The Time Variable
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 (Itime, IT). IT is most commonly used in Pressure Control ventilation, but since it is more intuitive than other “time” parameters, it is becoming more common in volume control as well. You should already understand that if the pressure of air to a closed system is applied for a longer time (longer IT) this will result in higher volumes.
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 1 second, but then you blew into Balloon 2 with the same force for 3 seconds, Balloon 2 would be more full.
Now, consider this the other way around: If you wanted to hit a specific volume, but Balloon 1 had 1 second to achieve the volume, while Balloon 2 had 3 seconds to get the volume in, which balloon would require a higher pressure to hit that same volume? Balloon 1 needs more force to push the air in faster to hit the volume needed in the shorter time.
In this way, the IT can have a direct impact on many factors of ventilation—impacting the volume delivered, or the peak pressures experienced by the lungs. Here, the clinician will not set a flow rate of the air. The ventilator can change the flow rate to achieve the set pressure and inspiratory time that the clinician has set.
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 Itime) 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
Some modes of ventilation do not utilize Itime (IT). Instead, the time in inspiration is affected by the peak flows that are utilized to deliver the breaths. Most commonly, this is seen with Volume Control Ventilation. The clinician will set the peak flow of air the ventilator will deliver when giving the set volume. How does this information relate to the time needed for inspiration? Returning once again to the balloon analogy, if a volume of 100mls is the overall target, Balloon 1 has a peak flow of 60Lpm vs Balloon 2 has a peak flow of 30Lpm. Obviously, the balloon with the air filling it at a faster rate will achieve the target volume in less time but at a higher pressure. The clinician will not set an Itime here. The ventilator IT will depend on the set parameters of the volume and flowrate of the air.
