کاپنوگراف (Capnograph)

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 CO₂partial 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.

Working mechanism

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.

Diagnostic usage

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.

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 CO₂ is typically

monitored at this junction.Capnography directly reflects the elimination of CO₂ by the lungs to the

anesthesia device. Indirectly, it reflects the production of CO₂ by tissues and the circulatory transport

of CO₂ to the lungs.When expired CO₂ is related to expired volume rather than time, the area

beneath the curve represents the volume of CO₂ in the breath, and thus over the course of a

minute, this method can yield the CO₂ per minute elimination, an important measure of

metabolism.Sudden changes in CO₂ 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.

برای اطلاعات بیشتر بر روی این لینک کلیک نمائید