Apex Anesthesia Machine

Where does each anesthesia-machine pressure system begin and end?

• High: cylinder → cylinder regulator

• Intermediate: pipeline → flowmeter valves

• Low: flowmeter tubes → common gas outlet

Pipeline pressure vs regulated cylinder pressure?

• Pipeline: 50 psi

• Cylinder after regulator: 45 psi

• Higher pipeline pressure makes it the primary source.

What components are in the low-pressure system?

Flowmeter tubes → vaporizers → check valve, if present → common gas outlet

How is the negative-pressure (low-pressure) leak test performed?

• Bulb attached to CGO

• Create −65 cm H₂O

• Bulb stays collapsed for 10 seconds

• Reinflation = leak/failure

What must be off during the negative-pressure leak test?

• Fresh gas flow

• Ventilator

• Machine, if it produces minimum flow

• Vaporizers initially; then test each one individually

How is the positive/high-pressure leak test performed?

• Close APL valve

• Pressurize to 30 cm H₂O

• Pressure remains constant = pass

• Pressure falls = leak

Best test for detecting a vaporizer leak?

Negative-pressure leak test, repeated with each vaporizer turned on individually.

How does a check valve affect the positive-pressure leak test?

A closed check valve prevents pressure from reaching the rest of the low-pressure system, so an upstream leak may be missed.

What signifies a failed-low pressure leak test

bulb reinflates within 10 seconds.

How much negative pressure should be created for a proper negative-pressure test?

-65 cm H2O

What does SPDD mean in the anesthesia machine?

• Supply: gases enter the machine

• Processing: gases prepared inside machine → CGO

• Delivery: breathing circuit brings gases to patient

• Disposal: scavenging removes waste gases

What are the five tasks of oxygen?

1. O₂ pressure failure alarm

2. O₂ pressure failure device/failsafe

3. O₂ flowmeter

4. O₂ flush valve

5. Ventilator drive gas

Memory: Alarm → Failsafe → Flowmeter → Flush → Ventilator

PISS vs DISS?

• PISS = cylinders: unique pin positions —> More than one washer can bypass PISS.

• DISS = hoses/pipelines: unique diameter and threads

What are the PISS pin configurations?

• Air = 1, 5

• O₂ = 2, 5

• N₂O = 3, 5

E-cylinder pressure and capacity

Air: 1,900 psi | 625 L

O₂: 1,900 psi | 660 L

N₂O: 745 psi | 1,590 L

CO₂ is also liquid in its cylinder: 838 psi, 1,590 L.

How do O₂ and N₂O cylinder contents differ?

• O₂ = gas: pressure falls proportionally with contents.

• N₂O = liquid + gas: pressure stays 745 psi until all liquid is gone.

• Determine N₂O contents by weight, not pressure.

How do you calculate O₂ cylinder duration?

1. Remaining liters = 660 L × gauge pressure ÷ 1,900 psi

2. Minutes remaining = remaining liters ÷ flow rate

Example: 500 psi at 2 L/min → 174 L → 87 minutes

What must you know about cylinder identification and colors?

• Identify the gas by its label—not color.

• US: O₂ green, air yellow, N₂O blue

• WHO: O₂ white, air black/white, N₂O blue

Essential cylinder-handling rules?

• Protect the valve—most delicate part

• Store upright and secured

• No replacement cylinder → insert yoke plug

• MRI → only nonmagnetic cylinder, such as aluminum

• Never use more than one washer because it can bypass PISS

What should you do if a cylinder connection hisses?

1. Tighten the connection

2. Replace the washer/gasket

3. Use a different cylinder

Never stack washers.

How are cylinder fires/explosions prevented?

• O₂ and N₂O are oxidizers

• Fire triad: oxidizer + fuel + ignition

• Never oil the valve

• Open cylinders slowly

• Avoid temperatures above 130°F/57°C

• “Cracking” briefly opens the valve to clear debris

What cylinder testing and regulatory facts are important?

• Tested at 1.66 × service pressure

• Tested every 5 years

• 5-point star permit = every 10 years

• ASTM: machine components/F1850

• FDA: pre-use checkout

• OSHA: occupational anesthetic exposure

• DOT: compressed-gas cylinders

What activates the O₂ pressure-failure device?

