Anesthesia Ventilator: Piston vs. Bellows
Basic, Basic Sciences
There are two primary types of drive mechanisms to deliver gas to the patient: bellows and piston.
Ascending bellows, which rise during expiration, are generally considered safer than descending bellows because they will not ascend if there is a leak in the system. Inspiration is driven pneumatically by the introduction of pressurized oxygen or air into the ventilator casing, which compresses the bellows in order to deliver tidal volumes. Piston ventilators, on the other hand, use an electric motor to drive a piston within a cylinder in order to deliver tidal volumes. Piston ventilators are generally considered more accurate because of the precise control over piston positioning. However, newer bellows systems with microprocessor controls have greatly improved accuracy over their predecessors.
Advantages of piston ventilators:
• Greater precision of tidal volume delivery due to rigid piston design, decreased compliance losses
• Greater precision of pressure control with the use of pressure sensors
• Electrical control instead of pneumatic control
• Economical use of wall oxygen (wall oxygen not used to compress a bellows, only used for patient delivery)
• A perforation in the bellows can allow the driving gas to be delivered to the patient. This can potentially cause barotrauma or unpredictable inspired gas concentrations, including oxygen and volatile anesthetic.
• No intrinsic PEEP (compared to ascending bellows)
Disadvantages of piston ventilators:
• Leak in piston diaphragm can lead to hypoventilation
• Possible entrainment of room air as piston returns to filled position