Often, the geometric analysis establishing the separation between the phase II and phase III is hardly seen and the rate of CO 2 raising of the phase III is nonlinear in patients with lung inhomogeneities (Fig. Thereafter, Vd aw can be calculated from the value obtained on the volume axis by back extrapolation from the first linear part of the VCO 2 versus volume curve.Īlthough these indexes are clinically useful, they are always bound to visual criteria for the definition of phase III of the expired capnogram. Briefly, VCO 2 is plotted versus expired breath volume. 2B are examples of Vd aw calculation using the Langley et al. is based on determination of the VCO 2 value, which corresponds to the area inscribed within the CO 2 versus volume curve (indicated in Fig. $$ \begin $$Īn alternative method to measure airway dead space introduced by Langley et al. Referring these values to the Vt, it is possible to single out several Vd components : By tracing a line parallel to the volume axis and equal to the PaCO 2, it is possible to determine the readings from areas y and z, which respectively represent the values of alveolar and airway dead space. Airway dead space is then measured from the beginning of expiration to the point where the vertical line crosses the volume axis. Fowler introduced a procedure for measuring Vd aw based on the geometric method of equivalent areas (p = q), obtained by crossing the back extrapolation of phase III of the expired CO 2 concentration over time with a vertical line traced so as to have equal p and q areas. Vd phys/Vt is the most commonly and commercially (volumetric capnographs) formula used to estimate pulmonary dead space at the bedside.Īdditional methods mostly used in research to calculate all the Vd components are shown in Fig. Since Vd phys/Vt measures the fraction of each tidal breath that is wasted on both Vd alv and Vd aw, the Vd aw must be subtracted from Vd phys/Vt to obtain the Vd alv/Vt. Physiologic dead space calculated from the Enghoff modification of the Bohr equation uses PaCO 2 with the assumption that PaCO 2 is similar to alveolar PCO 2, such that: Vd phys/Vt = (PaCO 2–P ECO 2)/PaCO 2, where P ECO 2 is the partial pressure of CO 2 in mixed expired gas and is equal to the mean expired CO 2 fraction multiplied by the difference between the atmospheric pressure and the water-vapour pressure. End-tidal CO 2 is used to approximate F ACO 2, assuming end-tidal and alveolar CO 2 fractions are identical. Therefore, the use of volumetric capnography is clinically more profitable than time-based capnography.īohr originally defined Vd/Vt as: Vd/Vt = (F ACO 2–F ECO 2)/F ACO 2, where F ACO 2 and F ECO 2 are fractions of CO 2 in alveolar gas and in mixed expired gas, respectively. 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.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |