CO2 retention
Encyclopedia
CO2 retention is a 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 CO2 is acidosis
(respiratory acidosis
when caused by impaired lung function); other effects include tachycardia
(rapid heart rate
) seizure
s, coma
, respiratory arrest
and death
.
CO2 retention is a problem in various respiratory diseases, particularly chronic obstructive pulmonary disease
(COPD). Patients with COPD who receive excessive supplemental oxygen
can develop CO2 retention, and subsequent hypercapnia. The mechanism that underlies this state is a matter of controversy. Some authorities point to a reduction in the hypoxic
"drive", a condition called carbon dioxide narcosis. When carbon dioxide levels are chronically elevated, the respiratory center becomes less sensitive to 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 CO2 concentration of the blood leaving the lungs.
As CO2 levels increase, patients exhibit a reduction in overall level of consciousness as well as respiratory effort. Severe increases in CO2 levels can lead to 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
.
(low oxygen level) is primarily of concern to the scuba diver due to "skip breathing". Other mechanisms 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 developed during breathing from open circuit equipment underwater is normally expelled from the apparatus in the exhaled breath as bubbles. The partial pressure of CO2 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 proportion of the CO2 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 "CO2 toxicity," which can lead to shortness of breath, headache
, confusion and drowning
(depending on how severe).
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 4 scfm
per person displacement.
diving, mainly by the military—but recently by more and more civilian divers, there is the possibility of hypercapnia (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 sweat
ing, mental impairment and finally, unconsciousness.
This hypercapnia comes about due to malfunction of the soda lime
CO2 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.
Pathophysiology
Pathophysiology is the study of the changes of normal mechanical, physical, and biochemical functions, either caused by a disease, or resulting from an abnormal syndrome...
process in which too little carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
is removed from the blood
Blood
Blood is a specialized bodily fluid in animals that delivers necessary substances such as nutrients and oxygen to the cells and transports metabolic waste products away from those same cells....
by the lungs. The end result is hypercapnia
Hypercapnia
Hypercapnia or hypercapnea , also known as hypercarbia, is a condition where there is too much carbon dioxide in the blood...
, an elevated level of carbon dioxide dissolved in the bloodstream. Various diseases may lead to this state; disturbed gas exchange
Gas exchange
Gas exchange is a process in biology where gases contained in an organism and atmosphere transfer or exchange. In human gas-exchange, gases contained in the blood of human bodies exchange with gases contained in the atmosphere. Human gas-exchange occurs in the lungs...
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 CO2 is acidosis
Acidosis
Acidosis is an increased acidity in the blood and other body tissue . If not further qualified, it usually refers to acidity of the blood plasma....
(respiratory acidosis
Respiratory acidosis
Respiratory acidosis is a medical condition in which decreased ventilation causes increased blood carbon dioxide concentration and decreased pH ....
when caused by impaired lung function); other effects include tachycardia
Tachycardia
Tachycardia comes from the Greek words tachys and kardia . Tachycardia typically refers to a heart rate that exceeds the normal range for a resting heart rate...
(rapid heart rate
Heart rate
Heart rate is the number of heartbeats per unit of time, typically expressed as beats per minute . Heart rate can vary as the body's need to absorb oxygen and excrete carbon dioxide changes, such as during exercise or sleep....
) seizure
Seizure
An epileptic seizure, occasionally referred to as a fit, is defined as a transient symptom of "abnormal excessive or synchronous neuronal activity in the brain". The outward effect can be as dramatic as a wild thrashing movement or as mild as a brief loss of awareness...
s, coma
Coma
In medicine, a coma is a state of unconsciousness, lasting more than 6 hours in which a person cannot be awakened, fails to respond normally to painful stimuli, light or sound, lacks a normal sleep-wake cycle and does not initiate voluntary actions. A person in a state of coma is described as...
, respiratory arrest
Respiratory arrest
Respiratory arrest is the cessation of breathing. It is a medical emergency and it usually is related to or coincides with a cardiac arrest. Causes include opiate overdose, head injury, anaesthesia, tetanus, or drowning...
and death
Death
Death is the permanent termination of the biological functions that sustain a living organism. Phenomena which commonly bring about death include old age, predation, malnutrition, disease, and accidents or trauma resulting in terminal injury....
