Deadspace and the single breath test carbon dioxide during anaesthesia and artificial ventilation. Respiratory dead space and arterial to end-tidal CO 2 tension difference in anesthetized man. Terminology and the current limitations of time capnography. Bhavani shankar K, Kumar AY, Moseley H, Hallsworth RA. The single breath test for carbon dioxide. The Netherlands:Kerckebosch-Zeist, 1989.Ģ. The improved C02 excretion is due to better perfusion of upper parts of the lung.18 Askrog found an inverse linear correlation between pulmonary artery pressure and (a-ET)PC02.18 Thus, under conditions of constant lung ventilation, PETCO 2 monitoring can be used as a monitor of pulmonary blood flow.17,19ġ Kalenda Z. Increases in cardiac output and pulmonary blood flow result in better perfusion of the alveoli and a rise in PETCO 2.17,18 Consequently alveolar dead space is reduced as is (a-ET)C02 The decrease in (a-ET)PC02 is due to an increase in the alveolar C02 with a relatively unchanged arterial C02 concentration, suggesting better excretion of C02 into the lungs. Reduction in cardiac output and pulmonary blood flow result in a decrease in PETCO 2 and an increase in (a-ET)PC02. For example, it has been observed during cardiac surgery that alveolar dead space was increased at the end of cardiopulmonary bypass but as the slope of phase III was also increased, there was no change in (a-ET)PCO 2.15,16 The (a-ET)PCO 2 may remain the same if there is an associated increase in the slope of the phase III. This implies that an increase in the alveolar dead space need not be always be associated with an increase in the (a-ET)PCO 2. Therefore, the (a-ET)PCO 2 is dependent both on alveolar dead space as well as factors that influence the slope of phase III. However, if phase III has a steeper slope, the terminal part of phase III may intercept the line representing PaCO 2, resulting in either zero or negative (a-ET)PCO 2 even in the presence of alveolar dead space. In this case, the area (blue shaded color in the figure above) rectangular and PaCO 2 > PETCO 2. Changes in alveolar dead space correlate with (a-ET)PCO 2 only when phase III is flat or has a minimal slope. However, (a-ET)PCO 2 does not correlate with alveolar dead space in all circumstances. Hence (a-ET)PCO 2 is an indirect estimate of V/Q mismatching of the lung. In healthy children, the (a-ET)PCO 2 gradient is smaller (-0.65-3 mm Hg) than in adults.9-14 This is due to a better V/Q matching, and hence a lower alveolar dead space in children than in the adults.9 The (a-ET)PCO 2 / PaCO 2 fraction is a measure of alveolar dead space, and changes in alveolar dead space correlate well with changes in (a-ET)PCO 2.4 An increase in (a-ET)PCO 2 suggests an increase in dead space ventilation. Under normal circumstances, the PETCO 2 (the CO 2 recorded at the end of the breath which represents PCO 2 from alveoli which empty last) is lower than PaCO 2 (average of all alveoli) by 2-5 mmHg, in adults.1-8 The (a-ET)PCO 2 gradient is due to the V/Q mismatch in the lungs (alveolar dead space) as a result of temporal, spatial, and alveolar mixing defects. (a-ET)CO 2 gradient as an index of alveolar dead space:
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