Trace fades after intubation
Think: Esophageal intubation until proven otherwise — also check the sampling line and circuit, and consider cardiac arrest.
You: Say it immediately: “I’m not seeing a sustained CO₂ trace.”
Respiratory physiology
Ventilation & capnography
Pulse oximetry tells you about oxygenation; the capnogram tells you about ventilation, circulation and where the tube is.
Read one breath
- Phase I — inspiratory baseline / dead-space gas (should be ≈ 0).
- Phase II — rapid upstroke as alveolar gas arrives.
- Phase III — alveolar plateau; ETCO₂ is the value at its end.
- Phase 0 — inspiratory downstroke as fresh gas enters.
Nine patterns to recognize
Look at the trace, decide what you’d think, then reveal. Schematic waveforms — guides at 0 and ≈ 40 mmHg.
Sudden drop to zero
Think: Disconnection, extubation, sampling-line kink or apnea.
You: Check the circuit from patient to machine; tell the team.
Sudden fall, not to zero
Think: ↓ Cardiac output, pulmonary embolism (air or thrombus), massive bleeding, partial leak.
You: Tell the team now; look at BP and HR.
Steady decline over breaths
Think: Hypotension or arrest, PE, hyperventilation.
You: Report the trend with BP.
“Shark fin” — sloped upstroke, no plateau
Think: Bronchospasm, COPD, kinked or obstructed tube.
You: Report it; check peak pressures and listen.
Curare cleft — notch in the plateau
Think: Patient breathing against the ventilator (paralysis wearing off, light anesthesia).
You: Tell your resident; check TOF.
Baseline doesn’t return to zero
Think: Rebreathing: exhausted CO₂ absorbent or a faulty expiratory valve.
You: Report it; the team checks the absorbent and valves.
Gradual rise
Think: Hypoventilation, CO₂ insufflation (laparoscopy), rising temperature, malignant hyperthermia (with tachycardia, rigidity).
You: Report with HR and temperature.
Ripples at end-expiration
Think: Cardiogenic oscillations — heartbeat transmitted at low respiratory rates. Usually benign.
You: Point it out as a teaching moment.
Ventilator basics
| Setting | Typical starting point | What it changes |
|---|---|---|
| Tidal volume (Vt) | 6–8 mL/kg predicted body weight | Ventilation; lung stretch |
| Respiratory rate | 10–14 /min → ETCO₂ 35–45 | Minute ventilation (Vt × RR) → CO₂ |
| PEEP | ≈ 5 cmH₂O | Oxygenation; prevents atelectasis |
| FiO₂ | Lowest that meets the SpO₂ target | Oxygenation |
| Mode | Volume control or pressure control; pressure support when breathing spontaneously | What’s fixed vs. what varies |
Oxygenation ← FiO₂ and PEEP. Ventilation (CO₂) ← rate and tidal volume. The team individualizes settings using mechanics, gas exchange and the procedure (e.g., pneumoperitoneum raises ETCO₂ and airway pressure).
Pimp questions
Pimp question 1
ETCO₂ suddenly falls from 38 to 18 mmHg with a normal-looking waveform. What are you worried about?
Answer
A sudden fall in cardiac output or pulmonary embolism (also massive hemorrhage or a large leak). Tell the team and check BP.
Pimp question 2
Why does ETCO₂ rise during laparoscopy?
Answer
CO₂ insufflated into the peritoneum is absorbed into the blood.
Pimp question 3
What does an elevated inspiratory baseline mean?
Answer
Rebreathing of CO₂ — classically an exhausted absorbent or an incompetent expiratory valve.
Pimp question 4
How do you increase CO₂ clearance on the ventilator?
Answer
Increase minute ventilation: rate and/or tidal volume.