Delivering only intended gases from the anaesthesia workstation
Delivering Only Intended Gases from the Anaesthesia WorkstationPresented By – Dr. Dhritiman ChakrabartiModerated By – Dr. Poonam Kalra
Introduction• Almost every piece of medical equipment carries some risk for misuse or failure.• Anesthetic gas delivery devices are a particular concern because they exhibit several basic features that may predispose to critical events and subsequent patient injury. These include -1. Presence of multiple connections2. The use of complex mechanical components3. Variations in manufacture and design.• They are thus a target of ever continuing research to help facilitate the delivery of anaesthesia and improve equipment safety.
Gas Delivery Equipment Gas delivery equipment will be classified by its parts (relevant in context) and safety features in each will be discussed accordingly.1. Cylinders.2. Pipelines.3. Oxygen Failure Warning Device.4. Oxygen Failure Safety Device.5. Flow Adjustment Control and Flowmeters.6. Vaporizer Manifold.7. Gas Monitoring.
Cylinder Safety to Deliver only Intended Gas Three safety features are usually incorporated:1. Colour Coding and Labelling of Cylinders.2. Valve Outlet Connections for Large Cylinders.3. Pin Index Safety Systems.
Colour Coding of Cylinders• Accidental confusion of cylinders has been a significant cause of mortality. Colour can be used to help identify gases.• The top and shoulder (the part sloping up to the neck) of each cylinder are painted the colour assigned to the gas it contains or the entire cylinder may be covered by using a nonfading, durable, water-insoluble paint.• In the case of a cylinder containing more than one gas, the colours must be applied in a way that will permit each colour to be seen when viewed from the top. In some countries, the body of the cylinder is painted with the colour of the major gas and the shoulder the colour of the minor gas.• An international colour code has been adopted by several countries.
Because of variations in colour tones, chemical changes in paint pigments, lighting effects, anddifferences in colour perception by personnel, colour should be not be used as the primary meansfor identification of cylinder contents. Cylinder labels are the best method to identify cylindercontents.
Labelling• Each cylinder must bear a label or decal on the side or, when space permits, the shoulder of the cylinder (but it may not cover any permanent markings).1. Diamond-shaped figure denoting the hazard class2. A white panel with the name of the contained gas3. A signal word (DANGER, WARNING, or CAUTION, depending on whether the release of gas would create an immediate, less than immediate, or no immediate hazard to health or property) is present.4. Statement of hazard5. Should contain the name and address of the cylinder manufacturer or distributor
Valve Outlet Connections for Large Cylinders• Larger cylinder valves have threaded outlet (bull nose) connections.• When the threads of this outlet mesh with those of the nut, the nut may be tightened, causing the nipple to seat against the valve outlet. In this way, the gas channel of the valve is aligned with the channel of the nipple.• The outlets and connections are indexed by diameter, thread size, right- and left-handed threading, external and internal threading, and nipple seat design.
Valve outlet connections for large cylinders.A: The valve outlet thread is external, i.e., the threads are on the outside of the cylinder valveoutlet and the nut screws over the valve outlet.B: The valve outlet thread is internal so that the nut screws into the outlet.The specification for cylinder connections are often shown as in the following example forOxygen: 0.903-14-RH EXT.The first number is the diameter in inches of the cylinder outlet.The next number gives the number of threads per inch.The letters following this indicate whether the threads are right hand or left hand and externalor internal. (Redrawn courtesy of the Compressed Gas Association.)
Pin Index Safety System• The Pin Index Safety System consists of holes on the cylinder valve positioned in an arc below the outlet port.• Pins on the yoke or pressure regulator are positioned to fit into these holes.• Unless the pins and holes are aligned, the port will not seat. Pin Index Safety System The figure shows the six positions for pins on the yoke. The pins are 4 mm in diameter and 6 mm long, except for pin 7, which is slightly thicker. The seven hole positions are on the circumference of a circle of 9/16 inch radius centered on the port.
