Mechanical ventilation can be a daunting subject for people to tackle. The sheer volume of information about the subject can put a lot of pressure on a student.
See what I did there? Volume? Pressure? I am _that_ good.
But getting a basic grip on mechanical ventilation can be troubling. That’s trouble and that starts with “T” and that rhymes with “P” and that stands for Proprietary naming conventions.
While all ventilators do basically the same things and have the same set of features, there is no agreed upon industry naming conventions or taxonomies that have been standardized for simplification. Instead, the opposite has happened. Vendors, driven by sales quotas and stockholder greed, fancify their vents with special modal aliases so now we are far more likely to push the wrong button and make our patients explode.
We are left with this unnecessary translative layer of abstraction between the basics of mechanical ventilation and the novelty nomenclatures on the knobs causing pneumos.
So to start, ignore the vents themselves and learn the basic modes and functions of mechanical ventilation as a science rather than as a specific machine. Then, once comfortable with that, you can translate that to apply it to a variety of machines later.
Starting with a sedated, paralyzed patient who has no spontaneous breathing we generally have two choices: Volume control or Pressure Control.
‘Control’ comes in two forms: Controlled Mechanical Ventilation (CMV) and Assist-Control Ventilation (ACV). ACV allows patients assisted spontaneous breaths. CMV does not. The only cases I have seen where CMV was used were in high spine injuries where the phrenic nerve had been severed and there was no chance of spontaneous breath, and in late-stage neuromuscular diseases such as ALS or Muscular Dystrophy. In other cases ACV is commonly used, giving the patient a soupçon of respiratory autonomy should they wake from their sedation/paralysis.
In both pressure and volume control ventilation, assisted or not, you set an FiO2 (fraction of inspired oxygen), anywhere from room air (21%) to pure oxygen (100%) depending on the patients needs. You also set the Positive End Expiratory Pressure (PEEP). This back pressure sets the delta between the alveolar pressure and atmospheric pressure and it helps prevent alveolar collapse, aka atelectasis, at the end of expiration. Generally, all mechanically ventilated patients start at 5 cm H2O. However, setting ‘optimal’ PEEP, the PEEP at which there is best oxygenation without any cardiovascular impediment, is a whole field of study in and of itself. I’ll write about that in later posts. Lastly, you set the breaths per minute (frequency), which again, depending on the patient and disease state, usually starts at anywhere from 10-16 breaths per minute.
Then, for volume control, you set the tidal volume: the volume of air per breath. Usually somewhere around 6-8 ml per kg of ideal body weight landing somewhere between 400-600 ml per breath. then, we titrate the volume down if the pressures are too high.
For pressure control, you set the peak pressure in cm of H2O, usually starting in a safe range, around 20 cm H2O and titrating from there to achieve a desired volume. Obviously the more pressure, the more volume.
So essentially ventilating with these two different control variables achieves the same result but in volume control we have to titrate to manage the pressures and in pressure control we titrate to manage the volumes.
The I:E ration in mechanical ventilation is the ration between the inspiratory and expiratory times. If you have a frequency set to 15 breaths per minute, then you have 4 seconds per breathing cycle. The percent of that cycle that is inspiratory and the percent that is expiratory is up to the technician. In general, again as always depending on the patient and disease state, we start with a 1:2 ratio. That means, with a 4 second total cycle, 1.33 seconds of inspiration and 2.67 seconds of expiration.
In volume control, on most ventilators, you also choose the I:E ratio directly. The shorter the inspiratory time, the higher the flow will be to achieve the set volume.
In pressure control you adjust the flow rate. The higher the flow, the faster the cycle pressure is reached, the shorter the inspiratory time. So here, flow is what determines the I:E ratio and this must be managed indirectly.
But Wait… There’s More
There’s always more. For patients who are spontaneously breathing there is Synchronized Intermittent MandatoryVentilation (SIMV) or Pressure Support (PS). Different vendors have different forms of ventilation that are closed loop dual-control and adjust the volume and frequency based on the pressures to achieve a constant minute ventilation. For the Hamilton G5 this is called Adaptive Support Ventilation (ASV) and Drager basically the same thing but with a different name because, I dunno, some CEO read Ayn Rand as a teenager and decided empathy was a character flaw.
Mechanical ventilation is a gigantic topic and it’s application varies from patient to patient and disease state to disease state. But to get started, you must understand the basics of pressure and volume controlled ventilation. Hope this helped.