Ventilator calculator

• Tidal volume by PBW, not actual weight. ATS (2017, retained in 2024): VT 4–8 mL/kg PBW and plateau pressure <30 cmH₂O for ARDS (strong recommendation). ARDSNet protocol: start at 8 mL/kg, reduce by 1 mL/kg at intervals ≤2 h to 6 mL/kg.
• Plateau pressure goal ≤30 cmH₂O (ARDSNet). Check at least every 4 h and after each PEEP or VT change. If >30: reduce VT by 1 mL/kg steps (minimum 4 mL/kg). If <25 and VT <6 mL/kg: increase VT by 1 mL/kg until Pplat >25 or VT = 6.
• Rate: set to approximate baseline minute ventilation, not >35/min. pH goal 7.30–7.45; if pH 7.15–7.30, raise RR (max 35); if pH <7.15 at RR 35, VT may be raised in 1 mL/kg steps (Pplat target may be exceeded).
• Oxygenation goal: PaO₂ 55–80 mmHg or SpO₂ 88–95%, minimum PEEP 5 cmH₂O (ARDSNet).
• PEEP: ATS 2024 suggests higher PEEP without lung recruitment maneuvers in moderate–severe ARDS (conditional) and recommends against prolonged recruitment maneuvers (strong).
• Severe ARDS: prone positioning >12 h/day (strong). Neuromuscular blockade in early severe ARDS (conditional). VV-ECMO in selected patients (conditional) — ATS cites P/F <80 mmHg or pH <7.25 with PaCO₂ ≥60 mmHg despite optimal conventional care, early (<7 days) in the course.
• Corticosteroids for ARDS: suggested (ATS 2024, conditional).

Predicted body weight (ARDSNet protocol card)

Metric form used in the 2000 ARMA trial: 50 (or 45.5) + 0.91 × (height cm − 152.4). The two differ by <0.2 kg across adult heights; this tool uses the protocol-card inch form (cm ÷ 2.54).

Driving pressure

Oxygenation ratios — FiO₂ as a fraction (0.21–1.0)

1 kPa = 7.50062 mmHg.

Berlin (2012): P/F bands above with PEEP or CPAP ≥5 cmH₂O (CPAP allowed for mild only). Global definition (2024): intubated ARDS uses the P/F or S/F bands above with PEEP ≥5; non-intubated ARDS requires P/F ≤300 or S/F ≤315 on high-flow ≥30 L/min or NIV/CPAP ≥5 cmH₂O, without severity grading. S/F is not valid above SpO₂ 97% or with suspected hemoglobin abnormality (CO, methemoglobin). Measure ≥30 min after changes in position, FiO₂ or flow.

Oxygenation is one criterion only: both definitions also require onset within 1 week, bilateral opacities (global definition also accepts ultrasound) and respiratory failure not fully explained by cardiac failure or fluid overload.

Validated in 191 adults with pneumonia on high-flow nasal cannula (35.6% intubated). Discrimination was modest (AUROC 0.68 at 2 h to 0.76 at 12 h). Values between the cutoffs are indeterminate — repeat the measurement. A trend that fails to rise over 12 h was associated with failure.

From the ARDSNet protocol card. Optional incremental combinations to reach the oxygenation goal; not required by the protocol.

• Use PBW, never actual weight. Lung size tracks height and sex. A 120 kg, 165 cm woman has a PBW of about 57 kg — 6 mL/kg is ~340 mL, not 720 mL.
• Measure height. Every inch (2.54 cm) of error moves PBW by 2.3 kg — about 14 mL of tidal volume at 6 mL/kg.
• Plateau and driving pressure need a passive patient and an inspiratory hold. Active effort makes both unreliable.
• Driving pressure target is observational. Lower ΔP tracked with survival in secondary analyses (Amato 2015); no trial has proven that titrating to a ΔP threshold improves outcome. 15 cmH₂O is a proposed safety limit, not a validated target.
• S/F above SpO₂ 97% is meaningless — the oxyhemoglobin curve is flat. Pulse oximetry can overestimate saturation, more often in patients with darker skin; get an ABG when the category would change management.
• ROX is for high-flow nasal cannula, derived in pneumonia. It informs, not dictates, the timing of intubation.