Problems with Pin Index Safety System1. If multiple sealing washers are used with a cylinder, the pins on the yoke or regulator may not extend far enough to engage the mating holes, and the Pin Index Safety System may be bypassed.2. Multiple mechanical problems can occur - Pins can be bent, broken, removed, or forced into the yoke or regulator; pin index holes may become worn.
User Precautions while using Cylinders:1. Regulators, hoses, gauges, or other apparatus designed for use with one gas should never be used with cylinders containing other gases.2. Adapters to change the outlet size of a cylinder valve should not be used, as this defeats the purpose of standardizing valve outlets.3. No part of the cylinder or its valve should be tampered with, painted, altered, repaired, or modified by the user. Cylinders should be repainted only by the supplier.4. When different types of gases are stored in the same location, containers should be grouped by contents and sizes (if different sizes are present).5. Transfilling should not be performed by unskilled, untrained person. It is best performed by a gas manufacturer or distributor.
Pipeline Safety Features• Pipelines are the backbone of institutional gas delivery systems.• Due to the multiple number of connections involved, reliance on personnel for maintenance of central supply as well as peripheral units and the propensity to accrue cumulative damage, pipeline systems are prone to unintended misconnections and crossconnections.
Anatomy of Pipeline SystemThe Branch lines end in Terminal Units which lead off to Hose Pipes which then finallyconnect to the end users – Anaesthesia Workstations or ICU Ventilators.
Sites Prone to Cross-connections• Cross connections are usually a result of personnel related errors or damage issues. They can occur at central supply and at peripheral sites beyond the terminal units.• Most pipeline systems are rigged to alarm based on pressures. Delivery of unintended gas in the pipeline within pressure range would not trigger alarm.• This necessitates inclusion of Oxygen analysers at end users of the gas delivery systems i.e. the Anaesthesia Workstation/ICU Ventilator.
Reported Cases of Wrong Gas Delivery• Although an uncommon event, accidental substitution of one gas for another at central supply can have devastating consequences. The most common cross overs have been between nitrous oxide and oxygen, but various other combinations have been reported.• Cases have been reported in which liquid oxygen tanks were filled with nitrogen or argon. Incorrect tanks have been placed on the central supply manifold.• There are numerous reports of outlets labeled for one gas that delivered another.• The wrong outlet connector may be installed. A terminal unit may accept an incorrect connector (due to connector pin breakage).
Diameter Index Safety System• The DISS was developed to provide noninterchangeable connections for medical gas lines at pressures of 200 psi or less.• Each DISS connector consists of a body, nipple, and nut combination.
• There are two concentric and specific bores in the body and two concentric and specific shoulders on the nipple.• The small bore mates with the small shoulder, and the large bore mates with the large shoulder.
• To achieve noninterchangeability between different connectors, the two diameters on each part vary in opposite directions so that as one diameter increases, the other decreases.• Only properly mated parts will fit together and allow the threads to engage.• The American Society for Testing and Materials (ASTM) anesthesia With increasing Compressed Gas Association (CGA) number, the small shoulder of the nipple workstation requires that becomes larger, and the large diameter becomes every anesthesia machine smaller. have a DISS fitting for each If assembly of a nonmating body and nipple is pipeline inlet. attempted, either small shoulder will be too large for small bore or large shoulder will be too large for large bore.
Quick Connectors• Quick connectors allow apparatus (hoses, flowmeters, etc.) to be connected or disconnected by a single action by using one or both hands without the use of tools or undue force. Quick connectors are more convenient than DISS fittings but tend to leak more.• Each quick connector consists of a pair of gas-specific male and female components. A releasable spring mechanism locks the components together. Hoses and other equipment are prevented from being inserted into an incorrect outlet by using different shapes and/or different spacing of mating portions.
Hose Pipes• Hose pipes are used to connect anesthesia machines and other apparatus to terminal units.• Each end has a permanently attached, noninterchangeable connector.• The connector that attaches to a terminal unit is called the inlet (supply) connector. The connector that attaches to equipment such as an anesthesia machine is the outlet (equipment) connector.• A colour-coded hose and the name and/or chemical symbol of the contained gas on each connector are desirable.
