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. Applications Assessing Airway Integrity Confirmation of Endotracheal Tube Placement Predictor of Outcomes in the Intensive Care Unit Intraoperative Complications (ie. air embolism, thromboembolism, etc.) CPR use in ACLS (Advanced Cardiovascular Life Support) Procedural Sedation Monitoring 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,