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However, carbon dioxide levels in a diver's bloodstream can rise under certain circumstances, and the effects can be disastrous. This article is not intended to frighten, but merely to inform. When a diver understands the risks of high concentrations of carbon dioxide, he is less likely to engage in behaviors that may predispose him to those risks.
Carbon Dioxide and Breathing:
The body needs a small amount of carbon dioxide for normal body functions. One of these functions is respiration. When a person inhales, he breathes in oxygen which his body metabolizes to create energy. One of the waste products of this metabolic reaction is carbon dioxide, which is eliminated from the body when the person exhales. Interestingly, it is the rising level of carbon dioxide in a person's bloodstream (not the falling level of oxygen) that signals the need for respiration.
How Does Your Body Maintain Safe Carbon Dioxide Levels During a Dive?:
A diver's body is constantly producing carbon dioxide, which it eliminates through exhalation. When a diver requires more energy, such as during moderate exercise, his body breaks down oxygen rapidly to provide that energy at a faster rate. This speeds the production of carbon dioxide. To keep the blood level of carbon dioxide steady, a diver's body increases his respiration rate to eliminate excess carbon dioxide, balancing its production and elimination.
Carbon Dioxide in a Diver's Body Can to Rise to an Unsafe Level:
Any factor that causes the concentration of carbon dioxide in a diver's body to rise to 45 mg Hg and above induces hypercapnia – a potentially dangerous excess of carbon dioxide. There are two situations in which this can happen.
• The concentration of carbon dioxide a diver inhales increases.
• A diver does not eliminate carbon dioxide as quickly as he produces it.
The Dangers of Hypercapnia:
1. Loss of Consciousness.
Carbon dioxide has an anesthetic effect on a diver's central nervous system. If the concentration of carbon dioxide rises to 75 mg Hg (depending upon the person), a diver may lose consciousness. Underwater, loss of consciousness is usually fatal – an unconscious diver generally loses his regulator and drowns.
2. Narcosis
The anesthetic properties of carbon dioxide at elevated concentrations can cause narcosis. Some of the common effects of carbon dioxide narcosis are the slowing of mental processes and the loss of dexterity. Only a very small increase in carbon dioxide levels is needed to produce these effects; carbon dioxide is four times more narcotic than nitrogen. In addition to producing narcosis on its own, carbon dioxide can also amplify the narcotic effects of nitrogen and other inert gasses.
3. Oxygen Toxicity
The human body has a programming glitch. It uses the level of carbon dioxide in the bloodstream to determine how much oxygen the body needs. In normal environments, this works well – as the carbon dioxide level increases, so does breathing rate, carbon dioxide elimination, and oxygen absorption. The higher the level of carbon dioxide, the harder a diver's body works to absorb oxygen. Unfortunately, in scuba diving high levels of oxygen can lead to oxygen toxicity, generally characterized by convulsions that result in drowning. High carbon dioxide levels cue the body to increase oxygen concentrations, speeding the onset of oxygen toxicity.
4. Decompression Sickness
Many hyperbaric physicians now believe that high levels of carbon dioxide may increase the risk of decompression sickness. One scenario is that high carbon dioxide levels interfere with the transport and elimination of nitrogen in the lungs. If the body is working hard to eliminate and exhale excess carbon dioxide, it will not be able to eliminate nitrogen as efficiently as it would if the carbon dioxide were not present. High levels of carbon dioxide lead to elevated levels of nitrogen in the body, which increases the risk of decompression sickness.
Avoid Elevated Carbon Dioxide Levels While Scuba Diving:
The most common behaviors and situations that increase the carbon dioxide concentration in a diver's bloodstream are listed below. By avoiding these situations, a diver nearly eliminates the risk of hypercapnia.
• Improper Breathing Techniques
Hyperventilation: When a diver hyperventilates, he fills only a small portion of his lungs and fails to fully exhale. This creates "dead" air spaces - spaces in the lungs and regulator in which air with high concentrations of carbon dioxide are not fully replaced with normally oxygenated air. For example, the first few mL of air a diver inhales from his regulator is "recycled," air with a high level of carbon dioxide from his previous exhalation. A diver must inhale fully to get past this air and receive fresh air. If a diver does not fully exhale, some of the of the old, carbon dioxide-filled air will remain in his lungs and he will breathe it again with his next breath. The concentration of carbon dioxide in a diver's lungs and regulator "dead" air space increases with each hyperventilated breath, leading to an increase in the level of carbon dioxide in his bloodstream.
