ANESTHETIC EQUIPMENT
Final Objective: On completion of this module you will have expert knowledge of the design, function and safety features of relevant anaesthetic equipment.
Enabling Objective: To achieve this goal, you should be able to:
Reference Reading:
There must always be alternative equipment available to ventilate the patient if the anaesthetic machine or oxygen supply fails. A self-inflating resuscitation bag does not need a source of oxygen. It should always be available whenever an anaesthetic is given.
The main functions of an anaesthetic machine are to:
ANAESTHETIC MACHINE.
The modern anaesthetic machine is the result of safety modifications of “Boyle’s” machine built in 1917.
Modern machines essentially have 6 components: high pressure gas supply, gas flow regulation and measurement, vaporizers, gas delivery systems, scavenging and monitors.
The high-pressure gas supply may be from individual cylinders or piped from a central supply. A spare full oxygen cylinder must be immediately available for both methods of supply. Full and empty cylinders should be kept separately. Cylinders should be handled carefully as they are heavy and may be a fire risk. Medical gas cylinders should be tested every 5 years.
All supply sources must have a safety design that prevents the gas supply from being connected to the wrong inlet on the anaesthetic machine. Cylinders have a pin index system.
Cylinders of oxygen and air contain gas. Their contents may be estimated from the pressure gauge. Nitrous oxide becomes a liquid at the pressure it is stored at in the cylinder. The pressure in a nitrous oxide cylinder will not begin to fall until all the liquid nitrous oxide has evaporated. As nitrous oxide cylinders contain a liquid, they should be kept vertical.
Oxygen is stored at approximately 14,000 kPa and nitrous oxide at 5,000 kPa. (An oxygen D cylinder contains 400 litres, an E cylinder 680 litre and a F cylinder 1400 litres. A nitrous oxide D cylinder contains 900 litres, an E cylinder 1800 litres and a F cylinder 3600 litres).
Cylinder gas supply pressure must be reduced to prevent damage to the anaesthetic machine (most machine have a working pressure of approximately 420 kPa), prevent barotrauma to the patient and to easily allow small adjustments to flow. Pressure regulators also maintain a constant pressure, otherwise as the cylinder empties the pressure falls and the anaesthetist would need to continually change the flow rate. (The pressure in a full oxygen cylinder may fall from 14,000 kPa when full to 150 kPa when empty). Piped gases are already at 400 kPa and therefore need no further reduction before reaching the flowmeters.
Gas cylinders are attached to the anaesthetic machine by a connecting yoke. The yoke supports the cylinder, provides a gas tight seal, allows unidirectional flow from the cylinder to the machine and prevents the cylinder being attached to the wrong inlet (pin index). The yoke consists of a body with a retaining screw that must be only “hand tightened” to secure the cylinder in place. A washer (Bodek) helps prevents leaks and a filter removes dirt. These should always be in place.
Flow meters reduce pressure from 400 kPa to just above atmospheric pressure and allow fine adjustment of flow. A delicate spindle valve controls the flow of gas. Over tightening the valve may damage it. As the flow increases, the ball or bobbin indicator rises in the vertical tube. (These flow meters are variable orifice and constant pressure. The tubes are tapered with the smallest diameter at the bottom). Flow rate is read at the top of the bobbin and in the middle of the ball. They are accurate within 5%. Each flow meter is made for a specific gas. They are not interchangeable. Modern anaesthetic machines are made so that oxygen is the last gas to be added to the fresh gas flow even if the oxygen flow meter is the first in line. This prevents oxygen escaping from a broken nitrous oxide or other flowmeter. The oxygen flow knob is different to all other flow knobs so that it can be easily recognised (even in the dark).
Modern anaesthetic machines have the oxygen flow linked to other gases so it is impossible to give less than 25% oxygen. Some anaesthetic machines have a minimum mandatory oxygen flow.
