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 (measured in kilopascals, "kPa" or millimeters of mercury, "mmHg") plotted against time, or, less commonly, but more usefully, expired volume (known as volumetric capnography). 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 (exhaling), it is called "end tidal" (PETCO 2 ). 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 (normal difference 4-5 mmHg). 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 which can be an indication of new pathology or change in the cardiovascular-ventilation system.

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 (measured in kilopascals, "kPa" or millimeters of mercury, "mmHg") plotted against time, or, less commonly, but more usefully, expired volume (known as volumetric capnography). 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 (exhaling), it is called "end tidal" (PETCO 2 ). 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 (normal difference 4-5 mmHg). 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 which can be an indication of new pathology or change in the cardiovascular-ventilation system.

Medical Use

Oxygenation and capnography, although related, remain distinct elements in the physiology of respiration. Ventilation refers to the mechanical process of which the lungs expand and exchange volumes of gasses, however respiration further describes the exchange of gasses (mainly and ) at the level of the alveoli. The process of respiration can be divided into two main functions: elimination of waste and replenishing tissues with fresh . Oxygenation (typically measured via pulse oximetry) measures the latter portion of this system. Capnography measures the elimination of which may be of greater clinical usefulness than oxygenation status. During the normal cycle of respiration, a single breath can be divided into two phases: inspiration and expiration. At the beginning of inspiration, the lungs expand and free gasses fill the lungs. As the alveoli are filled with this new gas, the concentration of that fills the alveoli is dependent on the ventilation of the alveoli and the perfusion (blood flow) that is delivering the for exchange. Once expiration begins to occur, the lung volume decreases as air is forced out the respiratory tract. The volume of that is exhaled at the end of exhalation is generated as a by product of metabolism from tissue throughout the body. The delivery of to the alveoli for exhalation is dependent on an intact cardiovascular system to ensure adequate blood flow from the tissue to the alveoli. If cardiac output (the amount of blood that is pumped out of the heart) is decreased, the ability to transport is also decreased which is reflected in a decreased expired amount of . The relationship of cardiac output and end tidal is linear, such that as cardiac output increases or decreases, the amount of is also adjusted in the same manner. Therefore the monitoring of end tidal can provide vital information on the integrity of the cardiovascular system, specifically how well the heart is able to pump blood. The amount of that is measured during each breath requires an intact cardiovascular system to delivery the to the alveoli which is the functional unit of the lungs. During phase I of expiration, the transported to the lungs gas occupies a given space that is not involved in gas exchange, called dead space. Phase II of expiration is when the within the lungs is forced up the respiratory tract on its way to leave the body, which causes mixing of the air from the dead space with the air in the functional alveoli responsible for gas exchange. Phase III is the final portion of expiration which reflects only from the alveoli and not the dead space. These three phases are important to understand in clinical scenarios since a change in the shape and absolute values can indicate respiratory and/or cardiovascular compromise.

Anesthesia

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 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, esophageal 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. p_D(t) = p_A (1 - e ^{-\alpha}e^{\alpha e^{-t/\tau}}) where p_D(t) represents the partial pressure of carbon dioxide measured by the capnogram as a function of time t since the beginning of exhalation. p_A represents the alveolar partial pressure of carbon dioxide. \alpha represents the inverse of the dead space fraction (i.e. the ratio of tidal volume to dead space volume). \tau represents the pulmonary time constant (i.e. the product of pulmonary resistance and compliance) In particular, this model explains the rounded "shark-fin" shape of the capnogram observed in patients with obstructive lung disease.

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