Skip Breathing: In an effort to reduce air consumption, many divers hypoventilate or skip breathe. This involves extremely slow breathing. The diver exhales fully and then holds his breath "out", keeping his lungs empty for a period of time before breathing in. Hypoventilation increases the carbon dioxide level in a diver's lungs. Skip breathing can be dangerous because of the risk of hypercapnia.
• Physical Exertion
On land, a person's body adequately deals with the increased production of carbon dioxide during physical exertion by raising respiration rates. Underwater, the excess carbon dioxide is more difficult to eliminate. The breathing resistance of the regulator and the greater density of inspired air at depth make it nearly impossible for a diver to increase his breathing rate to match strong physical exertion. Either the diver begins to hyperventilate, increasing the level of carbon dioxide in his lungs while the carbon dioxide level in his bloodstream also increases, or he maintains a slow and steady breathing rate which is insufficient to eliminate the huge excess of carbon dioxide in his body.
• Deep Diving Without Proper Instruction and Gases
Air and other breathing gases become denser as they compress with depth. The more dense the air, the more difficult it will be for a diver to properly empty and fill his lungs with each breath. The result is retained carbon dioxide in the diver's lungs, similar to the situations listed in "Improper Breathing Techniques" above. This is yet another reason for divers not to go beyond the depth limits of their certification level. Deep diving courses train scuba divers in proper breathing techniques and teaches them about the gas mixtures needed for deep diving. Proper training will help deep divers avoid the risk of hypercapnia.
• Poorly Functioning Breathing Equipment
Regulators that breathe "hard" or increase the resistance of breathing can increase the level of carbon dioxide in a diver's bloodstream. When breathing resistance increases, divers find it hard to draw a full breath and exhale fully. Again, this leads to unintentional hyper- or hypoventilation, which increases carbon dioxide levels. Keep in mind that some regulators will breath easily on shallow dives, but may be inappropriate at greater depths.
• Breathing Gas Contamination
In contemporary scuba diving, it is extremely unlikely to encounter contaminated breathing gas. Strict regulations governing compressor use have helped to standardize tank filling practices. However, if the intake of a compressor is close to the exhaust from an internal combustion engine or other source of carbon dioxide, the air from the tank may contain abnormally high levels of carbon dioxide. While a high level of carbon dioxide may not cause any ill-effects when the tank is tested on the surface, underwater the increased concentration of carbon dioxide may lead to hypercapnia. Carbon dioxide is an odorless and tasteless gas, but other contaminants from exhaust are noticeable. Smell and taste the air from a scuba tank before diving with it. Any unusual findings could indicate that the tank air is contaminated with exhaust or other pollutants which may be accompanied by carbon dioxide. Report strange odors and flavors to the fill station or dive shop and don't dive if you think the tank may be contaminated.
Signs and Symptoms of Hypercapnia:
One of the problems divers face when dealing with elevated levels of carbon dioxide is that the signs and symptoms indicative of an increasing concentration of carbon dioxide may be masked by the dive environment. For example, symptoms such as elevated breathing rate may be attributed to excitement or cold water. Symptoms like headache may be misattributed or absent due to the high partial pressure of oxygen at depth. Unfortunately, this means that the first sign of carbon dioxide toxicity may be sudden unconsciousness. Here are common signs and symptoms of hypercapnia.
• shortness of breath
• headache
• narcosis - confusion, slowed thought processing, loss of manual dexterity
• unconsciousness
Remember, Hypercapnia Is Avoidable:
Basic good diving practices can nearly eliminate the chance of hypercapnia. Hopefully after reading this article, you will be convinced to always . . .
• Use regulators appropriate for the depth of your dive
• Avoid exertion underwater
• Use proper breathing techniques
• Avoid deep diving without proper training, gear, and gas mixtures
• Smell and taste tank air to check for pollutants that may indicate an excess of carbon dioxide
Remember, knowledge is power. The more you know, the safer you will be underwater. Happy Diving!
Thanks to Natalie Gibb and About.com
Kathy Dowsett
www.kirkscubagear.com
Great article! Carbon dioxide is indeed one of the most dangerous gases to divers but by following a few simple rules, we can all dive safely!
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