The combination of airflow and mainstream capnography monitoring allows calculation of breath by breath CO 2 production and pulmonary dead space. The three phases of a volumetric capnogram are shown in Fig. Combined with the measurement of arterial PCO 2 (PaCO 2) it provides a precise quantification of the ratio of Vd phys to Vt. The integration of the volume signal using an accurate flow sensor (pneumotachograph) and CO 2 signal (with a very fast CO 2 sensor) is known as volumetric capnography. Time-based capnography expresses the CO 2 signal as a function of time and from this plot mean expiratory (Douglas bag method) or end-expiratory (end-tidal) CO 2 values can be obtained. The equation to transform FCO 2 into PCO 2 is PCO 2 = FCO 2 multiplied by the difference between barometric pressure minus water-vapour pressure. Ī device that measures partial pressures (PCO 2) or fractions (FCO 2) of CO 2 during the breathing cycle is called a capnograph.
Physiologic dead space (Vd phys) is comprised of Vd aw (instrumental and anatomic dead space) and Vd alv and it is usually reported in mechanical ventilation as the portion of tidal volume (Vt) or minute ventilation that does not participate in gas exchange. This instrumental dead space is considered to be part of the Vd aw.
Mechanical ventilation, if present, adds additional Vd as part of the ventilator equipment (endotracheal tubes, humidification devices, and connectors). The volume of dead space (Vd) reflects the sum of two separate components of lung volume: 1) the nose, pharynx, and conduction airways do not contribute to gas exchange and are often referred to as anatomical Vd or herein as airway Vd (Vd aw) 2) well-ventilated alveoli but receiving minimal blood flow comprise the alveolar Vd (Vd alv). The concept of dead space accounts for those lung areas that are ventilated but not perfused. The homogeneity between ventilation and perfusion determines normal gas exchange.
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