Anesthesiology APPLIED Exam › Pharmacology, equipment and physics

Ventilator modes, PEEP and IMV vs. pressure control

T1Pharmacology, equipment and physics·835 words · ~5 min read·v0.1✓ Sourced · 2DRAFT

Why examiners ask this

The examiner asks "what ventilator settings will you use?" to see whether you choose a mode for a reason and know what it trades away. The follow-ups target the patient whose peak pressures climb, whose oxygenation falls, or who breathes against the machine. It tests application of knowledge and adaptability, and the OSCE monitors station 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" in its outline.¹

The 60-second answer

For most patients I use volume control with a tidal volume of 6–8 mL/kg predicted body weight, PEEP of 5 cmH2O, a rate that keeps ETCO2 in the 35–40 mmHg range, and an FiO2 that keeps SpO2 at or above 94%. I measure plateau pressure and keep it under 30 cmH2O and driving pressure under 15. I choose pressure control when I need to limit airway pressure: a patient with poor compliance, a leaking uncuffed tube in a child, or one-lung ventilation. The trade-off is that tidal volume falls when compliance falls, so I set a minimum tidal volume alarm; the monitoring standard strongly encourages quantitative monitoring of expired gas and requires an audible disconnect alarm during mechanical ventilation.² In obesity, laparoscopy and steep Trendelenburg, I raise PEEP and use recruitment maneuvers, because the abdomen collapses dependent lung.

Decision points and the "why"

Volume control — guarantees minute ventilation; peak pressure rises with any increase in resistance or fall in compliance, which is useful diagnostic information. Peak and plateau pressures separate the problem: a high peak with normal plateau is resistance (tube, bronchospasm); both high is compliance (pneumothorax, abdominal pressure, endobronchial tube).
Pressure control — 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 and SIMV — pressure support augments spontaneous breaths during emergence and supraglottic airway cases; SIMV delivers mandatory breaths with spontaneous breaths between them, and I use it as a bridge to spontaneous ventilation.
Protective tidal volume — 6–8 mL/kg predicted body weight, not actual; volutrauma relates to lung size, which depends on height, not weight.
PEEP — prevents atelectasis and improves oxygenation; too much overdistends alveoli, raises intrathoracic pressure and drops venous return. I titrate to the lowest driving pressure and best compliance.
Recruitment — a sustained inflation of 30–40 cmH2O for 30 seconds opens collapsed lung; I warn the room, ensure volume status, and follow it with PEEP, because recruitment without PEEP is lost within minutes.
Obstructive disease — longer expiratory time with a lower rate and I:E of 1:3 or 1:4 to avoid auto-PEEP; permissive hypercapnia is acceptable.

The complication they will inject

During a robotic prostatectomy in steep Trendelenburg, the peak pressure in volume control rises from 22 to 42 cmH2O and the plateau to 36 cmH2O; the SpO2 is 92%. I tell the surgeon and check both: a high plateau means compliance, so I listen for asymmetric breath sounds because the head-down position and insufflation push the carina toward the tube, causing endobronchial intubation. I check the tube depth and withdraw it 1–2 cm if needed. If breath sounds are equal, I ask the surgeon to lower insufflation pressure, increase PEEP to counter the diaphragmatic push, and switch to pressure control at a reduced tidal volume, accepting higher CO2. If oxygenation does not recover or the pressures stay unsafe, I ask the surgeon to reduce the tilt.

Pitfalls that cost points

Quoting tidal volume per actual body weight in an obese patient.
Choosing pressure control "because it is safer" without naming the lost guarantee of minute ventilation.
Treating high peak pressure without measuring plateau.
Recruiting without PEEP afterward.
Ignoring auto-PEEP in a bronchospastic patient whose pressure falls when the circuit is disconnected.

Rapid-why

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

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.