Why set tidal volume by height?
The tidal volume is the amount of air delivered with each breath by a ventilator. Too large a volume overstretches the lungs and causes ventilator-induced lung injury. Lung size depends on a person’s height and sex, not on their weight, so tidal volume is set per kilogram of predicted body weight (PBW), calculated from height.
The ARDSNet formulas
- Men: PBW = 50 + 0.91 × (height in cm − 152.4) kg
- Women: PBW = 45.5 + 0.91 × (height in cm − 152.4) kg
The target tidal volume is then usually 6 mL per kg of PBW, within a range of 4 to 8 mL/kg.
The evidence
In 2000, the ARDS Network trial compared 6 mL/kg with the traditional 12 mL/kg in patients with acute respiratory distress syndrome. Mortality fell from about 40% to 31%. Low tidal volumes have since become the standard for ARDS and are widely used for other ventilated patients, including during surgery.
Worked example
A man 5 ft 10 in (70 in, 177.8 cm) tall:
| Result | Value |
|---|---|
| Predicted body weight | 73.1 kg |
| Tidal volume at 6 mL/kg | 439 mL |
| At 4 mL/kg | 292 mL |
| At 8 mL/kg | 585 mL |
| Minute ventilation at 16 breaths/min | 7.0 L/min |
A woman of the same height has a PBW of about 68.6 kg and a target of about 412 mL at 6 mL/kg.
Beyond the tidal volume
Lung-protective ventilation also involves:
- keeping the plateau pressure at or below 30 cmH₂O;
- choosing an appropriate PEEP to keep the alveoli open;
- accepting a moderately high carbon dioxide level (permissive hypercapnia) when needed;
- adjusting the respiratory rate to manage pH, often up to 35 breaths per minute in ARDS.
Limits
- The formulas are less reliable for very short adults, below about 152 cm (5 ft), and do not apply to children.
- Height should be measured or estimated carefully; a few inches of error change the volume noticeably.
- In obese patients, actual body weight can be far above PBW; using actual weight would overinflate the lungs.
This calculator is for general information and education. It is not medical advice and does not replace the judgment of a doctor, nurse or pharmacist who knows the full clinical picture.