Ventilator Basics

For some reason, ventilators is a subject that terrorizes medical students when they spend time in the ICU. While there are, in fact, many complex and difficult concepts involved in advanced ventilator management, the basics are easy to understand and to implement. We'll cover enough here to get you started and to help you understand why and how basic ventilation decisions are made. An excellent resource for learning more is The ICU Book by Paul Marino.


Lung Terms

The primary job of the lung is to take oxygen (O2) from the air and get it into the bloodstream, while at the same time removing carbon dioxide (CO2) from the bloodstream and expelling it into the air.

There are only a few factors that can affect how well this transfer of gases takes place:

With just these basics, you'll be able to make intelligent decisions regarding ventilators. If CO2 is too high, you'll increase the rate or the volume. If O2 is too low, you'll increase FiO2 or increase the PEEP (see below). [Now you know better than to suggest a rate increase for a low O2, right?]


In normal breathing, the force that moves air into the lungs is negative pressure. The diaphragm pulls down, creating low pressure inside the thorax and the lungs, and this draws air in (the iron lung used to work the same way). With a ventilator, the force used to move the air is positive pressure, supplied from a pump outside the body, pushing air into the lungs. Look at the following idealized waveform, representing the pressure the ventilator produces over time. X axis is time, Y is pressure. The dotted line represents 0 (atmospheric) pressure.

"Inspiration" begins with the sharp upslope and ends after the higher of the two horizontal lines. "Expiration" begins on the downslope after that plateau, and continues until the end of the lower of the two horizontal lines - when inspiration begins again. We put them in quotes to remind you that they're different from normal breathing, but we'll forget the quotes from now on, since you're starting to think like a ventilator.

In this example, the length of time spent on inspiration is about equal to the amount of time spent on expiration. This Inspiratory to Expiratory Ratio (I:E) is, therefore, 1:1. This ratio is one of the variables you can change on the ventilator. Typically, you'd begin with an I:E of 1:2. For people with COPD who have lost some of the elasticity of their lungs, you might start with 1:3 or even 1:4 in order to allow their lungs time to relax before you push in another breath.

Important terms that come up all the time are PIP, PEEP, and Plateau Pressure. All pressures are given in cm of H20.


Modes of Ventilation

The three primary modes you'll need to know are Assist Control (AC), Synchronous Intermittent Mandatory Ventilation (SIMV), and Pressure Support Ventilation (PSV). There are about 10 other possible modes, but we'll save those for another day.

Assist Control is the first setting you will use when you ventilate a patient. It's used for patients who don't (or can't) try to breathe at all on their own. This includes patients you've just paralyzed in order to place an ET (endotracheal) tube - they won't breathe at all, so you breathe for them.
      In assist control, you start by setting the rate, I:E, PEEP, FiO2, and volume. Classic settings are:

AC mode completely ignores a number of factors. If the patient starts to initiate breaths, the machine ignores the patient - it's going to push its 410 ml's 12 times a minute, regardless of what else is going on. You have little to no control as to how much force is being used to accomplish that volume - you can easily apply too much pressure and cause barotrauma (damage to lung tissue from too much pressure). Finally, it does nothing to encourage the patient to breathe on their own, so you can't "wean" (move a patient off the respirator) a patient from this mode.

Synchronous Intermittent Mandatory Ventilation is typically the mode of choice for patients who can initiate breaths, but don't have the strength to maintain a full respiratory effort. In this mode, the ventilator waits and watches for the patient to initiate a breath (it looks for negative, or sucking, pressure) - once the patient starts the breath, the ventilator does the rest of the work. The advantage of SIMV over AC is that it works with the patient, and encourages them to be "responsible" for their breathing, since they need to initiate each breath.

Of course, you'll need to set a backup rate. If for some reason the patient stops initiating breaths, or starts breathing at a rate lower than you'd like, you want the ventilator to kick in and help out. You can set a maximum interval (let's use 5 seconds as an example). If at any time 5 seconds have elapsed since the last breath, the ventilator starts a breath automatically. If all of the patient's breaths occur within 5 seconds of each other, this fallback setting is never utilized, since the "timer" will keep re-setting.

So, with the backup in place, you can see that if a patient isn't initiating any breaths at all, the rate will be every 5 seconds, or 12 times a minute. In this situation (with no effort by the patient), SIMV is functioning identically to AC. When the patient is doing some work, however, SIMV has the advantage of working with the patient, and pushing air only when the patient is receptive to it. It pushes during the inhalation phase of respiration only (it's synchronous, after all), while AC can be pushing at any point in a cycle - which might include a time when the patient is exhaling. This asynchronous pushing would cause two problems - first of all, it would be pushing air in against lungs that were actively pushing out, causing large pressures in the entire pulmonary tree. Secondly, the volume of air already in the lungs wouldn't be allowed to escape, and a completely new volume of air would be added to the existing air - this could potentially double the volume of air in the lungs, which defeats the purpose of setting a volume in the first place. This can obviously lead to not only patients "fighting the vent" and becoming anxious and agitated, but it can also lead to significant barotrauma.

SIMV is one of the settings on which patients are "weaned" from the vent - this simply means that their reliance on the ventilator is reduced until such time as they can do all their breathing by themselves. Settings for SIMV are calculated identically to those for AC.

Pressure Support Ventilation  is the final term with which you'll need to be familiar. Up until now, we've been dialing in a volume of air that we want, and not worrying about the pressure. In PSV, as the name suggests, all we worry about is pressure. The idea is similar to SIMV in that the ventilator watches for an inspiratory effort (you can also set a backup rate as we did with AC), but in this case it responds by providing pressure to aid with that inspiration. The pressure continues until the ventilator senses an expiratory effort (it watches for an increase in pressure to a certain level), at which time the pressure stops. 

The new settings you need to think about with PSV are pressure and sensitivity. Pressure should almost always be <35 - 20 would be a more typical setting, although many factors come into play (the more pressure, the more volume you end up pushing in -etc.). Sensitivity, which is the amount of pressure that the machine is watching for, is typically 1-2.

This mode requires that the patient be breathing spontaneously. It is becoming a more popular choice as the initial mode used in patients who can breathe on their own but are developing respiratory failure. Patients are also often weaned using this setting.

Because PSV requires a patient who can breathe spontaneously, and we only provide support, it is considered a Support Mode. AC is considered a Control Mode, since we actually control the parameters of the patient's breathing. SIMV is in-between, and can be considered as either. (Technically, control modes are modes in which volume is set, and support modes are modes in which pressure is set, but that's harder to remember. If anyone asks, well - now you know.)

Volume Calculations:
Enter a weight in either the Kilograms or the Pounds box (just the number, please):
If you have a specific rate in mind, please enter the number of breaths per minute here (just the number, please):
Minute Volume can be calculated as 7% of weight in Kg. 

Liters per minute is the predicted minute volume in this patient.
This minute volume needs to be divided evenly among the number of breaths per minute to determine a Tidal Volume. 

mL per breath is the predicted tidal volume in this patient at the rate you selected.
The simpler method is to simply calculate a Tidal Volume by using 5-10ml/Kg. 

mL per breath is the predicted tidal volume in this patient using this method, regardless of the rate.