A/C and SIMV will both ventilate the patient so either mode is good. A/C mode will assist/ control meaning it will give the patient a breath, it will pick up on the pt trying to take a breath and give the patient that breath. Simv mode will give the pt the set rate (f) and let the pt take a breath on their own. no need to worry about breath stacking the machine synchronizes the breaths. A/C is good for initial setting, SIMV is good for weaning. incase there is some Raw in SIMV mode just add pressure support :)