Vent settings

CMV? What ventilator are you using? Again as stated before, many different ventilators use similar terminology but to represent totally different modes. Many ventilators do not use the term CMV at all.

There are no longer ICU ventilators which deliver only a fixed number of breaths and nothing else used today in countries with advanced medical care. What you are describing is from the 1970s and earlier. The exception might be some of the very simple ventilators used by Paramedics. This is why you need to be careful about the sources you use as a reference. Outdated material is misleading.

CMV can be an Assist mode. It can deliver a set tidal volume and have a set rate. Both of these parameters can be exceeded by the patient.

SIMV can be more like what you where describing for a fixed rate and volume (or pressure) if not used correctly. Pressure support should be set at an adequate level for the spontaneous breaths. Some set it too low or fail to set it at all which makes the patient to struggle over the resistance of the tube.