Test for Cross Connections Testing for cross connections is done to ensure that the gas delivered at each terminal unit is that shown on the outlet label and that the proper connectors are present at station outlets.1. One gas system is tested at a time.2. Each gas is turned off at the source valve and the pressures reduced to atmospheric.3. The pipeline being tested is then filled with oil-free nitrogen at its working pressure.4. With appropriate adapters matching outlet labels, each station outlet is checked to ensure that test gas emerges only from the outlets of the medical gas system being tested.5. The cross-connection test is then repeated for each gas system in turn.
Problems• Most problems are caused by anesthesia providers being unaware that these systems can fail as well as because they are not sufficiently familiar with the system to make emergency adjustments.• Lack of communication between clinical and maintenance departments and commercial suppliers may also be a contributing factor.• Compliance with existing codes is not universal.• Intended tampering should not be overlooked.
Oxygen Failure Safety Device• The anaesthesia workstation standard requires that whenever the oxygen supply pressure is reduced below the manufacturer-specified minimum, the delivered oxygen concentration shall not decrease below 19% at the common gas outlet.• It is located in the intermediate pressure system just upstream of the flowmeter.• The oxygen failure safety shuts off or proportionally decreases and ultimately interrupts the supply of nitrous oxide if the oxygen supply pressure decreases. On many modern machines, the air supply is also cut off.
• Ohmeda machines are equipped with a fail- safe valve known as the Pressure-Sensor Shut-off .• The valve is threshold in nature, and it is either open or closed.
• Dräger uses a fail-safe valve known as the Oxygen Failure Protection Device (OFPD).• OFPD is based on a proportioning principle rather than a threshold principle. The pressures of all gases controlled by the OFPD decrease proportionally with the oxygen pressure.
Testing the functional status of OFSD To determine if a machine has a properly functioning oxygen failure safety device,1. The flows of oxygen and the other gas (usually nitrous oxide) are turned ON.2. The source of oxygen pressure is then removed.3. The fall in oxygen pressure is noted on the cylinder or pipeline pressure gauge.4. If the oxygen failure safety device is functioning properly, the flow indicator for the other gas will fall to the bottom of the tube just before the oxygen indicator falls to the bottom of its tube.
Oxygen Supply Failure Alarm• The anesthesia workstation standard specifies that whenever the oxygen supply pressure falls below a manufacturer-specified threshold (usually 30 psi), at least a medium priority alarm shall be enunciated within 5 seconds. It shall not be possible to disable this alarm.• The alarm is connected to the intermediate pressure system just downstream of the pressure regulator.
Because both the oxygen failure safety device and alarm depend on pressure and not flow, theyhave limitations such that do not offer total protection against a hypoxic mixture being delivered,because they do not prevent anesthetic gas from flowing if there is no flow of oxygen. Also anyoperator related errors or leaks downstream still have potential for delivering a hypoxic mixture.
Flow Adjustment Control• The flow adjustment controls regulate the flow of oxygen, nitrous oxide, and other gases to the flow indicators.• There are two types of flow adjustment controls: mechanical and electronic.• The flow adjustment control knobs are differentiated both by sight as well as by touch and feel.• The Oxygen control knob is usually White coloured, Larger in size and is Fluted.
Electronic Flow Control Devices Some electronic flow control devices differ from conventional pneumatic flowmeters in that the operator only has two parameters to work with:1. The total Flow of gas2. Concentration of Oxygen to be delivered.• The machine makes up the rest of the gas from either nitrous oxide or air as is preselected.• There is usually a mixing area that collects the gas mixture.• Flow and pressure transducers as well as temperature sensors are used to maintain accuracy.
