6.6 A Word of Caution on PSV
There are two “flaws” in pressure support ventilation that may lead to difficulty weaning patients. These flaws exist in how the breath is triggered, or the “on-switch”, and in the “off-switch” for the pressure delivery.
Once the ventilator senses a patient-triggered breath, the pressure support turns on and pushes air into the lungs at the set “help” pressure. The trigger is usually a set threshold of flow that patient must pull in to trigger a breath. A common flow trigger is anywhere from [latex]2\text{ - }5\text{ Lpm}[/latex]. When you break that amount of flow down to the fraction of a second it takes for the ventilator to sense a breath, it is essentially mere milliliters of air that the patient needs to pull in. Very little effort is needed to cause the pressure to start delivering.
This pressure then continues to be delivered until the “off-switch” is activated to tell the ventilator that the patient is done breathing. Most variations of PSV use a flow decay percentage to trigger the ventilator to stop delivering pressure. This means that the ventilator must sense a decrease in the air being delivered to the lungs. Normally, this decrease occurs naturally as the diaphragm is at its lowest point and the lungs are filling. As they fill, the air the patient is pulling in naturally slows down. This slowdown would stop the pressure and then the lungs would passively exhale. Remember, pressure delivered to a closed system always equals volume delivered.
Because of this fact, there are some instances where patients are able to “ride” PSV like a control mode. They trigger the vent with a small pull to start the pressure support but then allow the vent to deliver the air with the set pressure “help” without needing to pull any air themselves. The pressure will continue to push air in—similar to a controlled breath—until that flow decay is reached. Because the lungs will spontaneously decrease in compliance as they fill, this means that at the top of filling, the compliance pushes back to the point that it will naturally slow the air coming in. This triggers the expiratory phase on its own without the patient driving the breath. Essentially, the patient only has to satisfy a small inspiratory trigger or mere milliliters of air to essentially get “free pressure” until their lungs fill enough to terminate the breath on their own. High pressure supports are more likely to cause this scenario.
Object Lesson
Think of this scenario like a continuous motion ball pendulum, commonly called a Newton’s Cradle. With very little effort at the beginning, a movement is generated that continues with no effort.
One easy way to decrease the risk of pressure support acting like a control mode is to minimize the support pressure to ensure over-support is not possible. If the pressure support is kept on the lower side, then the patient will not be able to get an adequate tidal volume by passively allowing the pressure to push air into their lungs. Then, they will have to actively pull air in themselves to achieve the tidal volume their neurological drive to breathe demands. You can target minimum pressure support by ensuring the patient’s [latex]RR[/latex] is not too low (less than 12) and that the volumes they are pulling in are within their normal range (no more than [latex]8\text{ mL/Kg}[/latex] of IBW).
“A Word of Caution” from Basic Principles of Mechanical Ventilation by Melody Bishop, © Sault College is licensed under a Creative Commons Attribution-NonCommercial-ShareAlike 4.0 International License, except where otherwise noted.
