The Aeon 7200 platform: supplies, flowmeters, vaporiser, breathing system, ventilator and power

Name every assembly of the Aeon 7200 / 7200A from the gas inlets to the patient, know which ones are mechanical and which are under the ventilator's electronics, and know where each typical fault can live.

What kind of machine this is

The Aeon 7200A is a compact, largely mechanical anaesthesia workstation with an electronically controlled ventilator bolted onto it. That sentence decides how you service it. Gas supply, flow control, the vaporiser and the circle breathing system are pneumatic and mechanical: they have no self-test and no error log, and they tell you nothing unless you measure them. The ventilator is a small microprocessor unit with its own boards, sensors, display and battery, and it is the only part of the machine that raises alarms.

On the front you will find the castors with brakes on the front pair, a drawer, the absorber, the bellows assembly, the flowmeter bank, pipeline gauges for O2 and N2O, the ventilator panel, the manifold for the vaporisers, cylinder gauges where cylinders are fitted, the power switch, the O2 flush button and the common gas outlet. On the back: the optional O2 and N2O cylinder yokes, the gas inlets, the ventilator rear panel, the mains inlet with its fuses and the auxiliary mains outlets.

The service manual we work from covers the 7200A. Machines sold as Aeon 7200 without the suffix, and later configurations such as three-gas machines with an air flowmeter or with other agents and vaporisers, may differ in options, alarms and revisions, so confirm the model and serial plate and use the documents for that configuration before you trust any value. The serial number encodes the model, an export marker, and the year and month of manufacture, which helps when you need to know which revision of a part applies.

This course assumes the Level 1 anaesthesia course. We will not explain what a circle system or a vaporiser is; we will show where this machine does it its own way.

Gas supply: pipelines, cylinders and the first protections

The machine takes two gases: O2 and N2O, from pipelines and optionally from cylinders. There is no medical air on the flowmeters of the configuration described in the manual. Pipeline hoses connect with gas-specific fittings (DISS or NIST, depending on the market) and the manual expects pipeline pressure in a band of roughly 280 to 600 kPa. Cylinders hang on pin-indexed yokes, each with a sealing washer, and their pressure is reduced to about 400 kPa before it joins the pipeline path. Each inlet has a filter, a gauge and a check valve, so that a full cylinder does not back-feed the pipeline and a pipeline does not fill an empty cylinder.

After the inlets, O2 goes through a regulator to a lower working pressure for the flowmeters and to a small O2 reservoir. The reservoir is there for a reason: when O2 supply pressure falls, it keeps the whistle sounding long enough for the user to notice. In the configuration the service manual describes, the O2 supply failure warning is pneumatic: the whistle. The ventilator's electronic alarm list in that manual has no O2 supply pressure alarm, so the whistle is your first line. The ventilator alarms that follow (no tidal volume, low pressure, and an FiO2 alarm if its limits are set) are consequences, not a supply warning. Brochures for other configurations list more gas alarms, so check the User Manual of the machine in front of you.

N2O does not reach its flowmeter on its own authority. An N2O cut-off valve, piloted by O2 pressure, opens only while O2 pressure is present. Lose the O2 and N2O stops. This is the first of the two hypoxia protections; the second sits in the flowmeter bank.

The O2 flush takes oxygen from the supply side straight to the common gas outlet, bypassing flowmeters and vaporiser. The manual gives a flush flow range of about 35 to 75 L/min. Remember that the flush pushes high flow and pressure into the circuit; the bellows spill valve and the APL take the excess.

Flowmeters, the O2-N2O link and the vaporiser

Each gas has a pair of flowtubes in series: a fine tube for low flows up to about 1 L/min and a coarse tube up to about 10 L/min. The needle valves are made to seal with light torque. Overtightening them damages the seat and gives you a flowtube that never quite reaches zero, which is one of the commonest bench findings on mechanical machines.

The O2 and N2O controls are mechanically linked. When the user turns N2O up, the link drives O2 up with it so that the mixture never falls below about 25% O2. The link is a mechanism, so it wears, slips and goes out of proportion. It is checked by measurement with an O2 analyser at the outlet, not by looking at the floats.

The manifold carries one or two vaporisers of the manufacturer's VP300 type, for enflurane or isoflurane in the configuration the manual describes. Inside, a dial-operated bypass valve keeps the vaporiser out of the gas path when it is off. When it is on, fresh gas splits into a carrier stream through the wick chamber and a bypass stream through a temperature-compensating valve, and the setting dial changes a throttle groove that controls how much saturated gas leaves. With two vaporisers fitted, an interlock allows only one dial to be on. Vaporiser repair is not in this manual; it belongs to the manufacturer or an approved workshop. Your part is the mount, its seals, the interlock and an output check with an agent analyser.

