Figure 36-16 Monitoring a spontaneously breathing patient with a sidestream capnograph. A, Attachment of the sampling line to a non-rebreathing mask usually provides waveforms adequate for monitoring the respiratory rate, but end-tidal CO₂ tension is low because of mixing within the mask. B, Placement of a sample probe close to a nostril increases the accuracy of the PETCO₂ measurement. C, A specially designed probe (Oridion, Needham, MA) samples exhaled gas from the nose and mouth for assessing exhaled gas from mouth-breathers. A second port in the same device can be used to administer supplemental oxygen.