Capnography

Capnography is the monitoring of the concentration or partial pressure of carbon dioxide (CO2) 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 CO2 (measured in millimeters of mercury, 'mmHg') plotted against time, or, less commonly, but more usefully, expired volume. The plot may also show the inspired CO2, which is of interest when rebreathing systems are being used. Capnography is the monitoring of the concentration or partial pressure of carbon dioxide (CO2) 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 CO2 (measured in millimeters of mercury, 'mmHg') plotted against time, or, less commonly, but more usefully, expired volume. The plot may also show the inspired CO2, 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 CO2, and an indirect monitor of the CO2 partial pressure in the arterial blood. In healthy individuals, the difference between arterial blood and expired gas CO2 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 CO2 absorbs infrared radiation. A beam of infrared light is passed across the gas sample to fall on a sensor. The presence of CO2 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 CO2 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. Capnography provides information about CO2 production, pulmonary (lung) perfusion, alveolar ventilation, respiratory patterns, and elimination of CO2 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 CO2 and arterial blood CO2. Capnography can also be used to measure carbon dioxide production, a measure of metabolism. Increased CO2 production is seen during fever and shivering. Reduced production is seen during anesthesia and hypothermia. 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: p D ( t ) = p A ( 1 − e − α e α e − t / τ ) {displaystyle p_{D}(t)=p_{A}(1-e^{-alpha }e^{alpha e^{-t/ au }})}