Capnography
Encyclopedia
Capnography is the monitoring of the concentration or partial pressure
of carbon dioxide
in the respiratory gases. Its main development has been as a monitoring tool for use during anaesthesia and intensive care. It is usually presented as a graph of expiratory plotted against time, or, less commonly, but more usefully, expired volume. The plot may also show the inspired , which is of interest when rebreathing systems are being used.
The capnogram is a direct monitor of the inhaled and exhaled concentration or partial pressure of , and an indirect monitor of the partial pressure in the arterial blood
. In healthy individuals, the difference between arterial blood and expired gas partial pressures is very small. In the presence of most forms of lung disease, and some forms of congenital heart disease (the cyanotic lesions) the difference between arterial blood and expired gas increases and can exceed 1 kPa.
During anaesthesia, there is interplay between two components: the patient and the anaesthesia administration device (which is usually a breathing circuit and a ventilator). The critical connection between the two components is either an endotracheal tube
or a mask, and is typically monitored at this junction. Capnography directly reflects the elimination of by the lungs to the anaesthesia device. Indirectly, it reflects the production of by tissues and the circulatory transport of to the lungs.
When expired is related to expired volume rather than time, the area beneath the curve represents the volume of in the breath, and thus over the course of a minute, this method can yield the minute elimination, an important measure of metabolism. Sudden changes in elimination during lung or heart surgery usually imply important changes in cardiorespiratory function.
Capnographs usually work on the principle that absorbs infra-red radiation. A beam of infra-red light is passed across the gas sample to fall on a sensor. The presence of in the gas leads to a reduction in the amount of light falling on the sensor, which changes the voltage in a circuit. The analysis is rapid and accurate, but the presence of nitrous oxide
in the gas mix changes the infra-red absorption via the phenomenon of collision broadening. This must be corrected for. Measuring the in human breath by measuring its infra-red absorptive power was established as a reliable technique by John Tyndall
in 1864, though 19th and early 20th century devices were too cumbersome for everyday clinical use.
(lung) perfusion, alveolar
ventilation, respiratory patterns
, and elimination of from the anaesthesia breathing circuit and ventilator. The shape of the curve is affected by some forms of lung disease; in general there are obstructive conditions such as bronchitis
, emphysema
and asthma
, in which the mixing of gases within the lung is affected.
Conditions such as pulmonary embolism
and congenital heart disease, which affect perfusion of the lung, do not, in themselves, affect the shape of the curve, but greatly affect the relationship between expired and arterial blood . Capnography can also be used to measure carbon dioxide production, a measure of metabolism. Increased production is seen during fever and shivering. Reduced production is seen during anaesthesia and hypothermia
.
, oesophageal intubation and circuit disconnection; thus allowing patient injury
to be prevented. During procedures done under sedation, capnography provides more useful information, e.g. on the frequency and regularity of ventilation, than pulse oximetry
.
Capnography provides a rapid and reliable method to detect life-threatening conditions (malposition of tracheal tube
s, unsuspected ventilatory failure, circulatory failure and defective breathing circuits) and to circumvent potentially irreversible patient injury.
Capnography and pulse oximetry together could have helped in the prevention of 93% of avoidable anaesthesia mishaps according to an ASA (American Society of Anesthesiologists
) closed claim study.
A study in the March 2005 Annals of Emergency Medicine, comparing field intubations
that used continuous capnography to confirm intubations versus non-use showed zero unrecognized misplaced intubations in the monitoring group versus 23% misplaced tubes in the unmonitored group. The American Heart Association
(AHA) affirmed the importance of using capnography to verify tube placement in their 2005 CPR and ECG Guidelines.
The AHA also notes in their new guidelines that capnography, which indirectly measures cardiac output, can also be used to monitor the effectiveness of CPR and as an early indication of return of spontaneous circulation (ROSC). Studies have shown that when a person doing CPR tires, the patient's end-tidal (ETCO2, the level of carbon dioxide released at the end of expiration) falls, and then rises when a fresh rescuer takes over. Other studies have shown when a patient experiences return of spontaneous circulation, the first indication is often a sudden rise in the ETCO2 as the rush of circulation washes untransported from the tissues. Likewise, a sudden drop in ETCO2 may indicate the patient has lost pulses and CPR may need to be initiated.
