How much is the role of IRV in ARDS?

IRV is Inverse Ratio Ventilation

What is IRV?

ARDS is also known as non-cardiogenic pulmonary edema. Usually caused by an inflammatory process. It is characterized by decreasing compliance of the lungs and the need for increasing pressures (peep, bi level aprv, or inverse ratio ventilation) to keep Po2 levels up while allowing some permissive hypercapnia (elevated Co2) during ventilation.

Ok, inverse ratio ventilation comes in a couple different forms. It is used as a ventilation mode when conventional ventilation becomes dangerous due to high pressures needed to create an accepable tidal volume. In conventional ventilation the ratio of inspitory time to expitory time (I:E ratio) is about 1sec:3sec, When pressures become too high to continue in this mode (generaly over 30 cm/h2o mean airway pressure) you can go to inverse ratio ventilation. In the "old" days we had to paralize a pt to halt all spontanious breathing then switch to pressure controll vent mode, increase the I time to about 3-4 sec, and adjust the rate so that the E time was less than one, then set the inspitory pressure to about 20 cm/h2o. The theory is that with a lower over all peak pressure but a prolonged I time will "hold" open the lungs and the really short E time will allow for the release of co2, but not give the stiff lungs time to colapse into their atelictatic state. With some of the new vents (I have only done this with the PB 840) You can set up what is called APRV (airway pressure release ventilation) using the Bi-level mode. Best of all the pt needs only some sedation but gets to keep their spont. breaths. You set your pressures the same as the old IRV, but you also have pressure support, which you set so that when you add it to the low pressures (low peep) you are about 2-4 cm/h2o above the high peep. Then adjust the I time and rate for a 4:1 I:E ratio or something close, DON"T go below 2:1 you will lose the positive effect. When the pt breaths at high peep the vent will only add the 2-4 cm/h20 pressure above the high peep, when they breathe at low peep the vent will add the whole pressure support. Benifits: using less sedation/ paralitics will get the pt off the vent quicker. But when you make the switch you need to stay in the room for about 15-20 min as you will most likely have to make some changes to the settings. In my experience with in about 5 min I have had to drop the high peep (inspitory pressure) because the lung will open up and the tidal volumes will increase dramaticaly. and you will be able to start dropping the FIO2 as the pt's sats will improve within a few min. Don't forget to adjust the pressure support to stay about 2-4 cm/h2o above the insp. pressure.

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