Between the flowmeters and the common gas outlet is the back bar. Safety features on or downstream of the back bar are an oxygen failure device, spring-loaded non-return valve, pressure relief valve and emergency oxygen flush. There are different oxygen failure devices. The anaesthetist must know what type of oxygen failure device is on the anaesthetic machine and how it works. Early devices relied on batteries to light a red light and nitrous oxide to power a whistle (Bosun). Modern standards require that the alarm should be powered solely by the oxygen supply pressure. Some devices also discontinue the nitrous oxide supply and if the patient is breathing spontaneously, allow them to breath room air. Once the oxygen failure alarm sounds the only way to turn it off should be to restore the oxygen supply. The non-return valve and pressure relief valve (30-40 kPa) is used to minimise the effects of backpressure on the function of flowmeters and vaporizers. The emergency oxygen flush can deliver oxygen at a minimum of 35 l/min.
Modern vaporizers are agent specific, with accurate temperature compensation, can be safely tilted and can only be filled with the correct volatile agent. Older vaporizers have more variable output, will flood if tilted and can be filled with the wrong volatile agent. The greater the difference in SVP of volatile agents the greater the inaccuracy if a volatile agent is put in the wrong vaporiser. More than one vaporiser may be placed on the back bar. With modern vaporizers only one may be turned on at a time. Older vaporizers may permit more than one vaporiser to open at the same time. This is a dangerous situation.
The mixture of gases and vapour is finally delivered to a common outlet on the anaesthetic machine (22 mm male/15mm female tapered).
Adult “Mapleson” anaesthetic breathing systems consist of a connection for the common gas outlet, tubing, a reservoir bag and an adjustable expiratory valve. There are different ways of arranging the parts that will affect how they function with controlled and spontaneous ventilation. Mapleson classified all possible configurations, labelled A to F The reservoir bag (usually 2 litre capacity) supplies the patient’s peak inspiratory demands (30-40 l/min) with a lower constant fresh gas flow by storing the fresh gas flow during patient expiration. The adjustable expiratory valve vents expired gas. Closing the valve allows manual ventilation. There are no valves to direct flow and no carbon dioxide absorber. The fresh gas flow must wash out the expired carbon dioxide. Mapleson breathing systems require high fresh gas flow and do not conserve heat or moisture. Capnography is useful to verify sufficient washout of carbon dioxide.
Circle systems use less gas and volatile agent, conserve heat and moisture and are suitable for spontaneous and controlled ventilation. They are larger more complex (up to ten connections) and need a carbon dioxide absorber.
All breathing systems described and mechanical ventilators will vent varying volumes of excess gas into the theatre. Although there is no conclusive evidence to link exposure to low concentrations of anaesthetic agents with any risk, it is sensible to minimise pollution of the theatre with waste gas. Two types of scavenging systems are used: passive and active. Passive systems divert expired gas to the outside by the patient’s expiratory effort. Modern scavenging uses active systems with low-pressure suction.
CHECKING THE ANAESTHETIC MACHINE.
Oxygenation and ventilation are essential for every patient and because even a brief failure to maintain oxygenation or ventilation may cause harm or death, every anaesthetic machine must be regularly and thoroughly checked to ensure that all functions are correctly maintained.
There must be an emergency device to maintain oxygenation and ventilation of a patient should the anaesthetic machine fail.
To ensure early detection of any failure in the anaesthetic machine, it is essential that all alarms are working and all monitoring is used.
The anaesthetic machine check should be performed at the start of each anaesthetic session. The machine should be checked in a systematic way to ensure that no checks are accidentally omitted.
Always have an alternative resuscitation device (e.g. self-inflating bag).
Check that cylinders are full and attached to the anaesthetic machine. There must always be a reserve supply of oxygen. Never use a machine if there is no reserve supply of oxygen.
Check that piped gas supplies (where present) are at the correct pressure. Confirm the correct pipeline supply by using an oxygen analyser at the common gas outlet.
Disconnect piped gases. Turn off all cylinders.
Turn on all flow meters. There should be no flow. Check the flow meters for cracks.
