A capnograph that drops to zero suggests disconnection, oesophageal placement or complete obstruction. A sudden large fall with a preserved waveform suggests a fall in cardiac output or pulmonary embolism. A gradual decline points to hyperventilation, falling metabolic rate or slowly falling output.
At constant ventilation, a low end-tidal CO2 with a widening gap to arterial CO2 points to rising dead space; check a blood gas when the cause is not obvious.
- Zero trace: check circuit, tube position and obstruction first
- Sudden fall with waveform preserved: think low cardiac output or embolism
- Gradual fall: review ventilator settings and temperature
- Check an arterial gas to measure the arterial to end-tidal gap when unclear
Why it matters
The capnograph often signals low flow before the blood pressure does.
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