The Volume vs. Pressure Debate
Lately I can't help but notice a sudden momentum in the direction of pressure control (PC) mechanical ventilation. Somewhere along the way volume control (VC) became perceived as a subpar method of mechanical ventilation. There are even ventilators now that are strictly made for pressure (The Hamilton T1). This caused me to do some digging into why the process of delivering a breath based off pressure would be superior to delivering a breath based off a precise volume.
Resistance vs. Volume
When we look at ventilator settings there is typically a trigger to start the breath, and a determined method to stop the breath. Imagine you are blowing into a balloon, you can either inflate the balloon until you meet a desired volume, or you can blow into the balloon until you start to meet a predetermined amount of resistance. The difference may not seem that obvious in a balloon, but when we apply these methods to mechanical ventilation they can be vastly different. It's common knowledge amongst critical care providers that when we ventilate in a pressure mode we need to monitor exhaled tidal volumes (Vte). This is due to the fact that non compliant lungs will cause an earlier resistance to the delivered breath, and subsequently terminate the delivery. If not monitored closely this can result in hypoventilation in a patient with poor compliance. Volume mode is not without its need for astute monitoring as well. When we deliver a pre-determined volume to the same set of non-compliant lungs, we can quickly exceed safe pressures.
So Which One Is Superior?
To this date there, has been no evidence to support decreased mortality with one mode vs the other. The only study, that without doubt, showed improved patient outcomes, was the ARMA trial which was actually terminated early when the data demonstrated lower tidal volumes in the setting of ALI/ARDS led to a significant decrease in-hospital mortality. However, what we lack in evidence is quickly replaced with anecdotes and heuristics to support our preference.
Mode Of Choice In An Obstructive Patient
I recently released a podcast in which I proposed a ventilation strategy for the patient with acute injurious lungs and an underlying obstructive pathology. I purposely did not include the mode in which this strategy would take place because of the heterogeneous views and ventilation capabilities. I received a phone call from a respected provider who wanted me to clarify which mode I suggested be used in this hybrid patient. He sounded astonished to hear that I prefer to utilize volume control in a patient with obstructive pathology. With just recently switching to the Hamilton T1, it was no doubt he was of the pressure control gestalt. He brought up some fantastic points in which I would like to address:
"Quicker Time To Pplat In Pressure Mode"
My friend went on to explain that in a pressure mode he is able to adjust his Rise Time (time to plateau) in order to give the breath quicker and not compromise inspiratory flow rate which we know can be directly correlated to patient comfort. The idea is that by decreasing the inspiratory time we subsequently increase the exhalation time.
Here are my thoughts..
The idea of delivering a breath with a decreased inspiratory time can easily be done by decreasing your "I Time" on the vent when in a volume mode. If one wanted to remain at plateau for a predetermined amount of time to perform a recruitment maneuver, they could utilize the "Inspiratory Pause" function in VC. So I can basically do the exact same thing in a VC mode. So why do I prefer VC?
In order to actually ventilate a patient with an obstructive pathology, you will need to vastly increase your peak airway pressure if you are using a pressure control mode. The pressures needed to not only ventilate, but also get that breath in quickly to not compromise IFR, will be extremely high. As the patient improves you will slowly require more volume to meet those same high pressures that were needed at first. This can quickly result in unsafe volumes if not readily recognized & titrated. If using a volume mode the peak pressures will no doubt be elevated in the acute phase of obstructive ventilation (as they would in pressure mode), but as the patient improves with pharmaceutical therapy, the volumes will remain at the same safe range and peak pressures will come back down to safe parameters.
When titrating the "sweet spot" for PEEP in a patient with decreased parenchymal elasticity, one can usually assume that if they are adding extrinsic PEEP and seeing an increase in peak inspiratory airway pressure, they are contributing to auto PEEP and making the situation worse. This titration process can only be done in a mode where the peak inspiratory pressure is an independent variable, such as in VC mode. One could argue that you could utilize the same method in PC mode by watching your Vte. If extrinsic PEEP is added and the Vte decreases, you have contributed to auto PEEP. The problem I find with this technique is that the exhaled volume provides a looser and somewhat difficult guide for breath to breath titration.
CLEVELAND CLINIC JOURNAL OF MEDICINE VOLUME 72 • NUMBER 9 SEPTEMBER 2005
Ventilation is like religion to some providers, and I will never discount the idea that one should be familiar with as many options as possible for the patients we encounter. However, at this time I have not reasonably concluded as to why we see a sudden push towards pressure control ventilation.
What about PRVC? Isn't that like the best of both worlds?