Turn on the oxygen cylinder. There should only be flow in the oxygen flow meter. The bobbin should spin. Repeat with each oxygen cylinder. Check the pressure gauge for an adequate supply of oxygen. Set the oxygen flow to 4 litres/min.
Turn on the nitrous oxide cylinder. Check that there is flow in the nitrous oxide flow meter (the bobbin should spin) and that the oxygen flow meter is still at 4 litres/min.
Turn off the oxygen supply and push the oxygen flush button. The oxygen flush should operate. Where an anti-hypoxic device is present the nitrous oxide flow should automatically stop. The oxygen failure alarm should sound.
Turn on the oxygen cylinder again. The oxygen failure alarm should go off.
Reconnect piped gases where present.
Check that all vaporisers are full and correctly fitted. The controls should operate throughout their full range without sticking. Turn off the vaporisers. Check that vaporiser-filling ports are closed. Set a flow of oxygen of 5 litres/min and, with the vaporizer turned off, temporarily occlude the common gas outlet. There should be no leak from any of the vaporizers and the flowmeter bobbin (if present) should dip.
If the anaesthetic machine is fitted with a pressure relief valve it should be tested by occluding the common gas outlet whilst gas is flowing. (Never do this test if a pressure relief valve is not fitted).
Attach the breathing system. Check that it has been correctly assembled. Perform a leak test on the breathing system by closing the APL valve and occluding the patient end, applying a fresh gas flow of 300 ml/min and ensuring that a pressure of greater than 30 cm H20 is sustained. In a circle system the uni-directional valves need to be checked. Attach a reservoir bag to the end of the Y-piece and fill the system with gas. Squeeze each reservoir bag alternatively and observe the movement of the uni-directional valves. Check for normal resistance and compliance. Open the APL valve and ensure the breathing system empties.
Check that the ventilator operates. Check that high pressure alarms and disconnection alarms work.
Check the scavenging system
Check all airway equipment, suction equipment and drugs.
Check that all monitors work.
SELF-ASSESSMENT QUESTIONS
ASSIGNMENT
Draw a diagram of the circle breathing system. What is the importance of each component? What are the benefits and disadvantages of the circle breathing system?
ANAESTHETIC EQUIPMENT CASE STUDIES
Case 3.1
Hulan is a 16 year old boy who was riding in the back of a truck when he fell onto the road and was hit by a jeep. He has a compound fracture of his left tibia and multiple abrasions on his face and chest. He is fully conscious but in pain. Haemodynamically he is stable but needs to go to surgery.
When you do the anaesthetic machine test you discover a large leak. How do you determine the site of the leak?
You fix the leak and continue with the anaesthetic. Ten minutes after the rapid sequence induction, the oxygen failure alarm activates. How do you manage this situation?
A full oxygen cylinder is attached to the anaesthetic machine yoke but there is a large leak. What are the possible causes for the leak?
The surgeons have used alcoholic cleaning solution and are using diathermy. You are providing anaesthesia with oxygen, nitrous oxide, halothane and morphine. During the surgery a fire starts on the patient’s drapes.
What are the factors required to produce a fire? Identify these factors in your theatre.
What is your management of this fire?
What are the potential hazards of surgical diathermy?
At the completion of surgery as Hulan is waking up he begins to vomit. When you try to suction his mouth you have no suction.
What would you do?
Describe the features of an anaesthetic suction system.
Final Objective: On completion of this module you will have expert knowledge of the design, function and safety features of relevant anaesthetic equipment.
Enabling Objective: To achieve this goal, you should be able to:
- Describe the features and functions of the anaesthetic machine including the high pressures system, regulators, flow meters, and vaporizers and breathing systems (circuits).
- Outline the safety features incorporated in the design of an anaesthetic machine
- Perform a check of the anaesthetic machine
- Isolate and correct problems revealed by the machine check.
Reference Reading:
- Developing Anaesthesia Chapters 15, 16 and 17.
