High End-Tidal Oxygen Concentration Can Be a Misleading Sole Indicator of the Completeness of Preoxygenation
If one simply flows oxygen through an anesthesia circle system at 5 L/min into a room, the EtO2 monitor will display 98% to 100%. If the anesthesia circle system is then connected to the patient using a facemask, and there is no (zero) ventilation, then the EtO2 monitor will continue to display 98% to 100%. If the facemask seal is unsatisfactory and there is ventilation by the patient, the sampling line can continue to mainly detect the fresh O2 inflow and not the expired gas and the EtO2 value will remain misleadingly high and the patient’s alveolar space poorly preoxygenated. If the facemask seal is satisfactory and the patient inspires a tidal volume that is only slightly greater than the facemask and anatomical dead space (low ratio of alveolar ventilation to functional residual capacity), then the EtO2 value will decrease a small amount during the subsequent exhalation, but the absolute value of the EtO2 will remain misleadingly high. Then, in the presence of continuing the same hypoventilation, the EtO2 value will very slowly and exponentially rise from the already high value that occurred after the first small breath to higher values. The early high EtO2 level occurs because much of the exhaled gas is inspired gas residing in the anesthesia facemask and anatomical dead space and only a relatively small amount of the exhaled gas comes from the alveolar space; again, the patient’s alveolar space remains poorly preoxygenated.
When using EtO2 values as an indicator of adequate preoxygenation, each exhalation should result in a positive capnograph and the reservoir bag should expand and contract with each exhalation and inhalation, respectively. These additional findings will confirm that the EtO2 values represent alveolar gas rather than fresh O2 inflow or dead space gas.