Capnography is the monitoring of the concentration or
partial pressure
In a mixture of gases, each constituent gas has a partial pressure which is the notional pressure of that constituent gas as if it alone occupied the entire volume of the original mixture at the same temperature. The total pressure of an ideal gas ...
of
carbon dioxide
Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
() in the respiratory gases. Its main development has been as a monitoring tool for use during
anesthesia
Anesthesia is a state of controlled, temporary loss of sensation or awareness that is induced for medical or veterinary purposes. It may include some or all of analgesia (relief from or prevention of pain), paralysis (muscle relaxation), ...
and
intensive care
Intensive care medicine, also called critical care medicine, is a medical specialty that deals with seriously or critically ill patients who have, are at risk of, or are recovering from conditions that may be life-threatening. It includes pro ...
. It is usually presented as a graph of (measured in kilopascals, "kPa" or millimeters of mercury, "mmHg") plotted against time, or, less commonly, but more usefully, expired volume (known as volumetric capnography). The plot may also show the inspired , which is of interest when
rebreathing
A rebreather is a breathing apparatus that absorbs the carbon dioxide of a user's exhaled breath to permit the rebreathing (recycling) of the substantially unused oxygen content, and unused inert content when present, of each breath. Oxygen is ...
systems are being used. When the measurement is taken at the end of a breath (exhaling), it is called "end tidal" (PETCO
2).
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 is a body fluid in the circulatory system of humans and other vertebrates that delivers necessary substances such as nutrients and oxygen to the cells, and transports metabolic waste products away from those same cells. Blood in the c ...
. In healthy individuals, the difference between arterial blood and expired gas partial pressures is very small (normal difference 4-5 mmHg). In the presence of most forms of lung disease, and some forms of
congenital heart disease
A congenital heart defect (CHD), also known as a congenital heart anomaly and congenital heart disease, is a defect in the structure of the heart or great vessels that is present at birth. A congenital heart defect is classed as a cardiovascular ...
(the cyanotic lesions) the difference between arterial blood and expired gas increases which can be an indication of new pathology or change in the cardiovascular-ventilation system.
Medical Use
Oxygenation and capnography, although related, remain distinct elements in the physiology of respiration. Ventilation refers to the mechanical process of which the lungs expand and exchange volumes of gasses, however respiration further describes the exchange of gasses (mainly and ) at the level of the alveoli. The process of respiration can be divided into two main functions: elimination of waste and replenishing tissues with fresh . Oxygenation (typically measured via
pulse oximetry
Pulse oximetry is a noninvasive method for monitoring a person's oxygen saturation. Peripheral oxygen saturation (SpO2) readings are typically within 2% accuracy (within 4% accuracy in 95% of cases) of the more accurate (and invasive) reading of ...
) measures the latter portion of this system. Capnography describes the elimination of which may reveal greater clinical use than oxygenation status.
During the normal cycle of
respiration
Respiration may refer to:
Biology
* Cellular respiration, the process in which nutrients are converted into useful energy in a cell
** Anaerobic respiration, cellular respiration without oxygen
** Maintenance respiration, the amount of cellul ...
, a single breath can be divided into two phases: inspiration and expiration. At the beginning of inspiration, the lungs expand and free gasses fill the lungs. As the alveoli are filled with this new gas, the concentration of that fills the alveoli is dependent on the ventilation of the alveoli and the perfusion (blood flow) that is delivering the for exchange. Once expiration begins to occur, the lung volume decreases as air is forced out the respiratory tract. The volume of that is exhaled at the end of exhalation is generated as a by product of metabolism from tissue throughout the body. The delivery of to the alveoli for exhalation is dependent on an intact cardiovascular system to ensure adequate blood flow from the tissue to the alveoli. If cardiac output (the amount of blood that is pumped out of the heart) is decreased, the ability to transport is also decreased which is reflected in a decreased expired amount of . The relationship of cardiac output and end tidal is linear, such that as cardiac output increases or decreases, the amount of is also adjusted in the same manner. Therefore the monitoring of end tidal can provide vital information on the integrity of the cardiovascular system, specifically how well the heart is able to pump blood.
