Boyle's machine to Anesthesia Workstation
Welcome to the comprehensive guide
on understanding the anesthesia
machine and its evolution into the
modern anesthesia workstation.
! Historical Evolution of the Anesthesia Machine
''What is the basic function of an anesthesia machine?''
The basic function of an anesthesia machine
is to prepare a gas mixture of precisely
non but variable composition.
The gas mixture can then be
delivered to a breathing system.
''When did the first anesthetic machines appear and what were they like?''
The first anesthetic machines appeared
following the public demonstration of
anesthesia by Morton in 1846.
These initial devices were simple
inhalers based on the evaporation
of the anesthetic agent.
''What technological advances were incorporated into gas machines in the early 20th century?''
At the beginning of the 20th century,
gas machines emerged, incorporating
technological advances such as flow
meters, carbon dioxide absorption
systems, and fine adjustment vaporizers.
''Who invented the Boyle's machine and when?''
Boyle's machine was invented by
Henry Edmond Gaskin Boyle in 1917.
His machine was a modification of
the American Gwathmey apparatus of 1912
and became the best-known early
continuous flow anesthesia machine.
''Who were the other pioneers in manufacturing anesthesia machines before Boyle?''
Two other great men had done excellent
work before Boyle.
One was James Taylor Gwathmey,
practicing in New York, who invented
the Gwathmey machine in 1912.
Later, Jeffrey Marshall developed a
machine during the First World War
(1914-1918) based on the Gwathmey machine.
''What are some key landmarks in the evolution of the anesthesia apparatus?''
Key landmarks include: 1921 - Waters'
to-and-fro absorption apparatus;
1927 - Flow meter for CO2 and the
familiar back bar; 1930 - Plunge of
the vaporizer and introduction of
circle absorption system; 1933 -
Dry bobbin flow meters; 1952 -
Pin Index Safety System (PISS);
1958 - Introduction of Bourdon gauge.
''How is a modern anesthesia workstation defined?''
A modern anesthesia workstation is
defined as a system for the administration
of anesthesia to patients, consisting of
the gas delivery system, breathing system,
anesthetic gas scavenging system,
anesthetic vapor delivery system,
anesthesia ventilator, and associated
monitoring and protection devices.
! The Boyle's Machine: Foundational Pneumatic Systems
!! High Pressure System
''What is the function of the high-pressure system?''
The high-pressure system receives gases
from the cylinders at high variable
pressures and reduces those pressures
to a lower, more constant pressure
suitable for use in the machine.
''What are the components of the high-pressure system?''
It consists of the hanger yoke assembly,
cylinder pressure gauge, and primary
pressure regulator.
''What is the function of the hanger yoke?''
The hanger yoke orients and supports the
cylinder, provides a gas-tight seal,
and ensures a unidirectional gas flow.
''What safety feature is part of the hanger yoke assembly?''
The safety feature is the index pins
of the Pin Index Safety System (PISS),
which prevent attaching an incorrect
cylinder.
''How does the check valve in the hanger yoke assembly function?''
When cylinder pressure exceeds machine
pressure, the plunger slides away,
allowing gas to flow into the machine.
When machine pressure exceeds cylinder
pressure, the plunger moves to prevent
the escape of gas from the machine.
''What should be done when a cylinder is exhausted and a full one is not available?''
A yoke plug should be placed in the
empty yoke to form a seal and prevent
gas from escaping from the machine.
''Why should only one cylinder of a gas be open at a time?''
To prevent trans-filling between paired
cylinders, which can occur as a result
of a defective check valve.
''What is the purpose of the cylinder pressure gauge?''
The pressure gauge displays the cylinder
pressure for each gas supplied by the
cylinder, allowing the user to monitor
the remaining gas volume.
''What is the function of a pressure regulator?''
A pressure regulator reduces the high
and variable pressure found in a
cylinder to a lower, more constant
pressure suitable for use in an
anesthesia machine.
''What is a slave regulator in the context of nitrous oxide?''
In some machines, the nitrous oxide
pressure regulator is a slave regulator,
where the main spring is replaced by
oxygen pressure.
This means if the oxygen line is not
pressurized, there will be no output
from the nitrous oxide regulator,
acting as a safety feature.
!! Intermediate Pressure System
''What is the intermediate pressure system and what does it include?''
The intermediate pressure system receives
gases from the pressure regulator or
pipeline inlet.
Its pneumatic components include the
master switch, pipeline inlets, pressure
indicators, piping, gas power outlet,
oxygen pressure failure devices,
oxygen flush, secondary regulators,
and flow control valves.
''What is the Diameter Index Safety System (DISS) or NIST?''
These are non-interchangeable, threaded
fittings on pipeline inlets to ensure
gases are connected to the correct
machine inlet.
''Why is it important for a pipeline pressure indicator to be on the pipeline side of the check valve?''
If the indicator is on the pipeline side,
it monitors pipeline pressure only.
If the hose is disconnected, it reads zero,
even if a cylinder valve is open,
giving a true indication of pipeline
supply status.
''What is the correct pipeline pressure that should be indicated before machine use?''
The pressure should be between 50 to 55
PSIG (pounds per square inch gauge).
''What is the function of an oxygen pressure sensor shutoff system?''
It shuts off or proportionately decreases
and ultimately interrupts the supply of
nitrous oxide if the oxygen supply
pressure decreases, preventing hypoxic
mixtures.
