The effect of airway narrowing and dead space on the shape of the capnogram

We have investigated the effects of incomplete obstruction of the endotracheal tube and the amount of additional dead space between the endotracheal tube and the capnographic sampling adapter on the shape of the capnogram. A 9.0-mm endotracheal tube was connected to a 3-L reservoir bag filled from the bottom with 5% carbon dioxide and 95% oxygen. The narrowed adapter (internal diameter: 3.0, 4.0, 6.5, and 9.0 mm), the capnographic sampling adapter, and a semiclosed respiratory system were successively connected to this endotracheal tube. Additional dead space (0, 30, 62, 92, 124, or 154 ml) was inserted between the narrowed adapter and the capnographic sampling adapter. The reservoir bag was ventilated with the anesthesia ventilator (fresh gas flow, 6 L·min^-1; tidal volume, 500 ml; respiratory rate, 10 min^-1, and inspiratory-expiratory ratio, 1∶2). The capnogram from each initial ventilation was recorded and the peak carbon dioxide tension (P_peak CO₂) was also measured. The T_90% value was defined as the time it took for the capnograph output to respond from 0% to 90% of the P_peak CO₂. The T_90% value seen in a 3.0-mm adapter did not change compared with the value in a 9.0-mm adapter, when no additional dead space was connected between the endotracheal tube and the capnographic sampling adapter. Further, the slanting upstroke of the capnogram occurred only when the endotracheal tube narrowing and a large amount of dead space between the endotracheal tube and the capnographic sampling adapter coexisted. Thus, it is unlikely that incomplete obstruction of the endotracheal tube can easily be detected by the slanting upstroke of the capnogram.