- □ (A) Faulty inspiratory valve
- □ (B) Partially obstructed endotracheal tube
- □ (C) Leak in sampling line
The time capnogram (Figure 1) can provide evidence of certain disease states and equipment issues. A faulty inspiratory valve makes the inspiratory limb of the circuit bidirectional and increases dead space, allowing for rebreathing of exhaled gas. The characteristic finding on the capnogram is a slower (less steep) downslope that extends into the inspiratory phase (phase 0) (Figure 2, Panel A). An exhausted carbon dioxide absorber (or incompetent expiratory valve) will result in an elevated baseline carbon dioxide value on the capnogram (Figure 2, Panel B). A partially obstructed endotracheal tube or any condition that impairs exhalation (e.g., bronchospasm or chronic obstructive pulmonary disease) produces a capnogram that has a slowly rising phase III (Figure 2, Panel C). A leak in the gas sampling line will produce a capnogram that has a dual plateau in phase III. The sharp rise at the end of phase III is due to an increase in circuit pressure (decreased leak) that occurs at the onset of inhalation (Figure 2, Panel D).
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• Miller RD, ed Miller’s Anesthesia 8th ed. Philadelphia, PA Elsevier Saunders 2015 1552