Tidal Volume Calculation for 19-Year-Old Male at 8 mL/kg
For this 19-year-old male (57 kg, 168 cm), the tidal volume at 8 mL/kg predicted body weight (PBW) should be approximately 456 mL, calculated using PBW of 57 kg based on his height of 168 cm.
Predicted Body Weight Calculation
- PBW must be calculated from height, not actual body weight 1
- For males: PBW (kg) = 50 + 0.91 × (height in cm - 152.4) 1
- For this patient: PBW = 50 + 0.91 × (168 - 152.4) = 50 + 0.91 × 15.6 = 57 kg 1
- Coincidentally, his actual weight equals his PBW in this case
Tidal Volume at 8 mL/kg PBW
- Tidal volume = 8 mL/kg × 57 kg = 456 mL 2, 1
- This falls within the recommended range of 6-8 mL/kg PBW for intraoperative ventilation 2
Clinical Context for 8 mL/kg Setting
The 8 mL/kg setting is appropriate for specific clinical scenarios:
- Intraoperative ventilation during emergency laparotomy where 6-8 mL/kg PBW is recommended, with 8 mL/kg allowing use of stroke volume variability (SVV) and pulse pressure variability (PPV) for hemodynamic optimization when arterial line monitoring is available 2
- Non-ARDS mechanical ventilation where the Society of Critical Care Medicine permits initial tidal volumes up to 8 mL/kg PBW if 6 mL/kg is not tolerated, maintaining plateau pressure ≤30 cm H₂O 1
Critical Distinction: ARDS vs Non-ARDS Ventilation
For ARDS or sepsis-induced respiratory failure, 6 mL/kg PBW (342 mL for this patient) is mandatory, as this reduces mortality from 39.8% to 31.0% compared to traditional volumes 1
- The American College of Chest Physicians recommends starting with 6 mL/kg PBW for ARDS patients 1
- Tidal volumes >8 mL/kg PBW are associated with increased mortality and should be avoided 3
Monitoring Parameters
Essential monitoring includes:
- Plateau pressure must remain ≤30 cm H₂O regardless of tidal volume setting 2, 1
- Driving pressure (plateau pressure - PEEP) should be <15 cm H₂O, as this predicts outcomes better than any other ventilatory parameter 2, 1
- If plateau pressure exceeds 30 cm H₂O at 8 mL/kg, reduce tidal volume to 6 mL/kg or lower 1
Common Pitfalls to Avoid
Never use actual body weight instead of PBW for tidal volume calculation, as this leads to excessive volumes and increased mortality, especially in obese patients 1, 4
- Female patients and those with BMI >30 kg/m² are at highest risk of receiving inappropriately high tidal volumes when actual weight is used 5, 3
- Patients shorter than 165 cm are at increased risk of not receiving lung-protective ventilation 3
For this specific patient with height 168 cm and weight 57 kg, using actual weight happens to equal PBW, but the calculation must always be based on height-derived PBW to ensure consistency across all patients 1