Anesthesia ventilators are classified by where the patient's gas is stored and by what pushes it. A bellows ventilator stores the gas in a bellows inside a rigid housing and pushes it with a drive gas (O₂ or air); the drive gas has its own circuit separate from the patient's, so it is a double-circuit design. Bellows are ascending or descending according to the direction they move during expiration; a descending bellows keeps moving even when the circuit is disconnected, so nearly all current machines use an ascending bellows. A piston ventilator moves a piston in a cylinder with a motor; there is no drive gas circuit (single circuit) and the reservoir bag stores the gas. A blower (turbine) ventilator spins a small fan at high speed inside the circuit to create flow and pressure; it is also single-circuit with the reservoir bag as the store, and it has no part that pushes stored gas out and no need to refill like a piston (Dräger Perseus and Zeus, for example). A volume reflector ventilator stores the gas in a long tube (the reflector) and pushes it out with a flow of reflector gas (O₂); it has no moving parts. This page covers all four of them.
| Bellows (ascending) | Piston | Volume reflector | Blower (turbine) | |
|---|---|---|---|---|
| How fresh gas is made | Flowmeters with needle valves; variable-bypass vaporizer | Flowmeters with needle valves; variable-bypass vaporizer | An electronically controlled gas module for each gas; injection vaporizer | An electronically controlled gas mixer (a module for each gas); variable-bypass vaporizer |
| What drives the ventilator | Drive gas (O₂) in the housing pushes the bellows down | A motor moves the piston through a rod (no drive gas) | Reflector gas (O₂) pushes directly on the gas in the reflector, with no membrane between them | A motor-driven radial fan (blower) inside the circuit creates the flow and pressure (no drive gas) |
| How the next breath is collected | Exhaled and fresh gas push the bellows up from below | The piston descends and draws in gas from the reservoir bag and fresh gas | Exhaled and fresh gas (flowing backward through the absorber) enter the patient end of the reflector, pushing out the gas at the far end | Exhaled and fresh gas collect in the reservoir bag (during expiration the fresh gas rides the small circle flow through the inspiratory limb and round the Y-piece to get there; no refilling stroke is needed) |
| Fresh gas during inspiration | Joins the inspired gas and goes to the patient | The fresh gas decoupling valve closes; fresh gas goes to the reservoir bag | Joins the inspired gas and goes to the patient (the reflector gas pushes out correspondingly less). The gas modules deliver fresh gas mainly during inspiration (in this figure, two thirds of each breath’s fresh gas during inspiration and one third during expiration) | Joins the inspired gas and goes to the patient (there is no decoupling valve). A flow sensor measures the volume and the blower draws correspondingly less from the bag |
| Reservoir bag during ventilation | Taken out of the circuit by the switch; does not move | Stays in the circuit as a reservoir; inflates and deflates | Taken out of the circuit by the switch; does not move (in manual ventilation the switch opens, connecting it at the far end of the reflector, and the reflector gas is stopped) | Stays in the circuit as a reservoir; inflates and deflates |
| Negative pressure in the circuit | Unlikely, because the ventilator never draws gas in | Possible because the piston draws gas in; prevented by the PEEP/Pmax valve and the auxiliary air valve | Only pushing, no drawing in of gas | The fan does draw gas from the circuit, but its speed is regulated from the pressure upstream of the inspiratory valve, the PEEP/Pmax valve protects the expiratory side, and the auxiliary air valve opens if the circuit runs short of gas |
| Where excess gas leaves | Once the bellows is fully up, through the relief valve to the scavenging system | Once the bag is full and the pressure rises, through the APL bypass valve and exhaust check valve to the scavenging system (through the APL valve in manual ventilation) | From the far end of the reflector, through the PEEP/APL valve to the scavenging system (what leaves is O₂ and the oldest gas) | Once the bag is full and the pressure rises, through the APL bypass valve and the exhaust check valve to the scavenging system (through the APL valve in manual ventilation) |
| Tidal volume when fresh gas is reduced | Falls by the fresh gas no longer added during inspiration if the bellows delivers the same volume (modern machines compensate; in this figure the bellows pushes out more to keep it the same) | Unchanged (the decoupling valve means only the volume pushed by the piston reaches the patient) | Unchanged (the reflector gas pushes out more) | Unchanged (the flow sensor measures it and the blower draws more) |
| Circuit pressure at end-expiration | The opening pressure of the relief valve (2–3 cmH₂O) remains | Falls to the PEEP setting (near 0 if PEEP is 0) | The PEEP set on the PEEP/APL valve | Falls to the PEEP/Pmax setting (near 0 if PEEP is 0) |
The ascending bellows delivers a smaller breath if it does not rise fully, but in exchange it has no mechanism that actively draws gas in: the ventilator itself cannot make the patient’s airway pressure negative, and a disconnection or a large leak keeps the bellows from rising, so the problem is easy to notice. A design that puts safety first. The piston design can create negative pressure because it draws gas in; it compensates with fresh gas decoupling and valves, and in return achieves accurate tidal volumes and many ventilation modes. The volume reflector design pushes gas with a flow of O₂ instead of a piston, keeping the pushing gas and the patient’s gas side by side, almost unmixed, inside a long tube. Nothing draws gas in, so negative pressure is unlikely, and there are no moving parts. The blower type creates the flow and pressure with a small fan placed inside the circuit. It needs no part to push stored gas out and no refilling stroke like a piston, and because it is a pressure generator it suits pressure support and synchronization with spontaneous breathing.
