Anesthesia Machine and Workstation A Comprehensive Guide for Practical Exams

Introduction and Historical Context

When was anesthesia first introduced to the world, and what equipment was used?

Anesthesia was introduced to the world by William Thomas Green Morton on the 16th of October 1846.
He administered anesthesia for the removal of a vascular tumor; the patient was Gilbert Abbott, and the surgeons were Warren and Bigelow.
Morton used a simple ether flask, marking the birth of anesthesia.
For many years, anesthesiologists used only simple equipment like their own handkerchief, a Schimmelbusch mask, or a very simple apparatus for the open drop ether method.

Why was an anesthesia machine required, and who introduced the first one?

In the early part of the 20th century, gases like oxygen and nitrous oxide were introduced in compressed form in cylinders.
This created a requirement for a metal frame to support these cylinders, blend them with volatile anesthetic agents, and administer the mixture to the patient.
This need was fulfilled by Henry Boyle in 1917. The first manufacturers were the British Oxygen Company Limited, who named the machine after Boyle.
These were called Boyle's anesthesia machines.

What is the definition of a compressed gas?

A compressed gas is any gas stored in a cylinder with a pressure above 40 pounds per square inch (psi) at 70° Fahrenheit.
A liquid, such as nitrous oxide, can also be called a compressed gas if it has a pressure of more than 40 psi at 100° Fahrenheit.

What is the difference between an anesthesia machine and a modern anesthesia workstation?

An anesthesia machine is purely pneumatic, without electrical and electronic components.
An anesthesia workstation has electrical and electronic systems, a robotic system, an integrated ventilator, and integrated monitors.
The most important difference is the safety features: a machine has hardly any, while a workstation is full of safety features, mainly to prevent a hypoxic mixture from being delivered to the patient.
In practical exams, both may be kept side-by-side for comparison.

Who are the two main manufacturers of anesthesia workstations, and why is it important to know their differences?

The two giants that manufacture anesthesia workstations are Datex-Ohmeda (now GE) and Dräger.
There are differences in their structure and functioning.
In an exam, you might have either machine presented to you, so it is important to know the differences between GE and Dräger workstations.

Pneumatic System of the Anesthesia Workstation

How is the pneumatic system of the anesthesia machine classified?

It is classified based on the amount of pressure inside the different parts of the machine into three systems:

High Pressure System: From the cylinders up to the pressure regulator.
Intermediate Pressure System: From the pressure regulator downstream up to the flow control valves.
Low Pressure System: From the flow control valves up to the common gas outlet.

What are the typical pressures in each of these pneumatic systems?

High Pressure System: This mainly involves cylinder pressures. Oxygen pressure is roughly 2000 psig.
Nitrous oxide, at 20°C, has a vapor pressure of 750 psig.
Intermediate Pressure System: Pipeline pressure is 50-60 psig.
Cylinder pressures are reduced by pressure regulators to 45 psig before entering this system.
Low Pressure System: Pressure is just above atmospheric, about 5-8 psig at the common gas outlet.

If both a cylinder and the pipeline are connected and open, which gas source is used preferentially and why?

The workstation will preferentially use the pipeline gases.
This is a deliberate safety feature. The pipeline pressure enters the intermediate system at 50-60 psig, while the cylinder pressure enters the same system at a lower 45 psig.
This ensures that cylinders are kept as a backup source.
Despite this, it is crucial to close cylinders after checking them to prevent them from being used inadvertently during pipeline pressure fluctuations.

The High Pressure System: Medical Gas Cylinders

What are the parts of a medical gas cylinder?

A cylinder mainly consists of a body, the shoulder, the neck, and the cylinder valve.
The cylinder valve is a separate entity connected to the cylinder, not a part of the main body.
A key safety feature of the cylinder itself is its seamless body, which means no joints that could become a weak point under high pressure.

What materials are cylinders made of, and what are the advantages of modern materials?

Originally: Hand-forged steel, which had low tensile strength, requiring very thick, heavy cylinder walls and resulting in a smaller internal volume.
Presently: Molybdenum steel, which has excellent tensile strength.
This allows the cylinder walls to be thinner, making them 20% lighter and giving them a 20% increased gas capacity compared to older cylinders.
For MRI: Aluminum cylinders are also available.