The amount of that is measured during each breath requires an intact cardiovascular system to delivery the to the alveoli which is the functional unit of the lungs. During phase I of expiration, the transported to the lungs gas occupies a given space that is not involved in gas exchange, called dead space. Phase II of expiration is when the within the lungs is forced up the respiratory tract on its way to leave the body, which causes mixing of the air from the dead space with the air in the functional alveoli responsible for gas exchange. Phase III is the final portion of expiration which reflects only from the alveoli and not the dead space. These three phases are important to understand in clinical scenarios since a change in the shape and absolute values can indicate respiratory and/or cardiovascular compromise.
Applications
* Assessing Airway Integrity
* Confirmation of Endotracheal Tube Placement
* Predictor of Outcomes in the Intensive Care Unit
* Intraoperative Complications (ie. air embolism, thromboembolism, etc.)
* CPR use in ACLS (Advanced Cardiovascular Life Support)
* Procedural Sedation Monitoring
Anesthesia
![Capnometry parts together](https://upload.wikimedia.org/wikipedia/commons/f/f2/Capnometry_parts_together.JPG)
During anesthesia, there is interplay between two components: the patient and the anesthesia administration device (which is usually a breathing circuit and a
ventilator
A ventilator is a piece of medical technology that provides mechanical ventilation by moving breathable air into and out of the lungs, to deliver breaths to a patient who is physically unable to breathe, or breathing insufficiently. Ventilators ...
). The critical connection between the two components is either an
endotracheal tube
A tracheal tube is a catheter that is inserted into the trachea 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 a ...
or a mask, and is typically monitored at this junction. Capnography directly reflects the elimination of by the
lungs
The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of th ...
to the anesthesia 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 per minute elimination, an important measure of metabolism. Sudden changes in elimination during lung or heart surgery usually imply important changes in cardiorespiratory function.
Capnography has been shown to be more effective than clinical judgement alone in the early detection of adverse respiratory events such as
hypoventilation
Hypoventilation (also known as respiratory depression) occurs when ventilation is inadequate (''hypo'' meaning "below") to perform needed respiratory gas exchange. By definition it causes an increased concentration of carbon dioxide (hypercapnia ...
,
esophageal intubation and circuit disconnection; thus allowing patient
injury
An injury is any physiological damage to living tissue caused by immediate physical stress. An injury can occur intentionally or unintentionally and may be caused by blunt trauma, penetrating trauma, burning, toxic exposure, asphyxiation, o ...
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 is a noninvasive method for monitoring a person's oxygen saturation. Peripheral oxygen saturation (SpO2) readings are typically within 2% accuracy (within 4% accuracy in 95% of cases) of the more accurate (and invasive) reading of ...
.
Capnography provides a rapid and reliable method to detect life-threatening conditions (malposition of
tracheal tube
A tracheal tube is a catheter that is inserted into the trachea 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 a ...
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 anesthesia mishaps according to an ASA (
American Society of Anesthesiologists
The American Society of Anesthesiologists (ASA) is an educational, research and scientific association of physicians organized to raise the standards of the medical practice of anesthesiology and to improve patient care.
As of 2021, the organizat ...
) closed claim study.
Emergency medical services
Capnography is increasingly being used by
EMS personnel 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 tracheal tube is a catheter that is inserted into the trachea 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 a ...
or a
blind insertion airway device
A blind insertion airway device (BIAD or blind insertion device) is a medical device used for airway management that ensures an open pathway between a patient's lungs and the outside world, as well as reducing the risk of aspiration, which can ...