There must always be alternative equipment available to ventilate the patient if the anaesthetic machine or oxygen supply fails. A self-inflating resuscitation bag does not need a source of oxygen. It should always be available whenever an anaesthetic is given.
The main functions of an anaesthetic machine are to:
- Provide oxygen.
- Reduce the pressure of gases from the pipelines or cylinders to a safe pressure.
- Allow the accurate delivery of a gas mixture (fresh gas flow) of precisely known but variable composition (oxygen, air, nitrous oxide).
- Allow additional anaesthetic vapours to be added to the fresh gas flow.
- Enable patient ventilation, spontaneous or controlled.
- Minimize anaesthetic related risks to patients and staff.
ANAESTHETIC MACHINE.
The modern anaesthetic machine is the result of safety modifications of “Boyle’s” machine built in 1917.
Modern machines essentially have 6 components: high pressure gas supply, gas flow regulation and measurement, vaporizers, gas delivery systems, scavenging and monitors.
The high-pressure gas supply may be from individual cylinders or piped from a central supply. A spare full oxygen cylinder must be immediately available for both methods of supply. Full and empty cylinders should be kept separately. Cylinders should be handled carefully as they are heavy and may be a fire risk. Medical gas cylinders should be tested every 5 years.
All supply sources must have a safety design that prevents the gas supply from being connected to the wrong inlet on the anaesthetic machine. Cylinders have a pin index system.
Cylinders of oxygen and air contain gas. Their contents may be estimated from the pressure gauge. Nitrous oxide becomes a liquid at the pressure it is stored at in the cylinder. The pressure in a nitrous oxide cylinder will not begin to fall until all the liquid nitrous oxide has evaporated. As nitrous oxide cylinders contain a liquid, they should be kept vertical.
Oxygen is stored at approximately 14,000 kPa and nitrous oxide at 5,000 kPa. (An oxygen D cylinder contains 400 litres, an E cylinder 680 litre and a F cylinder 1400 litres. A nitrous oxide D cylinder contains 900 litres, an E cylinder 1800 litres and a F cylinder 3600 litres).
Cylinder gas supply pressure must be reduced to prevent damage to the anaesthetic machine (most machine have a working pressure of approximately 420 kPa), prevent barotrauma to the patient and to easily allow small adjustments to flow. Pressure regulators also maintain a constant pressure, otherwise as the cylinder empties the pressure falls and the anaesthetist would need to continually change the flow rate. (The pressure in a full oxygen cylinder may fall from 14,000 kPa when full to 150 kPa when empty). Piped gases are already at 400 kPa and therefore need no further reduction before reaching the flowmeters.
Gas cylinders are attached to the anaesthetic machine by a connecting yoke. The yoke supports the cylinder, provides a gas tight seal, allows unidirectional flow from the cylinder to the machine and prevents the cylinder being attached to the wrong inlet (pin index). The yoke consists of a body with a retaining screw that must be only “hand tightened” to secure the cylinder in place. A washer (Bodek) helps prevents leaks and a filter removes dirt. These should always be in place.
Flow meters reduce pressure from 400 kPa to just above atmospheric pressure and allow fine adjustment of flow. A delicate spindle valve controls the flow of gas. Over tightening the valve may damage it. As the flow increases, the ball or bobbin indicator rises in the vertical tube. (These flow meters are variable orifice and constant pressure. The tubes are tapered with the smallest diameter at the bottom). Flow rate is read at the top of the bobbin and in the middle of the ball. They are accurate within 5%. Each flow meter is made for a specific gas. They are not interchangeable. Modern anaesthetic machines are made so that oxygen is the last gas to be added to the fresh gas flow even if the oxygen flow meter is the first in line. This prevents oxygen escaping from a broken nitrous oxide or other flowmeter. The oxygen flow knob is different to all other flow knobs so that it can be easily recognised (even in the dark).
Modern anaesthetic machines have the oxygen flow linked to other gases so it is impossible to give less than 25% oxygen. Some anaesthetic machines have a minimum mandatory oxygen flow.