• O₂ supply pressure <20 psi

• Exhausted O₂ source/tank

What does the O₂ pressure-failure device NOT detect?

• Pipeline crossover

• N₂O accidentally flowing through the O₂ pipeline

• FiO₂ below 21%

• Leak distal to the flowmeters

Differentiate the two O₂ safety alarms.

• Pressure alarm: sounds at 28–30 psi

• Failsafe/pneumatic device: reduces or stops N₂O below 20 psi

• O₂ analyzer: alarms when FiO₂ <21%

• _{30 = pressure alarm}

• _{20 = failsafe}

• _{21% = oxygen analyzer}

Ohmeda failsafe vs Dräger protection device?

• Ohmeda: O₂ <20 psi → completely stops N₂O; all or nothing

• Dräger: decreasing O₂ pressure → proportional reduction in N₂O

Detects low-pressure causes:

• Depleted O₂ tank

• Drop in pipeline pressure

• Disconnected O₂ hose

Misses despite possible hypoxia:

• O₂-pipeline crossover

• Flowmeter leak

Reason: it senses O₂ pressure—not O₂ concentration.

What does the hypoxia-prevention/proportioning device do?

_{Proportioning device = Hypoxia prevention safety device}

• Links O₂ and N₂O flow

• Limits N₂O to 3× the O₂ flow

• Maximum N₂O = 75%

Minimum FiO₂ permitted = 25%

When can the proportioning device still allow a hypoxic mixture?

• O₂-pipeline crossover

• Leak distal to flowmeter valves

• Addition of a third gas

• Defective mechanical/pneumatic components

What must I know about the Thorpe-tube flowmeter?

• Begins the low-pressure system

• Variable-orifice tube: narrow bottom → wider top

• Gas flow pushes float up; gravity pulls it down

• Ball = read middle

• All other floats = read top/widest part

How do flow and Reynolds number relate?

• Low flow: laminar → depends on viscosity → Poiseuille

• High flow: turbulent → depends on density → Graham

• Re < 2,000 = laminar

• Re 2,000–4,000 = transitional

• Re > 4,000 = turbulent

Why is the O₂ flowmeter placed closest to the common gas outlet?

• O₂ is downstream/far right on U.S. machines.

• A leak in an upstream air or N₂O tube allows that gas to escape while downstream O₂ continues toward the patient.

• A leak in the O₂ flowmeter itself can still cause a hypoxic mixture.

• Flowmeters are distal to other safety devices; the O₂ analyzer is the final safeguard.

Reynolds numbers

• Re <2,000: Laminar flow

• Re 2,000–4,000: Transitional flow

• Re >4,000: Turbulent flow

Poiseuille’s equation vs Graham’s law

• Low flow → laminar → viscosity → Poiseuille

• High flow → turbulent → density → Graham

Memory: Low Loves Viscosity; High Has Density.

How do you calculate delivered tidal volume on a flow-coupled anesthesia machine?

look at formula

Convert I:E ratio into inspiratory fraction.

• 1:1 → 1/2

• 1:2 → 1/3

• 1:3 → 1/4

What increases tidal volume in a flow-coupled machine?

• ↑ Fresh gas flow

• ↑ Bellows volume/height

• Longer inspiration: 1:2 → 1:1

• ↓ Respiratory rate

What changes decrease delivered tidal volume in a flow-coupled machine?

• ↑ Respiratory rate

• Shorter inspiration: 1:2 → 1:3

• ↓ Fresh gas flow

• ↓ Bellows height

Memory: More time, flow, or bellows volume = larger Vₜ.

Circuit compliance

Compliance measures distensibility. During positive-pressure ventilation, some gas expands the circuit instead of reaching the patient.

How do you calculate volume lost to circuit compliance?

look at formula

Variable-bypass vaporizer

• Variable bypass

• Flow-over

• Temperature-compensated

• Out-of-circuit

• Agent-specific

Tipped vaporizer

_{Tipping may send liquid agent into the bypass chamber → dangerously increased output.}

Treatment:

1. Drain it.

2. Set dial to maximum.

3. Run high FGF for 20–30 minutes, until agent reading reaches zero.

Vaporizer numbers

• Vaporizing-chamber gas: 100% saturated

• 1 mL liquid agent ≈ 200 mL vapor

• Flow below 200 mL/min or above 15 L/min may decrease output

• Liquid used per hour:

mL/hr=Vol%×FGF (L/min)×3

Pumping effect and leaks

• Pumping effect → increased output from gas being forced backward into the vaporizing chamber.

