Ventilator modes, PEEP and IMV vs. pressure control
Stem
A 61-year-old man, 175 cm and 118 kg, with obstructive sleep apnea, is having a robotic-assisted radical prostatectomy. He will be in steep Trendelenburg with pneumoperitoneum for about four hours. After an uneventful intubation, the circulating nurse asks what to set on the ventilator.
Question
What ventilator mode and settings will you use, and why?
Ideal answer
HeadlineVolume control at 6–8 mL/kg predicted body weight, set from his height, not his 118 kg: about 70 kg, so roughly 420–560 mL.
- PEEP 5 cmH2O to start, raised, plus recruitment, because the abdomen collapses dependent lung in obesity, laparoscopy and steep Trendelenburg.
- Rate for ETCO2 35–40 mmHg, and FiO2 for an SpO2 at or above 94%.
- Plateau under 30 cmH2O, driving pressure under 15, measured, not assumed, because they track the stress on his lungs.
- Pressure control only to limit airway pressure: poor compliance, a leaking uncuffed tube in a child, one-lung ventilation; tidal volume then falls with compliance, so I set a minimum tidal volume alarm.
Watch for: peak and plateau after every position or insufflation change; they tell me where a problem lives.
Breaking down the answer
- What is tested — what each mode trades away; follow-ups bring climbing peaks or falling oxygenation. The OSCE outline lists airway pressure and flow waveforms, tidal volume, inspiratory and expiratory times, flow-volume loops and "ventilatory modes used in normal and critically ill patients."1
- Volume control as diagnosis — it guarantees minute ventilation, so peak pressure reports resistance or compliance: high peak with normal plateau is resistance (tube, bronchospasm); both high is compliance (pneumothorax, abdominal pressure, endobronchial tube).
- Pressure control's cost — decelerating flow distributes gas more evenly and limits peak pressure; tidal volume becomes the dependent variable, so a rising abdomen or bronchospasm silently drops ventilation. Pressure support augments spontaneous breaths at emergence and with supraglottic airways; SIMV bridges to spontaneous ventilation.
- Why predicted weight — volutrauma relates to lung size, which depends on height, not weight.
- Alarms — the standard strongly encourages quantitative expired-gas monitoring and requires an audible disconnect alarm during mechanical ventilation.2
- PEEP titration and recruitment — too much PEEP overdistends, raises intrathoracic pressure and drops venous return, so I titrate to lowest driving pressure, best compliance. Recruitment, a sustained 30–40 cmH2O for 30 seconds: room warned, volume ensured, then PEEP, or it is lost within minutes.
- Obstructive disease — lower rate and I:E of 1:3 or 1:4 to avoid auto-PEEP; permissive hypercapnia is acceptable.
Follow-up question
An hour into steep Trendelenburg, the peak pressure in volume control rises from 22 to 42 cmH2O and the plateau to 36 cmH2O. The SpO2 is 92%.
What do you do?
Follow-up answer
HeadlineA high plateau is compliance; I tell the surgeon and listen for asymmetric breath sounds: head-down and insufflation push the carina onto the tube.
I check tube depth for endobronchial intubation and withdraw it 1–2 cm if needed. With equal breath sounds, I ask for lower insufflation pressure, increase PEEP against the diaphragmatic push, and switch to pressure control at a reduced tidal volume, accepting higher CO2. If oxygenation does not recover or pressures stay unsafe, the surgeon reduces the tilt.
Pitfalls that cost points
- Choosing pressure control "because it is safer" without naming the lost guarantee of minute ventilation.
- Ignoring auto-PEEP in a bronchospastic patient whose pressure falls when the circuit is disconnected.
Rapid-fire
Q: Why measure plateau rather than peak pressure? A: Plateau reflects alveolar distension and lung injury risk; peak includes airway resistance.
Q: What is driving pressure? A: Plateau minus PEEP; it reflects strain on the functional lung and correlates with mortality in ARDS.
Q: How do you recognize auto-PEEP on the flow-time curve? A: Expiratory flow does not return to zero before the next breath begins; reading airway flow waveforms is an outline task.1
Q: Why add PEEP in the obese? A: The abdominal mass and supine position push FRC below closing capacity; PEEP of 8–10 cmH2O keeps alveoli open.
Numbers to know
| Item | Value |
|---|---|
| Protective tidal volume | 6–8 mL/kg PBW |
| Plateau pressure limit | <30 cmH2O |
| Driving pressure target | <15 cmH2O |
| Routine PEEP | 5 cmH2O (8–10 in obesity and laparoscopy) |
| Recruitment maneuver | 30–40 cmH2O for 30 s |
| PBW, male | 50 + 0.91 × (height cm − 152.4) kg |
Links
Sources
- 1.OSCE Content Outline · ABA · theaba.org
- 2.Standards for Basic Anesthetic Monitoring · ASA, 2025 · asahq.orgLast amended October 15, 2025; Standard II, ventilation.
Facts on this page were checked against these sources on Oct 14, 2026. Guidelines change; confirm doses against current institutional protocols.