Between the flowmeters and the common gas outlet is the back bar. Safety features on or downstream of the back bar are an oxygen failure device, spring-loaded non-return valve, pressure relief valve and emergency oxygen flush. There are different oxygen failure devices. The anaesthetist must know what type of oxygen failure device is on the anaesthetic machine and how it works. Early devices relied on batteries to light a red light and nitrous oxide to power a whistle (Bosun). Modern standards require that the alarm should be powered solely by the oxygen supply pressure. Some devices also discontinue the nitrous oxide supply and if the patient is breathing spontaneously, allow them to breath room air. Once the oxygen failure alarm sounds the only way to turn it off should be to restore the oxygen supply. The non-return valve and pressure relief valve (30-40 kPa) is used to minimise the effects of backpressure on the function of flowmeters and vaporizers. The emergency oxygen flush can deliver oxygen at a minimum of 35 l/min.
Modern vaporizers are agent specific, with accurate temperature compensation, can be safely tilted and can only be filled with the correct volatile agent. Older vaporizers have more variable output, will flood if tilted and can be filled with the wrong volatile agent. The greater the difference in SVP of volatile agents the greater the inaccuracy if a volatile agent is put in the wrong vaporiser. More than one vaporiser may be placed on the back bar. With modern vaporizers only one may be turned on at a time. Older vaporizers may permit more than one vaporiser to open at the same time. This is a dangerous situation.
The mixture of gases and vapour is finally delivered to a common outlet on the anaesthetic machine (22 mm male/15mm female tapered).
Adult “Mapleson” anaesthetic breathing systems consist of a connection for the common gas outlet, tubing, a reservoir bag and an adjustable expiratory valve. There are different ways of arranging the parts that will affect how they function with controlled and spontaneous ventilation. Mapleson classified all possible configurations, labelled A to F The reservoir bag (usually 2 litre capacity) supplies the patient’s peak inspiratory demands (30-40 l/min) with a lower constant fresh gas flow by storing the fresh gas flow during patient expiration. The adjustable expiratory valve vents expired gas. Closing the valve allows manual ventilation. There are no valves to direct flow and no carbon dioxide absorber. The fresh gas flow must wash out the expired carbon dioxide. Mapleson breathing systems require high fresh gas flow and do not conserve heat or moisture. Capnography is useful to verify sufficient washout of carbon dioxide.
Circle systems use less gas and volatile agent, conserve heat and moisture and are suitable for spontaneous and controlled ventilation. They are larger more complex (up to ten connections) and need a carbon dioxide absorber.
All breathing systems described and mechanical ventilators will vent varying volumes of excess gas into the theatre. Although there is no conclusive evidence to link exposure to low concentrations of anaesthetic agents with any risk, it is sensible to minimise pollution of the theatre with waste gas. Two types of scavenging systems are used: passive and active. Passive systems divert expired gas to the outside by the patient’s expiratory effort. Modern scavenging uses active systems with low-pressure suction.
CHECKING THE ANAESTHETIC MACHINE.
Oxygenation and ventilation are essential for every patient and because even a brief failure to maintain oxygenation or ventilation may cause harm or death, every anaesthetic machine must be regularly and thoroughly checked to ensure that all functions are correctly maintained.
There must be an emergency device to maintain oxygenation and ventilation of a patient should the anaesthetic machine fail.
To ensure early detection of any failure in the anaesthetic machine, it is essential that all alarms are working and all monitoring is used.
The anaesthetic machine check should be performed at the start of each anaesthetic session. The machine should be checked in a systematic way to ensure that no checks are accidentally omitted.
Always have an alternative resuscitation device (e.g. self-inflating bag).
Check that cylinders are full and attached to the anaesthetic machine. There must always be a reserve supply of oxygen. Never use a machine if there is no reserve supply of oxygen.
Check that piped gas supplies (where present) are at the correct pressure. Confirm the correct pipeline supply by using an oxygen analyser at the common gas outlet.
