“We need more minute volume so we turned up the rate. Don’t turn down the rate!” – RT
This one has happened to me more than a few times. We arrive at the bedside to some patient in some form of shock or ARDS that just is not behaving well for the ICU they are in and they need to be sent out. They can be found triple max pressed and the ever important bicarb drip is hanging (not always, but it is common to see), and perhaps some antibiotic hanging as well. Operationally it is complicated. Clinically it is complicated… The trick is to not make it worse.
Patient Background:
For this case, the patient is a male in his 20’s, IBW 75kg, in the ICU with aspiration pneumonia. He was found unconscious by his family after a “fall” (it is suspected that the patient suffered a syncopal episode). Vomit was noted in his mouth on scene. His head CT showed no ICH, but his CXR did reveal bilateral opacities consistent with aspiration pneumonia. UA was positive for cocaine and THC (perhaps explaining the source of the fall/syncope).
His clinical course has been complicated. He was intubated for altered mental status and airway protection in the field, and predictably he developed ARDS secondary to aspiration of gastric contents. In the ICU several attempts to optimize his oxygenation were made, which included prone positioning, but to no avail. Several ventilation maneuvers were attempted and he became most stable in a pressure control ventilation mode. He was placed on propofol, versed, fentanyl, and nimbex infusions. The nimbex was included because the patient had some difficulty behaving with the ventilator.
The CCT team gets the story and begins changing equipment over to facilitate transport. You begin to tinker with the vent and start setting it up for the transport…
An RT either looks over your shoulder while you are dialing things in or outright comes to you and says:
“Don’t turn down the rate, they need the minute volume…”
You take a look at the vent and see this:
Nope these are not from a pediatric patient…
Then you ask for the gas:
pH: 7.14
PCO2: 88
PO2: 81
HCO3: 30
Stop and Think. We need to zoom out and get a wider view on things.
Do the numbers add up?
Are they receiving the MV that they need?
Is there a deadspace issue?
Why are they not oxygenating despite PEEP/Fio2 titrations?
Maybe We Need To Start All The Way Over and Redo the Math…
1.) Measure the patient and get an IBW (based on gender).
2.) Calculate the needed MV (100-150cc/kg IBW). Take the physiologic picture into account,
3.) Calculate the vT and the RR. (I have become more of a fan of 8cc/kg with lower rates to meet MV demand UNLESS they are in ARDS already at which point I start at 6cc/kg).
4.) PEEP/Fio2 titrations if needed to improve oxygenation.
5.) Wait. Let the patient adjust to the new settings and wait for TRENDS. Blood gasses do not change immediately, the compensatory systems need time to do their thing. Respiratory compensation takes a few minutes once ventilatory action has been optimized. Resist the urge to twist dials just because there was not an immediate improvement. Trust your math, trust your knowledge, and let it work.
The Problem: Dead space loss due to high respiratory rate… and ARDS.
What you think is getting into the lungs and what is actually getting in to the lungs becomes more and more separated the higher and higher the rate gets rolled up. 150mL/breath is what is lost due to dead space, and it only gets worse the higher the more the rate is dialed up. It also gets worse the more attachments we have distal to the Y on the vent tubing. When coupled with low tidal volume strategies the patient can be dangerously under ventilated. The worse an acidosis on a gas looks, the more a clinician can be tempted to “compensate” by increasing the rate. Then it gets a little worse, and more and more until they are “failing conventional vent strategies” and must be sent out for an ECMO consult.
This particular patient with these vent settings had exactly ZERO time to exhale. A higher rate does not just lead to more deadspace loss, but it also has implications for the I:E ratio. With the selected rate of 32 breaths a minute, the cycle time does not allow enough time for exhalation. Cycle time is the total amount of time for inhalation and exhalation, calculated by dividing 60sec by the respiratory rate. At a rate of 32 the entire cycle time is 1.5 seconds (half of what it should be), meaning it is physically impossible for the patient to be ventilating.
What did the crew do?
Keep in mind that the patient was in ARDS, and that is their primary problem that had been complicated by this respiratory acidosis. Typically the vent strategy is oxygenation focused at the cost of perfect ventilation (sometimes). In this case, the crew had to protect the ground they had with oxygenation and do so while improving the ventilatory mechanics. The crew elected to use a PCV setting and targeted 6cc/kg for tidal volume and moved the rate down to 18… and they waited…
Time elapsed between images: 15:00
Vitals at drop off.
50:00 from the blood gas results previously mentioned in the scenario.
Things appear to have gotten at least a little better. The pH and PCO2 did improve as did the PO2. Better ventilation and better oxygenation. Which seems counterintuitive if the MV was reduced. The key was better ventilation which lead to better ventilation and gas exchange. Of note, there was also some increase in compliance and decrease in inspiratory resistance, leading to the conclusion that the new vent settings were allowing for some recruitment and better air flow through the diseased lungs. All of which are wins when treating a patient with ARDS.
Close Out
It was cases like this one that broke me from the mindset of just copying what the sending vent has on it and pasting it to mine. It simply is not always appropriate to do so. The expectation of you as a CCT team member is to be a force multiplier and be able to see when things need to be changed and/or optimized for the patient’s benefit. Are there maybe some contextual issues you will have to navigate in order to get this done? Of course, but you must advocate for and do what is best for the patient.
Do your own work with the ventilator. It may seem easy or expedient to copy what the sending facility has done and paste it to your vent for transport, but it may not be the most appropriate thing to do. In my experience, by going back and doing my own work I have seen more patients improve in all parameters (hemodynamics, oxygenation, and ventilation) than not.
Stop and think.
