CO₂ retention 1256680 210952600 2008-05-08T02:51:51Z Tenbergen 1283697 /* Closed circuit equipment */ '''CO<sub>2</sub> retention''' is a [[pathophysiology|pathophysiological]] process in which too little [[carbon dioxide]] is removed from the [[blood]] by the [[lungs]]. The end result is [[hypercapnia]], an elevated level of carbon dioxide dissolved in the bloodstream. Various diseases may lead to this state; disturbed [[gas exchange]] may lead to impaired excretion of the gas. In addition, breathing air with a high carbon dioxide concentration may also lead to hypercapnia. The principal result of the increased amount of dissolved CO<sub>2</sub> is [[acidosis]] ([[respiratory acidosis]] when caused by impaired lung function); other effects include [[tachycardia]] (rapid [[heart rate]]) [[seizure]]s, [[coma]], [[respiratory arrest]] and [[death]]. CO<sub>2</sub> retention is a problem in various respiratory diseases, particularly [[chronic obstructive pulmonary disease]] (COPD). Patients with COPD who receive excessive supplemental [[oxygen]] can develop CO<sub>2</sub> retention, and subsequent hypercapnia. The mechanism that underlies this state is a matter of controversy. Some authorities point to a reduction in the [[Hypoxia (medical)|hypoxic]] "drive", a condition called carbon dioxide narcosis. When carbon dioxide levels are chronically elevated, the respiratory center becomes less sensitive to CO2 as a stimulant of the respiratory drive, and the PaO2 provides the primary stimulus for respirations. Administering excess supplemental oxygen can potentially suppress the respiratory center. However, it is unclear whether such a hypoxic drive exists in the first place. An alternative explanation is that, in patients with COPD, the administration of oxygen leads to an increase in the degree to which diseased [[alveoli]] are perfused with blood relative to other, less-diseased [[alveoli]]. As a result, a larger fraction of blood passes through parts of the lung that are poorly-ventilated, with a resulting increase in the CO<sub>2</sub> concentration of the blood leaving the lungs. As CO<sub>2</sub> levels increase, patients exhibit a reduction in overall level of consciousness as well as respiratory effort. Severe increases in CO<sub>2</sub> levels can lead to respiratory arrest. CO<sub>2</sub> 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 CO<sub>2</sub>), this mechanism fails in type II. [[Mechanical ventilation]] (through [[intubation]], [[Continuous positive airway pressure|CPAP]] or [[BIPAP]]) may be indicated, or infusion of [[doxapram]]. ==In diving== CO<sub>2</sub> retention with its attendant dangers of death from convulsions and [[Hypoxia (medical)|hypoxia]] (low oxygen level) is primarily of concern to the scuba diver due to "skip breathing". Other mechanisms of CO<sub>2</sub> 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 CO<sub>2</sub> 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 CO<sub>2</sub> concentration on land. All of the CO<sub>2</sub> developed during breathing from open circuit equipment underwater is normally expelled from the apparatus in the exhaled breath as bubbles. The partial pressure of CO<sub>2</sub> produced by the body does not increase with depth as do other gases in breathing mixes, 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, or from inadequate breathing. If the diver takes shallow breaths or skip breathes, a larger porportion of the CO<sub>2</sub> is not completely expelled and is re-inhaled on the next breath. The medical term for high carbon dioxide in the blood is [[hypercapnia]]; when the level is high enough it can cause "CO<sub>2</sub> toxicity," which can lead to shortness of breath, [[headache]], confusion and [[drowning]] (depending on how severe). Elevated CO<sub>2</sub> levels play a significant role in [[oxygen toxicity]] and in [[nitrogen narcosis]]. The acceptable CO<sub>2</sub> 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. ===Closed circuit equipment=== With the increased usage of [[rebreather]] diving, mainly by the military-but recently by more and more civilian divers, there is the possibility of hypercapnia (high CO<sub>2</sub> 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 [[sweat]]ing, mental impairment and finally, unconsciousness. This hypercapnia comes about due to malfunction of the [[soda lime]] CO<sub>2</sub> absorbent canisters and can be avoided by decreasing the exercise rate, watching out for the operating limits of the canister, checking for leaks at the start of the dive and not reusing the absorbent. [[Category:Pulmonology]] [[Category:Diving medicine]]