Macaron Help - Hollows and Little Feet
The culprit would be the Italian meringue (unless you've got the name wrong). Italian meringue is made with egg whites and hot sugar syrup. The hot syrup cooks the whites, so there is no need to bake it afterwards. It is often a component in soft deserts like mouses, or just served as is, slightly browned with a burner. If you do bake Italian meringue, it comes out very dry and crisp - no at all like macarons are supposed to be.

Other problems that could be affecting your meringue:


Egg yolk in the whites (would refuse in the eggs refusing to get beaten into hard peaks, and then refusing to rise)
Temperature that's too hot. You know how pitas are made? A significant amount of water, and very hot oven. The water evaporates fast, but has no room to escape since the outer crust has already formed, so you get a pocket of air inside. The same might be happening to your meringues. (Consider that while French and Swiss meringue add sugar to the egg whites, Italian meringue has sugar syrup - sugar and water.