The Vitals: Control Modes of Mechanical Ventilation: Part 2

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The Pre-brief

In our last post, we broke down mechanical ventilation into simpler terms. The respiratory rate determines when a breath is given; the length of the breath is determined by the inspiratory time; and the breath is delivered by either a pressure controlled mode or a volume controlled mode. We will be exploring these modes in this post.

Let’s talk about pressure first. In Pressure Control Ventilation (PCV) the priority is pressure. The machine pushes gas into the patient with a given driving pressure (we will discuss driving pressure in a separate post). This pressure is set by the operator. Because of the concept of compliance (change in volume for a given change in pressure) this pre-set driving pressure will translate to different tidal volumes in different patients. If a patient has stiff, fibrotic lungs, it will take a much higher pressure to produce the same tidal volume than it would for a patient with healthy, more compliant lungs. (If you have blown up a few balloons in your life, you have probably experienced this:  some balloons are much easier because they are much more compliant.) The PCV take-home: you deliver a breath via a constant pressure with the ability to maintain safe airway pressures at the expense of a guaranteed tidal volume (or minute ventilation, since MV=TVxRR).



Easy-peasy…right?

Let’s move on to volume. I have to apologize because there is one slight nuance to Volume Control Ventilation (VCV). In VCV, tidal volume/minute ventilation is the priority. The operator sets the tidal volume he or she wants the machine to deliver to the patient. (Remember, in PCV the ventilation priority is pressure: the operator sets the driving pressure, and the breath is delivered with this set pressure.)  Here is where the nuance of VCV comes into play: in VCV, the ventilation priority is volume, the operator sets the volume, and the breath is delivered by…flow rate.

(Now you have lost trust in me. My whole soap box is making things simple and I’ve gone and mucked everything up with this last sentence. But let me explain. I’ll make it up to you. You’ll trust me again. I promise.) 

Remember, the machine will do whatever it is you tell it to do. You already told it how long it has for inspiration. Once you tell it how much gas you want it to move, it does a simple calculation. Think of a middle school math question:  If a train has to travel 500 miles and it has 2 hours to reach the destination, how fast does the train have to travel?  It’s the same thing. If a machine has to deliver a volume of gas of 500 mL in 2 seconds, how fast is the gas flow rate?  Flow rate is directly related to tidal volume because of the set inspiratory time.

(See, we’re friends again. I told you it wasn’t so bad.)

The VCV take-home: you deliver a breath with a constant flow rate in order to guarantee a given tidal volume and minute ventilation at the possible expense of safe airway pressures.

Regardless of the mode, expiration occurs at the end of inspiration passively down the pressure gradient. The next breath will be given at the predetermined time, regardless of whether expiration is complete.

And that’s it. Now you understand controlled modes of ventilation. Next up, we will talk about how ventilators become smarter and begin to interact with patients to create mixed and support modes of ventilation. Finally, we will talk about the more advanced modes of ventilation. Once you have all this, you’ll be able to break down ventilator waveforms and troubleshoot the machine without us saying a word. But we will probably have something to say about it anyway…

Is there a vent mode that confuses the dickens out of you? Let us know so that we can be sure to cover it!


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