by Dr Zambu (United Bulawayo Hospitals)

1. D R C A R O L Y N Z A M B U
The Anesthetic Machine.

2. The anesthetic machine

3. Overview
 The anesthetic machine comprises of 6 basic
subsystems:
1. Gas supplies: pipelines and cylinders
2. Gas flow measurement and control (flowmeters)
3. Vaporizers
4. Gas delivery: breathing system and ventilator
5. Scavenging
6. Monitoring

5. Overview
 The anesthetic machine is basically used to ventilate,
oxygenate and administer inhalational anesthetic to the
patient.
 It receives medical gases from a gas supply, controls the
flow, reduces the pressure to safe levels, vaporizes
volatile anesthetics into the final gas mixture and
delivers the gases at the common gas outlet to the
breathing circuit connected to the patient`s airway.
 Modern machines have become sophisticated with many
built-in safety features and devices, monitors and
microprocessors that can integrate and monitor all
components.

6. Supply of gases
 Most machines have gas inlets for oxygen, nitrous
oxide and air, others lack air inlets whilst other
machines have a 4th inlet for helium, heliox, CO2 or
nitric oxide.
 There are separate inserts for primary gas supply and
the secondary cylinder gas supply, therefore the
machine has 2 gas inlet pressure gauges for each gas.

7. Pipeline inlets
 Supply oxygen and nitrous oxide (and often air).
 Tubing is color coded connecting to the anesthesia machine
through a noninterchangeable diameter-index safety
system(DISS).
 The bore diameter of the body and that of the connection
nipple are specific for each gas.
 There is a filter that helps trap debris from the wall supply , a
one-way check valve to prevent retrograde flow of gases into
the pipeline supplies.
 Most modern machines have an oxygen (pneumatic) power
outlet that may be used to drive the ventilator or provide an
auxiliary oxygen flowmeter.
 The approximate pipeline pressure of gases delivered to the
machine is 45-60 psig (300-400kPa)

8. cylinder inlets
 Attach to the machine via hanger-yoke assemblies
with a pin-index safety system to prevent accidental
connection of the wrong gas cylinder.
 There`s also a washer, a gas filter and a check valve
to prevent retrograde flow
 The cylinders are also color coded
 The E-cylinders attached to the anesthetic machine
are a high-pressure source and generally only used as
backup supply in case of pipeline failure

9. Pressure regulators
 The gas issuing from medical gas cylinders is at a
much higher pressure than pipeline gas supply
necessitating the interposition of a pressure
regulator between the cylinder and the bank of flow
meters.
 There is a high-pressure relief valve for each gas that
opens when the pressure exceeds the machine`s
maximum safety limit.
 Some machines have a second regulator to drop both
pipeline and cylinder pressure further.

10. Flow valves and meters
 After pressure has been reduced to a safe level, gases
pass through flow control valves and are measured by
flowmeters before mixing and then entering the active
vaporizer and exiting the machine`s common gas outlet.
 The knobs are color coded to prevent turning the wrong
gas off or on
 The oxygen knob is fluted, larger and protrudes farther
than the other knobs and the oxygen flow meter is
positioned furthest to the right ,downstream to other
gases as a way of helping to prevent hypoxia if there is
leakage from a flowmeter positioned upstream.

11. Flow valves and meters
 Flow control knobs control gas entry into the flow meters
by adjustment using a needle valve.
 Flowmeters on anesthesia machine are either constant-
pressure variable-orifice(rotameter) or electronic.
 The former, an indicator ball, bobbin, or float is
supported by the flow of gas through a tube (Thorpe
tube) whose bore (orifice) is tapered.
 Flowmeters are calibrated for specific gases
 Some flowmeters have 2 glass tubes, one for low flows
another for high flows which are in series and controlled
by one valve

12. Flow valves and meters
 Some machines have electronic flow control and
measurement with a backup conventional (Thorpe)
auxiliary oxygen flowmeter.
 Other models have conventional flowmeters but
electronic measurement of gas flow along with
Thorpe tubes and digital or digital/graphic displays.

13. vaporizers
 Vaporize volatile anesthetics before they are
delivered to the patient.
 Vaporizers have concentration calibrated dials that
precisely add volatile anesthetic agents to the
combined gas flow from all flowmeters

14. vaporizers
 At operating room temperatures the molecules of a
volatile agent are distributed between liquid and gaseous
phases.
 The gas molecules bombard the walls of the container
creating the saturated vapor pressure of that agent which
depends on the characteristics of the volatile agent and
temperature.
 Vaporization requires energy(the latent heat of
vaporization) , which results in a loss of heat from the
liquid and hence a decrease in vapor pressure unless heat
is readily available to enter the system
 Vaporizers contain a chamber in which a carrier gas
becomes saturated with the volatile agent.

