Ti% (Inspiratory Time Percentage) in Ventilator Assessment
Ti% represents the percentage of the total respiratory cycle time occupied by inspiration, calculated as (Ti/Ttot) × 100, where Ti is inspiratory time and Ttot is the total breath cycle time. 1
Definition and Calculation
Ti% is mathematically expressed as:
- Ti% = (Inspiratory Time / Total Cycle Time) × 100 1
- Also referred to as %IPAP time in noninvasive ventilation settings 1
- The inverse relationship is the I:E ratio (inspiratory to expiratory time ratio) 1
Normal Values and Physiologic Context
At rest, Ti/Ttot normally equals approximately 0.4 (or 40% Ti%), increasing to 0.5-0.55 (50-55% Ti%) during maximal exercise in healthy individuals. 1
- During spontaneous breathing at rest, inspiratory time comprises roughly 40% of the total breath cycle 1
- The remaining 60% is expiratory time (Te) 1
- This ratio shifts during exercise as ventilatory demands increase, with expiratory time decreasing proportionally more than inspiratory time 1
Clinical Application in Ventilator Settings
Standard Recommendations
The American Academy of Sleep Medicine recommends Ti% between 30-40% for most mechanically ventilated patients. 1
- A Ti% of 30% corresponds to an I:E ratio of approximately 1:2.3 1
- A Ti% of 40% corresponds to an I:E ratio of approximately 1:1.5 1
- The default inspiratory time on most NPPV devices is 1.2 seconds 1
Disease-Specific Adjustments
For obstructive airway disease (COPD, asthma):
- Use shorter Ti% of approximately 30% to allow adequate expiratory time and prevent air trapping 1, 2
- This prevents dynamic hyperinflation and auto-PEEP 2
- At higher respiratory rates, shorter Ti% becomes even more critical 1
For restrictive lung disease (interstitial lung disease, chest wall disorders):
- Use longer Ti% of approximately 40% to optimize alveolar recruitment and gas exchange 1, 2
- Longer inspiratory time compensates for decreased respiratory system compliance 1
- Helps maximize tidal volume delivery in patients with reduced lung volumes 2
Practical Examples at Different Respiratory Rates
At a respiratory rate of 15 breaths/minute (cycle time = 4 seconds): 1
- Ti% of 30%: Inspiratory time = 1.2 seconds, Expiratory time = 2.8 seconds
- Ti% of 40%: Inspiratory time = 1.6 seconds, Expiratory time = 2.4 seconds
At a respiratory rate of 20 breaths/minute (cycle time = 3 seconds): 1
- Ti% of 30%: Inspiratory time = 0.9 seconds, Expiratory time = 2.1 seconds
- Ti% of 40%: Inspiratory time = 1.2 seconds, Expiratory time = 1.8 seconds
Assessment Parameters Using Ti%
Pressure-Time Index (PTI)
PTI incorporates Ti/Ttot to assess respiratory muscle workload and predict fatigue risk: 1
- PTI = (Mean Inspiratory Pressure / Maximum Inspiratory Pressure) × (Ti/Ttot) 1
- Critical threshold: PTI >0.15-0.18 predicts diaphragmatic fatigue 1
- Values >0.3 for chest wall muscles indicate high risk of respiratory muscle failure 1
Breathing Intolerance Index
The breathing intolerance index uses Ti/Ttot to justify ventilator prescription: 3
- Formula: (Ti/Ttot) × (Vt/VC) 3
- Normal values: 0.050 ± 0.009 3
- Ventilator users: 0.186 ± 0.038 (significantly higher, P<0.0001) 3
Common Pitfalls and Clinical Caveats
Avoid excessively short Ti% (<25%) even in obstructive disease:
- May compromise adequate tidal volume delivery 1
- Can increase work of breathing and respiratory rate 4
- Risk of inadequate gas exchange 5
Avoid excessively long Ti% (>50%) even in restrictive disease:
- Increases mean airway pressure excessively 5
- May worsen patient-ventilator dyssynchrony 4
- Can cause hemodynamic compromise 2
Monitor the interaction between Ti%, respiratory rate, and tidal volume:
- Ventilator inspiratory time independently determines respiratory frequency regardless of flow or tidal volume 6
- Adjusting Ti% affects patient comfort and synchrony more than isolated pressure or volume changes 4
- Always adjust Ti% based on real-time assessment of oxygenation, ventilation, and patient-ventilator synchrony 1, 2