Paramedics are also now beginning to monitor the ETCO2 status of nonintubated patients by using a special nasal cannula
that collects the carbon dioxide. A high ETCO2 reading in a patient with altered mental status or severe difficulty breathing may indicate hypoventilation
and a possible need for the patient to be intubated.
Capnography, because it provides a breath by breath measurement of a patient's ventilation, can quickly reveal a worsening trend in a patient's condition by providing paramedics with an early warning system into a patient's respiratory status. Clinical studies are expected into the uses of capnography in asthma
, congestive heart failure
, diabetes, circulatory shock, pulmonary embolus, acidosis
, and other conditions, with potential implications for the prehospital use of capnography.
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
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
in the respiratory gases. Its main development has been as a monitoring tool for use during anaesthesia and intensive care. It is usually presented as a graph of expiratory plotted against time, or, less commonly, but more usefully, expired volume. The plot may also show the inspired , which is of interest when rebreathing systems are being used.
The capnogram is a direct monitor of the inhaled and exhaled concentration or partial pressure of , and an indirect monitor of the partial pressure in the arterial 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....
. In healthy individuals, the difference between arterial blood and expired gas partial pressures is very small. In the presence of most forms of lung disease, and some forms of congenital heart disease (the cyanotic lesions) the difference between arterial blood and expired gas increases and can exceed 1 kPa.
During anaesthesia, there is interplay between two components: the patient and the anaesthesia administration device (which is usually a breathing circuit and a ventilator). The critical connection between the two components is either an endotracheal tube
Tracheal tube
A tracheal tube is a catheter that is inserted into the trachea in order for the primary purpose of establishing and maintaining a patent airway and to ensure the adequate exchange of oxygen and carbon dioxide. Many different types of tracheal tubes are available, suited for different specific...
or a mask, and is typically monitored at this junction. Capnography directly reflects the elimination of by the lungs to the anaesthesia device. Indirectly, it reflects the production of by tissues and the circulatory transport of to the lungs.
When expired is related to expired volume rather than time, the area beneath the curve represents the volume of in the breath, and thus over the course of a minute, this method can yield the minute elimination, an important measure of metabolism. Sudden changes in elimination during lung or heart surgery usually imply important changes in cardiorespiratory function.
Capnographs usually work on the principle that absorbs infra-red radiation. A beam of infra-red light is passed across the gas sample to fall on a sensor. The presence of in the gas leads to a reduction in the amount of light falling on the sensor, which changes the voltage in a circuit. The analysis is rapid and accurate, but the presence of nitrous oxide
Nitrous oxide
Nitrous oxide, commonly known as laughing gas or sweet air, is a chemical compound with the formula . It is an oxide of nitrogen. At room temperature, it is a colorless non-flammable gas, with a slightly sweet odor and taste. It is used in surgery and dentistry for its anesthetic and analgesic...
in the gas mix changes the infra-red absorption via the phenomenon of collision broadening. This must be corrected for. Measuring the in human breath by measuring its infra-red absorptive power was established as a reliable technique by John Tyndall
John Tyndall
John Tyndall FRS was a prominent Irish 19th century physicist. His initial scientific fame arose in the 1850s from his study of diamagnetism. Later he studied thermal radiation, and produced a number of discoveries about processes in the atmosphere...
in 1864, though 19th and early 20th century devices were too cumbersome for everyday clinical use.
Diagnostic usage
Capnography provides information about production, pulmonaryHuman lung
The human lungs are the organs of respiration in humans. Humans have two lungs, with the left being divided into two lobes and the right into three lobes. Together, the lungs contain approximately of airways and 300 to 500 million alveoli, having a total surface area of about in...