.
CO2 retention is a problem in various respiratory diseases, particularly chronic obstructive pulmonary disease
Chronic obstructive pulmonary disease
Chronic obstructive pulmonary disease , also known as chronic obstructive lung disease , chronic obstructive airway disease , chronic airflow limitation and chronic obstructive respiratory disease , is the co-occurrence of chronic bronchitis and emphysema, a pair of commonly co-existing diseases...
(COPD). Patients with COPD who receive excessive supplemental oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
can develop CO2 retention, and subsequent hypercapnia. The mechanism that underlies this state is a matter of controversy. Some authorities point to a reduction in the hypoxic
Hypoxia (medical)
Hypoxia, or hypoxiation, is a pathological condition in which the body as a whole or a region of the body is deprived of adequate oxygen supply. Variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise...
"drive", a condition called carbon dioxide narcosis. When carbon dioxide levels are chronically elevated, the respiratory center becomes less sensitive to 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 CO2 concentration of the blood leaving the lungs.
As CO2 levels increase, patients exhibit a reduction in overall level of consciousness as well as respiratory effort. Severe increases in CO2 levels can lead to respiratory arrest.
CO2 retention is the hallmark of type II respiratory failure
Respiratory failure
The term respiratory failure, in medicine, is used to describe inadequate gas exchange by the respiratory system, with the result that arterial oxygen and/or carbon dioxide levels cannot be maintained within their normal ranges. A drop in blood oxygenation is known as hypoxemia; a rise in arterial...
. While in type I any degree of hypoxia is compensated for by hyperventilation
Hyperventilation
Hyperventilation or overbreathing is the state of breathing faster or deeper than normal, causing excessive expulsion of circulating carbon dioxide. It can result from a psychological state such as a panic attack, from a physiological condition such as metabolic acidosis, can be brought about by...
(and a decrease in CO2), this mechanism fails in type II. Mechanical ventilation
Mechanical ventilation
In medicine, mechanical ventilation is a method to mechanically assist or replace spontaneous breathing. This may involve a machine called a ventilator or the breathing may be assisted by a physician, respiratory therapist or other suitable person compressing a bag or set of bellows...
(through intubation
Intubation
Tracheal intubation, usually simply referred to as intubation, is the placement of a flexible plastic or rubber tube into the trachea to maintain an open airway or to serve as a conduit through which to administer certain drugs...
, CPAP
Continuous positive airway pressure
Positive airway pressure is a mode of respiratory ventilation used primarily in the treatment of sleep apnea, for which it was first developed. PAP ventilation is also commonly used for those who are critically ill in hospital with respiratory failure, and in newborn infants...
or BIPAP) may be indicated, or infusion of doxapram
Doxapram
Doxapram hydrochloride is a respiratory stimulant. Administered intravenously, doxapram stimulates an increase in tidal volume, and respiratory rate.-Mode of action:...
.
In diving
CO2 retention with its attendant dangers of death from convulsions and hypoxiaHypoxia (medical)
Hypoxia, or hypoxiation, is a pathological condition in which the body as a whole or a region of the body is deprived of adequate oxygen supply. Variations in arterial oxygen concentrations can be part of the normal physiology, for example, during strenuous physical exercise...
(low oxygen level) is primarily of concern to the scuba diver due to "skip breathing". Other mechanisms 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
Partial pressure
In a mixture of ideal gases, each gas has a partial pressure which is the pressure which the gas would have if it alone occupied the volume. The total pressure of a gas mixture is the sum of the partial pressures of each individual gas in the mixture....
of carbon dioxide in a scuba diver's blood is a function only of metabolism
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
and the rate and depth of breathing—the same factors that determine blood CO2 concentration on land.
All of the CO2 developed during breathing from open circuit equipment underwater is normally expelled from the apparatus in the exhaled breath as bubbles. The partial pressure of CO2 produced by the body does not increase with depth as do other gases in breathing mixes, such as nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
, oxygen
Oxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
, carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
and hydrocarbons.
Abnormal carbon dioxide accumulation in the blood can occur from too high a level of metabolism
Metabolism
Metabolism is the set of chemical reactions that happen in the cells of living organisms to sustain life. These processes allow organisms to grow and reproduce, maintain their structures, and respond to their environments. Metabolism is usually divided into two categories...
