The ventilator's power source enables the machine perform the work of ventilating the patient. The power source may be electrical power, pneumatic (gas) power, or a combination of the two.
 
Electrically Powered Ventilators
Electrically powered ventilators rely entirely on electricity. The electrical source may be a wall outlet (alternating current [AC]) or a battery (direct current [DC]). Battery power usually is used for a short time, such as for transporting a ventilated patient. It also can be used in the home as a backup power source if the home's electricity fail.
 
Pneumatically Powered Ventilators
Some ventilators depend entirely on a compressed gas for power. These machines use 50 psi gas sources and have built in internal reducing valves so that the operating pressure is lower than the source pressure.
 
Combined Power Ventilators : Pneumatically Powered, Electronically or Microprocessor Controlled Models
Some ventilators use one or two 50 psi gas sources and an electrical power source.
In combined power ventilators, the pneumatic power (i.e., the 50 psi gas sources) provides the energy to deliver the breath. The electrical power, on the other hand, controls the internal function of the machine but does not provide the energy to deliver the breath. These internal controls use pneumatic input power to drive inspiration and electrical power to control the breath characteristics. For example the Servo ventilator.
Most current ICU ventilators are of this type.