15. Copper kettle vaporizer

16. Modern conventional vaporizers
 Agent specific and temperature corrected, are
capable of delivering a constant concentration of
agent regardless of temperature changes or flow
through the vaporizer.
 Turning the calibrated control knob to the desired
percentage diverts an appropriate small fraction of
the total gas flow into the carrier gas, which flows
over the liquid anesthetic in the vaporizing chamber.
 The rest exits the vaporizer unchanged so this type of
agent-specific vaporizer is also known as a variable
by-pass vaporizer.

17. Modern conventional vaporizer

18. Modern conventional vaporizers
 Temperature compensation is achieved by a bimetallic
strip as used in home thermostats.
 The metal strips expand and contract differently in
response to temperature changes, bending one way when
temperature drops allowing more gas to pass through the
vaporizer and the other when temperature rises
restricting gas flow to the vaporizer.
 Altering total fresh gas flow rates within a wide range
does not significantly affect anesthetic concentration.
 However the real output of an agent would be lower than
the dial setting at extremely high flow(>15l/min) and thr
converse is true at flow rates <250ml/min

19. Modern conventional vaporizers
 Avoid filling the vaporizer with the wrong anesthetic
as they are agent specific and using the incorrect
agent may lead to over or under dosage due to
differences in potency and vapor pressure of the
different agents
 Modern vaporizers have agent-specific , keyed, filling
ports.

20. Electronic vaporizers
 Must be utilized for desflurane and are used for all volatile
anesthetics in some sophisticated anesthesia machines.
 Desflurane has a high vapor pressure at sea level it almost
boils at room temperature.
 This high volatility plus low potency(1/5th that of other volatile
agents) presents unique delivery problems.
 The vaporization required for GA produces a cooling effect
that would overwhelm the ability of the conventional
vaporizers to maintain a constant temperature
 Because it vaporizes extensively, it would require
tremendously high fresh gas flow to dilute the carrier gas to
clinically relevant concentrations

21. Desflurane vaporizer
 A reservoir containing desflurane(desflurane sump) is
electrically heated to a temperature significantly higher than
its boiling point.
 The desflurane vapor joins the fresh gas mixture before
exiting the vaporizer rather than fresh gas flowing through the
desflurane sump.
 The amount of desflurane released from the sump depends on
the concentration selected and the fresh gas flow rate.
 Although this vaporizer maintains a constant desflurane
concentration over a wide range of fresh gas flow rates, it
cannot automatically compensate for changes in elevation as
do the variable by-pass vaporizers necessitating a manual
increase of desflurane concentration at high elevations as the
partial pressure of the agent decreses.

22. The breathing circuit

23. The breathing circuit
 The breathing system commonly used in adults is the circle
system.
 Gas composition at the common gas outlet can be controlled
precisely and rapidly by adjustments in flow meters and
vaporizers whereas;
 Gas composition esp volatile anesthetic concentration in the
breathing circuit is significantly affected by uptake in the pt`s
lungs, minute ventilation, total fresh gas flow, volume of the
breathing circuit and the presence of gas leaks .
 The effects of the above can be decreased by use of high gas
flow rates during induction and emergence
 Measuring anesthetic gas concentration of inspired and
expired mixtures also greatly facilitates anesthetic
management

24. The breathing circuit
 In most machines, the common gas outlet is attached
to the breathing circuit just past the exhalation valve
to prevent artificially high tidal volume
measurements.
 Newer machines have intergrated internalized
breatghing circuit components to reduce probability
of circuit misconnects, disconnects, kinks and leaks.

25. Oxygen analyzers
 Should be part of the breathing circuit before GA is administered.
 3 types available, polarographic(Clarke electrode), galvanic(fuel cell) and
paramagnetic.
 The first 2 utilize electrochemical sensors with cathode and anode
electrodes embedded in an electrolyte gel separated from the sample gas
by an oxygen-permeable membrane.
 A reaction between the oxygen and the electrodes generates a current that
is proportional to the oxygen partial pressure in the sample gas.
 Although the initial cost of paramagnetic sensors is greater, they are self-
calibrating and have no consumable parts plus their response time is fast
enough to differentiate between inspired and expired oxygen
concentrations.
 All oxygen analyzers should have a low-level alarm that is automatically
activated by turning on the machine.
 Sensor should be placed into the inspiratory or expiratory limb but not the
fresh gas line.

