Volume versus pressure control
In this video, you'll learn why volume control is the initial breath type of ventilation.
After selecting the ventilator mode for your patient, you need to be able to answer this question: should you select volume ventilation or pressure ventilation? In this video from our Mechanical Ventilation Essentials course, you'll learn why volume control is the initial breath type of ventilation and how to manage this in the clinical environment.
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[00:00:00] Now that we understand that the assist-control mode is the most common ventilator mode, as the primary initial mode used throughout the world, we need to discuss and answer a very important question. Why should we select volume ventilation instead of pressure ventilation? In other words, why should we inflate the lung with the same amount of volume, from breath to breath, rather than inflating the lung to a set pressure? Remember, in the previous lesson, we looked at a breath being controlled by a set
[00:00:30] tidal volume. In this case, the tidal volume was 500 mL. But is choosing a controlled volume to inflate the lung actually the best initial method of choice? In other words, can we inflate the lung to a controlled pressure instead of a controlled volume? Now, how to inflate the lung initially is an important question and choosing a controlled volume, as the initial mode of inflation, may not necessarily be what we're naturally used to, especially
[00:01:00] when you consider the way millions of people every day inflate a tire. Now, think about that. When you inflate a tire, you do not inflate with a fixed volume. Actually, when inflating a tire, the manufacturer recommends filling the tire until it reaches a safe pressure. And if we also have the option to choose to inflate the lungs to a set pressure, well then why not initially choose pressure ventilation instead of volume ventilation? Now, do you see why
[00:01:30] this was such a great question? And the truth is, there will be some patients who may benefit more from pressure ventilation, which we will definitely discuss later when we discuss how to modify our initial settings. But there are many more who will benefit from volume ventilation. So for now, let's look at why volume ventilation, is the preferred initial ventilation type. In other words, let's look at the advantages that volume ventilation offers over pressure ventilation, that makes it
[00:02:00] preferable as the initial type of ventilation. And the reason for that is minute ventilation. Minute ventilation is the result of the tidal volume, times the respiratory rate and it is expressed as VT times RR. Sometimes, it is expressed as VT times f, where f represents the frequency or the number of breaths in a minute, which is the same as respiratory rate. These expressions are
[00:02:30] often used interchangeably but minute ventilation represents the amount of volume used to ventilate or remove CO2 in one minute. So, that means this, in order to help ensure that we meet the patient's metabolic demand, which is directly related to how much CO2 should be exhaled. then we need to ensure that the patient is receiving adequate minute ventilation. If we do not maintain an adequate minute ventilation, this can lead to further
[00:03:00] respiratory complications. So, how do we know how much minute ventilation a patient needs? Well, we can calculate the amount of minute ventilation required for a patient based on several factors such as an individual's gender and body surface area. For example, we can estimate that the minute ventilation requirement for an adult male who is six-feet tall and weighs 85 kilograms would be about 8.4 L / min.
[00:03:30] So, now watch this, just hang in there. If I know that my patient has a minute ventilation requirement of 8.4, based on their gender and height. And let's say, for example, I wanted for them to breathe 14 breaths per minute, well then, I would know that my patient will require a tidal volume of 600 mL to meet the need of the patient's minute ventilation because minute ventilation is volume times rate.
[00:04:00] In this case, 600 mL, which is 0.6 L times 14 breaths a minute and this will equal 8.4 L / min, therefore, ensuring and setting a consistent minute ventilation is only possible in volume ventilation. It is not possible in pressure control ventilation. So, here's what I mean. Let's use the same patient with the same minute ventilation requirement of 8.4 L / min,
[00:04:30] but now, we'll use pressure control ventilation and the respiratory rate will still be set to 14. So, here's the question. With how much pressure should you inflate the lung? Well, it's actually impossible to know that. You cannot know if you should inflate the lungs to a pressure of 20 or whether you should inflate the lungs to a pressure of 30. Minute ventilation is not pressure times respiratory rate, it's volume times respiratory rate.
[00:05:00] And although pressure ventilation can help protect the lung from being overinflated, it cannot ensure that the patient is receiving adequate and consistent minute ventilation. Now, remember, this doesn't mean that you can never switch to pressure ventilation. You can, and some patients will benefit from it. And we'll discuss this in detail later but for now, in terms of the initial setup, the assist control with volume ventilation
[00:05:30] is the preferred initial mode of ventilation.