Gas Selector Switch Some machines have a gas selector switch that prevents air and nitrous oxide from being used together.Mechanical Gas Selector switch Electronic gas selector switch. Either nitrous oxide or air can be selected by pushing the appropriate button (lower left). Total gas flow and oxygen percentage are set by pushing the hard keys and rotating the wheel at the lower right. The balance of the fresh gas flow will be the other gas chosen (nitrous oxide or air).
Flowmeter Assembly• The flowmeter assembly located downstream from the flow adjustment control helps the anaesthesiologist visualize the gas flow and thus control it accurately.• It consists of the tube through which the gas flows, the indicator, a stop at the top of the tube, and the scale that indicates the flow.• Each assembly is clearly and permanently marked with the appropriate colour and name or chemical symbol of the gas measured.
Flowmeter Tube Arrangement• Flowmeter tube sequence can be a cause of hypoxia. The figures shows four different arrangements for oxygen, nitrous oxide, and air flowmeters. Normal gas flow is from bottom to top in each tube and then from left to right at the top.• In A/B, a leak is shown in the unused air flowmeter, showing potentially dangerous arrangements because the nitrous oxide flowmeter is located in the downstream position. A substantial portion of oxygen flow passes through the leak while all the nitrous oxide is directed to the common gas outlet.• Safer configurations are shown in C/D. By placing the oxygen flowmeter nearest the manifold outlet, a leak upstream from the oxygen results in loss of nitrous oxide rather than oxygen.
• Before discovering that flowmeter sequence was important in preventing hypoxia, there was no consensus on where the oxygen flowmeter should be in relation to the flowmeters for other gases.• To avoid confusion, the ASTM workstation standard requires that the oxygen flowmeter be placed on the right side of a group of flowmeter as viewed from the front.• It should be noted that having the oxygen flowmeter on the right is specific to North America. In many countries, the oxygen flowmeter is on the left with the outlet also on the left.• This sets the stage for potential operator error if a user administers anesthesia in a country other than where he or she was trained.• There is no consensus on the location of the air or nitrous oxide flowmeters as long as they do not occupy the location next to the manifold outlet.
Hypoxia Prevention Safety Devices1. Mandatory Minimum Oxygen Flow• Some anesthesia machines require a minimum (50 to 250 mL/minute) flow of oxygen before other gases will flow. This is preset by the manufacturer .• The minimum flow is activated when the master switch is turned ON.• It may be provided by a stop on the oxygen flow control valve or a resistor that permits a small flow to bypass a totally closed oxygen flow control valve.• Some machines activate an alarm if the oxygen flow goes below a certain minimum, even if no other gases are being used.• The minimum oxygen flow does not in itself prevent a hypoxic gas concentration from being delivered. A hypoxic gas mixture can be delivered with only modest anesthetic gas flows.
2. Minimum Oxygen Ratio The anesthesia workstation standard requires that an anesthesia machine be provided with a device to protect against an operator-selected delivery of a hypoxic mixture of oxygen and nitrous oxide having an oxygen concentration below 21% oxygen (V/V) in the fresh gas or the inspiratory gas.• These are of two types:1. Mechanical Linkage.2. Electronic Linkage.
Mechanical Linkage• A mechanical linkage between the nitrous oxide and oxygen flow control valves is shown in the Figure.• There is a 14-tooth sprocket on the nitrous oxide flow control valve and a 29-tooth sprocket on the oxygen flow control valve.• If the flow control valves are adjusted so that a 25% concentration of oxygen is reached, a pin on the oxygen sprocket engages a pin on the oxygen flow control knob.• This causes the oxygen and nitrous oxide flow control valves to turn together to maintain a minimum of 25% oxygen.• This minimum oxygen ratio device (proportioning system) permits independent control of each gas as long as the percentage of oxygen is above the minimum.
