End-Tidal CO₂ Capnography

The time capnogram: exhaled CO₂ against time. A normal breath climbs through phase I (dead-space baseline), phase II (expiratory upstroke), and phase III (alveolar plateau) to the end-tidal CO₂, then falls on inspiration. Load a pathology and read the shape — a sloping shark-fin for obstruction, a raised baseline for rebreathing, a curare cleft for a spontaneous effort, a flat line for an oesophageal tube — and the ETCO₂ value.

CAPNOGRAPHYNORMAL

CO₂--mmHg

ETCO₂

-- mmHg

RESP RATE

-- /min

INSPIRED CO₂ (FiCO₂)

-- mmHg

PATTERN

—

Pathology

Adjustments

Respiratory rate 14 /min

End-tidal CO₂ 40 mmHg

Inspired CO₂ 0 mmHg

Normal capnogram — phases

I — inspiratory / dead-space baseline (≈0) · II — expiratory upstroke · III — alveolar plateau → ETCO₂ · 0 — inspiratory downstroke.
The α angle (II→III) widens toward a slurred shark-fin in obstruction.