Decompression sickness, commonly known as the bends, represents one of the most significant physiological challenges faced by divers and aerospace personnel. This condition arises when inert gases, primarily nitrogen, form bubbles within the tissues and bloodstream due to a rapid reduction in environmental pressure. Understanding the specific causes of decompression sickness is essential for mitigating risk, as it allows individuals to move beyond simple symptom recognition to proactive prevention. The core issue lies in the delicate balance between gas solubility and pressure, a balance that can be easily disrupted by haste or inadequate planning.
Fundamental Physics of Gas Dissolution
At the heart of decompression sickness is Henry’s Law, a principle stating that the amount of gas dissolved in a liquid is directly proportional to the pressure of that gas above the liquid. When a diver descends, the surrounding pressure increases, forcing more nitrogen from the lungs into the blood and subsequently into the tissues. At a depth of 33 feet, the pressure doubles compared to the surface, effectively doubling the concentration of nitrogen in the blood compared to at the surface. This process is entirely normal and expected; the danger emerges when the diver ascends too quickly, allowing this dissolved gas to come out of solution too rapidly.
Rate of Ascent: The Primary Culprit
The most direct cause of decompression sickness is an ascent rate that exceeds the safe off-gassing limits of the body. If a diver rises to the surface without performing mandatory decompression stops, the pressure drops quickly, and the nitrogen that was safely dissolved under high pressure suddenly becomes a hazard. The tissues cannot eliminate the gas quickly enough, leading to supersaturation. This excess gas forms microscopic bubbles that can obstruct blood flow, damage cell membranes, and trigger inflammatory responses. A controlled, gradual ascent allows the lungs to act as a vent, expelling the nitrogen safely through exhalation rather than letting it form bubbles in the joints and organs.
Dive Profile and Repetitive Diving
It is not just a single dive that contributes to the causes of decompression sickness, but the cumulative effect of multiple dives. A dive profile—which includes depth, duration, and surface interval—dictates how much nitrogen is absorbed and subsequently cleared. A repetitive dive, conducted before the body has fully eliminated the nitrogen from a previous dive, places the diver at a significantly higher risk. The residual nitrogen loads the body like adding more fuel to a fire, reducing the margin for error on the subsequent dive. Ignoring surface interval times or underestimating the residual nitrogen burden is a frequent cause of delayed symptoms appearing hours after diving has ceased.
Physiological and Environmental Factors
While physics dictates the rules, human physiology determines how susceptible an individual is to breaking them. Dehydration is a major contributing factor, as it thickens the blood and reduces the efficiency of gas exchange. A well-hydrated diver allows for smoother off-gassing. Similarly, physical fitness and body composition play roles; fatty tissues tend to retain nitrogen longer than lean muscle, potentially increasing the risk for certain individuals. Environmental factors such as cold water can cause peripheral vasoconstriction, slowing blood flow and hindering the elimination of nitrogen, effectively trapping the gas in the extremities.
Individual Susceptibility and Medical History
Not every diver will get the bends after the same exposure, highlighting the role of individual susceptibility. Certain pre-existing conditions can alter blood flow or gas exchange, acting as silent co-conspirators in the development of sickness. A patent foramen ovale (PFO), a common heart defect where a small hole exists between the upper chambers of the heart, can allow venous blood bubbles to bypass the lung filter and enter the arterial system, potentially causing neurological symptoms. Additionally, medications like insulin or hormone replacement therapy can affect hydration and circulation, subtly increasing vulnerability.
