Typical capnogram. Expiration phase on the left, inspiration on the right.
Capnography is the monitoring of the concentration or partial pressure of carbon dioxide (Template:Chem/atomTemplate:Chem/atom) 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 graph of expiratory Template:Chem/atomTemplate:Chem/atom (measured in millimeters of mercury, "mmHg") plotted against time, or, less commonly, but more usefully, expired volume. The plot may also show the inspired Template:Chem/atomTemplate:Chem/atom, 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 Template:Chem/atomTemplate:Chem/atom, and an indirect monitor of the Template:Chem/atomTemplate:Chem/atom partial pressure in the arterial blood. In healthy individuals, the difference between arterial blood and expired gas Template:Chem/atomTemplate:Chem/atom 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.
Capnographs usually work on the principle that Template:Chem/atomTemplate:Chem/atom absorbs infra-red radiation. A beam of infra-red light is passed across the gas sample to fall on a sensor. The presence of Template:Chem/atomTemplate:Chem/atom 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 Template:Chem/atomTemplate:Chem/atom 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.
Capnography provides information about Template:Chem/atomTemplate:Chem/atom production, pulmonary (lung) perfusion, alveolar ventilation, respiratory patterns, and elimination of Template:Chem/atomTemplate:Chem/atom 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, 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 Template:Chem/atomTemplate:Chem/atom and arterial blood Template:Chem/atomTemplate:Chem/atom. Capnography can also be used to measure carbon dioxide production, a measure of metabolism. Increased Template:Chem/atomTemplate:Chem/atom production is seen during fever and shivering. Reduced production is seen during anesthesia and hypothermia.
Use in anaesthesia
During anesthesia, there is interplay between two components: the patient and the anesthesia 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 Template:Chem/atomTemplate:Chem/atom is typically monitored at this junction. Capnography directly reflects the elimination of Template:Chem/atomTemplate:Chem/atom by the lungs to the anesthesia device. Indirectly, it reflects the production of Template:Chem/atomTemplate:Chem/atom by tissues and the circulatory transport of Template:Chem/atomTemplate:Chem/atom to the lungs.
When expired Template:Chem/atomTemplate:Chem/atom is related to expired volume rather than time, the area beneath the curve represents the volume of Template:Chem/atomTemplate:Chem/atom in the breath, and thus over the course of a minute, this method can yield the Template:Chem/atomTemplate:Chem/atom minute elimination, an important measure of metabolism. Sudden changes in Template:Chem/atomTemplate:Chem/atom 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, 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 tubes, 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) closed claim study.
Capnography in 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 endotrachael tube or a blind insertion airway device. 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 will 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 (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 Template:Chem/atomTemplate:Chem/atom (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 Template:Chem/atomTemplate:Chem/atom 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. Low ETCO2 readings on patients may indicate hyperventilation.
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.
Use of capnography by Registered Nurses
Registered Nurses in critical care settings use capnography to determine if a nasogastric tube, which is used for feeding, has been placed in the esophagus as opposed to the trachea. 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.
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- Physical principles used in capnography graphically explained
- Capnography for Paramedics
- CapnoBase.org: Respiratory signal database that contains clinical and simulated capnogram recordings
- Research project on capnography