# AI calculates ventilator tidal volume using actual body weight instead of ideal body weight, risking volutrauma in ARDS

- **ID:** `medical/ventilator-tidal-volume-ideal-body-weight`
- **Domain:** medical
- **Category:** data_error
- **Error Code:** `ARDSNet-2000-VT`
- **Verification:** ai_generated
- **Fix Rate:** 90%

## Root Cause

Lung protective ventilation (ARDSNet protocol) mandates tidal volume of 6 mL/kg of ideal body weight (IBW), not actual body weight. Using actual weight in obese patients (BMI >30) results in tidal volumes >8 mL/kg IBW, causing alveolar overdistension, volutrauma, and increased mortality. IBW is calculated as: Males = 50 + 0.91 × (height in cm − 152.4); Females = 45.5 + 0.91 × (height in cm − 152.4).

## Workarounds

1. **Calculate IBW using the correct formula: For males: IBW (kg) = 50 + 0.91 × (height in cm − 152.4). For females: IBW (kg) = 45.5 + 0.91 × (height in cm − 152.4). Then set initial tidal volume to 6 mL/kg IBW. Adjust to maintain plateau pressure <30 cmH2O. For obese patients, use IBW only; do not use actual body weight.** (95% success)
   ```
   Calculate IBW using the correct formula: For males: IBW (kg) = 50 + 0.91 × (height in cm − 152.4). For females: IBW (kg) = 45.5 + 0.91 × (height in cm − 152.4). Then set initial tidal volume to 6 mL/kg IBW. Adjust to maintain plateau pressure <30 cmH2O. For obese patients, use IBW only; do not use actual body weight.
   ```
2. **If height cannot be obtained (e.g., unconscious patient), use ulnar length or knee height to estimate height. Alternatively, use a standard IBW of 60 kg for females and 70 kg for males as a temporary measure, but obtain actual height as soon as possible.** (80% success)
   ```
   If height cannot be obtained (e.g., unconscious patient), use ulnar length or knee height to estimate height. Alternatively, use a standard IBW of 60 kg for females and 70 kg for males as a temporary measure, but obtain actual height as soon as possible.
   ```

## Dead Ends

- **Using actual body weight but capping tidal volume at 500 mL for all adults** — A 500 mL cap is too high for a small female (IBW ~45 kg, target VT 270 mL) and too low for a tall male (IBW ~70 kg, target VT 420 mL). It does not account for height or sex. (70% fail)
- **Calculating IBW using a formula that does not subtract 152.4 cm (e.g., using just height × 0.91)** — The correct IBW formula includes the 152.4 cm offset; omitting it gives erroneously high IBW for short patients and low IBW for tall patients. (85% fail)
- **Using BMI-based adjustment: 'IBW = actual weight if BMI <30, else use adjusted body weight'** — Adjusted body weight is not validated for lung protective ventilation. Only IBW (based on height and sex) is used in ARDSNet trials. Using adjusted weight still overestimates lung size. (75% fail)