Electronic Linkage• An electronic system can be used to provide a minimum ratio of oxygen to nitrous oxide flow. An electronic proportioning valve controls the oxygen concentration in the fresh gas.• A computer continuously calculates the maximum allowable nitrous oxide flow given the oxygen flow.• If the nitrous oxide flow control valve is opened sufficiently to cause a flow higher than the maximum allowable, the proportioning valve reduces the nitrous oxide flow to supply a minimum of 25% oxygen. Alarms• Alarms are available on some machines to alert the operator that the oxygen:nitrous oxide flow ratio has fallen below a preset value.
Vaporizers• A vaporizer is a device that changes a liquid anesthetic agent into its vapor and adds a controlled amount of that vapor to the fresh gas flow or the breathing system.• Relevant to the context, the errors in intended gas delivery can occur at two levels:1. Wrong gas in wrong vaporizer.2. Simultaneous administration of multiple gases.• Failsafes to prevent such occurrences are in place in all vaporizers.
Prevent Wrong Gas in Wrong Vaporizer• Vaporizers are Colour coded and have Labels with names of the gas they deliver.• Same goes for the Bottles that agents are supplied in.
• Filling systems are a barrier to prevent wrong filling:1. Funnel Fill.2. Keyed Fill.3. Quik Fil.4. Easy Fil. They all have a Vaporizer end and a Bottle end which are specific to the agent.
Keyed Fill:• Each bottle of liquid anesthetic has a color-coded collar attached securely at the neck.• Each collar has two projections, one thicker than the other, which are designed to mate with corresponding indentations on the bottle adaptor.• Bottle adaptors are also color coded.• At one end, the adaptor has a connector with a screw thread to match the thread on the bottle and a skirt that extends beyond the screw threads and has slots that match the projections on the bottle collar.• At the other end is the male connector that fits into the vaporizer filler receptacle. It consists of a rectangular piece of plastic with a groove on one side and two holes on another surface. The groove is in different locations, depending on the agent that is to be used
Funnel Fill:• A color-coded adaptor is used.• At one end is a connector with a screw thread to match the thread on the bottle and a skirt that extends beyond the screw threads.• It has slots that match the projections on the bottle collar.• The adaptor for a different agent than the adaptor is intended for will not screw on either because of different threads or bottle opening size or because the projections will not line up with the slots on the adaptor.• A funnel-fill vaporizer can be converted to an agent-specific keyed filling system by the addition of an adaptor that screws into the vaporizer filler
Quik fil is for Sevoflurane only.• The vaporizer filler has a screw-on cap. The filler neck has three grooves that can accept only a special filler device, which comes attached to the bottle.• The bottle has a permanently attached, agent-specific filling device that has three ridges that fit into slots in the filler.Desflurane bottle adaptor has a spring-loaded valve that opens when the bottle is pushed into the filling port on the vaporizer.Easy Fill:The vaporizer filler channel are two keys (ridges) that fit grooves on the bottle adapter.The bottle adaptor attaches to the bottle by aligning the notches with the projections on the bottle collar.The adaptor has grooves that must be aligned with the projections on the vaporizer.
Some Manufacturing FallaciesThe collars on bottles of volatile inhaled anesthetics Similarly if the bottle collar foreach have large and small projections. The collars enflurane or halothane is upsidefor isoflurane and sevoflurane are symmetric mirror down on the bottle, the bottleimages of each other, as are the collars for adaptor for the other agent willhalothane and enflurane. fit.
Hazards of Incorrect Agent Filling• If an agent of low potency or low volatility is placed in a vaporizer designed for an agent of higher potency or volatility, the effect will be an output of low potency.• Conversely, if an agent of high potency or volatility is used in a vaporizer intended for an agent of low potency or volatility, a dangerously high concentration may be delivered.• If an incorrect agent is placed in a vaporizer, there will likely be a mixture of agents in the vaporizer.• Smelling cannot be relied on to tell which agent is in a vaporizer, because the smell of a small amount of one agent can completely mask the odor of a less-pungent agent, even if the second agent is present in much higher concentration
Prevent Simultaneous Administration of Multiple Gases• Interlock (vaporizer exclusion) systems prevent more than one vaporizer from being turned ON at a time.• For Datex-Ohmeda vaporizers, operating the dial release activates two extension rods that prevent operation of any other vaporizer installed on the manifold.
