Hypercapnia or Carbon Dioxide and Diving (part I)

By Dr. Sawatzky

Carbon-dioxide is present in the atmosphere in very small concentrations (0.03% or 0.30 mmHg). Humans (animals and most O2 using species) produce CO2 by the metabolism of the fats, proteins and carbohydrates that they eat (e.g. C6H12O6 + 6O2 = 6 CO2 + 6 H2O + energy). The amount of CO2 produced is roughly the same (actually slightly less) than the amount of O2 consumed (different for fats, proteins and carbohydrates). Carbon-dioxide is removed from the atmosphere by plants.

The human body is designed to maintain the PCO2 in arterial blood at 40 mmHg (the PCO2 of mixed venous blood is around 46 mmHg). The alveolar PCO2 is therefore also 40 mmHg but when alveolar gas is mixed with gas from the dead space, PCO2 in expired air is approximately 32 mmHg. Under normal conditions, each 100 ml of blood contains, 3 ml of CO2 dissolved, 3 ml attached to hemoglobin and other plasma proteins, and 44 ml carried as bicarbonate (HCO3-). At rest in a 70 kg male, approximately 200 ml of CO2 are produced each minute. At maximum work however, over 3,000 ml of CO2 can be produced per minute and a fit person can maintain work rates that produce over 2,000 ml of CO2 per minute for more than 30 minutes. Carbon-dioxide is the most potent stimulus to respiration. When we increase production of CO2 by exercising, the body quickly responds by increasing respiration so that arterial PCO2 is maintained very close to 40 mmHg. This mechanism is controlled by CO2 receptors in the medulla of the brain. The peripheral chemoreceptors (carotid and aortic bodies) primarily respond to levels of oxygen in the blood.

There are two ways in which the PCO2 in the body can be increased. Either the CO2 in the inspired gas is elevated or we fail to completely eliminate the CO2 produced in the body, thereby allowing it to accumulate. Both mechanisms can be a problem in divers.

As an aside, remember that hyperventilating before a breath-hold dive removes CO2 from the body so that it takes longer for the level of CO2 to rise to the point where you have to breathe. Also remember that you can lose consciousness from lack of oxygen before you have to breathe if you hyperventilate too much (more than three breaths) and/or work while holding your breath (see column in Diver Magazine, March 1997).

In open circuit (OC) diving (standard scuba), the only way to increase the inspired CO2 is by contamination of the breathing gas. This most commonly will occur if the intake from the compressor is too close to the exhaust of an internal combustion (gas, diesel, propane, etc.) engine (you also get CO). In rebreather diving (SCR or CCR) the most common problem is failure of the absorbent system. Most commonly this occurs when the diver forgets to change the CO2 absorbing material but it can also happen if the material is not inserted correctly (channelling), if the material is contaminated with sea water, if the material has too large a granule size or if the material does not absorb CO2 properly. In addition, there can be problems with the design of the rebreather. Finally, the ability of the material to absorb CO2 is highly variable depending on the temperature. It is much less effective if you are diving in cold water than warm water. When diving a full-face mask, helmet or diving in a chamber, CO2 can become elevated if the ventilation rates of the gas space are too low. These problems are all relatively easy to avoid.

The second mechanism of elevated CO2 is when the diver fails to completely eliminate all of the CO2 produced in his body. There are several ways in which this can occur. First, if the diver is anxious, they might breathe very rapidly, but take only very small breaths. The first few hundred ml that we inhale simply move the old gas that was sitting in the regulator, mouth and airways into the alveoli. If we are taking only shallow breaths, we will be moving NO NEW GAS into the alveoli but simply moves the samegas back and forth, thereby stopping the elimination of CO2 and the delivery of O2 to the body. Whenever you have a problem breathing, diving or not, it is important to take slow deep breaths to ensure you are moving the maximum amount of fresh gas in and out of the alveoli. A second way in which CO2 can accumulate in the body is if the person does not have a normal increase in respiration with increased CO2 production. Most people maintain arterial PCO2 at 40 mmHg by increasing respiration, even with heavy exercise and large increases in CO2 production. Some individuals do not. They allow the arterial PCO2 to increase, sometimes as high as 70 mmHg! These individuals are called CO2 retainers and there seem to be more CO2 retainers among divers than in the general population.

There are several theories for this. First, when breath-hold diving was a prerequisite for scuba diving, CO2 retainers would have been selected because they could hold their breath longer. Second, in some forms of diving (hard hat and old pendulum type rebreathers) the diver was chronically exposed to elevated CO2 in the inspired gas. Third, some divers chronically hypoventilate to try to use less gas, resulting in elevated PCO2 in their bodies. It is theorized that these divers adapt and learn to tolerate higher levels of CO2. Unfortunately, some individuals retain CO2 even though they have never been chronically exposed to elevated CO2 levels. The only sure way to identify these individuals is to measure their CO2 in a lab while they are diving or exercising. Warning signs however include divers who use very little gas and those who often end dives with a headache. These individuals are at risk for CO2 problems.

A third way in which CO2 can accumulate in the body is if the work of breathing is increased. The body maintains a balance between the arterial PCO2 and the work of breathing. If breathing is made more difficult, a higher level of arterial PCO2 is necessary to generate the required 'drive' for the increased work of breathing. Unfortunately, there are many reasons that the work of breathing is increased while diving. First, all regulators have some resistance to breathing, some much more than others. Second, all gases become denser as the pressure is increased. This increased density makes it much harder to move the gas through the regulator and the lungs. Most regulators are OK at shallow depths but many become too difficult to breathe as the gas density increases at depth (my old Sherwood Blizzards are great shallow but not recommended deeper than 100 fsw).

The largest problem is increased work of inspiration. The inspiratory muscles are very small and fatigue easily if worked too hard. Conversely, we can generate large expiratory pressures (blowing up balloons) by using some of the larger muscles in the body. Third, tight wet suits, dry suits, harnesses, buoyancy compensators, etc. all interfere with movement of the chest wall and thereby increase the work of breathing.

When diving at shallow depths with standard scuba gear, most divers will have an increased PCO2 if they exercise at more than 60 percent of their maximum. At deeper depths, this will occur at lower work levels because of the increased gas density and increased work of breathing. As can be seen from the preceding discussion, carbon-dioxide is very important and there are many reasons why PCO2 might be elevated in divers.

In the next column we will look at the signs and symptoms of elevated PCO2 and the dangers it poses for the diver.