Interactive teaching atlas · Chapter 1 · Machine
Anaesthesia Machine Atlas
Explore an anaesthesia workstation part by part: the path gas takes from the wall outlet to the alveolus, the physical laws that govern it, the bellows ventilator and the patient monitor. Rotate the model, tap the numbered parts and adjust the flows.
Chapter 2 · Gas path
From wall to alveolus: three pressure zones
The machine lowers gas pressure step by step. The 150 bar in the cylinder falls to around 20 cmH₂O in the patient's airway, a difference of roughly 7500-fold. At each step a different physical law and a different safety feature come into play.
High pressure
- Cylinder, hanger yoke, pin index system
- Cylinder pressure gauge (Bourdon)
- First-stage regulator: output ≈ 3–4 bar
- N₂O: liquid + vapour at 20 °C, constant ≈ 51 bar
Intermediate pressure
- Pipeline inlets and non-return valves
- O₂ flush (35–75 L/min), O₂ supply failure alarm
- Hypoxic guard, ventilator driving gas
- Flow control needle valves
Low pressure
- Flowmeter tubes (Thorpe)
- Vaporisers and interlock
- Common gas outlet → circle system
- Most frequent leak site: negative-pressure test
Chapter 3 · Ventilator
Bellows ventilator and the equation of motion
Most anaesthesia ventilators are double-circuit: driving gas squeezes the bellows from outside while patient gas stays inside it. The bellows below moves in step with the lung in the simulator. Try the six modes (VCV, PCV, PRVC, SIMV, PSV, CPAP), increase the patient's breathing effort and watch the triggering.
Patient type
Lung presets
Ventilation mode
Patient's breathing effort
Trigger and cycling
0 = patient under neuromuscular block, not breathing. As effort increases the patient triggers the ventilator (▲ above the waveform).
Pressure–volume loop. The slope shows compliance, the width of the loop shows the resistive pressure.
What is the screen telling you?
The model is a single-compartment linear lung solved in real time in 2.5 ms steps. VCV uses square-wave flow with a 10% inspiratory pause; triggering uses an adjustable flow threshold (adult default 2 L/min). Labels above the waveform show the breath type: VC, PC, PS, SP; ▲ patient-triggered, BACK-UP apnoea ventilation. Circuit compliance and fresh gas coupling are ignored.
Patient–ventilator asynchrony
Mismatches between the patient's effort and the ventilator's breaths. Markers on the waveform screen: ✕ IE ineffective triggering, DT double triggering, FS flow starvation, DC delayed cycling, AP flow not returning to zero at end-expiration (auto-PEEP).
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Chapter 4 · Monitor
Monitor: reading the waveforms
A real-time physiology model runs behind the monitor: circuit gas, alveolar O₂ and CO₂ change over minutes. If FiO₂ or ventilation fails, SpO₂ falls slowly; arrhythmia, bradycardia and arrest develop only with prolonged severe hypoxaemia. NIBP measures blood pressure intermittently; insert an arterial line for continuous monitoring. Start a machine fault or clinical event from the catalogue below, read the findings and choose the correct intervention.
Monitor and simulation
Abbreviations: HR heart rate · PR pulse rate · RR respiratory rate · NIBP non-invasive blood pressure · ART invasive arterial pressure · MAC minimum alveolar concentration.
NIBP only shows the pressure at the moment it measures; only an arterial line continuously captures a fall between two readings.
Patient: what the monitor cannot see
Case mode
Hidden events arrive in sequence during an operation. Read the findings and intervene from the event card above. At the end you get a debriefing report and a score.
Event catalogue
Choose an event. In hidden fault mode the name and description are hidden; you have to diagnose from the findings.
ECG
The electrical activity of the heart is measured as a potential difference of a few millivolts between surface electrodes. In theatre, lead II (rhythm, P waves) and V5 (left ventricular ischaemia) are most often monitored together. Diathermy interference and filter settings can artificially alter the ST segment: diagnostic mode (0.05–150 Hz) and monitoring mode (0.5–40 Hz) are not the same.
Pulse oximetry and plethysmography
Oxyhaemoglobin absorbs more infrared, deoxyhaemoglobin more red light. The device isolates arterial blood by using only the pulsatile (AC) component. Carboxyhaemoglobin makes SpO₂ falsely high, and methaemoglobin pulls it towards ≈ 85%. Respiratory variation of the plethysmograph can indicate hypovolaemia.
Capnography
A side-stream capnometer draws 50–200 mL/min of sample from the circuit; a mainstream sensor fits directly on the Y-piece. Normally EtCO₂ is 2–5 mmHg lower than PaCO₂; the gap widens with dead space.
NIBP, temperature and gas analysis
Oscillometric NIBP tracks arterial wall oscillations as the cuff deflates; the pressure at maximum oscillation is mean arterial pressure, and systolic and diastolic values are calculated by algorithm. Cuff width should be ≈ 40% of arm circumference. O₂ is measured with a paramagnetic or galvanic cell and anaesthetic agents by infrared absorption; the MAC value is the sum of the agents' contributions.
Chapter 5 · Safety
Fail-safe design
Most of the safety systems on an anaesthesia machine are a response to an accident in the past. None of them is sufficient on its own: the last line of defence against a hypoxic mixture is the O₂ analyser in the inspiratory limb and the clinician watching it.
Pin index safety system
The holes in the cylinder valve and the pins on the yoke are specific to each gas, so the wrong cylinder cannot be fitted. A missing pin or two washers stacked together defeats the system.
DISS / NIST connectors
Pipeline hoses connect with gas-specific diameters and thread profiles. Wall outlets also use gas-specific quick connectors.
Hypoxic guard
The N₂O and O₂ flow knobs are linked mechanically (chain and sprockets) or pneumatically. The Datex-Ohmeda Link-25 system limits the N₂O:O₂ ratio to 3:1, so the fresh gas contains at least 25% O₂; this does not guarantee the FiO₂ the patient inspires. Air or a third gas is outside this protection.
Oxygen failure protection device
When O₂ pressure falls (fail-safe valve) the flow of N₂O and other gases is cut off or reduced in proportion; current standards require an alarm within 5 seconds once pressure falls below the manufacturer's threshold (usually ≈ 2 bar / 30 psig) (older mechanical systems sounded for at least 7 seconds). This system monitors pressure, not flow or concentration.
O₂ flowmeter last
The O₂ tube is positioned nearest the common manifold (last, downstream). If an upstream tube leaks, the gas lost is N₂O or air, not O₂.
Vaporiser interlock and agent-specific filling
The interlock allows only one vaporiser to be turned on at a time. Keyed filling ports prevent filling with the wrong agent; if an agent with a high SVP is put into the vaporiser of an agent with a lower SVP, an overdose results.
Hands-on machine check
Find the faulty machine
A hidden fault is randomly assigned to the machine (or the machine is fine). Carry out the tests in ASA 2008 order, mark each step "Pass" or "Fail", then choose the faulty component.
Pin index positions
Positions of the yoke pins by gas (ISO 407). Choose a gas.
Machine checklist
Abridged from the ASA 2008 recommendations. Your ticks are stored only in this browser. The manufacturer's checkout procedure always takes priority.
Chapter 6 · Test
Test yourself
Each question builds on a concept from the earlier chapters. When you tap an option, the correct answer and a short explanation appear.
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