View Full Version : pressure control vs. volume control?
05-11-2009, 10:42 AM
Can anyone explain the difference to me? Alexander's first vent clinic appt was today and they've changed him from volume to pressure, claiming "smaller lungs" fill better this way.
He's been on volume control since being trached 6 months ago. This pulmo saw him when we had him admitted last month and upped his volume then. So I'm not sure what to think, other than she's just changing things because he's coming from another doctor and hospital and she's changing just to change.
They also upped his rate - since his rate when he's awake is well over the min settings, and was only hitting his 18 rate when sound asleep (like, heart rate 30-40 points below waking heart rate asleep) I'm not sure I see why changing his rate higher was going to help.
She was supposed to be lowering his settings, since we're so low on O2. Not raising them. *grumbles*
I researched this issue a while back and found this:
"Volume-controlled ventilation (VCV) and pressure-controlled ventilation (PCV) are not different ventilatory modes, but are different control variables within a mode. Just as the debate over the optimal ventilatory mode continues, so too does the debate over the optimal control variable. VCV offers the safety of a pre-set tidal volume and minute ventilation but requires the clinician to appropriately set the inspiratory flow, flow waveform, and inspiratory time. During VCV, airway pressure increases in response to reduced compliance, increased resistance, or active exhalation and may increase the risk of ventilator-induced lung injury. PCV, by design, limits the maximum airway pressure delivered to the lung, but may result in variable tidal and minute volume. During PCV the clinician should titrate the inspiratory pressure to the measured tidal volume, but the inspiratory flow and flow waveform are determined by the ventilator as it attempts to maintain a square inspiratory pressure profile. Most studies comparing the effects of VCV and PCV were not well controlled or designed and offer little to our understanding of when and how to use each control variable. Any benefit associated with PCV with respect to ventilatory variables and gas exchange probably results from the associated decelerating-flow waveform available during VCV on many ventilators. Further, the beneficial characteristics of both VCV and PCV may be combined in so-called dual-control modes, which are volume-targeted, pressure-limited, and time-cycled. PCV offers no advantage over VCV in patients who are not breathing spontaneously, especially when decelerating flow is available during VCV. PCV may offer lower work of breathing and improved comfort for patients with increased and variable respiratory demand."
Jack was on volume controlled ventilation for the first 3+ years of his life. When we moved from St. Louis to Phoenix, the pulmonologist changed Jack to pressure controlled ventilation to help accommodate his air leak. He has been on PCV ever since (7+ years). However, Jack's situation is different because he is vent dependent for life with no hopes of weaning from the vent.
If you are uncomfortable with the changes made, I would suggest calling your doctor and asking for an explanation as to why she made the change she did and explain why you feel like it's going in the wrong direction. Your pulmonologist will hopefully have an explanation that will give you some comfort in what she is doing. I used to always make a stink when Jack's pulmonologist would go the "wrong way" on the vent. She was always very good at explaining why and also at appreciating why it was difficult on me when changes were made.
05-11-2009, 12:00 PM
Thanks for the explaination. So....it sounds to me like they don't really know why one works better, and it's just trial and error to get whatever works best for the child, combined with what the doc is most comfortable with :)
I made the mistake of letting my husband take Alexander to this first appointment - we had so many appointments this month that we couldn't both be at all of them, and it's just killing me now that I didn't go. :(
Actually....the pulmo upped his volume last month because he has a leak - his own breaths were much bigger than the set value before, and they didn't change size when she made that change. So maybe the mode doesn't matter so much?
Who knows. DH says his CO2 values have dropped (at least, according to the monitor they have) so it might help. I know we've been running higher O2 requirements (including the fact that we haven't gotten him back to room air at night) since switching to his home vent. It wasn't much higher (28-30% vs 25%) so no one was really worried at the time and figured it was just an adjustment he needed to make.
05-12-2009, 10:17 AM
I know at times they change to pressure control because they want to limit the PIP that the child received. Where as in volume control the child may draw/need bigger PIPs to reach the set volume.
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