What are the important markings found on a medical gas cylinder?

DOT Specifications: For example, '3AA' indicates it is made of steel. '2015' represents the service pressure (2015 psia).
Service Pressure: The pressure to which the cylinder is filled for use, roughly 2015 pounds per square inch absolute (psia).
Elastic Expansion (EE): For example, 'EE 17.5'.
This is the result of hydraulic testing, showing minimal expansion (17.5 cc) when the cylinder is pressurized to 1.6 times its service pressure, indicating good tensile strength.
Tare Weight: The weight of the empty cylinder, crucial for determining the content of liquified gases like nitrous oxide.
Test Date: Marked on a plastic or metal disc between the cylinder valve and the body, indicating when the cylinder was last tested.

What is the color coding for medical gas cylinders, and what is the new international standard?

Traditional Color Coding (1949): Oxygen: Black body with white shoulders.
Nitrous Oxide: French blue for both body and shoulders. Carbon Dioxide: Gray. Helium: Brown. Cyclopropane: Orange. Entonox: Blue body with blue and white stripes.
New International Color Coding (2010): The body of all medical gas cylinders is colored white, signifying medical grade.
The specific gas is identified by the shoulder color. For example: Oxygen: white body and white shoulder; Nitrous Oxide: white body and blue shoulder.
A capital 'N' on the shoulder denotes the new international coding.

What are cylinder valves made of and how do they function?

Cylinder valves are separate parts made of brass and chromium-plated.
They are known as direct cylinder valves or pack valves.
A Teflon packing is present, and when the spindle is rotated anticlockwise 3-4 times, the port opens, allowing gas to flow directly into the machine.
It is crucial to open the spindle, not the packing nut, which can cause a leak.

What is the Pin Index Safety System, and what are its specifications?

Introduced by Philip Woodbridge in 1952, it is a safety system to prevent connecting the wrong gas cylinder to a yoke.

Holes are located on the cylinder valve at a radius of curvature of 9/16 of an inch from the center of the port.
The holes are numbered 1-6 from right to left.
Diameter of holes: 4.75 mm
Diameter of pins on yoke: 4 mm
Length of pins and depth of holes: 6 mm
Common configurations: Oxygen is 2-5, Nitrous Oxide is 3-5, and Air is 1-5.

What are Pressure Relief Devices on cylinders and why are they needed?

If cylinders are exposed to heat, the gas inside expands.
Because the cylinder's tensile strength prevents volume expansion, pressure increases, potentially causing the cylinder to burst.
Pressure relief devices prevent this catastrophe.

Fusible Plug: Contains Woods Metal (an alloy of cadmium, bismuth, lead, and tin), which melts at 150-170°F, venting the gas. It is non-closable.
Rupture Disc: A copper disc designed to break at a specific pressure. It is also non-closable.
Closable/Spring-Loaded Valve: Opens at high pressure and closes when pressure normalizes, but is prone to malfunction and not commonly used.

In Indian cylinders, Woods Metal is often incorporated between the cylinder valve and the shoulder as a safety measure.

What is meant by "cracking" a cylinder, and how is it done safely?

"Cracking" a cylinder is the process of briefly opening it before connecting it to the machine to blow out any dust particles that might be inside the port.
These particles could block fine valves, obstruct flow, or even catch fire and cause an explosion.
To do it safely, incline the cylinder forward, away from you, and never keep your head above the cylinder while opening it.

Why should a cylinder be opened slowly, and what is an adiabatic process in this context?

Opening the cylinder rapidly allows a large volume of high-pressure gas to accumulate in the small area between the yoke and pressure regulator.
This sudden compression of gas generates a large amount of heat energy in a process called an adiabatic process, where heat is gained so quickly it cannot be dissipated.
This heat can cause dust particles to catch fire, potentially leading to an explosion.

What is the hydraulic test for cylinders, and why is it the most common?

It is a test done every 5-10 years to check a cylinder's tensile strength.
An empty cylinder is placed in a water jacket.
It is then pressurized hydraulically to 1.6 times its service pressure.
Any expansion is measured by a rise in the water column.
The expansion should be minimal (e.g., 15 cc), indicating the cylinder is safe.
It is the most common test because, unlike flattening, impact, or bend tests, it does not require the cylinder to be destroyed.

How are oxygen and nitrous oxide cylinders filled differently?

