The Pre-brief
Mechanical ventilation is an example of a topic that is often made unnecessarily complex. This is compounded by the proprietary nature of mechanical ventilators, each company describes the same functions using different terms. In the end, how you talk about mechanical ventilation is influenced by the specific company’s machine on which you were raised or how those that taught you were raised. Think soda, pop, sprinkles, Jimmies, hoagie, hero, etc. All the same things. But also quite different things. It’s even worse in the world of mechanical ventilation than it is for regional differences in food names: limit, cycle, trigger, control. Not only do there exist different words for the same things, but we have also allowed a system that uses the same word to mean different things; we are endlessly confused as a result. Each time you think you have a concept mastered, someone uses a different term, and you are back to square one feeling lost and confused.
We have a solution. Let’s forget the semantics and break down mechanical ventilation into its basic parts. For this exercise, forget about the ventilators you use every day which are highly sophisticated machines that can interact with the patient in a myriad of ways. Instead, imagine a very simple machine. This simple machine does not interact with the patient at all. It only delivers a breath based on parameters that you, the operator set, no more and no less. What happens with that breath is of no concern to the machine. What the patient can or can’t do is of no concern to the ventilator. The operator, through the machine, is in full control. We call this control mode.
In a control mode, the ventilator asks for only limited input:
- When do I give a breath?
- How long is the breath?
- How do I actually deliver the breath?
Let’s break down each of these answers:
When do I give a breath?
The machine gives a breath because it is time to give a breath. You set the respiratory rate. Based on this rate, the machine knows that after “x” seconds, it’s time to give another breath. For example, if the respiratory rate is set at 15 breaths per minute, the machine will deliver a breath every 4 seconds.
How long is the breath?
You set the inspiratory time. Often this is done indirectly by setting the Inspiratory:Expiratory (I:E) ratio. If the machine knows the respiratory rate, it knows how long each respiratory cycle is. For example, if the respiratory rate is 10 breaths per minute, each respiratory cycle is 6 seconds. If the I:E ratio is 1:2 (a standard value), it knows that it will spend twice as much time in expiration as it will in inspiration. In this example, it follows that the inspiratory time is 2 seconds and the expiratory time is 4 seconds.
How do I deliver the breath?
You have two choices. The choice depends on your priorities for this specific patient. The common choices are:
- Pressure
- Volume
In the next post, we will cover each of these modes of ventilation.
References
1. Title image by NASA/JPL-Caltech via Wikimedia Commons
