Tidal Index in Mechanical Ventilation
Tidal index is not a standard term in mechanical ventilation; the key ventilation parameter is tidal volume, which should be set at 4-8 ml/kg predicted body weight to minimize ventilator-induced lung injury. 1
Understanding Tidal Volume in Mechanical Ventilation
- Tidal volume refers to the volume of air delivered to a patient's lungs with each breath during mechanical ventilation 1
- It should be calculated based on predicted body weight (PBW), not actual body weight, as lung size correlates better with height 2, 3
- For males, PBW is calculated as: 50 + 0.91[height (cm) - 152.4] kg 1, 2
- For females, PBW is calculated as: 45.5 + 0.91[height (cm) - 152.4] kg 1, 2
Evidence-Based Recommendations for Tidal Volume Settings
- For patients with ARDS, tidal volumes should be limited to 4-8 ml/kg PBW with plateau pressures <30 cmH2O 1
- For patients without ARDS, tidal volumes of 6-10 ml/kg PBW are appropriate, though there is growing evidence that lower volumes (6-8 ml/kg) may be beneficial for all mechanically ventilated patients 3, 4, 5
- Meta-regression analysis shows a significant inverse association between larger tidal volume gradient and mortality risk, with trials using larger differences between control and intervention groups showing greater benefits of lower tidal volumes 1
Clinical Significance of Driving Pressure
- Driving pressure (DP = plateau pressure - PEEP) is a better predictor of outcomes in ARDS than either tidal volume or plateau pressure alone 1, 3
- Target driving pressure should ideally be kept below 15 cmH2O 2, 3
- Recent observational data from multiple RCTs demonstrates that driving pressure is more strongly associated with survival than other ventilatory parameters 1
Monitoring and Adjusting Ventilation
- Work of breathing progressively increases as tidal volume decreases (from 0.86 ± 0.32 J/L at 8 ml/kg to 1.57 ± 0.43 J/L at 5 ml/kg) 6
- The Global Inhomogeneity (GI) index, measured by electrical impedance tomography, can assess ventilation distribution and may be affected by tidal volume settings, particularly at low PEEP levels 7
- Higher PEEP levels may be required when using lower tidal volumes to maintain oxygenation and prevent atelectasis 2, 3
Common Pitfalls to Avoid
- Using actual body weight instead of predicted body weight leads to excessive tidal volumes, especially in obese patients 3, 8
- Failure to adjust ventilation parameters based on clinical response may result in persistent hypoventilation or ventilator-induced lung injury 8
- Delaying implementation of lung-protective ventilation strategies due to unfamiliarity with PBW calculations or resistance to changing practice patterns 4
By implementing appropriate tidal volume settings based on predicted body weight, clinicians can significantly improve outcomes for mechanically ventilated patients, with evidence showing reduced mortality and increased ventilator-free days in patients with ARDS 9.