, such as from exercise at depth, or from inadequate breathing. If the diver takes shallow breaths or skip breathes, a larger proportion of the CO2 is not completely expelled and is re-inhaled on the next breath. The medical term for high carbon dioxide in the blood is hypercapnia
Hypercapnia
Hypercapnia or hypercapnea , also known as hypercarbia, is a condition where there is too much carbon dioxide in the blood...
; when the level is high enough it can cause "CO2 toxicity," which can lead to shortness of breath, headache
Headache
A headache or cephalalgia is pain anywhere in the region of the head or neck. It can be a symptom of a number of different conditions of the head and neck. The brain tissue itself is not sensitive to pain because it lacks pain receptors. Rather, the pain is caused by disturbance of the...
, confusion and drowning
Drowning
Drowning is death from asphyxia due to suffocation caused by water entering the lungs and preventing the absorption of oxygen leading to cerebral hypoxia....
(depending on how severe).
Elevated CO2 levels play a significant role in oxygen toxicity
Oxygen toxicity
Oxygen toxicity is a condition resulting from the harmful effects of breathing molecular oxygen at elevated partial pressures. It is also known as oxygen toxicity syndrome, oxygen intoxication, and oxygen poisoning...
and in nitrogen narcosis
Nitrogen narcosis
Narcosis while diving , is a reversible alteration in consciousness that occurs while scuba diving at depth. The Greek word ναρκωσις is derived from narke, "temporary decline or loss of senses and movement, numbness", a term used by Homer and Hippocrates...
. The acceptable CO2 level for diving operations is 1.5% surface equivalent (10.5 mmHg); the acceptable level for hyperbaric oxygen therapy
Hyperbaric oxygen therapy
Hyperbaric medicine, also known as hyperbaric oxygen therapy , is the medical use of oxygen at a level higher than atmospheric pressure. The equipment required consists of a pressure chamber, which may be of rigid or flexible construction, and a means of delivering 100% oxygen...
operations is one that allows a vent schedule of 4 scfm
SCFM
Standard cubic feet per minute is the volumetric flow rate of a gas corrected to "standardized" conditions of temperature and pressure. It is equivalent to the molar flow rate by the ideal gas law. Conversion of SCFM to mass flow requires knowledge of the mixture averaged gas molecular weight. ...
per person displacement.
Closed circuit equipment
With the increased usage of rebreatherRebreather
A rebreather is a type of breathing set that provides a breathing gas containing oxygen and recycled exhaled gas. This recycling reduces the volume of breathing gas used, making a rebreather lighter and more compact than an open-circuit breathing set for the same duration in environments where...
diving, mainly by the military—but recently by more and more civilian divers, there is the possibility of hypercapnia (high CO2 levels), among other medical considerations.
Signs and symptoms that need to be observed are hyperventilation
Hyperventilation
Hyperventilation or overbreathing is the state of breathing faster or deeper than normal, causing excessive expulsion of circulating carbon dioxide. It can result from a psychological state such as a panic attack, from a physiological condition such as metabolic acidosis, can be brought about by...
, shortness of breath and tachycardia
Tachycardia
Tachycardia comes from the Greek words tachys and kardia . Tachycardia typically refers to a heart rate that exceeds the normal range for a resting heart rate...
(rapid heart beat), headache and excessive sweat
SWEAT
SWEAT is an OLN/TSN show hosted by Julie Zwillich that aired in 2003-2004.Each of the 13 half-hour episodes of SWEAT features a different outdoor sport: kayaking, mountain biking, ice hockey, beach volleyball, soccer, windsurfing, rowing, Ultimate, triathlon, wakeboarding, snowboarding, telemark...
ing, mental impairment and finally, unconsciousness.
This hypercapnia comes about due to malfunction of the soda lime
Soda lime
Soda lime is a mixture of chemicals, used in granular form in closed breathing environments, such as general anaesthesia, submarines, rebreathers and recompression chambers, to remove carbon dioxide from breathing gases to prevent CO2 retention and carbon dioxide poisoning.It is made by treating...
CO2 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.