As we covered in a previous article, the assist-control (AC) mode
is the most common primary initial ventilator mode used throughout the world. Still, we need to discuss and answer an essential 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, we previously looked at a breath being controlled by a set tidal volume (VT). In this case, the tidal volume was 500 mL. But is choosing a controlled volume to inflate the lung the best initial method of choice? In other words, can we inflate the lung to a controlled pressure instead of a controlled volume?
Can I use controlled pressure instead of controlled volume?
How to inflate the lung initially—pressure or volume—is an important question. Choosing to use a controlled volume as the initial mode of inflation may not seem like the natural choice, especially when you consider the way millions of people inflate a tire every day. Now, take a moment to think about that. When you inflate a tire, you do not inflate with a fixed volume. Instead, tire manufacturers recommend you fill 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, then why do we not initially choose pressure ventilation instead of volume ventilation. So, do you see why this was such a great question?
The truth is, there will be some patients who may benefit more from pressure ventilation—which we’ll take a closer look at when we discuss how to modify our initial settings—but generally speaking, there are many more who will benefit from volume ventilation. So, for now, let’s look at why volume ventilation is the preferred initial type by considering its advantages when compared to pressure ventilation.
Why is volume ventilation preferred over pressure ventilation?
The main reason volume ventilation is preferred over pressure ventilation is because of minute ventilation. Minute ventilation is the result of the tidal volume (VT) times the respiratory rate (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 often used interchangeably, but either way, minute ventilation represents the amount of volume used to ventilate or remove CO2 in one minute. So that means, to help ensure we meet the patient’s metabolic demand—which is directly related to how much CO2 should be expelled—then we need to ensure that the patient is receiving adequate minute ventilation. If we do not maintain adequate minute ventilation, we can cause further respiratory complications.
How much minute ventilation is needed?
So, how do we know how much minute ventilation a patient needs? Well, we can calculate the amount required 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 liters per minute.
And then, let’s say, for example, I wanted for them to breathe 14 breaths per minute. Well, then, because minute ventilation is volume times rate, I would know that my patient would require a tidal volume of 600 mL to meet their minute ventilation need. In this case, 600 milliliters, which is 0.6 liters times 14 breaths a minute, equals 8.4 liters per minute. Pressure doesn’t even factor in. Therefore, ensuring and setting a consistent minute ventilation is only possible in volume ventilation.
So, let’s test that out. Let’s use the same patient with the same minute ventilation requirement of 8.4 liters per minute. The respiratory rate will still be set to 14 but now we’ll use pressure-controlled ventilation. So, with how much pressure should you inflate the lung?
Well, turns out it’s a trick question because it’s impossible to know if you should inflate the lungs to a pressure of 20 or to a pressure of 30. Minute ventilation is not pressure times the respiratory rate; it’s volume times respiratory rate. And although pressure ventilation can help protect the lungs 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’t utilize pressure ventilation effectively. You can, and as we’ll cover in detail later, some patients will benefit from it. But for now, in terms of the initial setup, the assist-control with volume ventilation is the preferred initial mode of ventilation.
That’s it for now. If you want to improve your understanding of key concepts in medicine, and improve your clinical skills, make sure to register for a free trial account, which will give you access to free videos and downloads. We’ll help you make the right decisions for yourself and your patients.
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- Mechanical ventilation protocol summary. NIH-NHLBI ARDS Clinical Network. http://www.ardsnet.org