Just had a thought, back in my youth I worked my way through scuba diving class and eventually got into advanced diving. To do the advanced dive coarse I had to really dig into the physics of the human and also the equipment. The medical ventilator; I'm thinking the scuba regulator device is very similar to a medical ventilator. The mouth held regulator is designed to create a slight atmospheric pressure increase such that when you take in a breath the diaphragm opens a valve and air is forced into your lungs. The word forced is a small force, it was designed to overcome the pressure change between the regulator and your lungs while under water, a fraction of a PSI.
Maybe these could be modified to make ventilators.