5.2 The Time Variable
The time variable is an important parameter in all ventilation modes. We have just discussed how volume and pressure have a linear and direct relationship if all other variables are the same—meaning that if the speed and time that the air is being delivered are the same and the lungs (balloons) have not changed. Now let’s discuss these “other variables” we have alluded to—time and speed of air.
All control ventilation has a “time” element that is set, which refers to the speed by which air is delivered. At its most basic explanation, mechanical ventilation is the application of a positive pressure of air over a period of time to a closed system. This time aspect can be expressed in a few different ways; we will discuss inspiratory time and flow.
Inspiratory Time
The most direct way of manipulating how long the air is pushed into the lungs is by setting an inspiratory time (I-time,
Object Lesson

Think about balloons again. If you blew into two different balloons with the same force (pressure), but you only blew into Balloon 1 for
Now, consider this the other way around: If you wanted to hit a specific volume, but Balloon 1 had
In this way, the
Object Lesson
Yet again, we have two identical balloons (which will represent two copies of the same lungs). Let’s think about blowing up Balloon 2 twice as big (double the volume) as Balloon 1. What if we didn’t want to blow too hard (high pressure) because we were worried about damage to the balloon? What if, instead, we use the same blowing force that we used to inflate Balloon 1, but just blew for a longer time? We would be able to inflate Balloon 2 to twice the volume by just blowing longer (longer I-time) at the same pressure.
Manipulating how long the air is being pushed into the lungs for can directly affect the volume without having to change the pressure. Remember: volume delivered is just air being pushed into the lungs for a set amount of time—regardless of whether you are in pressure control or volume control.
Patient comfort should also be observed while setting the inspiratory time. If the patient has any spontaneous drive, try to match the patient’s spontaneous inspiratory time by watching chest-rise and respiratory efforts.
Flow as Related to the Time Variable
Some modes of ventilation do not utilize I-time (
“The Time Variable” from Basic Principles of Mechanical Ventilation by Melody Bishop, © Sault College is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.