WALL REF 01 · FIELD REFERENCE

Ventilator Theory

How the machine breathes — and how to read it back
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GoldOxygenation — FiO₂ · PEEP
OliveVentilation & compliance — Rate · Vt
RustDanger / limits / resistance
1Two Dialsthe whole model
OXYGENATION VENTILATION RR × Vt = minute vent FiO₂ PEEP · mean Paw Rate Vt Two nearly-independent dials. Wronggas → turn the matching knob.

Low O₂? raise FiO₂ / PEEP (open & hold alveoli). High CO₂? raise rate / Vt (flush more gas). CO₂ is a flow problem; O₂ is a pressure-and-surface problem.

2Compliance vs Resistance
SOFT STIFF
Compliance — how easily the lung stretches. Stiff lung → high PLATEAU. C = Vt ÷ (Pplat−PEEP), nl 50–70
OPEN NARROW
Resistance — friction through the airways. Narrow airway → high PEAK. Bronchospasm · secretions · kinked/bitten tube
!The Three Ceilingsmemorize these
6
mL/kg PBW tidal volume
≤30
plateau cmH₂O
≤15
driving pressure ΔP

ΔP = Pplat − PEEP = the stretch the remaining "baby lung" feels each breath — the vent number most tied to survival in ARDS (Amato 2015).

MModes in a Linewhat's held constant
MODEHOLDSONE-LINE
VC / AC-VCVolumeSet Vt every breath; pressure floats — watch Pplat.
PC / AC-PCPressureSet pressure & time; Vt floats — watch exhaled Vt.
SIMVVol/PressMandatory rate + supported spontaneous breaths between.
PSVPressurePatient triggers all; vent boosts. No backup rate.
PRVC / ASVAdaptiveTargets Vt at the lowest pressure, breath to breath.
APRVPressureTwo CPAP levels; brief releases ventilate. Open-lung.
3Read the Scalarspressure · flow · volume
vs time — the vent's ECG
ONE VOLUME-CONTROL BREATH inspiratory pause shown
INSPIRATION PAUSE EXPIRATION PRESSURE cmH₂O PIP Pplat PEEP PIP−Pplat RESISTANCE ΔP driving ≤ 15 FLOW L/min 0 inspiratory flow (set) normal: returns to 0 next breath starts before flow hits 0 → AUTO-PEEP VOLUME mL Vt 6 mL/kg TIME →
One high-peak alarm → do an inspiratory pause → two answers
PEAK ↑ · PLATEAU normal · wide gap
Resistance
Airway problem. Suction, unkink/bite-block, bronchodilator. Compliance is fine.
PEAK ↑ · PLATEAU ↑ (>30)
Compliance
Stiff lung/chest. ARDS, edema, pneumothorax, mainstem, auto-PEEP. ↓Vt, treat cause.
The pause trick: hold inspiration → flow = 0 → resistance drops out → what's left (plateau) is pure lung. That gap is the diagnosis.
Auto-PEEP: buy exhale time

↓ Rate · ↓ Vt · ↑ inspiratory flow (shorten Ti) · treat obstruction. Trapped gas ↑ intrathoracic pressure → ↓ venous return → PEA.

I:E ratio

Normal ~1:2. Obstructive lungs (COPD/asthma) need a longer expiratory time — aim 1:3–1:4 so the next breath doesn't stack on the last.

4P–V Loopsslope · width · beak
NORMALSTIFF (ARDS) OBSTRUCTIVEOVER-DISTEND
slope = compliance width = resistance beak = overdistension

x = pressure, y = volume. Ride the steep middle; avoid the flat bottom (collapse) and the beak (injury).

5Worked Caseswalk the numbers
Set it from height♂ 5'10"
PBW = 50 + 2.3(70−60) = 73 kg Vt 6 mL/kg → ~440 mL · RR 16–18 Minute vent ≈ 0.44 × 16 = 7 L/min Check Pplat ≤30, ΔP ≤15. Old 12 mL/kg way ≈ 880 mL — nearly double, and deadly.
Fix the gasABG
pH 7.28 · PaCO₂ 58 · PaO₂ 92 on 40% Acidosis + O₂ fine = a ventilation problem. Turn the CO₂ knob: ↑ RR ≈16×(58/40) → ~22. Keep Vt at 6. Don't touch FiO₂/PEEP — wrong dial. Watch for auto-PEEP.
Same alarm, two fixesPIP 45
A · Pplat 20 → gap 25 = resistance: suction, unkink, bronchodilator. B · Pplat 32 → both high, ΔP 22: stiff lung — rule out pneumothorax, ↓Vt 6→5→4.
VtTidal Volume6 mL/kg PBW by height
HEIGHT♀ 6mL♂ 6mL♀ 8mL♂ 8mL
5'0"275300365400
5'4"330355440475
5'8"385410515550
6'0"440465585620
6'4"495520660695

Target Vt in mL from height & sex (PBW, not scale weight). Start at 6; 8 is the ceiling, not a goal.

6Safe EnvelopeARDSNet ladder
FiO₂.3.4.5.6.7.81.0
PEEP↓55–88–101010–141418–24
PEEP↑12141618202224

Climb FiO₂ & PEEP together. Targets: SpO₂ 88–95%, pH ≥7.20 (permissive hypercapnia beats big-Vt injury). Higher-PEEP table for moderate–severe ARDS.

!Crashing on the Vent
  • Dislodged tube
  • Obstruction / plug
  • Pneumothorax
  • Equipment failure
⚠ If they code — DISCONNECT

Take them off the circuit, let the chest fully deflate (auto-PEEP obstructs venous return), then bag slowly.

REMEMBER — Oxygen = FiO₂ + PEEP · Ventilation = Rate × Vt · Peak-without-plateau = airway · Peak-with-plateau = lung.
Sheet
Ventilator Theory
Series / No.
WALL REF 01
Part No.
CC-WR-01
Rev
B
Status
FIELD REFERENCE — NOT A PROTOCOL. VERIFY LOCALLY.

SOURCES: ARDSNet/ARMA, NEJM 2000;342:1301 (Vt 6 mL/kg, Pplat ≤30) · Amato, NEJM 2015;372:747 (driving pressure & survival) · PROSEVA, NEJM 2013;368:2159 (proning) · Devine PBW · West, Respiratory Physiology; Marino, The ICU Book. © 2026 Claustrum LLC d/b/a Crash Card · crashcard.co · Printed in Minnesota.