Oxygen: Its critical temperature is -118°C, so it remains a gas at room temperature.
It is filled based on pressure (up to its service pressure of 2000 psig).
Nitrous Oxide: Its critical temperature is 36.5°C, so it exists as a liquid at room temperature.
It is filled based on a "filling density" (filling ratio), which is the weight of nitrous oxide divided by the weight of water the cylinder could hold.
In tropical countries, this ratio is 0.68. The cylinder is not completely filled; 5-10% of the top is left for vaporization.

How can you estimate the contents of an oxygen cylinder versus a nitrous oxide cylinder?

Oxygen: As a gas, its pressure is directly proportional to its contents at a constant temperature (Boyle's Law).
A full 'E' cylinder has about 660 liters. If the pressure gauge shows half of the full pressure, the cylinder is half full.
Nitrous Oxide: As a liquid, the pressure gauge reads the vapor pressure until all liquid is gone.
To know the content, you must weigh the cylinder and subtract its tare weight.
Using Avogadro's hypothesis (1 gram molecular weight of any gas occupies 22.4 L at STP), you can calculate that 1 gram of nitrous oxide yields about 0.55 L at room temperature.
A full 'E' cylinder contains roughly 3.4 kg of nitrous oxide, yielding about 1850 liters.

Hanger Yoke Assembly

What is the function of the Hanger Yoke Assembly?

The Hanger Yoke Assembly has three main functions:

It supports and orients the cylinder to the machine.
It maintains a leak-proof connection between the cylinder and the machine (using a Bodok seal).
It maintains a unidirectional flow of gases towards the machine (via check valves).

In modern workstations, the yoke assemblies are often located at the back of the machine, below the pipeline inlets.

What is a Bodok seal, and why should only one be used?

A Bodok seal is a washer placed around the nipple of the yoke to ensure a leak-proof connection.
It is made of neoprene rubber with a metal aluminum ring, giving it a long life and resistance to wear and tear.
Only one Bodok seal should be used because using more than one can nullify the Pin Index Safety System.

What is a yoke plug and a yoke block, and what is their difference?

Yoke Plug: A small metal piece used to plug a yoke assembly that is not in use.
This prevents leaks if the check valve malfunctions and prevents gas transfer between cylinders.
Yoke Block: In older machines, this device was used to connect a pipeline directly to the yoke assembly.
It resembles a cylinder valve and has pin index holes on both sides to prevent the Pin Index System from being nullified if the block is inverted.

Intermediate Pressure System

What is the Diameter Index Safety System (DISS)?

DISS is a safety system for pipeline inlets on the workstation and outlets on the wall.
It uses specific diameters for the nut and stem (or body and probe) to ensure that a pipeline for a specific gas can only be connected to the correct inlet.
In American-made workstations, the nuts are diameter-indexed.
In British-made workstations, Non-interchangeable Screw Threads (NIST) connectors are used, where the nuts are the same diameter, but the probe is diameter-indexed.

What are pressure gauges, and what safety features are associated with them?

Pressure gauges, often Bourdon-type, display the gas pressure.
They consist of a C-shaped copper tube that straightens under pressure, moving an indicator.
Safety features include:

In older machines: A thick, break-proof glass front and a thin metal sheet at the back so that if the tube bursts, gases vent backward, away from the user.
Color coding: The gauges are color-coded for the gas (e.g., white for oxygen, blue for nitrous oxide).
In modern workstations: Electronic pressure sensors display pressures directly on a monitor.

Modern workstations also have pressure gauges for pipeline gases, providing an early warning if the pipeline supply is being exhausted.

Why were there initially no pressure gauges for nitrous oxide in some older machines?

Oxygen remains a gas in the cylinder, so its pressure gauge directly reflects the cylinder's contents.
However, nitrous oxide is a liquid, and its pressure gauge will show a constant vapor pressure (around 745 psig at 20°C) until all the liquid has evaporated.
Only then does the pressure start to fall, indicating the cylinder is nearly empty.
Despite this, modern machines include them to provide a warning when the cylinder is being exhausted.

What is the basic principle and function of a pressure regulator?