(lung) perfusion, alveolar
Pulmonary alveolus
An alveolus is an anatomical structure that has the form of a hollow cavity. Found in the lung parenchyma, the pulmonary alveoli are the dead ends of the respiratory tree, which outcrop from either alveolar sacs or alveolar ducts, which are both sites of gas exchange with the blood as well...
ventilation, respiratory patterns
Respiration (physiology)
'In physiology, respiration is defined as the transport of oxygen from the outside air to the cells within tissues, and the transport of carbon dioxide in the opposite direction...
, and elimination of from the anaesthesia breathing circuit and ventilator. The shape of the curve is affected by some forms of lung disease; in general there are obstructive conditions such as bronchitis
Bronchitis
Acute bronchitis is an inflammation of the large bronchi in the lungs that is usually caused by viruses or bacteria and may last several days or weeks. Characteristic symptoms include cough, sputum production, and shortness of breath and wheezing related to the obstruction of the inflamed airways...
, emphysema
Emphysema
Emphysema is a long-term, progressive disease of the lungs that primarily causes shortness of breath. In people with emphysema, the tissues necessary to support the physical shape and function of the lungs are destroyed. It is included in a group of diseases called chronic obstructive pulmonary...
and asthma
Asthma
Asthma is the common chronic inflammatory disease of the airways characterized by variable and recurring symptoms, reversible airflow obstruction, and bronchospasm. Symptoms include wheezing, coughing, chest tightness, and shortness of breath...
, in which the mixing of gases within the lung is affected.
Conditions such as pulmonary embolism
Pulmonary embolism
Pulmonary embolism is a blockage of the main artery of the lung or one of its branches by a substance that has travelled from elsewhere in the body through the bloodstream . Usually this is due to embolism of a thrombus from the deep veins in the legs, a process termed venous thromboembolism...
and congenital heart disease, which affect perfusion of the lung, do not, in themselves, affect the shape of the curve, but greatly affect the relationship between expired and arterial blood . Capnography can also be used to measure carbon dioxide production, a measure of metabolism. Increased production is seen during fever and shivering. Reduced production is seen during anaesthesia and hypothermia
Hypothermia
Hypothermia is a condition in which core temperature drops below the required temperature for normal metabolism and body functions which is defined as . Body temperature is usually maintained near a constant level of through biologic homeostasis or thermoregulation...
.
Use in anaesthesia
Capnography has been shown to be more effective than clinical judgement alone in the early detection of adverse respiratory events such as hypoventilationHypoventilation
In medicine, hypoventilation occurs when ventilation is inadequate to perform needed gas exchange...
, oesophageal intubation and circuit disconnection; thus allowing patient injury
Injury
-By cause:*Traumatic injury, a body wound or shock produced by sudden physical injury, as from violence or accident*Other injuries from external physical causes, such as radiation injury, burn injury or frostbite*Injury from infection...
to be prevented. During procedures done under sedation, capnography provides more useful information, e.g. on the frequency and regularity of ventilation, than pulse oximetry
Pulse oximetry
Pulse oximetry is a non-invasive method allowing the monitoring of the oxygenation of a patient's hemoglobin.A sensor is placed on a thin part of the patient's body, usually a fingertip or earlobe, or in the case of an infant, across a foot....
.
Capnography provides a rapid and reliable method to detect life-threatening conditions (malposition of tracheal tube
Tracheal tube
A tracheal tube is a catheter that is inserted into the trachea in order for the primary purpose of establishing and maintaining a patent airway and to ensure the adequate exchange of oxygen and carbon dioxide. Many different types of tracheal tubes are available, suited for different specific...
s, unsuspected ventilatory failure, circulatory failure and defective breathing circuits) and to circumvent potentially irreversible patient injury.
Capnography and pulse oximetry together could have helped in the prevention of 93% of avoidable anaesthesia mishaps according to an ASA (American Society of Anesthesiologists
American Society of Anesthesiologists
The American Society of Anesthesiologists is an association of physicians, primarily anesthesiologists, that share a common goal of raising the standard of the medical specialty of anesthesiology and the improvement of patient care by fostering and encouraging education through research and...
