![]() The resulting volumetric capnogram is composed by three phases. With this technique volume and CO 2 are simultaneously measured and the latter is plotted against expired volume. When Bohr in 1891 calculated dead space fraction of expired tidal volume for the first time as Vd/Vt = (PACO 2 − PeCO 2)/PACO 2, where Vt was total exhaled volume, PACO 2 the amount of carbon dioxide at the alveolar level and PeCO 2 the partial pressure of mean expired carbon dioxide, it was immediately evident that, if in an ideal lung arterial PCO 2 (PaCO 2) would be the equivalent of PACO 2, this perfect condition was unachievable in clinical practice, where PACO 2 was always less than PaCO 2.įor this reason and for the difficulties with measurement of PACO 2, Enghoff in 1938 used PaCO 2 instead of PACO 2 and adapted the Bohr’s equation as Vd/Vtphys = (PaCO 2 − PECO 2)/PaCO 2, where PECO 2 was obtained using volumetric capnography. Moreover, over the years different methods of calculation have been proposed ( 3, 4). Nevertheless, monitoring dead space at the bedside in this kind of patients is infrequent, especially because the capnograms are influenced by many factors related both to the patient and to the ventilator, but also to the monitoring system used, which inevitably complicate their interpretation. ![]() In the continuous search for an index of the efficiency of the gas exchange in critical care patients, dead space is the only parameter that reflects the alterations in V/Q ratios and any type of V/Q mismatch affects it. Dead space (Vd) is the portion of each tidal volume that does not take part in gas exchange and includes: anatomical dead space (Vd aw), that is the part of airways that do not contribute to gas exchange (nose, pharynx, conduction airways and ventilator equipment if mechanical ventilation is present) and alveolar dead space (Vd alv) or alveoli which are well-ventilated but poorly perfused ( 1, 2). Physiological gas exchange occurs only in presence of ventilation and perfusion (V/Q) homogeneity nevertheless an easy and accurate indicator that can measure V/Q alterations is far from being available. ![]() Keywords: Dead space acute respiratory distress syndrome (ARDS) volumetric capnography lung recruitment Different capnographic indices can be useful to evaluate therapeutic interventions or setting mechanical ventilation. Dead space measurement is a reliable method that provides important clinical and prognostic information. Different patterns of ventilation affect also CO 2 elimination in fact, end-inspiratory pause prolongation reduces dead space, increasing respiratory system compliance plateau pressure and consequently driving pressure increase accordingly. Dead space guided recruitment allows avoiding regional overdistension or reduction in cardiac output in critical care patients with ALI or ARDS. Lung recruitment is a dynamic process that combines recruitment manoeuvres (RMs) with positive end expiratory pressure (PEEP) and low Vt to recruit collapsed alveoli. Different dead space indices can provide useful information in acute lung injury (ALI) and acute respiratory distress syndrome (ARDS) patients, where changes in microvasculature are the main determinants for the increase in dead space and consequently a worsening of the outcome. Dead space is not routinely measured in critical care practice, because the difficulties in in interpreting capnograms and the different methods of calculations.
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