''What is an Oxygen Ratio Controller (SORC) system?''
These are devices that shut off the
supply of gases other than oxygen or
alarm when oxygen pressure falls to a
dangerous level.
It is a mechanical interlock designed
to maintain a ratio of no less than
25% oxygen, regardless of operator input.
''What is the purpose of the oxygen flush?''
The oxygen flush receives oxygen from
the pipeline or cylinder regulator and
directs a high, unmetered flow directly
to the common gas outlet.
This is used to rapidly deliver 100%
oxygen.
''Why might there be secondary pressure regulators in the intermediate pressure system?''
They are used just upstream of flow
indicators to reduce pressure further
and eliminate fluctuations from the
pipeline, ensuring more constant flow.
!! Low Pressure System
''What defines the low-pressure system?''
The low-pressure system is downstream
of the flow control devices.
The pressure in this section is only
slightly above atmospheric and varies
depending on the flow from the flow
control valves.
''What components are found in the low-pressure system?''
It includes the flow meters, hypoxia
prevention safety systems, pressure
relief valves, unidirectional valve,
and the common gas outlet.
''How does a traditional mechanical flow meter (rotameter) work?''
It is a constant pressure, variable
orifice flow meter.
Gas enters a vertically tapered tube,
pushing a bobbin or float up.
The height of the bobbin is proportional
to the flow rate.
''How should the flow reading be taken with different float shapes?''
For a ball float, read at the center
(the widest portion).
For a bobbin, read at the highest and
widest portion of the float (the top rim).
''How is low flow measurement (below 1 L/min) achieved?''
This is achieved by using two flow meter
tubes for the same gas in a cascade.
A long, thin tube is accurate for flows
from 0-1000 mL/min, complementing a
second conventional tube for higher flows.
''Why is the oxygen flow meter positioned as the most downstream one?''
Placing the oxygen flow meter closest to
the common gas outlet is an anti-hypoxia
safety feature.
In the event of a leak in another flow
meter, oxygen will still flow directly
to the outlet, making a hypoxic mixture
less likely.
''What is the purpose of the pressure relief valve in the low-pressure system?''
It prevents buildup of excessive pressures
upstream of the outlet check valve by
venting gas to the atmosphere if a preset
pressure is exceeded.
''What is the purpose of the unidirectional (check) valve near the common gas outlet?''
Its purpose is to prevent reverse gas
flow, which would permit fresh gas to
re-enter the vaporizer.
This prevents the "pumping effect."
! The Modern Anesthesia Workstation: Components and Advances
''What defines a modern integrated anesthesia workstation?''
It is designed to be a complete anesthesia
and respiratory gas delivery and monitoring
system, combining advanced ventilation,
gas and vapor delivery with patient
monitoring and information management.
''What are the key components of a modern workstation?''
Components include the gas delivery and
scavenging system, vaporizers, electronic
flow meters, the ventilator, and monitors.
''How do electronic flow meters work?''
Many use a mass flow sensor.
Gas flows past a heated chamber, and
the electrical energy required to maintain
a set temperature is proportional to the
specific heat and flow rate of the gas.
Flow is extrapolated from this energy.
''What is an advantage of electrically driven piston or turbine ventilators?''
They do not require pressurized drive
gases, making them economical in their
use of oxygen and usable when pressurized
oxygen is in short supply.
''What major advance in intraoperative ventilation do modern workstations provide?''
They can deliver very low tidal volumes
accurately (e.g., 20 mL) due to compliance
compensation, making neonatal ventilation
possible on circle systems.
''How does fresh gas decoupling work to ensure accurate tidal volume?''
During inspiration, circuit pressure
closes the fresh gas decoupling valve,
directing fresh gas to the breathing bag
so it doesn't interfere with the delivered
tidal volume.
During expiration, the valve opens, allowing
fresh gas and gas from the bag to refill
the ventilator chamber.
''What additional monitoring waveforms are now standard on workstations?''
In addition to pressure-time and volume-time
curves, modern machines display flow-time
and flow-volume waveforms, essential for
ventilating diseased lungs.
''What is Target Controlled Anesthesia (TCA) in the context of a workstation?''
TCA is a fully automatic system where the
clinician sets target end-tidal oxygen
and agent values.
The system constantly monitors and
automatically adjusts gas delivery and
total flow to achieve and maintain these
targets, which is economical and
environmentally friendly.
''How do modern workstations facilitate Total Intravenous Anesthesia (TIVA)?''
Some workstations are equipped with syringe
pumps that have an integrated drug database
actively linked to a software system,
automatically setting default values and
dosage ranges for various drugs.
''What is an Anesthesia Information Management System (AIMS)?''
AIMS is a specialized electronic health
record system that allows automatic,
reliable collection, storage, and
presentation of peri-operative data for
management, quality assurance, and research.
! Conclusion and Key Takeaways
''Are modern anesthesia workstations completely foolproof?''
No.
Although gas delivery systems and monitoring
have progressed, newer technology can also
be associated with new problems of their own.
''What is the key message for anesthesiologists using modern workstations?''
It is the user's responsibility to go
through the manufacturer's instructions
on features and simple troubleshooting.
A thorough understanding is needed for
optimal anesthesia practice, as all
advancements are aimed at improving safety
and addressing environmental concerns.