A capnograph waveform shows a slanted rather than flat expiratory plateau (shark-fin pattern) in a mechanically ventilated patient. What is the diagnosis?
- A Pulmonary embolism — reduced ETCO₂
- B Normal waveform in patients with high respiratory rates
- C Obstructive airways disease (bronchospasm or COPD) — slow uneven CO₂ exhalation causes a rising expiratory phase instead of a flat plateau ✓
- D Oesophageal intubation — no capnograph waveform
Explanation
The normal capnograph waveform has a flat phase III (alveolar plateau) representing uniform CO₂ exhalation from alveoli. In obstructive diseases (bronchospasm, COPD, asthma), heterogeneous lung units with different time constants empty at different rates, causing slow progressive CO₂ rise during phase III — the 'shark fin' or 'crocodile jaw' pattern (also called slanting upstroke). This is a sensitive indicator of bronchospasm and can precede clinical signs. Pulmonary embolism reduces ETCO₂ with widened PaCO₂–ETCO₂ gradient. Oesophageal intubation produces a rapidly diminishing waveform or absent capnograph.
Reference: Morgan & Mikhail's Clinical Anesthesiology, 6th ed.
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Written and medically reviewed by the StethoPrep medical team.