The Relationship Between Pressure and Volume

So, we know that pressure control and volume control are essentially the same concept explained in different ways—air being pushed into the lungs. Let’s explore this concept further to explain why this is true.

When air is mechanically delivered into lungs via a positive pressure breath, two things occur at the same time inside the lungs:

• The volume of air inside the lungs increases
• The pressure inside the lungs increases.

These effects happen because, with a positive pressure breath, it is a closed system with the patient connected to the ventilator through an endotracheal tube. The air has nowhere else to go but to inflate the lungs. In a sealed system, volume and pressure go hand in hand and have a direct, linear relationship. As one increases, the other increases as well.

As we introduce this concept, we are going to think of the lungs as an unchanging environment. We know that lungs can be damaged—due to illness or other factors, which can affect how easily lungs inflate, but let’s talk about a short-term situation where the lungs themselves are not changing. We are going to look at how the ventilator settings impact each other if inflating the same set of lungs, using the following object lesson.

Object Lesson

Let’s go back to that analogy of lungs as balloons to understand this concept fully. Imagine you have two identical balloons side by side. If you blew for 1 second at a soft pressure into Balloon 1 and then blew for 1 second at a hard pressure into Balloon 2, which balloon do you think would have a larger volume in it at the end? When you think of balloons, it makes it easy to see that if you blow harder, there will be more air (or volume) in Balloon 2.

Let’s look at this another way, using our two identical balloons again. If you have the same amount of time to blow up both balloons, but Balloon 2 you only want to blow up to half the size of Balloon 1, which balloon do you think you will have to blow harder (more pressure) to achieve the volume you want in the time given? If balloon 2 only needs to get blown up half the amount, if blowing for the same length of time, you would blow much softer (less pressure)

The correlation  between volume and pressure is a very important principle to understand when dealing with ventilation, as it is critical to always monitor the opposite parameter (either volume or pressure) being experienced by the lungs when you do not set them. This is because the opposite variable (pressure or volume) will be affected by the volume or pressure that you set. To explain, when in a Volume Control mode, you do not set pressure directly. Instead, you set volume and the pressure changes as a direct outcome based on the volume that is set by the medical provider. In pressure control, you do not set the volume directly, but the volume changes as a direct outcome based on what pressure is set by the medical provider. Remember talking about how lungs can be damaged by high pressures or volumes? Monitoring the opposite parameter to make sure they do not go too high is essential to ensure safe ventilating practices.