Dive Physiology Part 2 — Barotrauma, DCS and Lung Overexpansion Injuries

Barotrauma — Pressure Injuries During Descent and Ascent

Baro means pressure, trauma means injury. A barotrauma is a pressure-related injury. As you descend, increasing pressure can squeeze air spaces in the body. As you ascend, decreasing pressure allows trapped gas to expand — and if it cannot escape, it causes injury. Barotraumas can happen in both directions.

During Descent — The Squeeze

• Ear squeeze — the most common. Failure to equalise causes pressure to build in the middle ear against the eardrum.
• Mask squeeze — failure to exhale through the nose into the mask during descent. The result looks like two black eyes.
• Lung squeeze — extremely rare. Can occur in breath-hold divers who forcefully exhale all air before a rapid descent.

During Ascent — Expansion Injuries

• Reverse block — air trapped in the ear or sinuses that cannot expand out during ascent, causing pain.
• Lung overexpansion injuries — the most serious ascent barotrauma. Caused by breath-holding, chest congestion blocking airflow, or loss of surfactant (the protective lung coating, most commonly from smoking).

Circulation and Carbon Monoxide Poisoning

The circulatory system transports oxygen to the cells and removes carbon dioxide. The key protein that carries oxygen is haemoglobin, found in red blood cells. Oxygen bonds readily with haemoglobin — but one gas bonds with it even more strongly: carbon monoxide.

When CO is present, it takes up the haemoglobin binding sites that oxygen needs, effectively blocking oxygen transport to the body's cells.

Carotid Sinus Reflex

The carotid arteries in the neck supply blood to the brain. Sensors called carotid sinus receptors monitor blood pressure in these arteries. If they detect high pressure, they signal the heart to slow down.

A tight wetsuit neck seal or hood can press against the carotid arteries from outside the body. The receptors interpret this as high blood pressure and instruct the heart to slow — repeatedly, because the external pressure never goes away. Blood pressure drops, blood supply to the brain falls, and the diver can lose consciousness without warning.

Lung Overexpansion Injuries

When a lung overexpansion injury occurs, air escapes from the lungs and must go somewhere. There are four possible destinations, each producing a different injury.

Decompression Sickness (DCS)

DCS can produce dozens of seemingly unrelated symptoms. It is almost easier to remember what is not a symptom — a fever is not a symptom of DCS. If a diver presents with joint pain and headache but also has a fever, look for another cause.

Silent Bubbles

After a dive, a diver may have small nitrogen bubbles in their body but show no symptoms. These are called silent bubbles — present but producing no signs or symptoms. Detectable only by ultrasound.

Factors That Increase DCS Risk

• Age — circulation degrades over time
• Poor physical fitness
• Obesity — fatty tissue has poor blood supply
• Smoking — impairs circulation
• Alcohol — alters blood viscosity and circulation
• Dehydration — thickens the blood
• Injury or illness — alters blood supply
• Heavy exercise before or after a dive
• Cold water — slows circulation to the extremities
• High CO₂ levels
• A very hot shower immediately after a cold dive — rapidly increases blood flow to the skin

Why Pure Oxygen Helps with DCS

Breathing air normally, the lungs contain 79% nitrogen. Nitrogen in the blood slowly equalises with nitrogen in the lungs and off-gasses. When a diver breathes 100% oxygen, the partial pressure of nitrogen in the lungs drops to zero. The pressure difference between blood nitrogen and lung nitrogen is now much larger — nitrogen off-gasses much faster and is exhaled safely.

The Ear — Equalisation Injuries

Delayed Equalisation

Descending without equalising squeezes the eustachian tube closed. Once closed, further equalisation becomes very difficult — continuing to descend and trying harder rarely works. The correct response is to ascend slightly to allow the eustachian tube to reopen, equalise, then descend again.

Blood and Fluid in the Middle Ear

Diving slightly under-equalised creates low pressure in the middle ear, drawing blood and fluid out of the surrounding tissues — similar to how a bruise forms under suction. This feels like water trapped in the ear but is on the wrong side of the eardrum. Tilting the head will not help. Anti-inflammatory medication may be needed to reduce swelling in the eustachian tube and allow the fluid to drain.

Round Window Rupture

A very forceful Valsalva equalisation can generate enough pressure to rupture the round window — a membrane between the middle and inner ear. The result is immediate severe vertigo and nausea. The Frenzel technique (using throat muscles rather than lung pressure) cannot generate enough force to cause this injury.

Aquatic Life Injuries

There are four types: bites, stings, cuts, and punctures.

• Bites — signs and symptoms are obvious
• Stings — range from minor to life-threatening. Some species (such as the Irukandji jellyfish off northern Australia) can cause severe shock leading to respiratory and circulatory failure. Stings are the aquatic life injury type most likely to cause respiratory or circulatory arrest.
• Cuts — risk of infection
• Punctures — often associated with severe localised swelling (e.g. lionfish spines)