Capnography shows an end-tidal CO2 waveform with an ascending phase slope (phase 2) that is nearly vertical in a normally ventilated patient. This changes to a gradual upward slope on the following case. The MOST likely explanation for the sloped phase 2 is:

• A Endobronchial intubation with atelectasis of the contralateral lung
• B Hyperventilation with a reduced ETCO2 and altered alveolar dead space
• C Air entrainment into the sampling line diluting the CO2 signal
• D Obstructive airways disease causing non-uniform lung emptying (increased phase 3 slope also present) ✓

Explanation

In a normal capnograph, phase 2 (the ascending phase from dead-space washout to alveolar plateau) is steep, indicating simultaneous emptying of all alveoli with similar CO2 content. In obstructive airways disease (asthma, COPD), airway resistance is heterogeneously increased; slow-emptying alveoli with high CO2 continue emptying after fast units have finished, causing a gradual mix of gas and sloping phase 2. Additionally, phase 3 (alveolar plateau) acquires a characteristic upward slope ('shark-fin' pattern), because slow lung units keep delivering CO2 throughout exhalation. This appearance is distinct from the normal flat phase 3.

Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.