Īlthough simple to apply, standard time-based capnography does not allow identification of the volume components of the signal, which is necessary for determination of the anatomical source of CO 2 and understanding the pathological processes. 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.

Positive pressure ventilation (i.e.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.

Neck extension and jaw protrusion (can increase it twofold).

General anesthesia – multifactorial, including loss of skeletal muscle tone and bronchoconstrictor tone.

The ratio of physiologic dead space to tidal volume is usually about 1/3. Alveolar dead space is the volume of gas within unperfused alveoli (and thus not participating in gas exchange either) it is usually negligible in the healthy, awake patient. Anatomic dead space is the volume of gas within the conducting zone (as opposed to the transitional and respiratory zones) and includes the trachea, bronchus, bronchioles, and terminal bronchioles it is approximately 2 mL/kg in the upright position. Physiologic or total dead space is the sum of anatomic dead space and alveolar dead space. Dead space is the volume of a breath that does not participate in gas exchange.

Increase in dead space volume in lungs