26. spirometers
 A.k.a respirometers
 Measure exhaled tidal volume in the breathing
circuit, typically near the exhalation valve.
 Some machines also measure the inspiratory tidal
volume just past the inspiratory valve or the actual
delivered and exhaled tidal volumes at the Y-piece
connector that attaches to the patient`s airway.

27. spirometers
 Vane anemometer or Wright respirometer is a common
method which employs a rotating vane of low mass a the
expiratory limb in front of the expiratory valve.
 The flow of gas across the vanes within the respirometer
causes their rotation, which is measured electronically ,
photoelectrically or mechanically.
 Another variation using this turbine principle, the
volumeter or displacement meter is designed to measure
the movement of discrete quantities of gas over time

28. spirometers
 Changes in exhaled tidal volume during positive pressure
ventilation usually represent changes in ventilator settings but can
also be due to circuit leaks, disconnections or ventilator
malfunction.
 These spirometers are prone to errors cased by inertia, friction and
water condensation.
 The measurement of exhaled tidal volume at this location in the
expiratory limb includes gas that had been lost to the circuit.
 The difference between volume of gas delivered to the circuit and
the volume of gas actually reaching the patient becomes significant
with long, compliant breathing tubes, rapid RR and increased
airway pressures.
 These problems are at least partially overcome by measuring the
tidal volume at the Y- connector piece of the pt`s airway.

29. spirometers
 Hot-wire anemometer
 Ultrasonic flow sensors
 A pneumotachograph-:fixed-orificeflowmeter that
can function as a spirometer
 There are machines with variable-orifice flowmeters
which usually employ 2 sensors, one at the
inspiratory port and another at the expiratory port.

30. Circuit pressure
 A pressure gauge or electronic sensor measures breathing circuit
pressure somewhere between the expiratory and inspiratory
unidirectional valves.
 If measured as close to the pt`s airway as possible (the Y-
connection), breathing circuit pressure usually reflects airway
pressure.
 A rise in airway pressure may signal worsening
pulmonarytcompliance, an increase in tidal volume or an
obstruction in the breathing circuit, tracheal tube or the pt`s airway,
whilst a drop in pressure may indicate improved compliance, a
decrease in tidal volume or a leak in the circuit.
 If thee pressure is being measure at the CO2 absorber, it will not
always mirror the pressure in the pt`s airway.
 Some machines have incorporated auditory feedback for pressure
changes during ventilator use.

31. Adjustable pressure-limiting valve
 Pressure relief or pop-off valve, spill valve.
 Designed to vent gas when there is a positive pressure in the
system
 Usually fully open during spontaneous ventilation but must be
partially closed during manual or assisted bag ventilation.
 Often requires fine adjustments.
 If not closed sufficiently, excessive loss of circuit volume due
to leaks prevents manual ventilation.
 If closed too much or full closed, a progressive rise in pressure
could result in pulmonary barotrauma or hemodynamic
compromise or both.
 The APL valves on modern machines can never be completely
closed with an upper limit of 70-80cmH20.

32. humidifiers
 Inhaled gases in the operating room are normally
administered at room temperature with little or no
humidification, they therefore must be warmed to body
temperature and saturated with water in the upper
respiratory tract, tracheal intubation and high fresh gas
flows however bypass this normal humidification system
exposing lower airways to dry room temperature gases.
 Prolonged humidification of gases by the lower
respiratory tract leads to dehydration of mucosa, altered
ciliary function and if excessively prolonged could
potentially lead to thickening of secretions, atelectasis
and even V/Q mismatching particularly in patients with
underlying lung disease

33. humidifiers
 Body heat is lost when gases are warmed and water
is lost when they are humidified’
 The heat loss is not significant foe short
procedures(<1hr) and can usually be compensated
for by use of a forced air warming blanket.
 Humidification and heating of inspiratory gases may
be most important for small pediatric pts and older
pts with severe underlying lung pathology.

34. Passive humidifiers
 Condenser humidifiers or heat and mositure
exchanger(HME) units.
 Contain a hygroscopic material that traps exhaled
humidification and heat which is then released upon
subsequent inhalation.
 Some condenser humidifiers also act as effective
filters that may protect the breathing circuit and
anesthesia machine from bacterial or viral cross
contamination

35. Problems of passive humidifiers
 May substantially increase apparatus dead space
which can cause significant rebreathing in pediatric
pts.
 Can increase breathing circuit resistance and the
work of breathing during spontaneous respirations.
 Excessive saturation of the HME with water and
secretions can obstruct the breathing circuit