The basic principle is Force = Pressure x Area.
By keeping the force constant and increasing the area on which a gas acts, its pressure drops.
The main function of a pressure regulator is to provide a constant, reduced pressure of gases to the flow meters, ensuring that flows remain accurate even as cylinder pressure drops.
Other functions include preventing wear and tear on fine needle valves and preventing barotrauma.

What are Master and Slave regulators in older machines?

This was an early fail-safe mechanism where the oxygen pressure regulator acted as a 'master' and the nitrous oxide regulator as a 'slave'.
Oxygen pressure had to be present to open the nitrous oxide regulator, theoretically preventing nitrous oxide flow without oxygen.
However, this system was not foolproof, as it still allowed a hypoxic mixture if nitrous oxide was added.

What are Fail-Safe valves, and what is the difference between GE and Dräger systems?

Fail-safe valves are pneumatic devices that cut off or reduce the flow of other gases (like nitrous oxide) if the oxygen pressure falls below a certain threshold.

GE (Shut-Off Valve): If oxygen pressure drops below 25 psig, it completely shuts off the nitrous oxide flow.
Dräger (Oxygen Failure Protection Device - OFPD): This device proportionately reduces nitrous oxide pressure as oxygen pressure falls.
It maintains a ratio until oxygen pressure drops to 10 psig, at which point it completely shuts off nitrous oxide.

What is an Oxygen Supply Failure Alarm, and how does it work?

Standards require an audible alarm to activate within 5 seconds if oxygen pressure drops below 30 psig.

Older machines: Used pneumatic whistles (e.g., Ritchie whistle, Bosun's whistle) powered by a small internal reservoir of oxygen.
A problem was that the whistle would stop once this small oxygen reserve was exhausted.
Modern machines: Use electronic pressure sensors and alarms that continue to sound until the pressure is restored.

What is the Oxygen Flush, and what are its uses and risks?

The oxygen flush is a valve that delivers a high flow (35-75 L/min) of oxygen directly from the source, bypassing the flow meters and vaporizers.
Uses:

Emergency ventilation when there is a large leak around a mask.
Leak testing the machine and circuit.

Risks:

Accidental activation can cause barotrauma.
It dilutes the anesthetic concentration, potentially causing patient awareness (light planes).

What is the Fresh Gas Decoupling valve in Dräger machines?

This is a safety feature in Dräger workstations.
During inspiration, when the oxygen flush is activated, the fresh gas decoupling valve diverts the flush volume to the reservoir bag, not directly to the patient.
The gas is only delivered to the patient during the expiratory phase, preventing barotrauma from accidental flush activation.

What is the purpose of a Second Stage Pressure Regulator in GE machines?

Found only in GE machines, these regulators further reduce the pressure of gases before they reach the flow meters.
Oxygen is reduced to 14 psig and nitrous oxide to 26 psig.
This difference in pressures helps maintain a 3:1 nitrous oxide-to-oxygen ratio (25% oxygen) through the Link-25 proportioning system.
It ensures accurate flow even with pressure fluctuations upstream.

What do Dräger machines use instead of Second Stage Pressure Regulators?

Dräger machines use simple flow restrictors upstream of the flow control valves.
Based on Bernoulli's principle, a narrowing in the tube causes a pressure drop.
This is less costly but has the disadvantage that upstream pressure fluctuations can cause less accurate flows compared to second-stage regulators.

How many connections are there for the oxygen source inside GE vs. Dräger machines?

GE Machines (6 connections): Flow meters, fail-safe valve, oxygen supply failure alarm, auxiliary oxygen flow meter, oxygen flush, and anesthesia ventilator driving gas (since GE uses bellows ventilators).
Dräger Machines (5 connections): Dräger uses piston-driven ventilators, which do not require a driving gas, hence they have one fewer connection.

Low Pressure System

What is the arrangement of flow meters in modern workstations and why?

Modern workstations have two flow meters arranged in series for each gas: one for low flows (e.g., up to 1 liter) for accuracy in low-flow anesthesia, and another for higher flows.
Crucially, the oxygen flow meter is always placed downstream of all other gases.
This ensures that if a flow meter upstream (e.g., for air or nitrous oxide) is broken, that gas leaks out, but the oxygen flow to the patient is preserved, preventing a hypoxic mixture.

What are the safety features on flow control knobs and flow tubes?

