Frequently asked questions
These answers summarize the ARDSNet ARMA protocol and the Devine formula. They are educational background, not clinical guidance for any individual patient.
How do I calculate tidal volume for ventilator settings?
Multiply ideal body weight by 6 to 8 mL/kg. First compute IBW with the Devine formula from sex and height, then use 6 mL/kg IBW as the lung-protective starting point (the ARDSNet ARMA protocol used 6 mL/kg of predicted body weight), up to 8 mL/kg IBW for patients with healthier lungs. For example, an IBW of 70.6 kg gives about 423 mL at 6 mL/kg and about 565 mL at 8 mL/kg.
What is the Devine formula for ideal body weight?
Devine (1974): for men, IBW (kg) = 50 + 0.91 × (height in cm − 152.4); for women, IBW (kg) = 45.5 + 0.91 × (height in cm − 152.4). In inches it is 50 kg (men) or 45.5 kg (women) plus 2.3 kg per inch over 5 feet. The ARDSNet ARMA trial used this same formula as predicted body weight to set tidal volumes.
Why is tidal volume based on ideal body weight instead of actual body weight?
Lung size correlates with height, not with body weight. In an obese patient, dosing Vt by actual body weight delivers far more volume than the lungs can accommodate, which can cause volutrauma and barotrauma. That is why ventilator tidal volume is conventionally set from IBW, which depends only on sex and height.
What did the ARDSNet ARMA trial show about tidal volume?
The 2000 ARDSNet ARMA trial (861 patients, funded by the NIH National Heart, Lung, and Blood Institute) compared 6 mL/kg of predicted body weight against 12 mL/kg. The 6 mL/kg strategy lowered hospital mortality by about 22% relative to the traditional 12 mL/kg strategy, and patients were extubated on average two days earlier. It became the foundation of lung-protective ventilation.
What is the difference between 6, 7, and 8 mL/kg?
6 mL/kg IBW is the standard lung-protective starting target from ARDSNet. 7–8 mL/kg IBW is the upper end used for initial settings in patients with relatively normal lungs (some protocols start at 8 and wean down). Whatever the starting point, the final setting is titrated to plateau pressure (≤ 30 cmH2O in ARDSNet), arterial pH, PaCO2, and oxygenation — not fixed by the mL/kg number alone.
What is adjusted body weight, and when is it used?
Adjusted body weight = IBW + 0.4 × (actual body weight − IBW). It is commonly referenced for weight-based drug dosing in obese patients. For ventilator tidal volume, IBW remains the conventional basis; this calculator shows AdjBW alongside so you can see how the different weight bases change the resulting Vt range.
How can I check whether a current ventilator setting is within the lung-protective range?
Divide the set tidal volume in mL by the patient's IBW in kg. A result between 6 and 8 mL/kg IBW falls in the conventional lung-protective band. Use the "Check current setting" tab above: enter the set Vt and the patient's height, and it computes the mL/kg value and compares it against the band.
What is permissive hypercapnia?
When low tidal volumes reduce CO2 clearance, clinicians may allow PaCO2 to rise — and arterial pH to fall to about 7.20 — rather than increase Vt, accepting mild respiratory acidosis to keep volumes lung-protective. It was part of the ARDSNet strategy and requires close monitoring.
What about plateau pressure?
ARDSNet targeted a plateau pressure of 30 cmH2O or less. If plateau pressure exceeded 30, the protocol allowed reducing tidal volume to 4 mL/kg of predicted body weight. Plateau pressure reflects alveolar distension, so it is the key bedside check that a chosen Vt is not overdistending the lungs.
Is this calculator medical advice?
No. It is an educational tool for healthcare professionals, students, and researchers. Ventilator settings involve many patient-specific factors — lung mechanics, gas exchange, hemodynamics — and must be decided by qualified clinicians. Always verify results independently before any clinical use.