36. Active humidifiers
 Are more effective at preserving moisture and heat.
 They add water to a gas through various methods:-
passing the gas over a water chamber(passover
humidifier) or through a saturated wick(wick
humidifier), bubbling it through water(bubble-through
humidifier), or mixing it with vaporized water(vapor-
phase) humidifier.
 Use particularly valuable in children as they help prevent
hypothermia and plugging of small tracheal tubes by
dried secretions
 Of course ,any design that increases airway dead space
should be avoided in pediatric pts

37. Active humidifiers
 Heated humidifiers with thermostatically controlled
elements are more effective because increasing
temperature increases the capacity of gas to hold water,
but they are not without hazards which include:-
 Thermal injury
 Nosocomial infection
 Increased airway resistance from excess water
condensation in the breathing circuit
 Interference with flow meter function
 Increased likelihood of circuit disconnection

38. ventilators
 Generate gas flow by creating a pressure gradient
between the proximal airway and the alveoli.
 Their function is best described in relation to the
four phases of the ventilatory cycle: inspiration,
transition from inspiration to expiration, expiration
and transition from expiration to inspiration.

39. Inspiratory phase
 Tidal volume is generated by producing gas flow
along a pressure gradient.
 The machine generates either a constant
pressure(constant-pressure generator) or constant
gas flow rate(constant-flow generators) during
inspiration, regardless of lung mechanics.
 Nonconstant generators produce pressures or gas
flow rates that vary during the cycle but remain
consistent from breath to breath.

40. Transition phase from inspiration to
expiration(cycling)
 Termination of the inspiratory phase can be
triggered by a preset limit of time(time-cycled), a set
inspiratory pressure that must be achieved(pressure-
cycled) or a predetermined tidal volume that must be
delivered.
 Modern ventilators incorporate secondary cycling
parameters or other limiting mechanisms e.g. time-
cycled and volume-cycled ventilators usually
incorporate a pressure limiting feature that
terminates inspiration when a preset adjustable
safety pressure limit is reached.

41. Expiratory phase
 Airway pressure is reduced to atmospheric levels or
some preset PEEP.
 Exhalation is therefore passive
 Flow out of the lungs is determined primarily by
airway resistance and lung compliance
 Expired gases fill up the bellows and are relieved to
the scavenging system

42. Transition phase from expiration to inspiration
 May be based on a preset time interval or a change in
pressure.
 Behavior of the ventilator during this phase together
with the type of cycling determines ventilator mode.

43. Ventilator circuit design
 Double-circuit system design which is pneumatically
powered and electronically controlled.
 Newer machines also incorporate microprocessor
controls and sophisticated and precise pressure and
flow sensors to achieve multiple ventilatory modes,
PEEP, accurate tidal volumes and enhanced safety
features.

44. Double-circuit system ventilators
 Tidal volume is delivered from a bellows assembly.
 The bellows take the place of the breathing bag in the
anesthesia circuit.
 Pressurization compresses the pleated bellows inside
forcing the gas inside into the breathing circuit and
patient.
 During exhalation they ascend.
 A ventilator flow control valve regulates drive gas
flow into the pressurizing chamber.

45. Double-circuit system ventilator
 A leak in the bellows can transmit high gas pressure
to the pt`s airway, potentially resulting in pulmonary
barotrauma.
 This may be indicated by a higher than expected rise
in inspired oxygen concentration.
 Double-circuit design ventilators also incorporate a
free breathing valve that allows air to enter the rigid
drive chamber and the bellows to collapse if the pt
generates negative pressure by taking spontaneous
breaths during mechanical ventilation.

46. Spill valve
 Ventilator`s own pressure relief “pop-off” valve.
 It is pneumatically closed during inspiration so that
positive pressure can be generated.
 During exhalation the pressurizing gas is vented out and
the ventilator spill valve is no longer closed .
 The bellows refill during expiration ,when the bellows is
completely filled the increase in the circle system
pressure causes the excess gas to be directed to the
scavenging system through the spill valve.
 Sticking of the valve can result in abnormally elevated
airway pressure during exhalation.

47. Pressure and volume monitoring

48. Causes of increased PIP

49. Ventilator alarms
 Anesthesia work stations have at least 3 disconnect
alarms:- low peak inspiratory pressure, low exhaled tidal
volume and low exhaled CO2.
 The first is always built into the ventilator whereas the
last 2 may be in separate modules
 A small leak or partial breathing circuit disconnection
may be detected by subtle decreases in peak inspiratory
pressure, low exhaled tidal volume or end-tidal CO2
before alarm thresholds are reached.
 Other built in ventilator alarms include high PIP, high
PEEP, sustained high airway pressure, negative pressure
and low oxygen-supply pressure

