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 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. When the measurement is taken at the end of a breath, it is called "end tidal" .
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 difference between arterial blood and expired gas increases and can exceed 1 kPa.

Medical use

Anaesthesia

During anesthesia, there is interplay between two components: the patient and the anesthesia administration device. 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 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, 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 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 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 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 affirmed the importance of using capnography to verify tube placement in their 2005 CPR 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. Studies have shown that when a person doing CPR tires, the patient's end-tidal 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. 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.

Registered nurses

s in critical care settings may use capnography to determine if a nasogastric tube, 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.

Working mechanism

Capnographs usually work on the principle that absorbs infrared radiation. 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 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 in 1864, though 19th and early 20th century devices were too cumbersome for everyday clinical use.

Diagnostic usage

Capnography provides information about production, pulmonary perfusion, alveolar 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, 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 anesthesia and hypothermia.

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
In particular, this model explains the rounded "shark-fin" shape of the capnogram observed in patients with obstructive lung disease.