ACE 20B Sample Question

Which of the following is MOST likely represented by the time-based capnogram below?

A. A leak in the sidestream sampling line ✔

B. A patient with a single lung transplant X

C. Recovery from muscle relaxant in a mechanically ventilated patient X

Read the discussion below.

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Discussion

In order to understand the pathology causing abnormalities in the time-based capnogram, anesthesiologists need to know the phases of a normal capnogram trace (Figure 1).

Figure 1. Normal capnogram. Phase 0 represents inspiration. The next phase is the inspiratory baseline (I). Anatomic and alveolar dead space combine to create the expiratory upstroke (phase II). Phase III is the alveolar plateau. ETCO₂ is normally measured at the end of phase III. © 2020 American Society of Anesthesiologists.

A leak in the sampling line enables room air entrainment during expiration when the pressure is low in the breathing circuit and the sampling line. This dilutes the carbon dioxide (CO₂) in the breathing circuit, resulting in a lower-than-expected value during the plateau phase. When positive pressure inspiration begins, it prevents any further air entrainment and pushes the last CO₂ in the sampling line towards the analyzer, creating the distinctive “church steeple” shape (Figure 2).

Figure 2. A leak in the sidestream sample line is indicated by the so-called “church steeple” shape in phase III of the capnogram. Illustration © 2023 Jennifer Gentry.

In single lung transplant patients, the capnogram shows 2 peaks in phase III because of the heterogenicity of both lungs. The first peak is from the normal transplanted lung, while the second is from the diseased native lung (Figure 3).

Figure 3. The double peaks in phase III suggest 2 heterogeneous lungs emptying in sequence (eg, in a patient with a single lung transplant). Illustration © 2023 Jennifer Gentry.

Spontaneous breathing efforts in the middle of controlled mechanical ventilation are indicated by an interruption of the plateau phase by small downward clefts (known as curare clefts) (Figure 4).

Figure 4. The curare clefts interrupting the plateau of phase III suggest spontaneous breathing efforts during controlled mechanical ventilation. Illustration © 2023 Jennifer Gentry.

REFERENCES

1. Gropper MA, Cohen NH, Eriksson LI, Fleisher LA, Leslie K, Wiener-Kronish JP, eds. Miller’s Anesthesia. 9th ed. Elsevier; 2020:1308-1309.
2. Ehrenwerth J, Eisenkraft JB, Berry JM, eds. Anesthesia Equipment: Principles and Applications. 3rd ed. Elsevier; 2021:239.
3. Kodali BS. Capnography outside the operating rooms. Anesthesiology. 2013;118(1):192-201. doi:10.1097/ALN.0b013e318278c8b6

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