Hello guys. Before I initiate new discussion on this, could anyone guide me, maybe this has been discussed already. I will wait for the response for a month before I go on specifics. Recently the RTs in our hospital revisited this topic backed up with books, clippings and actual experiments by curious RTs. You can comment if this is also a worthwhile topic.
ric mandanas jr
c/o rt dept.
hamad medical corp.
p.o. box 3050
It is all about flow. Low flow devices (NC, SImple mask, PRB mask and yes even NRB mask, HighOx 80, Oxymask)) FiO2 will vary based on the patients inspiratory flow demands. We have done some bench test and human tests recently (presented at nationals in 2006 and 2007) and found that the NRB mask only delivers around 65% with a normal breathing patient and can drop dramatically as the inspiratory flow increases. So basically with one of these devices, you can not how much the patient is actually recieving.
Highflow devices (anything that produces >30 L/min) still have variability in delivered FiO2 based once again on the cursed four letter word FLOW. If the device is metting or exceeding the patients inspiratory flow demands that you can be pretty sure at the delivered FiO2, if they exceed the flow of the device than all you can be assured is that the FiO2 is less than what is being provided by the device.
Just my thoughts.
thanks light., point well taken. in the first part of your post you mentioned that you've made some human test on the FiO2 of the NRB mask. Can you share me the method that you have used?. Well these are few issues that the RTs in the department would like to look into. Thereby convincing physicians that what's writen in their textbook on the approx FiO2 per liter of a low flow device cannot be guaranteed.
Other specifics would deal on humidifications and some device alterations that makes work confusing to RTs.
We got some college students to have their Ve measured, then they were placed on a NRB mask for twenty min after that time we drew a bloodgas. All the students had no history of asthma, COPD or smoking. Since their was no lung history we assumed a normal A-a gradient. From this normal A-a gradient (10%) we were able to calculate what the alveolar pressure of oxygen was based on the PaO2. Now we were able to backwards work the alveolar air equation to find the FiO2 delivered by the NRB on these healthy normally breathing college students.
The text book answers even when you look at the answers it will state "on a normal breathing patient" and how many normal breathing patients need a NRB mask? So if we start at 60-65% for a normal person we can work our way down for that hyperpneic, tachypneic patient. How far? Don't know, depends on inspiratory flow. Our students look at Flow as a bad four letter word by the time they get out school from hearing it so much.