Flow Control Knobs: They are touch-coded and color-coded. Oxygen is the largest knob with widely spaced, large flutes, making it easily identifiable by touch.
Nitrous oxide and air knobs are smaller. Oxygen knobs are fluted, while nitrous oxide knobs are conical.
Guard: A projecting part above the knobs prevents inadvertent movement.
Flow Tubes (Thorpe Tubes): Made of hard glass (Plexiglas) and coated internally with an anti-static material (Croxtine) to prevent static electricity buildup, which could cause the bobbin to stick to the wall.
Tapered Tube: The tube is tapered to reduce its overall length. At the bottom, the annular space is small, and at the top, it is larger.
Stop: A stop at the top prevents the bobbin from being lost into the manifold.

What is the principle of the flowmeter bobbin, and what is its fluorescent dot for?

The bobbin, made of lightweight aluminum, floats due to equilibrium between the downward force of gravity and the upward force of the gas flow.
Oblique flutes on the bobbin cause it to spin, indicating flow.
A fluorescent dot on the bobbin allows its movement to be seen in dark environments, such as radiology suites.

Why is the concept of 'annular space' in a flow tube important?

The annular space is the gap between the bobbin and the inner wall of the flow tube.
At low flows, this space is small, making the flow laminar (tubular) and obeying the Hagen-Poiseuille law (flow is inversely proportional to viscosity).
At higher flows, the space is larger, making the flow turbulent (orificial) and obeying Graham's law (flow is inversely proportional to the square root of density).
This is clinically significant because flow meters are calibrated at sea level.
At high altitudes, the density of gases changes, affecting the accuracy of higher flows and potentially leading to a hypoxic mixture.

What is the Link-25 proportioning system in GE machines?

Link-25 is a mechanical proportioning device in GE machines.
It consists of sprockets (29 teeth for oxygen, 16 for nitrous oxide) connected by a chain behind the flow control knobs.
The 'link' is established when the oxygen flow falls to 25% of the total flow, preventing the delivery of a hypoxic mixture with less than 25% oxygen.
It maintains a 3:1 nitrous oxide-to-oxygen ratio.
A simple way to check for Link-25 is to open the nitrous oxide knob; if the oxygen knob also rotates, the machine has Link-25.

What is the Oxygen Ratio Monitor Control (ORMC) in Dräger machines?

ORMC (or Sensitive Oxygen Ratio Control - SORC) is a purely pneumatic proportioning device in Dräger machines.
It uses resistors and diaphragms in the oxygen and nitrous oxide flow paths to ensure a minimum 25% oxygen concentration.
If you try to increase nitrous oxide flow beyond the set ratio, the device creates a back pressure that closes a slave valve on the nitrous oxide side, preventing its flow.
Unlike Link-25, turning the nitrous oxide knob will not move the oxygen knob.

What are the check valve and pop-off valve for?

Located downstream of the flow meters, these valves protect the low-pressure system from back pressure, which can occur during positive pressure ventilation or if the common gas outlet is obstructed.

Check Valve: Prevents back pressure from reaching the vaporizers, which could cause a 'pumping effect' and alter agent delivery.
Pop-off Valve (Pressure Relief Valve): Opens at a set pressure (e.g., 300 cm H2O) to vent excess pressure to the atmosphere, protecting the flow control valves and tubes from damage.

What are the two types of leak tests for the low-pressure system?

Positive Pressure Leak Test: The common gas outlet is occluded, and the system is pressurized to 20 cm H2O using the oxygen flush.
The pressure should remain stable for at least 10 seconds.
This test cannot detect leaks upstream of a check valve.
Negative Pressure Leak Test (Universal Leak Test): A suction bulb is used to create a vacuum in the system.
If the system is leak-free, the bulb will remain collapsed.
This test works regardless of check valves and is therefore considered the universal test.

The most important safety measure is always to have an oxygen analyzer at the patient's end to detect any hypoxic mixture, as there are limited safety features between the flow meters and common gas outlet.

Conclusion and Important Questions

What is the most important thing for patient safety despite all machine advancements?

Despite all the technological advancements and safety features, an oxygen analyzer and a vigilant anesthesiologist are the most important factors for managing any kind of anesthesia safely.

Who tests the cylinders (e.g., hydraulic testing)?

The testing of cylinders, including hydraulic tests, is usually carried out by the company or the manufacturers themselves.