Breathing system, absorber and the bellows

The breathing system module is a classic circle: an expiratory unidirectional valve, the absorber, a fresh gas inlet, the bag/ventilator switch, the APL valve, the airway pressure gauge and an inspiratory unidirectional valve. The absorber holds two canisters of about 1500 ml each and has a drain. The module clamps to the machine with a knob and is released with a handle, which is why it is the part most often reassembled wrongly after cleaning.

Follow the gas with the switch on bag: exhaled gas passes the expiratory valve, goes through the absorber, picks up fresh gas, and reaches the bag; the APL lets the excess out; on inspiration the user's squeeze sends gas through the inspiratory valve to the patient. With the switch on ventilator, the bag and APL are out of the loop and the bellows takes their place.

The bellows assembly has three ports you must not confuse: the breathing system connection, the drive gas connection and the exhaust port. Inside sits a spill-over valve that releases excess circuit gas at the end of expiration at a low pressure, in the order of a few tenths of a kPa. The manual carries a firm warning: never connect the exhaust port directly to a sub-atmospheric (active) scavenging system, because the suction then pulls on the circuit and creates a leak. The scavenging interface must break the vacuum.

The ventilator and its drive gas

This ventilator is pneumatic with electronic timing, and it is driven by oxygen. Supply O2 passes a filter and a first regulator set at the factory to about 0.25 MPa. Downstream sits a gas cut-off solenoid valve, which the main board opens for inspiration and closes for expiration. When it is open, gas goes two ways. One path goes through a second regulator, factory-set to about 0.05 MPa, to the diaphragm of the ventilator's expiratory valve and holds it shut. The other path goes through the flow valve, a motor-driven proportional valve with a potentiometer that reports its position to the main board; its opening sets the inspiratory flow and therefore the tidal volume.

The drive gas does not stay pure oxygen. It enters a mixing chamber where a venturi entrains room air through a one-way valve with a sponge filter, so the bellows is driven by an O2-air mixture. A safety valve on the chamber opens at about 6 kPa and protects the patient side from excessive drive pressure. On expiration the solenoid closes, the pressure on the expiratory valve diaphragm disappears, the chamber vents to atmosphere and the bellows refills from the circuit.

Three consequences matter in the field. First, the ventilator consumes oxygen and stops ventilating when the O2 supply fails, and the patient must then be ventilated by hand: with the bag only while gas still reaches the circuit, otherwise with a self-inflating resuscitator and an independent O2 source, which belong next to every machine. Second, a dirty sponge filter or a sticking venturi one-way valve changes delivered volume. Third, a leak anywhere between the solenoid and the bellows housing shows up as wrong volume, not as an alarm with a clear name.

Monitoring, electronics and power

Monitoring is basic and you must know each sensor. Airway pressure comes from a solid-state pressure sensor on a sampling line. Volume comes from a differential-pressure flow sensor connected by two parallel sampling tubes to a probe near the patient. FiO2 comes from a galvanic O2 cell with a 3.5 mm mono jack, which in room air gives a signal of about 9 to 13 mV. The ventilator offers volume-controlled IPPV, pressure-controlled PCV and SIMV, plus manual; SIMV is a purchased option, so a machine without it is not necessarily faulty.

Electronics: a main control board with the microcontroller, the pressure and flow front ends, the valve drivers and test points; a power board with the charge management and the switching between mains and battery; a switching power supply feeding the power board; a rechargeable battery; a 5.7-inch TFT display with its controller and backlight inverter; a membrane keypad, a rotary encoder, a buzzer and a calibrating valve. An RS-232 port on the main board connects to the manufacturer's PC service tool. That tool is used only by authorised personnel per the manufacturer's procedure.

Mains enters through a fused inlet; the auxiliary mains outlets on the back have their own fuses and a limited total current. The machine is Class I, Type B. Two consequences: the protective earth matters, and anything plugged into the auxiliary outlets becomes part of a medical electrical system whose leakage must be measured as a whole. The battery keeps the ventilator running when mains is lost; when the battery indicator reaches empty without mains, the ventilator shuts itself down.

On-site checklist

• Model and serial plate read; revision of the manual matched
• Gases fitted and connector type (DISS or NIST) noted
• Cylinder yokes, washers and gauges present where fitted
• Number of vaporisers, agent and interlock noted
• Bellows exhaust port connection and scavenging interface inspected
• Options noted (SIMV fitted or not) before judging a missing mode
• Anything plugged into the auxiliary mains outlets listed

Check yourself