The Fabius platform: models, assemblies and the gas path

Identify which Fabius variant and configuration is in front of you, name its main assemblies and boards, and follow its gas from the inlet to the scavenging port, knowing what makes this machine different from the generic workstation of anes-l1.

One platform, several badges

The Fabius family shares one idea: a compact anaesthesia workstation whose ventilator is a piston driven by an electric motor, sitting next to a compact breathing system that clips onto the machine. The service manual this course is built on is the Fabius GS Technical Service Manual, revision L, from 2004. The GS is our reference machine. The Fabius Plus, the Fabius Tiro and the Fabius MRI are built on the same platform, and the field cases in this course come mostly from the Plus and the Tiro, so we will keep saying which statements are proven for the GS and which you must check in the documentation for your variant.

In anes-l1 you learned the generic gas path and the generic tests. This course does not repeat them. It adds what is specific to this platform: where its regulators and switches sit, what the manual expects from them, how the piston ventilator and the compact breathing system work together, which calibrations the service software offers and in what order, and what engineers actually meet in the field.

Before you touch anything, establish exactly what you have. Read the data plate for model and serial number. Watch the power-up screen: it lists the self-tests, the software version and its checksum, and ends with FUNCTIONAL, CONDITIONALLY FUNCTIONAL or NON-FUNCTIONAL. On the GS the system service screen also shows the hardware part number of the controller PCB, and two controller versions exist with small differences in what the service software can do. Note the gas configuration (two or three gases), cylinder yokes or cylinder hoses, vaporisers and their mount, the auxiliary O2 flowmeter, the breathing system variant, the O2 sensor and the scavenger type. Write it down: which parts are in the kit, which tests apply and which calibrations are available all depend on it.

How the variants differ

Keep the differences general and confirm them in the documentation for the exact machine. The manual's own scope tells you how close the GS and the Tiro are in this generation: the service software has a model type setting with GS and Tiro as the two choices, the event log has entries for switching between them, and the one-year and three-year maintenance kits are listed as GS/Tiro kits. That is why a Tiro whose settings were lost can start up looking like a GS in English: the controller falls back to its defaults.

The Tiro is the compact member, offered as a cart version and a wall-mount version. The Plus is a later member with its own service documentation; do not assume its boards, flowmeter design or menus match the GS manual. The MRI version is built for use near an MR scanner: its restrictions on where it may stand, which tools may be used near it and which parts may be fitted come from its own documentation, and you never improvise a part on it.

Older and regional variants exist too, and manuals for them circulate. A manual for a different variant can look almost right and still have different hoses, sensors or limits. Match the manual to the data plate, the software version and the serial number range before you trust any number in it.

The GS manual itself changes with serial number: the fresh gas flow sensor and filter assembly is shown in two versions, one for earlier and one for later serial numbers, and the breathing system comes in two part-number families with slightly different internal routing. These are small differences, but they decide which O-ring you order and which diagram you read.

Supplies and the gas box

The gas box is the gas inlet assembly and what hangs off it. Pipeline gases enter through filters and non-return valves. Cylinders, where fitted, connect through yokes with their own check valves and regulators; machines without yokes take cylinders through hoses. Pipeline pressure gauges are on the flowmeter assembly at the front; cylinder gauges are on the trolley.

Downstream of the inlets, regulators on the gas inlet assembly set the pressure delivered to the flow control valves. The PM procedure checks the O2 and N2O inlet regulators at 28 to 32 psi (roughly 1.9 to 2.2 bar) with 4 L/min flowing, and the adjustment procedure sets them to 30 psi. Because the inlet regulator sits after the cylinder regulator, the flow controls see a stable pressure whichever source is feeding them.

The cylinder regulators are adjusted against a test gauge with a compensation table: the correct output depends on the cylinder pressure at the moment of adjustment, and the N2O regulator is not compensated. You will not set them often, but you must know that a cylinder regulator reading 'wrong' may simply have been tested with a cylinder at a different pressure from the table row you looked at.

The O2 supply pressure alarm switch is part of the gas inlet assembly. When O2 pressure falls below its set point, the low O2 supply alarm appears; the manual's adjustment procedure expects activation between 16 and 24 psi on a test gauge as the pressure falls. An electronic O2 whistle can be enabled in the configuration to add an audible signal. Note that the switch is an alarm, not a cut-off: the protection that stops N2O when O2 is lost is pneumatic and sits in the flowmeter assembly.

Flow control, S-ORC and fresh gas

On the GS, each gas passes a needle-type flow control valve in a valve manifold and then an electronic flow sensor. These sensors work on the specific heat of the gas: a heated chamber loses heat in proportion to the mass flow, and the current needed to keep it at temperature is scaled into L/min. The fresh gas display PCB shows the result as digital readings and virtual flowtubes; the operator chooses on which side O2 appears. A mechanical total fresh gas flowmeter shows the summed flow as a cross-check. So a missing or wrong flow reading on the display does not necessarily mean the gas is not flowing: look at the total flowmeter.

