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Capnometry is a measurement of end-tidal carbon dioxide (etCO2). EtCO2 represents a partial pressure or maximal concentration of CO2 at the end of exhalation. Capnography is a measurement and a graphic display of the characteristic waveform against time or volume, known as the capnogram. CO2 reflects cellular metabolism. There are four main stages of normal CO2 physiology: production, transport, buffering and elimination. The principal determinants of etCO2 are alveolar ventilation, pulmonary perfusion (cardiac output) and CO2 production. Capnography is most commonly used during endotracheal intubation to identify correct placement of an endotracheal tube. During acutely low cardiac output, as in cardiac arrest, decreased pulmonary blood flow becomes the primary determinant resulting in abrupt decrease of partial pressure of etCO2 (petCO2). If ventilation and chest compressions are constant — with the assumption that CO2 production is uniform — then the change in petCO2 reflects the changes in systemic and pulmonary blood flow. Ultimately, it can be used as a quantitative index of evaluating adequacy of ventilation and pulmonary flow during cardiopulmonary resuscitation (CPR) [1, 2 |