. A properly positioned tube in the
trachea
The trachea, also known as the windpipe, is a Cartilage, cartilaginous tube that connects the larynx to the bronchi of the lungs, allowing the passage of air, and so is present in almost all air-breathing animals with lungs. The trachea extends ...
guards the patient's airway and enables the
paramedic
A paramedic is a registered healthcare professional who works autonomously across a range of health and care settings and may specialise in clinical practice, as well as in education, leadership, and research.
Not all ambulance personnel are p ...
to breathe for the patient. A misplaced tube in the
esophagus
The esophagus (American English) or oesophagus (British English; both ), non-technically known also as the food pipe or gullet, is an organ in vertebrates through which food passes, aided by peristaltic contractions, from the pharynx to the ...
can lead to the patient's death if it goes undetected.
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
The American Heart Association (AHA) is a nonprofit organization in the United States that funds cardiovascular medical research, educates consumers on healthy living and fosters appropriate cardiac care in an effort to reduce disability and death ...
(AHA) affirmed the importance of using capnography to verify tube placement in their 2005
CPR
Cardiopulmonary resuscitation (CPR) is an emergency procedure consisting of chest compressions often combined with artificial ventilation in an effort to manually preserve intact brain function until further measures are taken to restore sponta ...
and Emergency Cardiovascular Care 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 (PETCO2, 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 PETCO2 as the rush of circulation washes untransported from the tissues. Likewise, a sudden drop in PETCO2 may indicate the patient has lost pulses and CPR may need to be initiated.
Paramedics are also now beginning to monitor the PETCO2 status of nonintubated patients by using a special nasal
cannula
A cannula (; Latin meaning 'little reed'; plural or ) is a tube that can be inserted into the body, often for the delivery or removal of fluid or for the gathering of samples. In simple terms, a cannula can surround the inner or outer surfaces ...
that collects the carbon dioxide. A high PETCO2 reading in a patient with altered mental status or severe difficulty breathing may indicate
hypoventilation
Hypoventilation (also known as respiratory depression) occurs when ventilation is inadequate (''hypo'' meaning "below") to perform needed respiratory gas exchange. By definition it causes an increased concentration of carbon dioxide (hypercapnia ...
and a possible need for the patient to be
intubated
Intubation (sometimes entubation) is a medical procedure involving the insertion of a tube into the body. Patients are generally anesthetized beforehand. Examples include tracheal intubation, and the balloon tamponade with a Sengstaken-Blakemo ...
. Low PETCO2 readings on patients may indicate
hyperventilation
Hyperventilation is irregular breathing that occurs when the rate or tidal volume of breathing eliminates more carbon dioxide than the body can produce. This leads to hypocapnia, a reduced concentration of carbon dioxide dissolved in the blood. ...
.
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. When compared to oxygenation which is measured by pulse oximetry, there are several disadvantages that capnography can help address to provide a more accurate reflection of cardiovascular integrity. One shortcoming of measuring pulse oximetry alone is that administration of supplemental oxygen (ie. via nasal cannula) can delay desaturation in a patient if they stopped breathing, therefore delaying medical intervention. Capnography provides a rapid way to directly assess ventilation status and indirectly assess cardiac function. Clinical studies are expected to uncover further uses of capnography in
asthma
Asthma is a long-term inflammatory disease of the airways of the lungs. It is characterized by variable and recurring symptoms, reversible airflow obstruction, and easily triggered bronchospasms. Symptoms include episodes of wheezing, cou ...
,
congestive heart failure
Heart failure (HF), also known as congestive heart failure (CHF), is a syndrome, a group of signs and symptoms caused by an impairment of the heart's blood pumping function. Symptoms typically include shortness of breath, excessive fatigue, a ...
,
diabetes
Diabetes, also known as diabetes mellitus, is a group of metabolic disorders characterized by a high blood sugar level ( hyperglycemia) over a prolonged period of time. Symptoms often include frequent urination, increased thirst and increased ap ...
, circulatory shock,
pulmonary embolus
Pulmonary embolism (PE) is a blockage of an artery in the lungs by a substance that has moved from elsewhere in the body through the bloodstream (embolism). Symptoms of a PE may include shortness of breath, chest pain particularly upon breathing ...