The S-ORC (Sensitive Oxygen Ratio Controller) is the hypoxic guard of this machine, and it is purely pneumatic. Restrictors at its outlets generate back-pressures that act on control diaphragms: O2 back-pressure opens the N2O path, N2O back-pressure closes it. The result is that the fresh gas never falls below about 23% O2 in O2/N2O mixtures. The manual's function chapter says N2O stops when the O2 control is shut or O2 is at about 200 mL/min or less, and returns from about 300 mL/min; the PM check, which is what you test against, expects N2O to start at 0.1 to 0.2 L/min of O2 (lesson 2). A bypass lets O2 skip the S-ORC restrictor when more than about 10 L/min of O2 is needed. The S-ORC only knows O2 and N2O: it does not protect against a hypoxic mixture made with air, and it does not know what is actually in the O2 pipeline. That is the job of the O2 monitor.

The gases then pass a safety relief valve set at about 10 psi (0.7 bar) that protects the low-pressure side, the vaporiser mount with its interlock, and the fresh gas outlet. The O2 flush delivers O2 to the fresh gas outlet through an ejector, which prevents flush gas from flowing back into the vaporiser and pushing up the agent concentration. The optional auxiliary O2 flowmeter takes O2 for a separate outlet.

The piston ventilator and the pneumatic assembly

This is the part that most differs from the bellows machines you may know. The ventilator sits in a swing-out compartment on the left side. A motor turns a ball screw that moves a piston up and down in a cylinder; a sight window lets the user see it move. There is no drive gas. Losing the O2 supply does not stop the ventilator; losing power and battery does.

The piston is sealed by a rolling diaphragm made of two diaphragms, upper and lower. Vacuum is applied between the outside of this seal and the cylinder wall so that the diaphragm rolls cleanly instead of folding. That vacuum comes from the pneumatic assembly in the controller housing. The controller knows where the piston is from two signals: an incremental encoder on the motor shaft counts revolutions, and a position (home) sensor tells it when the piston reaches its lower end. A transducer on the controller PCB measures pressure in the ventilator chamber.

The top of the ventilator, the patient assembly, carries two valves. The high pressure safety valve opens at about 75 cmH2O if pressure limiting fails; the PM test accepts 70 to 80. The negative pressure relief valve lets ambient air in if the patient tries to breathe and there is not enough gas, for example a failed supply or an empty bag. Its opening threshold is about -3 or -8 mbar depending on software version, and when the controller senses the chamber pressure reaching that point it can raise a Fresh Gas Low alarm, if that alarm is enabled in the configuration.

The pneumatic assembly has a small pump, reservoirs, an electronic PEEP valve and a variable restrictor. In automatic ventilation the pump runs. The current through the electronic PEEP valve coil is proportional to the set PEEP and sets the control pressure on the PEEP/Pmax valve in the breathing system. The same pump provides the vacuum that holds the APL bypass valve open, set to -150 to -240 cmH2O by the restrictor. In manual mode the pump and the ventilator stop and the spring-loaded APL bypass valve closes, so exhaled gas goes through the APL valve.

The compact breathing system and scavenging

The Cosy II compact breathing system gathers the circle into one block: inspiratory and expiratory valves under domes, the O2 sensor port on the inspiratory valve, the Spirolog flow sensor on the expiratory side, the PEEP/Pmax valve, the MAN/SPONT lever APL valve (adjustable from 5 to 70 cmH2O), the APL bypass valve, the fresh gas decoupling valve, the absorber, the bag terminal, the ventilator port, the scavenging port and the pressure sensor connector.

The fresh gas decoupling valve is the key to how it ventilates. During mechanical inspiration the pressure from the piston closes it, so fresh gas arriving during inspiration goes to the bag and is not added to the tidal volume. During expiration the piston moves down and draws its next charge from the bag and the absorber, while exhaled gas passes the flow sensor, the PEEP/Pmax valve and the expiratory valve. Pmax is not a separate valve: the PEEP/Pmax valve opens when airway pressure reaches the set limit. Excess gas leaves through the open APL bypass valve to the scavenger.

In manual ventilation the piston rests at its upper end to reduce the volume in the system, the APL valve limits pressure and excess goes through a non-return valve to the scavenger. In SPONT the APL valve is open whatever its setting. Because the breathing system, its control hoses (PEEP/Pmax control and APL bypass) and the pressure pilot line all plug into the interface panel at the back, a kinked or swapped hose here produces ventilator symptoms that have nothing to do with the ventilator.

Scavenging is either an active AGS unit with a reservoir canister, a silencer and a flow indicator float, or a passive scavenger with an anti-occlusion cage and a filter. Both are in the PM procedure. On non-US systems the breathing system may also carry an exhaust return port for the sample gas from an anaesthetic gas monitor.

On-site checklist

• Model, serial number and data plate recorded
• Power-up result, software version and checksum recorded
• Controller PCB hardware number noted from the system service screen
• Gas configuration, yokes or cylinder hoses, vaporisers and auxiliary O2 noted
• Breathing system variant, O2 sensor and scavenger type noted
• Manual revision matched to variant and serial number range
• Interface panel hoses (PEEP/Pmax, APL bypass, pressure line) traced and seated

Check yourself