber attendant.

The person in charge of a diving operation, responsible for planning, gas supply, personnel, and safety, and for directing the dive from the surface. Authority to stop a dive is central to the role.

A medical assessment judging whether a person can dive without unacceptable risk to themselves or others. It considers cardiovascular, respiratory, neurological, ear, and psychological conditions, plus medications.

The interval required between diving and ascent to altitude or commercial flight to reduce decompression sickness risk from residual inert gas. Recommended delays increase with dive intensity and repetitiveness.

A person working in a pressurised environment such as a tunnel, caisson, or shaft, exposed to hyperbaric conditions and decompression requirements. Medical surveillance and controlled decompression are essential safeguards.

The time spent at surface pressure between two dives, during which residual inert gas is eliminated. Longer intervals reduce the decompression obligation of the next, repetitive dive.