,
acidosis
Acidosis is a process causing increased acidity in the blood and other body tissues (i.e., an increase in hydrogen ion concentration). If not further qualified, it usually refers to acidity of the blood plasma.
The term ''acidemia'' describes t ...
, and other conditions, with potential implications for the prehospital use of capnography.
Registered nurses
Registered nurse
A registered nurse (RN) is a nurse who has graduated or successfully passed a nursing program from a recognized nursing school and met the requirements outlined by a country, state, province or similar government-authorized licensing body to o ...
s in critical care settings may use capnography to determine if a
nasogastric tube
Nasogastric intubation is a medical process involving the insertion of a plastic tube (nasogastric tube or NG tube) through the nose, down the oesophagus, and down into the stomach. Orogastric intubation is a similar process involving the inserti ...
, which is used for feeding, has been placed in the trachea as opposed to the esophagus. Usually a patient will cough or gag if the tube is misplaced, but most patients in critical care settings are sedated or comatose. If a nasogastric tube is accidentally placed in the trachea instead of the esophagus, the tube feedings will go into the lungs, which is a life-threatening situation. If the monitor displays typical waveforms then placement should be confirmed.
Diagnostic usage
Capnography provides information about production,
pulmonary
The lungs are the primary organs of the respiratory system in humans and most other animals, including some snails and a small number of fish. In mammals and most other vertebrates, two lungs are located near the backbone on either side of th ...
(lung) perfusion,
alveolar Alveolus (; pl. alveoli, adj. alveolar) is a general anatomical term for a concave cavity or pit.
Uses in anatomy and zoology
* Pulmonary alveolus, an air sac in the lungs
** Alveolar cell or pneumocyte
** Alveolar duct
** Alveolar macrophage
* ...
ventilation,
respiratory patterns, and elimination of from the anesthesia 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 is inflammation of the bronchi (large and medium-sized airways) in the lungs that causes coughing. Bronchitis usually begins as an infection in the nose, ears, throat, or sinuses. The infection then makes its way down to the bronchi. ...
,
emphysema
Emphysema, or pulmonary emphysema, is a lower respiratory tract disease, characterised by air-filled spaces ( pneumatoses) in the lungs, that can vary in size and may be very large. The spaces are caused by the breakdown of the walls of the alve ...
and
asthma
Asthma is a long-term inflammatory disease of the airways of the lungs. It is characterized by variable and recurring symptoms, reversible airflow obstruction, and easily triggered bronchospasms. Symptoms include episodes of wheezing, cou ...
, in which the mixing of gases within the lung is affected.
Conditions such as
pulmonary embolism
Pulmonary embolism (PE) is a blockage of an pulmonary artery, artery in the lungs by a substance that has moved from elsewhere in the body through the bloodstream (embolism). Symptoms of a PE may include dyspnea, shortness of breath, chest pain p ...
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
Metabolism (, from el, μεταβολή ''metabolē'', "change") is the set of life-sustaining chemical reactions in organisms. The three main functions of metabolism are: the conversion of the energy in food to energy available to run cell ...
. Increased production is seen during fever and shivering. Reduced production is seen during anesthesia and
hypothermia
Hypothermia is defined as a body core temperature below in humans. Symptoms depend on the temperature. In mild hypothermia, there is shivering and mental confusion. In moderate hypothermia, shivering stops and confusion increases. In severe h ...
.
Working mechanism
![Capnometer-schema](https://upload.wikimedia.org/wikipedia/commons/e/e4/Capnometer-schema.jpg)
Capnographs work on the principle that is a polyatomic gas and therefore absorbs
infrared radiation
Infrared (IR), sometimes called infrared light, is electromagnetic radiation (EMR) with wavelengths longer than those of visible light. It is therefore invisible to the human eye. IR is generally understood to encompass wavelengths from around ...
