Oct 1, '05
Simple answers mostly. Tidal Volume: When the machine cycles it is set to give a specific amount of air (well--gas, since it meters in a certain Oxygen content, but think of it as air for now.) Say we set the vent for a TV of 800ml. Then every breath given by the vent is--you guessed it!!--800ml.
AC: Assist Control. There are two really 'normal' or 'popular' ways of using the vent. One is to set the machine so that every breath gets the FULL TIDAL VOLUME. This means that say we set the vent to give 8 breaths per minute and 800ml/breath and it set on ASSIST CONTROL. OK? The very least the patient's gonna get is 8 full breaths/min. What if the pt is awake and takes a couple of breaths more than 8/min? (We say--he 'breaths over' the vent.) Well, the vent senses his breath and gives him the FULL TIDAL VOLUME of 800ml with each breath whether or not there's much of a breathing effort on his part.
Constrast this with IMV (Intermittent Mechanical Ventilation). Imagine the same breaths/min are programmed into the vent: 8/min and the same volume of 800ml. But we're on IMV mode istead. He gets 8breaths/min of 800ml. But this setting means that his own spontaneous breaths are ONLY AS BIG AS HE CAN BREATH.
Usually the pt in crisis gets put on AC, then is 'weaned' from the vent through the IMV settings--because that requires him to work harder on his own.
PEEP: Positive End-Expiratory Pressure. Remember that air is pulled by the lungs into tiny little air sacs called 'alveolus'. These are very very small little balloons where the blood swooshs around the outside--giving up its CO2, absorbing O2. You know they're wet inside. You know the coughing and suctioning and mucous production of the chest can collapse these balloons. Yes? It's not hard to imagine a sick persons chest has lots of aveoli that are collapsed like shriveled up little balloons.
Now if your kid (nephew, neighbors kid) brings this poor very wet shriveled up balloon to you and says: Blow up my balloon! You're going to have to puff very hard to get the walls of the wet balloon to seperate so you can blow it up. OK? You've had that experience? Well, why? The answer is 'surface tension' which means the fluid in the balloon clings to the walls of the balloon.
Same thing happens inside the alveoli. The wet walls cling together and make the alveolus resistant to re-opening. (Therefore the importance in non-vented pts of 'turn, DEEP BREATH, and cough.')
PEEP is a constant elevation of the pressure inside the pt's lungs that minimizes the collapse of the alveolus. Since the lowest pressure in the lungs is at the 'end of expiration' (DUH!!)--we apply--you guessed it "POSITIVE END-EXPIRATORY PRESSURE".
Little hint: the lungs are soft and actually expand a little bit when we 'blow them full' by using PEEP. And what else is in the chest? THE HEART!! So PEEP can lower the cardiac output and lower the blood pressure.
FiO2: "Fraction Inspired Oxygen". It's the oxygen setting on the vent.
Pressure Support: It is an extra 'push' of pressure delivered with each breath above the minimum needed to get the pre-set volume in. So each breath is bigger than it would ordinarily be. It's less important when your pt's on AC mode. It increases the volume of breaths and works with PEEP to increase the number of full alveoli--and since pressure helps the Oxygen in the 'air' dissolve through the interstitial fluid into the blood--you get higher pO2 levels and higher Sats.
AG: Don't use that abbreviation except for 'attorney general' (on political websites).
Hope this helps
Last edit by papawjohn on Oct 1, '05