When The Breathing Gas Becomes a Trap: Gas Density, Flow, and Dynamic Airway Collapse in Diving

When divers talk about underwater risks, the conversation often centers on decompression sickness or oxygen toxicity. But there’s another, more subtle threat that can creep up unnoticed—dynamic airway compression. This physiological phenomenon becomes especially critical for those venturing into deeper waters, where breathing gas density increases dramatically.

Bildvorlage: Karin Aggeler

What Is Dynamic Airway Collapse?

Dynamic airway collaps (DAC) occurs when the pressure inside your airways during exhalation drops below the pressure outside them—specifically, the pressure in the surrounding chest cavity. When this happens, particularly in the flexible, non-cartilaginous segments of the airway, the walls begin to collapse inward. This significantly limits airflow and, in some cases, can obstruct it altogether.

On the surface, this isn’t a problem. During maximal effort—such as a forced expiratory maneuver in a lung function test—some flow limitation naturally occurs, but only at very high exhalation rates. We barely notice it and can exercise hard without any issue because we’re always able to eliminate the carbon dioxide (CO₂) our bodies produce.

In diving, however, this changes completely. The flow limitation sets in at much lower effort and lower flow rates, making it a significant concern during exertion.

Why Does This Happen?

The culprit is gas density, which increases in direct proportion to ambient pressure. Because divers must breathe gas delivered at a pressure matching the surrounding water pressure (within a few cm H₂O), the deeper you go, the denser the gas becomes.

This increase in density greatly amplifies airway resistance and sets the stage for dynamic collapse at flow rates that would be safe on the surface.

The Airplane Analogy: Flow, Suction, and Collapse

To understand this better, let’s borrow a concept from aerodynamics:

An airplane generates lift through the Bernoulli effect: fast airflow over the curved wing reduces pressure on top, creating suction that lifts the plane. The denser the air (as at sea level), the less speed is needed to generate enough lift. At higher altitudes (thinner air), the plane must go faster.

In your lungs, the analogy works like this: the faster gas flows through your airways, the more suction it creates on the airway walls—just like on the airplane wing. In denser gas, this suction effect becomes stronger at lower flow rates. The result? Your airway walls are pulled inward, increasing the risk of collapse.

In diving, this becomes a serious limitation. As depth and gas density increase, even normal breathing efforts can cause dynamic compression, potentially impairing your ability to eliminate CO₂.

Why Effort Doesn’t Help—And Can Make It Worse

Here’s the critical—and counterintuitive—truth: you cannot overcome dynamic airway compression by breathing harder.

Once the airway begins to collapse, increasing your breathing effort raises the flow rate. That faster flow causes an even greater drop in pressure, worsening the suction effect and pulling the airway walls inward even more.

This phenomenon is known as effort-independent flow limitation. No matter how strong your respiratory muscles are, or how determined you feel, you’ve hit a hard physiological ceiling. Ventilation cannot increase beyond this point.

This is the kind of physiological trap that claimed divers like David Shaw. At extreme depth, breathing gas becomes so dense that airflow limitation occurs even at rest. With elevated CO₂ levels under effort, the natural urge is to breathe harder—but that only speeds up collapse, leading to a downward spiral of CO₂ retention, panic, unconsciousness, and, ultimately, drowning.

The Vicious Loop

Let’s break down how this process unfolds:

1.     You begin to accumulate CO₂ due to exertion.

2.     You try to breathe harder to compensate.

3.     Increased flow causes dynamic airway collapse.

4.     Ventilation becomes limited.

5.     CO₂ continues to rise, leading to distress, panic, and further effort.

6.     The denser the gas, the earlier this happens and the more difficult it is to

escape.

Resting Helps... Until It Doesn’t

At moderate depths (around 40–50 meters), divers can avoid this trap simply by resting. Reducing effort keeps flow rates low, allowing the lungs to ventilate enough CO₂—even if the airways are partially collapsed.

But that safety margin is thin. A slightly denser gas, or just a little more effort, can push you over the edge into a ventilatory trap. Once you hit that wall, no amount of resting will help, because the airways are already too collapsed to allow effective gas exchange.

Dense gas + high flow rate = early collapse.

Recommended Limits for Gas Density

To reduce the risk of dynamic airway compression and CO₂ retention, several research groups and diving authorities have proposed the following gas density guidelines:

• Ideal gas density during exertion: ≤ 5.2 g/L

• Absolute maximum limit: 6.2 g/L

Breathing gases denser than this—especially while exercising—can lead to serious ventilatory limitations and CO₂ toxicity.

Real-World Example: Air Diving at 40 Meters

Let’s apply these numbers to a common scenario: compressed air diving at 40 meters (ambient pressure = 5 bar).

Compressed air is roughly:

• 79% nitrogen

• 21% oxygen

• Surface density: ~1.2 g/L

At 5 bar:

• 1.2 g/L × 5 = 6.0 g/L

At 40 meters, air has a gas density of approximately 6.0 g/L, which puts it right at the danger threshold. This means:

Any significant exertion—like swimming against a current, rescuing a buddy, or carrying heavy equipment—can push you into a CO₂ retention zone due to dynamic airway collapse.

Who’s Most at Risk?

Some divers are particularly vulnerable to the effects of dynamic airway compression:

• Compressed air or nitrox dives beyond 40 meters

• Older or less fit individuals with reduced lung capacity

• Divers with asthma or other respiratory conditions

• Smokers are even more prone to dynamic airway collapse, because their lung structure is damaged.

Key Takeaways

• Higher flow = higher risk: Increased airflow causes stronger suction on airway walls, encouraging collapse.

• Greater gas density magnifies this effect, causing airway collapse at lower effort.

• Flow limitation is effort-independent—you can’t “power through” it.

• More effort makes it worse—you may be pushing yourself straight into failure.

Final Thoughts

Dynamic airway compression is an invisible threat. You won’t feel it coming, but once it sets in, it’s often too late. It doesn’t matter how strong, fit, or experienced you are—the laws of physics will win.

In diving, understanding gas density and flow limitation is more than theoretical—it’s a matter of survival. Smart divers respect the limitations of their own physiology.

More effort is not the solution. In dense gas diving, it might be the problem.