volume vs pressure ventilation

Pediatric Critical Care Columnist · Jul 24, 2011

The main reason for the use of different ventilation strategies is the different physiologic needs of the patients. In NICU many of the babies have stiff, non-compliant, surfactant-poor lungs. You want to minimize volutrauma and barotrauma in these kids to try and protect their lungs for the future. If their lungs weren't stiff and they were breathing spontaneously their volumes would naturally fluctuate the same way they do on pressure-control ventilation.

In the CICU you're balancing cardiovascular and respiratory physiology much more. There are similar concerns about volutrauma and barotrauma, but there's also the cardiac output component, which will be negatively affected by high intrathoracic pressures. Hearts that have been handled tend to be cranky. They don't contract with the same force as untouched ones, and they need a lot of room to swell. So any change in pressure exerted on that heart can cause problems with perfusion. If the child's sternum is open there's much less constraint on the lungs and voutrauma becomes an issue at much lower pressures. Another issue that affects ventilation in CICU is the alterations in circulation that occur in conjunction with the corrective surgery done. Where before the lungs may not have gotten enough blood flow, maybe now they're getting too much and pulmonary edema creeps in. Now you've got less effective surface area for gas exchange and will need a higher PEEP to expose more alveoli to the FiO2. Alternatively, if there was pulmonary overcirculation pre-surgery and now it's "normal" the pulmonary vasculature will have been highly sensitive to blood pressure and clamped down to reduce the blood flow but haven't gotten the message that it can relax now. In that case, high PEEP is counterproductive. Is this making sense?

PRVC is an effective mode that my unit doesn't use much but the unit where I worked before used it all the time. It's similar to the volume guarantee mode found on Drager's older ventilators and provides some safeguards. By controlling both volume and pressure, the ventilator acts as an early warning system of worsening lung pathology. If the set volumes cannot be delivered within the desired pressure range, the ventilator will tell you with high-pressure alarms.

HFOV isn't pulled into the orificenal simply on the basis of rate. It's very effective for CO2 clearance in patients who are hypercarbic despite adequate volumes, high PEEP and high rate and for patients whose oxygenation index is low despite high FiO2, adequate volumes and high rate. Do you understand how HFOV works? There's a great thread on it, https://allnurses.com/picu-nursing-pediatric/oscillator-vent-203931.html if you'd like a bit better background.

Extubating from a rate is rarely done on my unit. We get them down to a rate of 6 for several hours, watch for tachypnea, hypoxia, hypercarbia, fatigue and signs of distress, then put them on PSV for a while. If they're looking good on PSV and have a good ABG then we'll extubate. The typical progression is PSV at 0600, ABG at 0800, extubate at 1000 after rounds. The reason for all of this is that we use a lot of sedation and analgesia on these kids and need to be sure that they're actually GOING to breathe once the irritant of the tube is gone. We have very few failures with this schedule. My experience with NICU suggests a much lower sedation factor and a reliance on assessment of respiratory drive in response to CO2 (breathing above a rate of 18 in a neonate is a good indication that they have a normal CO2 response) so extubation from a rate of 18 will work.

Has this helped?

umcRN, BSN, RN · Jul 24, 2011

thanks Jan! I always know i can count on some good info from you. Understanding the reason why certain ventilation modes are used helps alot, especially puting it in relation to the pathophysiology of the situation. That was what I was having trouble with and my preceptor, who started in the cicu the same year I started in the nicu (both of us as new grads 2 yrs ago), couldn't seem to understand what I was trying to figure out and having trouble with.

I do understand a bit more about the HFOV than I let on, I only thought of that as an afterthought when one of the docs had made a comment to me about in the nicu going to hfov at a high rate when they go earlier, after I had asked about why the rate was weaned down so low prior to extubation. And yes, they also did a pressure support trial before pulling the tube as well. I had actually thought prior to my transfer that HFOV wasn't used that often in the CICU because of the increased stress it can place on the heart is that not the case?

I obviously have a lot to learn over the next couple of months, and am trying not to compare everything to how I learned it in the nicu, its just that the nicu is the only knowledge base I have right now so its hard not to wonder why one place does one thing when another does it differently.

Thanks for the help!

Pediatric Critical Care Columnist · Jul 24, 2011

Oh, no! Never let go of what you've learned in NICU. Build on it! It's all valuable education. There will be a lot of differences between the two, even for patients in the same age group. And it always comes back to what's underlying... why the child needs the ICU in the first place. When I first started on my current unit, after having worked neonatal and had certain principles drilled into my head, I was appalled - Day 2 post-op we're putting in small bowel feeding tubes and starting feeds at 2 mL an hour and increasing by 2 an hour q6h to a goal of X. We're gonna what?? But to my surprise, despite the profound hemodynamic instability and hypoxia most of these babies experience, and the risk factors they have for infection, our NEC rates are infinitestimal. Our very new neos aren't monitored for bili... unless they look overtly jaundiced. We fluid restrict to 50% of PEDS TFIs (100 mL/kg for the first 10 kg, 50 mL/kg for the next 10 and 20/kg for the rest) and we really don't worry about growth. It was a huge learning curve.

There are definitely hemodynamic effects to HFOV. When we're putting a cardiac kid on (which isn't all that often) we're ready with a fluid bolus and anything else we might need to overcome that. And of course, it's the intrathroacic pressure shift that causes the problem. It's always interesting to see a kid on ECLS who is also on the oscillator. Put the pieces of THAT one together!

I know what you mean about your preceptor. S/he may be very competent at the practical part of CICU nursing without totally understanding what's going on when different things are initiated. I see that all the time. I'll ask somebody why we're doing something while I'm getting report from our newer (anybody newer than me :lol2: - and I've been there 9 years) and they won't be able to tell me. Missed opportunities! You've gotta grab those teachable moments.

umcRN, BSN, RN · Jul 27, 2011

Thanks for the wise words! I definitely won't be forgetting my NICU knowledge, I am looking at this as a wonderful new learning experience and way to broaden my practice. I am very excited about it, but I won't forget my baby basics. And I know about the feeds! Had a little 2.9kg ex 33wkr the other day, full cont feeds ND with a sump to LIS putting out green bile, and a distended (but soft) belly...omg I was nervous! :rolleyes: (however we have had NICU of ours go to CICU because nicu wasn't "growing" them fast enough, and then die of NEC there...so I will forever be watching for it)

Can't wait to see though, what the next few weeks bring :-)

Pediatric Critical Care Columnist · Jul 27, 2011

Seriously, I saw more NEC in the IMCN in 2 years than I've seen in our P/CICU in 9! And that counts the post-NEC short guts and the NICU survivors with ASDs or VSDs that come in for repair. It boggles my mind. I've been chewed a new one for testing stools for OB and for requesting a flat plate/LLD. Doesn't stop me, but... You can take the nurse out of the nursery but you can't take the nursery out of the nurse.