Quote from poppy07
what confuses me with the A/C mode is that you will read the tidal volume by the pt on a set volume of let's say 500 reading 460, 520, 730, 340...etc with a rate of 18-24 on set rate of 12. So, if it truly is A/C then why are each of these breaths determined by the pt and not the set volumes?
Poppy...........I am both an RRT and a Critical care RN, so from both standpoints I will try to explain this the best that I can. With A/C Rate of 12, Vt of 500 as you mentioned, the ventilator is set at a MINIMUM of 500 cc's to be delivered 12 times a minute.........MINIMUM. Now, if the patient "decides" to do some work on his/her own, they can take in a larger tidal volume and a faster rate if desired. So, the patient can breath as fast as they want, they just cant breathe any slower then 12 times a minute. The same with tidial volume (Vt). The patient can breathe as much volume as they want theoreticlally, however they cant breathe less than 500 cc's or the set tidal volume.........whatever that may be. You listed several ovserved volumes above in your post (460, 520, 730, 340). I think there is either the possibility that the patient wasnt actually in A/C, they had a leak or they were high pressuring the ventilator and the set volume wasnt being delivered. In A/C of 500, and no problems were present, the tidal volume would NEVER go below the set volume, so you would never see a tidal volume of 460 or 340 as you mentioned. This can get a bit confusing, but sometimes, depending on what ventilator you are using and how old it is, it will display settings of A/C or SIMV on the screen, however the vent itself might be set on something different. A popular ventilator, the PB 7200 often shows on the display screen all of the settings for A/C but can acutally be in pressure support. Back to the tidal volumes you listed above such as the 520 and 730. In the case of the ventilator being set on 500 cc's, the patient will be given 500 cc's by the vent, however if the patient decides " hey, I want to take a deeper breath in and do a little of the work on my own", the patient can breathe in deeper.........sucking in 100,200,300,400, ect..........cc's of more tidal volume. The volume that you are seeing on the screen that is changing IS NOT measuring what is delivered to the paient. It is measuring what the patient exhales with each breath.
SIMV and A/C both have their benefits and downfalls, and I personally would argue the fact that its not good for more then a day or two. It is really a physicians preference. SIMV is better for Post op hearts due to the risk of the cyclic build-up of intrathoracic pressure and subsequently the risk of compression of the vessles and additional strain on the heart. SIMV allows for a periodic release of pressure during spontaneous breathing. A/C is usually preferred in patients who are sedated and/or patients who cannont tolerate spontaneous breathing such as in in acute respiratory distress, ARDS, ALI, ect. I hope this helps.