Synchronization of spontaneous breathing with breaths supplied by the ventilator is essential for providing optimal ventilation to patients on mechanical ventilation. Some ventilation techniques such as Adaptive Support Ventilation (ASV), Proportional Assist Ventilation (PAV), and Neurally Adjusted Ventilatory Assist (NAVA) are designed to address this problem. In PAV, the pressure support is proportional to the patient's ongoing effort during inspiration. However, there is no guarantee that the patient receives adequate ventilation. The system described in this article is designed to automatically control the support level in PAV to guarantee delivery of patient's required ventilation. This system can also be used to control the PAV support level based on the patient's work of breathing. This technique further incorporates some of the features of ASV to deliver mandatory breaths for passive subjects. The system has been tested by using computer simulations and the controller has been implemented by using a prototype.