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Setting tidal volume from predicted body weight
Two patients of the same height get the same breath, whether one of them weighs 56 kilograms and the other 116.
The calculation
Height and sex, nothing else. You cannot derive it from weight, and a tidal volume set from actual weight in an obese patient is the most reliably harmful setting on the ventilator.
If the height is not recorded, get it. Estimating it from the bed is better than using the weight.
Why weight is the wrong input
Lungs do not get bigger when a patient gains weight. They are sized to the thoracic cage, which is sized to height. Adipose tissue adds load to the chest wall and makes ventilation harder, but it does not add a single alveolus to receive the breath.
| Patient | Actual weight | Height | PBW | Volume at 6 mL/kg |
|---|---|---|---|---|
| Woman, BMI 22 | 56 kg | 163 cm | 55 kg | 330 mL |
| Woman, BMI 45 | 116 kg | 163 cm | 55 kg | 330 mL, the same |
| Man, BMI 24 | 84 kg | 188 cm | 82 kg | 490 mL |
| Man, BMI 18 | 56 kg | 188 cm | 82 kg | 490 mL, the same |
Set by actual weight, the second patient in that table receives 700 mL instead of 330. That is not a cautious margin, it is roughly double.
What to set
- Start at 6 mL/kg of predicted body weight for most patients.
- Four to 8 mL/kg is the working range. Go to 4 in moderate or severe ARDS, and up toward 8 only in a patient with healthy lungs and a reason.
- Above 8 mL/kg deserves a justification written down. Above 10 is hard to defend at all.
- Then check the plateau pressure, because the volume is a starting assumption and the pressure is the measurement.
Volume is the parameter that matters
Of everything on the ventilator you can set, tidal volume is the one robustly connected to survival. Overdistension injures lung directly, and the injury is not confined to the lung: it drives the inflammatory response that damages other organs.
The companion measurement is plateau pressure, which should stay at or below 30 cmH2O, and driving pressure, which indexes the breath to the lung volume actually available to take it. A protocol-correct 6 mL/kg can still be too much breath for a lung that has lost most of its aerated volume.
The one tension worth knowing about
Lung-protective volumes put most patients below the tidal volume at which pulse pressure variation and stroke volume variation were validated. Those fluid-responsiveness numbers become unreliable in exactly the population most likely to be ventilated this way, and reading them anyway is a common error.
Nobody ever mastered a ventilator by reading about one.
Set the same patient height with two very different weights and watch the protective volume stay where it is. The Ventilator Workbook puts a live ventilator in front of you with a simulated patient behind it, so you can break things that cannot be harmed. Three modules and time on the simulator are free, and no card is needed.
Keep reading
- Driving pressure: the ventilator number that tracks mortality
- Setting PEEP in ARDS
- Ventilator glossary, for the terms in this article
- Every topic in the library