Veterinary Capnography (ETCO₂) Reference
Capnography reference for veterinary anaesthesia: normal ETCO₂, interpretation of abnormal values, and common waveform abnormalities.
Normal ETCO₂
| Patient | Range |
|---|---|
| Dog (anaesthetised) | 35–45 mmHg |
| Cat (anaesthetised) | 30–40 mmHg |
| Awake patient | 35–45 mmHg (slightly lower in cats) |
Interpreting low ETCO₂ (< 30 mmHg)
| Cause | Action |
|---|---|
| Hyperventilation (mechanical) | Decrease minute volume; check vent settings. |
| Decreased cardiac output / hypotension | Treat hypotension - fluids, vasopressor. |
| Increased dead space (e.g. PE, occluded ETT) | Check tube position and patency; rule out PE. |
| Disconnection / sampling line leak | Check connections. |
Interpreting high ETCO₂ (> 45-50 mmHg)
| Cause | Action |
|---|---|
| Hypoventilation | Increase respiratory rate or tidal volume. |
| Increased CO₂ production (hyperthermia, MH, sepsis) | Treat underlying cause. |
| Exhausted CO₂ absorbent | Replace soda lime. |
| Faulty valve in breathing circuit | Check inspiratory/expiratory valves. |
Waveform abnormalities
| Waveform | Likely cause |
|---|---|
| Sloping plateau (rising shark fin) | Bronchoconstriction, partial airway obstruction. |
| Cleft in plateau | Patient attempting breath during mechanical ventilation. |
| Baseline above zero | Inspired CO₂ - check absorbent, valves, fresh gas flow. |
| Sudden loss to 0 | Disconnection, ETT displacement, cardiac arrest. |
| Cardiogenic oscillations (small dips at end-expiration) | Normal in small patients. |
ETCO₂ is the most useful monitor during anaesthesia - it confirms ventilation, perfusion (cardiac output), and circuit integrity. Sudden drop to 0 is a CPA until proven otherwise.
Frequently asked questions
What is a normal ETCO₂ during anaesthesia?
35-45 mmHg for dogs and cats under inhalant anaesthesia. ETCO₂ approximates PaCO₂ minus 2-5 mmHg in patients without significant pulmonary disease.
What does a sudden drop in ETCO₂ mean?
Sudden drop to near-zero suggests circuit disconnect, ETT obstruction or extubation. A gradual drop suggests cardiac arrest, severe hypotension, pulmonary embolism, or hyperventilation. Always check the patient and circuit before adjusting the ventilator.
What does ETCO₂ tell you during CPR?
ETCO₂ correlates with cardiac output during compressions. Values < 10 mmHg predict poor outcome; > 15 mmHg suggests adequate forward flow. A sudden rise to > 35 mmHg often signals ROSC.