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" (PETCO2). 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 describes the elimination of which may reveal greater clinical use 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