) closed claim study.
Capnography in emergency medical services
Capnography is increasingly being used by paramedics to aid in their assessment and treatment of patients in the prehospital environment. These uses include verifying and monitoring the position of an endotracheal tube. A properly positioned tube in the trachea guards the patient's airway and enables the paramedic to breathe for the patient. A misplaced tube in the esophagus can lead to death.A study in the March 2005 Annals of Emergency Medicine, comparing field intubations
Tracheal 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...
that used continuous capnography to confirm intubations versus non-use showed zero unrecognized misplaced intubations in the monitoring group versus 23% misplaced tubes in the unmonitored group. The American Heart Association
American Heart Association
The American Heart Association is a non-profit organization in the United States that fosters appropriate cardiac care in an effort to reduce disability and deaths caused by cardiovascular disease and stroke. It is headquartered in Dallas, Texas...
(AHA) affirmed the importance of using capnography to verify tube placement in their 2005 CPR and ECG Guidelines.
The AHA also notes in their new guidelines that capnography, which indirectly measures cardiac output, can also be used to monitor the effectiveness of CPR and as an early indication of return of spontaneous circulation (ROSC). Studies have shown that when a person doing CPR tires, the patient's end-tidal (ETCO2, the level of carbon dioxide released at the end of expiration) falls, and then rises when a fresh rescuer takes over. Other studies have shown when a patient experiences return of spontaneous circulation, the first indication is often a sudden rise in the ETCO2 as the rush of circulation washes untransported from the tissues. Likewise, a sudden drop in ETCO2 may indicate the patient has lost pulses and CPR may need to be initiated.
Paramedics are also now beginning to monitor the ETCO2 status of nonintubated patients by using a special nasal cannula
Cannula
A cannula or canula is a tube that can be inserted into the body, often for the delivery or removal of fluid or for the gathering of data...
that collects the carbon dioxide. A high ETCO2 reading in a patient with altered mental status or severe difficulty breathing may indicate hypoventilation
Hypoventilation
In medicine, hypoventilation occurs when ventilation is inadequate to perform needed gas exchange...
and a possible need for the patient to be intubated.
Capnography, because it provides a breath by breath measurement of a patient's ventilation, can quickly reveal a worsening trend in a patient's condition by providing paramedics with an early warning system into a patient's respiratory status. Clinical studies are expected into the uses of capnography in asthma
Asthma
Asthma is the common chronic inflammatory disease of the airways characterized by variable and recurring symptoms, reversible airflow obstruction, and bronchospasm. Symptoms include wheezing, coughing, chest tightness, and shortness of breath...
, congestive heart failure
Congestive heart failure
Heart failure often called congestive heart failure is generally defined as the inability of the heart to supply sufficient blood flow to meet the needs of the body. Heart failure can cause a number of symptoms including shortness of breath, leg swelling, and exercise intolerance. The condition...
, diabetes, circulatory shock, pulmonary embolus, 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....
, and other conditions, with potential implications for the prehospital use of capnography.
See also
- Medical equipmentMedical equipmentMedical equipment is designed to aid in the diagnosis, monitoring or treatment of medical conditions.-Types:There are several basic types:* Diagnostic equipment includes medical imaging machines, used to aid in diagnosis...
- Medical testMedical testA diagnostic test is any kind of medical test performed to aid in the diagnosis or detection of disease. For example:* to diagnose diseases, and preferably sub-classify it regarding, for example, severity and treatability...
- Integrated Pulmonary IndexIntegrated Pulmonary IndexIntegrated Pulmonary Index is a patient pulmonary index which uses information from capnography and pulse oximetry to provide a single value that describes the patient’s respiratory status...
- Respiratory monitoringRespiratory monitoringMonitoring a patient's respiratory status usually takes place in a hospital setting and may be the primary purpose for a patient being observed or admitted to a medical setting....