Of the exhaled gas, that at the end of a large tidal volume best represents alveolar gas. After all, we want information about alveolar gas, because that Ls the best way to represent arterial gas. Indeed, sampling gas in the airway itself would be ideal. It is often recommended to sample gas for analysis as close to the mouth as possible. The measurement of carbon dioxide in respired gas has its vicissitudes. They also asked whether hypercarbia, as observed by them during induction of anesthesia in children, has deleterious effects.

The colleagues wondered why the capnometer gave such misleading information during inhalation anesthesia with mask and bag. It shows large PaCO2-PETC02 gradients during general anesthesia with use of a mask, and quite normal gradients once the children had been intubated and their lungs mechanically ventilated. Depending on the disease condition, additional mechanisms that can contribute to an elevated physiological dead space measurement include shunt, a substantial increase in overall V'A/Q' ratio, diffusion impairment, and ventilation delivered to unperfused alveolar spaces.Recently, two colleagues wrote to the APSF that they had observed a discrepancy between end-tidal and arterial carbon dioxide tensions (PETC02 and PaCO2) in children during inhalation anesthesia. For the range of physiological abnormalities associated with an increased physiological dead space measurement, increased alveolar ventilation/perfusion ratio (V'A/Q') heterogeneity has been the most important pathophysiological mechanism. Although a frequently cited explanation for an elevated dead space measurement has been the development of alveolar regions receiving no perfusion, evidence for this mechanism is lacking in both of these disease settings. An elevated physiological dead space, calculated from measurements of arterial CO2 and mixed expired CO2, has proven to be a useful clinical marker of prognosis both for patients with acute respiratory distress syndrome and for patients with severe heart failure.

Dead space ventilation anesthesiology