How does a standard mechanical ventilator — a breathing machine — work?

December 2, 2009  |  General

Often children in the PICU need a mechanical ventilator — a breathing machine — so parents are confronted with this machine. When do we use them and how do they work?

There are three main reasons for using a ventilator: 1.) the child’s lungs are not working well, not getting needed oxygen into the body and carbon dioxide waste out (severe pneumonia is a good example); 2.) the child is not awake and aware enough to breathe and cough properly on his own (a head injury is an example); 3.) the child’s lungs and level of awareness are fine, but he is too weak to breath properly (various muscle problems are examples).

There are many different kinds of ventilators made by different companies, and superficially they may look very different from one another. They have an array of dials and flashing indicators on them, and most draw complicated graphs on a screen as the ventilator works. But in spite of this variability, at root all common ventilators are alike (there are exceptions — special machines we use in special situations). They give a child a breath of air down through a breathing tube, called an endotracheal tube, which we place through a child’s mouth or nose. That air nearly always has extra oxygen in it. We express this additional oxygen as a percent — ordinary room air is 21% oxygen (most of the rest is nitrogen) and pure oxygen is 100%.

We use all the knobs on the machine to set what kind of breaths we want — how big and how often. We also tell the machine what to do if the child takes a breath on her own, usually telling it to help the child in one of several ways with her spontaneous breathing attempts. Finally, we tell the machine what to do between breaths, typically to maintain some air pressure in the system (called PEEP, for positive end-expiratory pressure).

The whole point of using a ventilator is to take over the job of breathing from the child. The machine can do it all, or it can assist the child while she does some of the work. A mechanical ventilator is not an all or nothing device. After the child has healed and no longer needs the ventilator, we can progressively turn down the settings on the machine, in this way asking the child to take over her own breathing more and more. When she can do all the work herself, we pull out the breathing tube.

These principles are straightforward. But if your child, or one you know, needs a ventilator machine, you should know that managing a ventilator is a fine art — it can’t be done by cookbook. We have general guidelines we follow, but most pediatric intensivists have their own personal wrinkles in how they apply the guidelines, using what has worked for them over the years. Most importantly, each child is different and reacts to the ventilator differently, so its use needs to be tailored to each child.

Mechanical ventilators in the PICU are an excellent example of something that appears, on the surface, to be high-tech modern medicine in action. Using one successfully, though, takes some low-tech skills of the sort good physicians have used for generations.

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