Monitor atlas: perioperative respiratory events
Stem
Monitors station, scenario 2 of 3. The card reads: a 29-year-old woman, 64 kg, with asthma, intubated two minutes ago for an appendectomy. The monitor now shows:
Question
What does the monitor show, and what do you do?
Ideal answer
What the monitor showed: a new shark-fin capnogram; peak pressure 20 to 38 cmH2O with the plateau steady at 18; SpO2 99% to 93% with a clean pleth.
HeadlineBronchospasm: peak up, plateau steady, a resistance problem in an asthmatic just intubated. I hand-ventilate on 100% oxygen to feel the compliance.
- Read in a fixed order. Capnograph first, as it proves circuit and perfusion; oximeter and pleth; then peak, plateau, delivered against set tidal volume, and the loops.
- Name the pattern. Phase III slopes up, the desaturation is real because the pleth is good, and peak rose while plateau did not.
- Exclude the tube. Suction catheter through it, because a kinked or obstructed tube is the alternative.
- Treat the cause. Deepen the anesthetic, give a bronchodilator, lengthen expiration.
Watch for: capnograph plateau back, peak falling toward plateau, SpO2 recovering: the graded reassessment.
Breaking down the answer
The outline lists capnography and pulse oximetry (waveform and value), airway pressure with peak and PEEP, airway flow, tidal volume and flow-volume loops;1 almost every airway crisis has a signature:
| Pattern | Identifying features | What it changes |
|---|---|---|
| Capnograph flat, zero | No waveform, SpO2 normal for a minute | Disconnect, extubation, obstruction, esophageal tube, or arrest; hand-ventilate, look, listen, check the pulse2 |
| Sudden fall to a low plateau | Shape preserved, value drops over a few breaths | Loss of pulmonary blood flow: embolism, massive hypotension, arrest |
| Rising end-tidal, normal shape | Climb over minutes, tachycardia | Hypoventilation, absorbent exhaustion, laparoscopic CO2, or malignant hyperthermia if it outpaces ventilation |
| Upsloping phase III, "shark fin" | Prolonged expiratory upstroke, no plateau | Bronchospasm, kinked tube, COPD2 |
| Baseline does not return to zero | Inspired CO2 above zero | Rebreathing: exhausted absorbent, incompetent valve, inadequate fresh gas2 |
| Curare cleft | Notch in the plateau | Patient breathing against the ventilator; relaxant wearing off2 |
| Cardiac oscillations | Downslope ripples | Low rate, long expiration; benign |
| SpO2 falls, pleth good | Clean pleth, normal capnograph | True hypoxemia: FiO2, atelectasis, endobronchial tube, shunt |
| SpO2 falls, pleth poor | Noisy or flat pleth | Perfusion or artifact: cuff, cold, vasoconstriction, movement; check the pulse before treating |
| SpO2 reads 85% and will not move | Normal PaO2 on a gas | Methemoglobinemia; methylene blue 1–2 mg/kg |
| Peak up, plateau up | Both rise together | Compliance: pneumothorax, endobronchial intubation, abdominal insufflation, pulmonary edema |
| Peak up, plateau normal | Peak-plateau gap widens | Resistance: bronchospasm, secretions, kinked or bitten tube |
| Flow-volume loop, expiratory limb scooped | Concave expiratory flow | Small airway obstruction, bronchospasm |
| Flow-volume loop, flattened inspiratory limb | Truncated inspiratory flow | Variable extrathoracic obstruction, kinked tube, cuff herniation |
| Pressure-volume loop, beaking | Volume stalls, pressure climbs | Overdistension; lower tidal volume |
| Pressure-volume loop fails to close | Expired volume less than inspired | Leak: cuff, circuit, bronchopleural fistula |
- Capnograph first — the standard requires continual end-tidal CO2 with an audible alarm and an audible variable-pitch pulse tone;3 the capnograph changes within breaths, the oximeter lags a minute or more after preoxygenation, so the absent trace is the earliest warning.
- Inspiratory hold — separates resistance from compliance, because the treatments differ: a bronchodilator for one, a chest tube or tube withdrawal for the other.
- A gas when the numbers disagree — a fixed oximeter reading beside a normal capnograph needs a PaO2 and co-oximetry, not more FiO2.
- Surgical context — insufflation explains both a pressure rise and an end-tidal rise; I ask what the surgeon is doing.
Follow-up question
Scenario 3, a different patient: a laparoscopic case with a central line. The monitor now shows:
What is it, and what do you do?
Follow-up answer
What the monitor showed: peak pressure 22 to 40 cmH2O with the plateau rising to 36, end-tidal CO2 40 to 26, SpO2 88%, pressure 76/42 with a new respiratory swing.
HeadlineA compliance problem with hypotension: tension pneumothorax or capnothorax until proven otherwise, and I say so.
100% oxygen, hand-ventilate, surgeon desufflates; I examine the chest, and if pressure keeps falling I decompress at the second interspace midclavicular or the fourth to fifth interspace anterior axillary line before the radiograph. The subtle inject: an inspired CO2 baseline of 4 mmHg on a long case means an exhausted absorbent or stuck valve; I raise fresh gas flow above minute ventilation and change the absorbent.
Pitfalls that cost points
- Quoting "36 mmHg" and stopping: the grader scores the shape named aloud.
- Listing causes without an order of action.
Rapid-fire
Q: Why does end-tidal CO2 fall with pulmonary embolism but rise with malignant hyperthermia? A: Embolism creates dead space, so ventilated alveoli carry no CO2; MH raises production, so every alveolus carries more.
Q: Why does the oximeter read 85% in methemoglobinemia? A: Methemoglobin absorbs red and infrared light nearly equally, driving the ratio toward about 85% whatever the true saturation.
Q: What does a flattened inspiratory flow-volume limb mean in an intubated patient? A: Inspiratory obstruction above the carina: a kinked tube, a herniated cuff, or a tube against the tracheal wall.
Numbers to know
| Item | Value |
|---|---|
| Normal end-tidal CO2 | 35–45 mmHg, a few mmHg (about 2–5) below PaCO22 |
| Plateau pressure target | <30 cmH2O |
| Lung-protective tidal volume | 6–8 mL/kg predicted body weight |
| Methylene blue dose | 1–2 mg/kg IV over 5 min |
| Needle decompression sites | 2nd interspace midclavicular or 4th–5th interspace anterior axillary |
| Oximeter lag after desaturation begins | 30–60 s, longer with poor perfusion |
Links
- The monitors and echo station: format and timing, Capnography: waveforms and sudden loss of ETCO2, Pulse oximetry: principles and limitations, Intraoperative bronchospasm, Pneumothorax and tension pneumothorax
- Atlas: Monitor atlas: ventilator modes and loops; Monitor atlas: perioperative cardiac events
- ASA Standards for Basic Anesthetic Monitoring (current edition)
Sources
- 1.OSCE Content Outline · ABA · theaba.org
- 2.Quick Reference Guide: Capnography Waveforms · Masimo · masimo.comManufacturer education sheet; gradient given as 1–5 mmHg with normal lungs.
- 3.Standards for Basic Anesthetic Monitoring · ASA, 2025 · asahq.orgLast amended October 15, 2025.
Facts on this page were checked against these sources on Oct 14, 2026. Guidelines change; confirm doses against current institutional protocols.