Today, SIMV is quite different than the IMV first described in 1973. The mandatory breaths can be either volume control, pressure control, or adaptive pressure control. The spontaneous breaths can be pressure support (PS) or adaptive pressure control, and tube compensation can be used. The PS spontaneous breaths can be altered with rise time and flow cycle controls. It has always been the case that IMV and CMV are synonymous if the patient is making no spontaneous breathing efforts. But, with the current generation of ventilators, the selection of breath parameters might also result in little difference between SIMV and CMV during spontaneous breathing. Imagine, for example, if the mandatory breaths are pressure control, the spontaneous breaths are PS, and the pressure control and PS levels are identical.