50. Problems with anesthesia vents
 Ventilator-fresh gas flow coupling
 Excessive positive pressure
 Tidal volume disrepancies

51. Scavenging systems
 Waste-gas scavengers dispose of gases that have been vented from
the breathing circuit by the APL valve or the ventilator spill valve
 Both valves should be connected to transfer tubing leading to the
scavenging interface which may be inside the machine or an
external attachment
 The scavenging interface can be described as either open or closed
 An open interface is open to the outside atmosphere
 A closed interface is closed to the outside atmosphere and requires
negative and positive pressure relief valves that protect the patient
from the negative pressure of the vacuum system and positive
pressure from an obstruction in the distal tubing, respectively.
 Risk of occupational exposure to health care providers is higher
with an open interface.

52. Scavenging systems
 The outlet of the scavenging system may be a direct line to the
outside via a ventilation duct beyond any point of
recirculation(passive scavenging) or
 A connection to the hospital`s vacuum system(active
scavenging)
 A chamber or reservoir bag accepts waste-gas overflow when
the capacity of the vacuum is exceeded.
 The vacuum control valve on an active system should be
adjusted to allow the evacuation of 10-15l/min of waste gas as
this rate is adequate for periods of high fresh gas flow yet
minimizes the risk of transmitting negative pressure to the
breathing circuit during lower flow conditions
 Some machines come with both active and passive scavenger
systems.

53. Safety features of modern anesthetic machines
 Specificity of probes on flexible hoses between
terminal outlets and connections with the anesthetic
machine, the hoses are color-coded and have non-
interchangeable screw-threaded connectors to the
machine.
 Pin index system to prevent incorrect attachment of
gas cylinders to the anesthetic machine, cylinders are
color-coded and labelled with the type of gas they
contain.
 Pressure relief valves on the downstream side of
pressure regulators.

54. Safety features
 Flow restrictors on the upstream side of flow meters.
 Arrangement of flow meters such that the oxygen
flow meter is on the right(i.e. downstream side) or
oxygen is the last gas to be added to the gas mixture
being delivered to the back bar.
 Non-return valves to prevent refilling of an empty
cylinder by the reserve cylinder when a single
regulator and contents meter is used for 2 cylinders
in machines that have 2 cylinders attached.
 Pressure gauges indicate the pressures in the
pipeline and cylinders.

55. Safety features
 An oxygen by-pass valve(emergency oxygen) delivers
oxygen directly to a point downstream of the vaporizers,
should give a flow rate of at least 35l/min.
 Mechanical linkage between the nitrous oxide and
oxygen flow meters which prevent the delivery of <25%-
30% oxygen
 Mounting of vaporizers at the back bar, the newer TEC
Mark 4 & 5 vaporizers have the interlocking Selectatec
system which has locking rods to prevent more than one
vaporizer being used at the same time. When a vaporizer
is mounted at the back bar, the locking lever needs to be
engaged , otherwise the dial cannot be moved.

56. Safety features
 A non-return valve situated downstream of the
vaporizers prevents back-pressure which might
otherwise cause output of high concentrations of
vapor.
 A pressure relief valve may be situated downstream
of the vaporizer, opening at 34kPa to prevent
damage to the flow meters or vaporizer if the gas
outlet from the anesthetic machine is obstructed.
 A pressure relief valve set to open at a low pressure
of 5kPa may be fitted to prevent the patient`s lungs
from being damaged by high pressure.

57. Safety features
 Oxygen failure warning systems.
 The reservoir bag in an anesthetic breathing system
is highly distensible and seldom reaches pressures
exceeding 5kPa.

58. Oxygen failure protection devices
 Nitrous oxide, air(in some machines) and other gases
must first pass through safety devices before reaching
their respective control valves unlike the oxygen supply
which passes directly to its flow control valve.
 These devices permit flow of other gases only if there is
sufficient oxygen pressure in the safety device.
 This helps prevent accidental delivery of a hypoxic
mixture in the event of oxygen supply failure.
 Safety devices sense oxygen pressure via a small “piloting
pressure” line that may be derived from the gas inlet or
secondary regulator. In some machine designs, if the
piloting pressure line falls below a threshold, the shut-off
valve close, preventing administration of any other gases.

59. Oxygen failure protection devices
 Proportionately reduce the pressure of nitrous oxide
and other gases except for air(they completely shut
off nitrous oxide and other gas flow only below a set
minimum oxygen pressure(e.g. 0.5 psig nitrous oxide
and 10 psig for other gases.
 All machines have an oxygen supply low-pressure
sensor that activates alarm sounds when inlet
pressure drops below a threshold value(usually 20-
30 psig) .

60. the End.