. A beam of infrared 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 (dinitrogen oxide or dinitrogen monoxide), commonly known as laughing gas, nitrous, or nos, is a chemical compound, an oxide of nitrogen with the formula . At room temperature, it is a colourless non-flammable gas, and has a ...
in the gas mix changes the infrared absorption via the phenomenon of collision broadening. This must be corrected for measuring the in human breath by measuring its infrared absorptive power. This was established as a reliable technique by
John Tyndall
John Tyndall FRS (; 2 August 1820 – 4 December 1893) was a prominent 19th-century Irish physicist. His scientific fame arose in the 1850s from his study of diamagnetism. Later he made discoveries in the realms of infrared radiation and the p ...
in 1864, though 19th and early 20th century devices were too cumbersome for everyday clinical use. Today, technologies have since improved and are able to measure the values of near instantaneously and has become a standard practice in medical settings. There are currently two main types of sensors that are used in clinical practice: main-stream sensors and side-stream sensors. Both effectively serve the same function to quantify the amount of that is being exhaled in each breath.
Capnogram model
The capnogram waveform provides information about various respiratory and cardiac parameters. The capnogram
double-exponential model attempts to quantitatively explain the relationship between respiratory parameters and the
exhalatory segment of a capnogram waveform.
According to the model, each exhalatory segment of capnogram waveform follows the analytical expression:
where
*
represents the
partial pressure
In a mixture of gases, each constituent gas has a partial pressure which is the notional pressure of that constituent gas as if it alone occupied the entire volume of the original mixture at the same temperature. The total pressure of an ideal gas ...
of
carbon dioxide
Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...
measured by the capnogram as a function of time
since the beginning of exhalation.
*
represents the alveolar partial pressure of carbon dioxide.
*
represents the inverse of the
dead space fraction (i.e. the ratio of
tidal volume
Tidal volume (symbol VT or TV) is the volume of air moved into or out of the lungs during a normal breath. In a healthy, young human adult, tidal volume is approximately 500 ml per inspiration or 7 ml/kg of body mass.
Mechanical vent ...
to
dead space
''Dead Space'' is a science fiction/horror fiction, horror media franchise created by Glen Schofield and Michael Condrey, developed by Visceral Games, and published and owned by Electronic Arts. The franchise's chronology is not presented in a lin ...
volume).
*
represents the pulmonary time constant (i.e. the product of
pulmonary resistance and
compliance)
In particular, this model explains the rounded "shark-fin" shape of the capnogram observed in patients with
obstructive lung disease
Obstructive lung disease is a category of respiratory disease characterized by airway obstruction. Many obstructive diseases of the lung result from narrowing (obstruction) of the smaller bronchi and larger bronchioles, often because of excessive ...
.
See also
*
Integrated pulmonary index
*
Medical equipment
A medical device is any device intended to be used for medical purposes. Significant potential for hazards are inherent when using a device for medical purposes and thus medical devices must be proved safe and effective with reasonable assura ...
*
Medical test
A medical test is a medical procedure performed to detect, diagnose, or monitor diseases, disease processes, susceptibility, or to determine a course of treatment. Medical tests such as, physical and visual exams, diagnostic imaging, genetic te ...
*
Respiratory monitoring
Mechanical ventilation, assisted ventilation or intermittent mandatory ventilation (IMV), is the medical term for using a machine called a ventilator to fully or partially provide artificial ventilation. Mechanical ventilation helps move air i ...
*
Colorimetric capnography
Colorimetric capnography is a qualitative measurement method that detects the presence of carbon dioxide (CO2, a relatively acidic gas) in a given gaseous environment. From a medical perspective, the method is usually applied by exposing litmus ...
Citations
External links
CapnoBase.org: Respiratory signal database that contains clinical and simulated capnogram recordings
{{Emergency medicine
Anesthesia
Breath tests
Diagnostic emergency medicine
Diagnostic intensive care medicine
Diagnostic pulmonology