Capnography under anaesthesia: high EtCO2 in the dog
25 kg dog under isoflurane anaesthesia, breathing spontaneously on an FiO2 close to 1. Over 30 minutes, EtCO2 rises progressively from 40 to 62 mmHg; the waveform is normal and the baseline stays at zero. SpO2 is 99%.
What is the most appropriate interpretation and action?
chevron_right Show answer and explanation
A. Hypoventilation from anaesthetic depression: lighten the plane and start positive-pressure ventilation
Correct. EtCO2 reflects PaCO2 (normal 35–45 mmHg), and a progressive rise with a zero baseline indicates hypoventilation. SpO2 is normal because the dog is breathing almost pure oxygen.
B. Rebreathing due to exhausted soda lime
Rebreathing raises inspired CO2, so the capnogram baseline does not return to zero; here it does.
C. Malignant hyperthermia: give dantrolene
It is very rare in dogs and causes a rapid CO2 rise with hyperthermia, tachycardia and rigidity; hypoventilation, which is common, should be considered first.
D. None: with an SpO2 of 99% ventilation is adequate
SpO2 assesses oxygenation, not ventilation. On 100% oxygen, severe hypercapnia and respiratory acidosis can coexist with a normal SpO2.
lightbulb Key takeaway
EtCO2 is usually 2–5 mmHg below PaCO2; the gap widens with dead space (hypotension, pulmonary thromboembolism). A sudden fall to zero suggests disconnection, extubation or arrest; during CPR, a sudden rise in EtCO2 suggests return of spontaneous circulation.
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Updated: October 31, 2026 · Educational content for veterinary professionals. It does not replace current clinical guidelines or the judgement of the attending veterinarian.