• Promoted by PPV/O₂ flush, low FGF, low dial setting, and low liquid level.

• Most common cause of vaporizer leak: loose filler cap.

• Test for a vaporizer leak with the vaporizer turned on.

What is the “T” setting?
Transport mode—prevents liquid anesthetic from entering the bypass chamber if the vaporizer is tipped during transport/removal.

Most common cause of a vaporizer leak?
Loose filler cap.

How does the vaporizer compensate for cooling?

The temperature-compensating valve changes the ratio of vaporizing-chamber flow to bypass flow to maintain constant output.

• variable bypass vaporizer does NOT require recalibration for elevation

Why does desflurane need a special vaporizer?

• Low potency: MAC 6.6%

• Boiling point: approximately 22°C

• Vapor pressure: 3–4× higher than other agents

TEC 6 numbers and function

• Heats desflurane to 39°C

• Pressurizes it to 2 atmospheres

• Injects vapor directly into the FGF

• Does not use variable bypass or flow-over

• Dual circuit, requires: power

Memory: Des = 39 and 2.

TEC 6 delivers the dialed volume percent, but it does not compensate for atmospheric pressure.

• Higher altitude → lower atmospheric pressure → increase dial

• Hyperbaric environment → higher pressure → decrease dial

Required dial= (Normal dial×760​)/ (Ambient pressure)

Oxygen analyzer

• Located in the inspiratory limb

• Measures O₂ concentration, not pressure

• Final defense against a hypoxic mixture

• First device to detect an O₂ pipeline crossover or O₂ flowmeter leak

Analyzer types:

• Galvanic

• Paramagnetic

closed-circuit anesthesia

O₂ delivery is matched to O₂ consumption.

• Average adult O₂ consumption: 250 mL/min

• If consumption rises but delivery stays constant → FiO₂ falls

• Causes: fever, sepsis, pain, sympathetic stimulation, thyrotoxicosis

• Only the oxygen analyzer detects this developing hypoxic mixture

Immediate response to O₂ pipeline failure/crossover

1. Open the O₂ cylinder

2. Disconnect the O₂ pipeline

3. Confirm FiO₂ is increasing on the oxygen analyzer

_{Why disconnect? Pipeline pressure can prevent cylinder oxygen from entering the machine—even if the pipeline contains the wrong gas.}

If FiO₂ does not increase—assume machine malfunction

• Ventilate with an Ambu bag

• Use an independent O₂ cylinder or room air

• Convert to TIVA

• Use low flows to conserve cylinder O₂

• Avoid the machine’s auxiliary O₂ flowmeter—it may be supplied by the contaminated pipeline

• Reconnect only after the pipeline is tested

Memory: Cylinder ON → Pipeline OFF → Verify FiO₂.

• O₂ analyzer alarms → assume pipeline crossover until proven otherwise.

• Pipeline failure → cylinder ON, pipeline OFF.

Oxygen flush valve

• Intermediate-pressure system → breathing circuit

• Bypasses flowmeters and vaporizers

• Delivers 35–75 L/min at approximately 50 psi

• Dilutes volatile anesthetic → possible awareness

Why avoid O₂ flush during inspiration?

During mechanical inspiration, the spill valve is closed. Pressing the flush sends its high flow/pressure toward the patient → barotrauma.

Ventilator spill valve

• Inspiration: drive gas compresses bellows and closes spill valve

• Expiration: exhaled gas refills bellows; spill valve opens around 2–4 cm H₂O and sends excess gas to scavenging

• This pressure creates intrinsic PEEP

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Memory: Inspiration = spill shut; expiration = spill excess.

What occurs during inspiration?

• Drive gas compresses the bellows

• Drive gas closes the spill valve

• Fresh gas flows to the patient

• O₂ flush during this phase → barotrauma risk

What occurs during expiration?

• Drive gas stops/exits

• Patient’s expired gas refills the bellows

• Once circuit pressure exceeds 2–4 cm H₂O, the spill valve opens

• Excess gas flows to the scavenging system