Computing the Inspiratory-to-Expiratory (I:E) Ratio
The I:E ratio is computed by dividing the inspiratory time by the expiratory time during a complete respiratory cycle, typically expressed as a ratio such as 1:2, where inspiration lasts 1 second and expiration lasts 2 seconds. 1
Basic Calculation Method
The I:E ratio calculation follows this straightforward approach:
Determine the total respiratory cycle time by dividing 60 seconds by the respiratory rate (breaths per minute). For example, at 15 breaths/minute, the total cycle time = 60/15 = 4 seconds 1
Measure or set the inspiratory time (Ti), which normally comprises 30-40% of the total respiratory cycle 1, 2
Calculate the expiratory time (Te) by subtracting inspiratory time from total cycle time: Te = Total cycle time - Ti 1
Express as a ratio by dividing both values by the smaller number to create the conventional format (e.g., 1:2,1:1.5,2:1) 1
Practical Example
For a patient with respiratory rate of 15 breaths/minute:
- Total cycle time = 4 seconds 1
- If inspiratory time = 1.2 seconds (30% of cycle) 2
- Expiratory time = 4 - 1.2 = 2.8 seconds 2
- I:E ratio = 1.2:2.8, which simplifies to approximately 1:2.3 1
Standard Initial Settings
The American Academy of Sleep Medicine recommends starting with an I:E ratio of 1:2 for most mechanically ventilated patients, with inspiratory time set at 30-40% of the total respiratory cycle 1. At a respiratory rate of 15 breaths/minute, this translates to an inspiratory time of approximately 1.2-1.6 seconds 1, 2.
Disease-Specific Adjustments
The calculation remains the same, but target values differ:
Obstructive airway disease: Use shorter inspiratory times with I:E ratios of 1:2 to 1:3, allowing adequate expiratory time to prevent air trapping 1
Restrictive lung disease: Consider longer inspiratory times with I:E ratios closer to 1:1.5 for improved alveolar recruitment 1
Common Pitfall
A critical error is confusing the I:E ratio with the percentage of inspiratory time (%Ti/Ttot). The I:E ratio is the relationship between two time periods, while %Ti/Ttot represents inspiratory time as a fraction of the total cycle 2. These are related but distinct measurements that should not be used interchangeably.