Monitor atlas: perioperative respiratory events
Why examiners ask this
Respiratory clips are the bread and butter of the monitors station because the capnograph, the pulse oximeter and the ventilator screen carry a signature for almost every airway and breathing crisis; the outline lists capnography waveform and value, pulse oximetry waveform and value, airway pressure with peak and PEEP, airway flow, tidal volume and flow-volume loops.1 The facilitator shows a waveform and asks what it is and what I do. It tests application of knowledge and organization, and it is where candidates who know the physiology but have never looked carefully at a flow-volume loop lose points.
The 60-second answer
I read respiratory clips in a fixed order: capnograph first, because it tells me the circuit is intact and the patient is perfused; then the pulse oximeter number and the pleth shape; then the ventilator, where I look at peak and plateau pressure, tidal volume delivered against set, and the shape of the pressure-volume and flow-volume loops. I describe the waveform in words, name the mechanism in one sentence, and give the actions in order: 100% oxygen, hand-ventilate to feel the compliance, confirm the tube, treat the cause. I say what I expect to see on the monitor when the treatment works, because that is the reassessment step the grader is looking for.
Decision points and the "why"
The patterns in the outline, each with the features that identify it and the action it drives.
| Pattern | Identifying features | What it changes |
|---|---|---|
| Capnograph flat, zero | No waveform at all, SpO2 still normal for a minute | Disconnect, extubation, obstruction, esophageal tube, or arrest; hand-ventilate, look, listen, check the pulse2 |
| Sudden fall to a low plateau | Waveform shape preserved, value drops over a few breaths | Loss of pulmonary blood flow: embolism, massive hypotension, arrest |
| Rising end-tidal with normal shape | Steady climb over minutes with 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, COPD; deepen, bronchodilator, lengthen expiration2 |
| 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 | Small ripples on the downslope | Low rate, long expiration; benign |
| SpO2 falls, pleth good | Number falls with a clean pleth and a 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 pressure up, plateau up | Both rise together | Compliance problem: pneumothorax, endobronchial intubation, abdominal insufflation, pulmonary edema |
| Peak pressure up, plateau normal | Gap between peak and plateau widens | Resistance problem: 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 stops rising while pressure climbs | Overdistension; lower tidal volume |
| Pressure-volume loop fails to close | Expired volume less than inspired | Leak: cuff, circuit, bronchopleural fistula |
- Capnograph before oximeter — the standard requires continual end-tidal CO2 with an audible alarm and an audible variable-pitch pulse tone on the oximeter;3 the capnograph changes within breaths; the oximeter lags by a minute or more after preoxygenation, so the absent trace is the earliest warning.
- Peak versus plateau is the discriminator — I use an inspiratory hold to separate resistance from compliance, because the treatments are different: a bronchodilator for one, a chest tube or tube withdrawal for the other.
- A gas when the numbers disagree — a pulse oximeter that will not move and a capnograph that looks fine means I need a PaO2 and a co-oximeter, not a bigger FiO2.
The complication they will inject
The clip shows a laparoscopic case on which the peak pressure climbs from 22 to 40 cmH2O, the plateau rises with it, the end-tidal CO2 falls, the SpO2 drifts to 88% and the arterial trace begins to swing. I say the picture: a compliance problem with hypotension, and in a laparoscopic patient with a central line that is tension pneumothorax or capnothorax until proven otherwise. I state the actions: 100% oxygen, hand-ventilate, tell the surgeon to desufflate, listen and look at the chest, and if the pressure keeps falling decompress at the second interspace midclavicular or the fourth to fifth interspace anterior axillary line before the chest radiograph. The other inject is the subtle one: an inspired CO2 baseline of 4 mmHg on a long case, where the answer is an exhausted absorbent or a stuck valve, and the action is to raise fresh gas flow above minute ventilation and change the absorbent.
Pitfalls that cost points
- Reading the end-tidal number without describing the waveform shape.
- Treating a low oximeter number with a poor pleth as hypoxemia before checking the patient.
- Saying "high airway pressure" without separating peak from plateau.
- Forgetting the surgical context, where insufflation explains both the pressure and the end-tidal rise.
- Listing causes without an order of action.
Rapid-why
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: What does a widening peak-to-plateau gap tell me? A: Resistance is rising while compliance is unchanged; the problem is in the tube or the airways, not the lung or chest wall.
Q: Why does the pulse oximeter read 85% in methemoglobinemia? A: Methemoglobin absorbs red and infrared light nearly equally, which drives the two-wavelength ratio toward a fixed reading of about 85% regardless of the true saturation.
Q: What does a flattened inspiratory flow-volume limb mean in an intubated patient? A: Obstruction above the carina on inspiration: 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 (coming), Intraoperative bronchospasm, Pneumothorax and tension pneumothorax
- Atlas: Monitor atlas: ventilator modes and loops (coming); 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.