Anesthesiology APPLIED Exam › Thoracic and respiratory

ARDS and lung-protective ventilation

Thoracic and respiratory·~5 min read· Physician-reviewed✓ Sourced · 4

Stem

A 58-year-old woman, 165 cm (65 inches) and 92 kg, has been intubated in the ICU for two days with pneumonia and bilateral infiltrates. She now needs an exploratory laparotomy for a perforated colon. On the ICU ventilator she is on volume control, tidal volume 500 mL, PEEP 12 cmH2O, FiO2 0.7; PaO2 is 77 mmHg, plateau pressure is 31 cmH2O, and she is on norepinephrine 0.08 mcg/kg/min.

Question

How will you ventilate her in the operating room?

Ideal answer

HeadlineModerate ARDS, so I stop injuring her lung: 6 mL/kg predicted body weight, plateau below 30, no disconnections.

Tidal volume from 500 down to roughly 340 mL. Predicted body weight is 45.5 plus 2.3 times 5 inches, about 57 kg; 500 mL is nearly 9 mL/kg.¹
Plateau below 30 cmH2O, rate up to 35, to hold pH at 7.30 or above, accepting higher CO2.¹
PEEP at least where it is, because ATS suggests higher PEEP in moderate to severe ARDS; FiO2 to a PaO2 of 55–80 or SpO2 of 88–95%.²^,1
Clamp the tube for every circuit change and transport, as each disconnect derecruits her; or the ICU ventilator if our machine cannot deliver her settings.

Watch for: Plateau, driving pressure and gas exchange, as the abdomen opens and closes; arterial line, goal-directed, not liberal, fluids.

Breaking down the answer

Classify before you treat — moderate: bilateral infiltrates within a week of pneumonia, P/F 110 on PEEP above 5. Berlin also needs edema not fully explained by heart failure and PEEP of at least 5; mild 201–300, moderate 101–200, severe 100 or less.³
Tidal volume by predicted, not actual, weight — the lung is sized by height; the guideline recommends 4–8 mL/kg PBW with a plateau below 30 cmH2O.² Using actual weight in an obese patient is the classic error.
Permissive hypercapnia — I trade CO2 for lung protection, down to a pH of about 7.30. Changes if: raised intracranial pressure.¹
The conditional part — ATS 2024 suggests higher PEEP, without recruitment maneuvers;² ESICM 2023 makes no recommendation either way.⁴ Changes if: plateau reaches 30 or pressure falls as PEEP rises.
Every disconnect undoes hours of recruitment — protecting against derecruitment is the anesthesia-specific judgment examiners look for.
No prolonged recruitment maneuvers — both guidelines advise against holding 35 cmH2O or more for a minute or longer;²^,4 on a pressor, a sustained breath also drops her pressure.

Follow-up question

After the abdomen is opened and packs are placed, SpO2 falls to 84% on FiO2 1.0, peak pressure jumps from 28 to 42 cmH2O, and plateau pressure is 36 cmH2O. Blood pressure is 82/50.

What do you do?

Follow-up answer

HeadlineAn acute change, not just her ARDS. I tell the surgeon to stop packing while I sort it out.

I hand-ventilate briefly to feel compliance, pass a suction catheter for a blocked or displaced tube, listen for bilateral breath sounds and check the capnogram for bronchospasm. With a pressor running and high airway pressure, I look for tension pneumothorax with ultrasound and decompress it. If packs and retractors push the diaphragm up, they are repositioned and I go toward 4 mL/kg PBW to get the plateau under 30.² Norepinephrine, a blood gas, reassess. After surgery, a PaO2/FiO2 still under 150 once settings are optimized meets the ESICM threshold for prone sessions of 16 hours or more.⁴

Pitfalls that cost points

Dropping her PEEP to 5 because "that is what the anesthesia machine is set to."
Chasing a normal PaCO2 at the cost of plateau pressure.

Rapid-fire

Q: What is the predicted body weight formula? A: Men 50, women 45.5, plus 2.3 kg for each inch above 60 inches.¹

Q: When is prone positioning recommended? A: Severe ARDS for more than 12 hours a day (ATS);² ESICM uses PaO2/FiO2 below 150 after stabilization, in sessions of 16 hours or more.⁴

Q: Is ECMO recommended for severe ARDS? A: Yes, in selected patients: ATS 2024 suggests venovenous ECMO, and ESICM 2023 recommends it for EOLIA-criteria patients in an ECMO center.²^,4

Q: What oxygenation target do you use? A: PaO2 55–80 mmHg or SpO2 88–95%.¹

Q: What is the highest set rate in the ARDSNet protocol? A: 35 breaths per minute.¹

Numbers to know

Item	Value
Tidal volume	4–8 mL/kg PBW, start at 6²
Plateau pressure	<30 cmH2O²
Berlin severity (P/F on PEEP ≥5)	Mild 201–300, moderate 101–200, severe ≤100³
Oxygenation goal	PaO2 55–80 mmHg or SpO2 88–95%¹
pH goal	7.30–7.45¹
Prone positioning	Severe ARDS, >12 h/day (ATS);² P/F <150, sessions ≥16 h (ESICM)⁴

Sources

1.NIH NHLBI ARDS Clinical Network Mechanical Ventilation Protocol Summary · NHLBI ARDS Network, 2008 · adventhealth.com
2.An Update on Management of Adult Patients with Acute Respiratory Distress Syndrome: An Official American Thoracic Society Clinical Practice Guideline · ATS (Am J Respir Crit Care Med 2024;209:24-36), 2024 · europepmc.orgFull text read. Carries forward 2017: VT 4-8 mL/kg PBW and plateau <30 (strong), prone >12 h/day in severe ARDS (strong). New: corticosteroids, VV-ECMO in selected severe ARDS, NMB in early severe ARDS (all conditional); higher PEEP without recruitment maneuvers in moderate-severe (conditional); against prolonged recruitment maneuvers, PEEP >=35 cmH2O for >60 s (strong).
3.Clinical review: Acute respiratory distress syndrome - clinical ventilator management and adjunct therapy (Table 1, Berlin definition) · Critical Care (BMC), 2013 · ccforum.biomedcentral.com
4.ESICM guidelines on acute respiratory distress syndrome: definition, phenotyping and respiratory support strategies · ESICM (Intensive Care Med 2023;49:727-759), 2023 · europepmc.orgFull text read. Rec 5.1 VT 4-8 mL/kg PBW; 6.1 no recommendation for higher vs lower PEEP/FiO2; 6.3 against prolonged high-pressure recruitment; 7.1-7.2 prone if PaO2/FiO2 <150 on PEEP >=5 after stabilization, sessions of about 16 h; 8.1 against routine continuous NMBA infusion; 9.1 ECMO for EOLIA-criteria severe ARDS in an ECMO center.

Facts on this page were checked against these sources on Nov 2, 2026. Guidelines change; confirm doses against current institutional protocols.