The patient is ventilated by squeezing the reservoir bag instead of using the ventilator. With the bellows type, the switch is turned to Bag, and the L-shaped channel of the plug connects the expiratory junction to the APL valve and bag (the bellows is taken out of the circuit and stops). The piston machine has no such switch: the piston stops and you squeeze the bag that is always in the circuit. In both, during inspiration the squeezed gas passes through the absorber, loses its CO₂, joins the fresh gas and goes through the inspiratory valve to the patient (through the fresh gas decoupling valve on the piston machine). During expiration, exhaled and fresh gas refill the bag; once it is full and the circuit pressure exceeds the APL setting, excess gas leaves through the APL valve to the scavenging system. With the volume reflector, the switch opens to connect the bag at the far end of the reflector, and the reflector gas is stopped: the squeezed gas passes through the reflector and the absorber to the patient, and exhaled gas returns through the reflector to the bag. Once the bag is full and the pressure exceeds the setting, excess gas leaves through the PEEP/APL valve (acting as the APL valve) to the scavenging system. On the blower type the blower stops and lets gas pass through passively; as on the piston type, you squeeze the reservoir bag. The gas passes through the absorber and the blower to the patient, exhaled gas returns to the bag, and the excess leaves through the APL valve.
In the Too low scene, the patient’s uptake (particles that vanish from the alveoli into the blood, 4 per breath) exceeds the fresh gas (3 per breath). The gas in the circuit falls by one particle each breath; that much reflector gas (O₂) stays at the far end, the boundary moves toward the patient, and within a few breaths O₂ passes through the absorber, joins the inspired gas and reaches the alveoli. On the real machine too, when fresh gas is less than uptake plus leaks, the shortfall is made up by O₂ from the reflector gas module. Tidal volume is maintained, but inspired O₂ rises above the setting and the anesthetic is diluted. In the other scenes, uptake is set to zero for clarity.
Each type shows it differently. On the bellows (ascending) type the bellows no longer rises fully, and if it does not rise the next breath cannot be delivered. After a disconnection or with a leak, watch the bellows and, if needed, refill the circuit with the O₂ flush during expiration (pressing it during inspiration puts the pressure on the lungs). On the piston and blower types the reservoir bag, which moves even during ventilation, collapses flat. If the circuit goes further negative, both open their auxiliary air valve and room air enters the circuit, diluting the anesthetic. On the volume reflector type the tidal volume is maintained, so it is easy to miss: the reflector gas (O₂) advances toward the patient, the inspired O₂ rises and the anesthetic is diluted (the Too low scene).
Exhaled gas enters the absorber from below, so the absorbent is used from the inlet (bottom), and the purple band moves toward the outlet (top). A change that takes hours or more in reality is compressed into minutes here.
With Fresh gas: High, the fresh gas flow is close to the minute ventilation. With the bellows type, the exhaled CO₂ leaves as excess gas before reaching the absorber, so in this figure the absorbent is hardly used. With the piston, exhaled gas first collects in the bag and the piston draws it through the absorber, so the absorbent is used up in about 35 breaths, after which a little CO₂ returns to the inspiratory side. On the blower type, exhaled gas goes straight from the expiratory limb into the reservoir bag, while fresh gas rides the circle flow through the inspiratory limb and round the Y-piece and arrives later. The bag therefore holds mostly exhaled gas, which the blower draws through the absorber on the next inspiration, so as on the piston type the absorbent is used up in a little over 20 breaths even with high fresh gas flow (not much different from low flow).
With Fresh gas: Low flow, the fresh gas is cut to one quarter. Almost all exhaled gas passes through the absorber and is breathed again, so the absorbent is used up faster (about 20–25 breaths). Once it is exhausted, orange particles return to the patient through the inspiratory limb and inspired CO₂ rises (the capnogram baseline no longer returns to zero).
With the volume reflector, exhaled and fresh gas enter the reflector and are pushed out on the next breath in reverse order. In the High scene what is pushed out is the fresh gas that entered after the exhaled gas, and the exhaled CO₂ leaves from the far end of the reflector through the PEEP/APL valve, so the absorbent color does not change. In the Low flow scene the reflector pushes out more, and the absorbent is used up in about 23 breaths.
The difference in breaths between the types is not a comparison of real machines. The indicator color can fade back to white after the absorbent rests, so decide when to replace it from inspired CO₂ as well as the color.
Machines of every type can have a way to change the absorbent without stopping ventilation. On some bellows (ascending) machines (GE, Mindray) and on the Flow-i volume reflector machine, the circuit switches to a bypass around the absorber while the absorber is removed. On Dräger piston and blower machines, valves in the absorber mount (the CLIC adapter) close and the gas goes round the circuit without passing through the absorbent. With any of these, exhaled CO₂ is not removed while the gas skips the absorbent, and gas containing CO₂ reaches the patient (the bypass valve is drawn closed on the bellows figure).
This is a schematic. The shape and position of parts and the valve arrangement vary between machines (the piston machine is modeled on the Dräger Fabius layout, the blower machine on the Dräger Perseus, and the volume reflector machine on the Getinge (Maquet) Flow-i). Each breath is shown as 6 s (2 s inspiration, 4 s expiration). Gas volumes, timing and absorbent capacity are adjusted for clarity.