CO2 retention

CO₂ retention is a pathophysiological state in which too much carbon dioxide is accumulated in the blood. It is the result of hypercarbia . Various diseases may lead to this state; disturbed gas exchange may lead to impaired excretion of the gas. In addition, breathing in too much carbon dioxide may cause this problem.

A result of the increased CO₂ is acidosis (respiratory acidosis when caused by impaired lung function), as well as tachycardia (rapid heart rate) seizures, coma, respiratory arrest and death.

CO₂ retention is a problem in various respiratory diseases, such as chronic obstructive pulmonary disease (COPD). Administering too much oxygen decreases the hypoxic "drive", with impaired blowing off of CO₂ and risk of respiratory arrest.

CO₂ retention is the hallmark of type II respiratory failure. While in type I any degree of hypoxia is compensated for by hyperventilation (and a decrease in CO₂), this mechanism fails in type II. Mechanical ventilation (through intubation, CPAP or BIPAP ) may be indicated, or infusion of doxapram.

In diving

CO2 retention with its attendant dangers of death from convulsions and hypoxia (low oxygen level) is primarily of concern to the scuba diver due to "skip breathing". Other sources of CO2 retention are breath-hold diving, breathing in a sealed environment, faulty regulator, exercise at extreme depth and using contaminated air.

Symptoms include rapid respiration in 4-6%, rapid pulse rate, shortness of breath in 7-10% and convulsions and unconsciousness in 11-20%.

The CO2 level in the blood is unchanged by the ambient pressure (i.e., the depth) per se, since the partial pressure of carbon dioxide in a scuba diver's blood is a function only of metabolism and the rate and depth of breathing - the same factors that determine blood CO2 concentration on land.

All of the CO2 that's developed during breathing underwater is exhaled in the bubbles from scuba apparatus and does not increase with depth as do other gases, such as nitrogen, oxygen, carbon monoxide and hydrocarbons. Abnormal carbon dioxide accumulation in the blood can occur from too high a level of metabolism (such as from exercise at depth) and/or inadequate breathing (usually not breathing deep enough or skip breathing). The medical term for high carbon dioxide in the blood is hypercapnia; when the level is high enough it can cause "CO2 toxicity," which can lead to shortness of breath, headache, confusion and drowning (depending on severity).

Elevated CO2 levels play a significant role in oxygen toxicity and in nitrogen narcosis. The acceptable CO2 level for diving operations is 1.5% surface equivalent (10.5 mmHg); the acceptable level for hyperbaric oxygen therapy operations is one that allows a vent schedule of 4scfm/person displacement.

With the increased usage of rebreather diving, mainly by the military-but recently by more and more civilian divers, there is the possibility of hypercarbia (high CO2 levels), among other medical considerations.

Signs and symptoms that need to be observed are hyperventilation, shortness of breath and tachycardia (rapid heart beat), headache and excessive sweating, mental impairment and finally, unconsciousness.

This hypercarbia comes about due to malfunction of the soda lime CO2 absorbent canisters and can be avoided by decreasing the exercise rate, watch out for the operating limits of the canister, checking for leaks at the start of the dive and not reusing the absorbent.