The airway paradigm: what really changed?

not flow in blocks). For example, if malfunction of the inspiratory valve results in decreased resistance, it is conceivable that more than half of the expiratory gases might enter the inspiratory limb. This, in conjunction with the gas mixing between carbon dioxide–containing expiratory gases and carbon dioxide–free fresh gases may cause the final portion of the inspiratory tidal volume to contain certain amounts of carbon dioxide. Thus, the resulting downstroke of the capnogram (phase 0) will not reach the zero baseline. This was illustrated by me and my colleagues in a case report where we recorded capnograms during inspiratory valve malfunction and the subsequent inspiratory downstrokes did not reach the baseline.2 However, it was also demonstrated by our group that capnograms can apparently appear normal despite substantial rebreathing resulting from inspiratory valve malfunction. However, when respiratory gas flows were superimposed on the capnograms, the significant rebreathing was obvious.3 Regarding capnogram 3A–D illustrated in the original article,1 the morphology of capnograms depends, once again, on several factors. These include patient’s respiratory rate, tidal volume, supplementary oxygen flow, gas leaks from the mask resulting in the carbon dioxide washout by the oxygen flow, and more importantly, the site of the carbon dioxide sampling. In capnograms 3A–D, the site of the sampling was adjacent to the inside wall of the mask via an adaptor, and not at the nostril. Therefore, the recorded carbon dioxide concentration does not represent the carbon dioxide concentration at the nostril. The morphology of the capnograms depends on the location of carbon dioxide sampling within the mask and on the washout of carbon dioxide by the supplementary oxygen flow. Unless carbon dioxide measurements are performed at the nostril, it may be difficult to ascertain whether there is rebreathing (although minimal). For example, figure 1A and B from this reply shows a patient undergoing upper gastroinstestinal endoscopy with supplementary oxygen provided via the mask, and end-tidal carbon dioxide monitoring was performed within the nostril using carbon dioxide sampling nasal cannula. The endoscope was inserted via a “U-shaped flap cut” in the mask. In this case, the carbon dioxide rebreathing was zero (fig. 1B) due to the carbon dioxide washout by supplementary oxygen at the nostril. Capnograms during sedation is a good subject for future discussion.