Disconnect piped gases. Turn off all cylinders.
Turn on all flow meters. There should be no flow. Check the flow meters for cracks.
Turn on the oxygen cylinder. There should only be flow in the oxygen flow meter. The bobbin should spin. Repeat with each oxygen cylinder. Check the pressure gauge for an adequate supply of oxygen. Set the oxygen flow to 4 litres/min.
Turn on the nitrous oxide cylinder. Check that there is flow in the nitrous oxide flow meter (the bobbin should spin) and that the oxygen flow meter is still at 4 litres/min.
Turn off the oxygen supply and push the oxygen flush button. The oxygen flush should operate. Where an anti-hypoxic device is present the nitrous oxide flow should automatically stop. The oxygen failure alarm should sound.
Turn on the oxygen cylinder again. The oxygen failure alarm should go off.
Reconnect piped gases where present.
Check that all vaporisers are full and correctly fitted. The controls should operate throughout their full range without sticking. Turn off the vaporisers. Check that vaporiser-filling ports are closed. Set a flow of oxygen of 5 litres/min and, with the vaporizer turned off, temporarily occlude the common gas outlet. There should be no leak from any of the vaporizers and the flowmeter bobbin (if present) should dip.
If the anaesthetic machine is fitted with a pressure relief valve it should be tested by occluding the common gas outlet whilst gas is flowing. (Never do this test if a pressure relief valve is not fitted).
Attach the breathing system. Check that it has been correctly assembled. Perform a leak test on the breathing system by closing the APL valve and occluding the patient end, applying a fresh gas flow of 300 ml/min and ensuring that a pressure of greater than 30 cm H20 is sustained. In a circle system the uni-directional valves need to be checked. Attach a reservoir bag to the end of the Y-piece and fill the system with gas. Squeeze each reservoir bag alternatively and observe the movement of the uni-directional valves. Check for normal resistance and compliance. Open the APL valve and ensure the breathing system empties.
Check that the ventilator operates. Check that high pressure alarms and disconnection alarms work.
Check the scavenging system
Check all airway equipment, suction equipment and drugs.
Check that all monitors work.
SELF-ASSESSMENT QUESTIONS
- What is the pressure of a full oxygen cylinder used in theatre? How long will each type of oxygen cylinder last with your usual gas flow rate? When should an oxygen cylinder be replaced?
- How do the Mapleson breathing systems differ in design and function?
- Describe the safety features on your anaesthetic machine. Which safety features are missing? What are the potential hazards of your anaesthetic machine?
- Outline how you check your anaesthetic machine.
ASSIGNMENT
Draw a diagram of the circle breathing system. What is the importance of each component? What are the benefits and disadvantages of the circle breathing system?
ANAESTHETIC EQUIPMENT CASE STUDIES
Case 3.1
Hulan is a 16 year old boy who was riding in the back of a truck when he fell onto the road and was hit by a jeep. He has a compound fracture of his left tibia and multiple abrasions on his face and chest. He is fully conscious but in pain. Haemodynamically he is stable but needs to go to surgery.
When you do the anaesthetic machine test you discover a large leak. How do you determine the site of the leak?
You fix the leak and continue with the anaesthetic. Ten minutes after the rapid sequence induction, the oxygen failure alarm activates. How do you manage this situation?
A full oxygen cylinder is attached to the anaesthetic machine yoke but there is a large leak. What are the possible causes for the leak?
The surgeons have used alcoholic cleaning solution and are using diathermy. You are providing anaesthesia with oxygen, nitrous oxide, halothane and morphine. During the surgery a fire starts on the patient’s drapes.
What are the factors required to produce a fire? Identify these factors in your theatre.
What is your management of this fire?
What are the potential hazards of surgical diathermy?
At the completion of surgery as Hulan is waking up he begins to vomit. When you try to suction his mouth you have no suction.
What would you do?
Describe the features of an anaesthetic suction system.