This creates a slight delay between collection and gas analysis. In the side-stream technique gas is continuously aspirated from the primary airway through a sampling line that is placed between the patient and the Y-piece of the ventilator. In the mainstream approach the infrared light source and sensor are placed in the primary airflow tube so that expired gas is sampled directly during expiration and the CO 2 signal is in-phase with the air-flow and pressure signals. Expired CO 2 can be obtained by sampling either mainstream or side-stream expiratory flow. Time-based capnography refers to the elimination of CO 2 over time and gives an indication of ventilation inefficiency. Capnography is the measurement of expired PCO 2. When CO 2 production and total ventilation (VE) are constant, arterial PCO 2 (partial pressure of carbon dioxide) increases in proportion to the increase in VD. Ventilation dead space (VD) refers to the parts of the lung and airways that do not partake in the clearance of carbon dioxide (CO 2) and indicates the inefficient portion of ventilation.
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