The Level 2 method: event logs, self-test results, the subsystem map, test lung and analyser

Turn an alarm, a failed self-test step or a clinical complaint into a located fault, using the event log, the subsystem map, a test lung and a gas flow analyser, and know what you may and may not change.

Where Level 1 ends

At Level 1 you learned to keep a ventilator safe, to run its self-test, to fit the PM kit and to verify it with an analyser. Level 2 begins when the machine fails anyway: a self-test step will not pass, the measured volume is lower than the set volume, PEEP drifts, the oxygen reading wanders, the machine reboots on battery or shows a technical alarm nobody understands. Your job now is to say which part of the machine has failed, prove it with a test, and either repair it within your authorisation or hand over a clear, evidence-based request to whoever is authorised.

The expensive mistake at this level is to replace the part the machine named. A ventilator judges itself with its own sensors. When the self-test says 'expiratory flow sensor', it means only that a comparison involving that sensor did not come out right. The sensor may be faulty. So may the tube that carries its pressure signal, the board that reads it, the valve that made the test flow, the gas supply that fed that valve, or the test lung and circuit you connected. Change the sensor without asking which of these is guilty and you may have spent money, lost a day and kept the fault.

Read the event log before you restart anything

Almost every modern ventilator keeps a record of technical events and alarms with a time stamp, and many keep a separate log of self-test and calibration results. Some show it to the operator, others only in a service screen your access level allows. Before you power-cycle the machine, read or photograph that record, because some machines overwrite older entries and a restart may add a burst of new ones that hide the first.

Work backwards from the moment the problem was noticed. A supply pressure warning followed by a delivered-volume alarm and then an 'inoperative' message is one story with one cause, not three faults. The first technical event in a burst is usually closest to the cause; what follows is consequence. Note also what the machine was doing: mode, set volume or pressure, FiO2, whether it was on mains or battery, and whether a nebuliser, humidifier or suction was in use.

Check the clock before trusting the order of events. A flat backup cell can reset the date and scramble the log; on some machines the real-time clock failure itself appears as an error after long storage. And learn how each machine grades its messages. Neonatal nCPAP drivers, for example, may distinguish a full lock-out from a restricted mode in which only basic CPAP remains available; the Infant Flow SiPAP service manual describes exactly this split. A message's grade tells you how urgent it is and what the machine still trusts about itself.

• Photograph the log and the failed test screen before any restart.
• Write down mode, settings, power source, accessories in use and the time of the event.
• Ask the clinical staff what they saw and heard, and what changed: a new circuit type, a move, a cleaning visit, a gas supply problem, a software update.

A failed self-test step is a question, not an answer

Each step of a self-test or pre-use check makes the machine do something and compares two of its own measurements. A leak test pressurises the circuit and watches pressure fall. A flow sensor test sends a known flow through one sensor and compares it with another. A valve test drives a valve and watches pressure or flow respond. When a step fails, ask three questions. What did this step make the machine do? Which parts took part in that action? Which of them could have produced this exact result?

Many failures are not the machine at all. The test was run with the wrong accessory, the circuit was not occluded when the machine asked, or released when it asked to open it; the Oxymag operations manual lists several failed autotest steps whose only cause is exactly that. A test bag of the wrong size can fail a leak or compliance step on an anaesthesia machine. A low or fluctuating oxygen supply can fail a proportional valve test. Repeat the step once with the correct circuit, the correct test lung, both gases at normal pressure and the instructions followed literally. If it then passes, you have found an operator or accessory problem, and that is a valid result to write down.

If it fails again in the same way, you now have a reproducible fault and a list of suspects. Order them by likelihood and cost of checking: accessories and circuit first, then supply, then consumables such as filters, membranes and sensors, then tubing and connections inside, and only then valves, boards and modules.

The subsystem map

Level 1 lesson 2 walked the pneumatic chain from inlet to exhaust. At Level 2 you use that chain as a map for fault-finding, and you add the electronics beside it. Every ventilator, whatever its brand, has the same groups: a gas source (wall supply, internal turbine, piston pump or compressor), a way of mixing oxygen, a way of metering flow, sensors for flow, pressure and oxygen, an expiratory valve that sets PEEP, safety and ambient valves, and electronics that tie them together: control and monitoring processors, power supply and batteries, display and controls.

Place the symptom on that map before you open a cover. Wrong oxygen with correct volumes points to mixing, the oxygen supply or the oxygen sensor. Correct delivered volume on the analyser but a wrong displayed volume points to monitoring: sensor, tubing, calibration, board input. Wrong delivered volume with correct monitoring of what was delivered points to the gas source or the metering valve. PEEP that is too high or too low points to the expiratory side. Faults that change with power source, temperature or time point to electronics and power.

Test lung and analyser: your independent witnesses

Two tools turn a suspicion into evidence. The test lung gives the machine a known, repeatable load, so that you are no longer guessing whether a patient's lungs changed. Use one of known compliance and resistance, suited to the patient range, and note which one you used: a small lung with a large volume produces high pressures that are physics, not a fault, and a lung without its intended airway restrictor can upset flow sensor readings, as lesson 7 shows.

The gas flow analyser is the witness that does not belong to the ventilator. With it you can split every volume or pressure complaint into two questions: is the machine delivering what was set, and is it displaying what it delivers? Write three numbers for each point: set, displayed, measured. Set against measured is delivery; displayed against measured is monitoring. The VELA service manual makes the same point in its own way: judge monitored volumes only after you have confirmed that delivered volumes are correct, because one is calculated on the basis of the other. Use the analyser in a stated gas standard and with its own calibration valid, and never adjust a ventilator to agree with an analyser you have not checked.

Change one thing at a time, and know your limits

Swapping a suspect part with a known-good one is a strong test on ventilators, because flow sensors, oxygen cells, expiratory valve assemblies and even whole gas modules are often interchangeable between machines of the same model and revision. Swap only identical parts with the same part number and a revision the manufacturer allows, change one thing at a time, and write each change down. Return parts that proved nothing. Remember that many replacements are not finished until a calibration has been run: a swapped flow sensor, expiratory valve or oxygen cell usually needs one, and a test done without it proves nothing.

Your authority is set by your employer, your contract, the manufacturer's training and national law. In Russia, technical maintenance of medical equipment for third parties is a licensed activity; in Kazakhstan national rules make service of medical devices in use mandatory, and the work follows the manufacturer's documentation. Check the rules that apply to you before you work.

Inside that authority the usual territory is: reading logs and running the tests your access level offers, replacing consumables and modules by the manufacturer's procedure, user and service calibrations you are trained for, and verification. Outside it and outside this course: service passwords or keys you were not given, activating options or modes, changing configuration that alters how the machine ventilates, loading software other than by the manufacturer's procedure, and disabling any alarm, safety valve or protection, even for a test. And the rule from Level 1 does not change at Level 2: no fault-finding on a ventilator connected to a patient. The patient goes onto another means of ventilation first.

On-site checklist

• Patient disconnected and on another means of ventilation.
• Log and failed step photographed before any restart; clock checked.
• Step repeated once with correct circuit, test lung and instructions.
• Supplies and mains confirmed before blaming valves.
• Set, displayed and measured values written for each test point.
• One change at a time, identical parts only, calibration after each replacement.
• Fault diary kept: as-found, tests, changes, as-left.

Check yourself