Colorimetric capnography is a qualitative measurement method that detects the presence of

carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...

(CO2, a relatively acidic gas) in a given gaseous environment. From a medical perspective, the method is usually applied by exposing litmus paper/film to an environment containing a patient's airway gases (i.e. placing it into their breathing circuit/airway circuit), where it will then change color depending on the amount of exhaled CO2 within the circuit. Although there are multiple uses, the colorimetric capnography method is most frequently used to quickly confirm that an advanced airway device such as an endotracheal tube (ETT) or nasotracheal tube (NTT) has been placed in the desired location. Correct placement is evidenced by sufficient color change of the

litmus paper Litmus is a water-soluble mixture of different dyes extracted from lichens. It is often absorbed onto filter paper to produce one of the oldest forms of pH indicator, used to test materials for acidity. In an acidic medium, blue litmus pape ...

while exposed to the airway gases which, if the device is properly placed, will contain relatively high amounts of exhaled CO2. Conversely, lack of detection of CO2 suggests many possible issues, including improper placement of the advanced airway device, most commonly into the

esophagus The esophagus (American English), oesophagus (British English), or œsophagus (Œ, archaic spelling) (American and British English spelling differences#ae and oe, see spelling difference) all ; : ((o)e)(œ)sophagi or ((o)e)(œ)sophaguses), c ...

. There are no contraindications to

capnography 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 anesthesia and intensive care. It is usually presented as a g ...

Equipment

Colorimetric capnography devices do not require electricity, change color reversibly (breath-by-breath), and add a small amount of

dead space ''Dead Space'' is a science fiction horror franchise created and directed by Glen Schofield. ''Dead Space'' was developed by Visceral Games and published and owned by Electronic Arts. The franchise's chronology is not presented in a linear format ...

. A variety of devices are available from multiple manufacturers. Specific devices on the market include the Covidien Nellcor EasyCap or PediCap (manufactured by Medtronic), and the StatCO₂, Mini StatCO₂, and Neo-StatCO₂ (made by Mercury Medical). Metacresol purple is an example of a pH-dependent dye commonly used in such devices. The colour change of the EasyCap is from purple to yellow/gold, leading to the phrase '' or 'gold is good', as an

aide-mémoire In diplomatic correspondence, an aide-mémoire is a proposed agreement or negotiating text circulated informally among delegations for discussion without committing the originating delegation's country to the contents. It has no identified sourc ...

for successful tracheal intubation.

Medical applications

Airway device placement

Colorimetric capnography is most frequently used in medical contexts to determine whether exhaled CO2 is present in a patient's airway circuit after placement of an advanced airway device (e.g. ETT or NTT), but the meaningful significance of CO2 detection is in reality much more broad. Detection of sufficient CO2 suggests that a patient is metabolically active and capable of eliminating CO2, while lack of color change or insufficient color change suggests an issue at one or more of many possible levels including issues with

cellular metabolism Metabolism (, from ''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 cellular processes; the co ...

, airway device integrity and placement, circuit integrity, mucous plugging, cardiopulmonary function, device malfunction, and more.

Nasogastric tube placement

Inadvertent placement of a nasogastric tube (NGT) into the airway rather than the stomach can lead to complications such as

pneumothorax A pneumothorax is collection of air in the pleural space between the lung and the chest wall. Symptoms typically include sudden onset of sharp, one-sided chest pain and dyspnea, shortness of breath. In a minority of cases, a one-way valve is ...

and

pneumonia Pneumonia is an Inflammation, inflammatory condition of the lung primarily affecting the small air sacs known as Pulmonary alveolus, alveoli. Symptoms typically include some combination of Cough#Classification, productive or dry cough, ches ...

. The use of colorimetric capnography to detect proper placement of 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 esophagus, and down into the stomach. Orogastric intubation is a similar process involving the insertion ...

(NGT) has shown promise, especially in mechanically ventilated patients. With this method, rather than exposing the litmus paper to the patient's airway gases via connection with the airway circuit, the litmus paper is exposed to the gases transmitted via the nasogastric tube. Put simply, just as this method can detect the desired ''presence'' of significant CO2 in an airway, it can also detect the desired ''lack'' of CO2 in a properly placed nasogastric tube. Further research on this particular use of colorimetric capnography and the widespread incorporation of adapters will likely be required if this utilization is to become more commonplace.

Patient monitoring

Data inputs from multiple sources (e.g. blood pressure cuff (sphygmomanometer),

pulse oximeter Pulse oximetry is a noninvasive method for monitoring blood 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 art ...

thermometer A thermometer is a device that measures temperature (the hotness or coldness of an object) or temperature gradient (the rates of change of temperature in space). A thermometer has two important elements: (1) a temperature sensor (e.g. the bulb ...

, etc.) are used to characterize a patient's vital condition, and the more meaningful data that is available, the more accurate and precise a clinician can be in addressing a patient's health status.

Capnography 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 anesthesia and intensive care. It is usually presented as a g ...

as a whole represents a significantly useful data point in monitoring a patient's respiratory and metabolic status in situations including but not limited to

cardiac arrest Cardiac arrest (also known as sudden cardiac arrest CA is when the heart suddenly and unexpectedly stops beating. When the heart stops beating, blood cannot properly Circulatory system, circulate around the body and the blood flow to the ...

metabolic acidemia Metabolic acidosis is a serious electrolyte disorder characterized by an imbalance in the body's acid-base balance. Metabolic acidosis has three main root causes: increased acid production, loss of bicarbonate, and a reduced ability of the kidn ...

mechanical ventilation Mechanical ventilation or assisted ventilation is the Medicine, medical term for using a ventilator, ventilator machine to fully or partially provide artificial ventilation. Mechanical ventilation helps move air into and out of the lungs, wit ...

, and

procedural sedation Procedural sedation and analgesia (PSA) is a technique in which a sedating/dissociative medication is given, usually along with an analgesic medication, in order to perform non-surgical procedures on a patient. The overall goal is to induce a decr ...

Comparison to quantitative capnography

One disadvantage of qualitative capnography methods such as colorimetry is that they do not produce a direct numeric or waveform readout as can a quantitative method such as infrared capnography. The colorimetric method rather presents CO2 simply as a color most commonly on the spectrum of purple (lower CO2) to gold (higher CO2) and leaves the clinician to interpret anything beyond this single returned value. Despite being a more rudimentary method of capnography as compared to quantitative methods such as infrared capnography, colorimetric capnography has proven to remain beneficial in multiple contexts in modern medicine as mentioned in the above section. Evidence also suggests that colorimetric capnography is just as effective as infrared capnography at determining correct tracheal airway device placement, but may fall short of infrared capnography when detecting inadvertent esophageal airway device placement.

Naming controversy

There has been debate related to the term 'colorimetric capnography,' especially with the growing employment of this method during the

COVID-19 pandemic The COVID-19 pandemic (also known as the coronavirus pandemic and COVID pandemic), caused by severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), began with an disease outbreak, outbreak of COVID-19 in Wuhan, China, in December ...

due to an increased volume of patients requiring

. The use of the term 'capnography' technically implies some type of quantitative result or readout which, as described above, is not produced by the colorimetric method. Descriptors such as "carbon dioxide colorimetry," "colorimetric carbon dioxide detection," and "chemical colorimetric analysis" have been suggested as potentially more accurate replacements.

References

{{Reflist

External links

LITFL.com information on capnography
Anesthesia