• A switch on the back bar may be used to direct gas flow through only one vaporizer at a time, e.g. the Fraser Harlake Selectatec back bar and the Vapour changeover switch used with Dräger 19.1 vaporizers.
• The Dräger Interlock 1 system for Vapour 19.2 vaporizers features a rotating bar on the manifold with teeth that fit into a cut-out on the back of the control dial.
• A mechanical locking system may be used that only allows one vaporizer to be switched on, e.g. Ohio selector manifolds and Dräger 19.3 vaporizers.• The Ohio triple selector manifold allows the left, centre or right vaporizer to be used. Slots in the selector (arrowed) line up with flanges on the vaporizer control dials.
• If none of the above is possible, mounting the vaporizer for the most volatile agent downstream will prevent release of high concentrations of a volatile agent owing to contamination of a vaporizer designed for an agent with a low saturated vapour pressure. vaporizers should be arranged in the order:• This will, of course, do nothing to prevent the patient being inadvertently exposed to more than one anaesthetic at a time.
Gas Monitoring• Reliable, affordable, and user-friendly monitors to measure respiratory and anesthetic gas concentrations are now available.• Discussion will be concerned with:1. Oxygen and N2OAnalysers.2. Volatile Anaesthetic Agent Analyser.
Oxygen Analysers• The standards for basic anaesthesia monitoring of the ASA and AANA state that the concentration of oxygen in the patient breathing system shall be measured by an oxygen analyzer with a low oxygen concentration alarm in use.• Measured by using electrochemical or paramagnetic technology.• Electrochemical analysis provides only mean concentrations. Paramagnetic technology has a sufficiently rapid response time to measure both inspired and end-tidal levels.
Applications of Oxygen Analyzer1. Detecting Hypoxic or Hyperoxic Mixtures2. Detecting Disconnections and Leaks (not reliable)3. Detecting Hypoventilation (Steady state difference of > 5% b/w Inspired and Expired O2).4. Measure the adequacy of preoxygenation (EtO2).5. Expired O2 conc. – Estimated O2 Consumption – Diagnosis Malignant Hyperthermia.6. Concentration of nitrous oxide can be estimated from the concentration of oxygen.7. Detect Air Embolism (decreased diff. b/w IO2 and EtO2)
Volatile Anesthetic Agents Analyzer• The volatile anesthetic agents can be measured by using infrared analysis, refractometry, or piezoelectric analysis.• When an agent is used for which an analyzer is not programmed, it may be possible to apply a conversion factor so that the analyzer may be used to monitor that agent.• Uses:1. Ability to assess vaporizer accuracy.2. Incorrect Agent detection: Agent-specific analyzers can detect an incorrect agent, and non-agent-specific analyzers will usually exhibit unusual readings when an agent error is made.3. Alert the user when a vaporizer has become empty or when a vaporizer not in use is allowing significant amounts of vapor to leak into the fresh gas line.• There is usually a difference between the two values, with the inspired concentration being lower at the beginning of a case and higher at the end. This discrepancy results from the time needed to equilibrate the concentration in the relatively large volume of gas in the breathing system as well as by agent uptake by the patient.
Caplan et al (1997), conducted a review of the Closed Claims Project database to determine the contribution of Gas delivery equipment (GDE) to patient morbidity and mortality.1. GDE accounted for 2% of all Claims.2. 76% of the adverse events resulted in death or permanent brain damage.3. Misuse of equipment (75%) was three times more common than equipment failure (24%).4. Misconnects and disconnects of the breathing circuit made the largest contribution to injury (35%).5. 78% were deemed preventable with the use or better use of monitors (Pulse Oxymeters, Capnographs).
Summary As a take home message,• adequate knowledge of the machinery that surrounds,• adequate perception of something that might go wrong and,• adequate reflexive action on part of the anaesthesiologist if something does go wrong are the most important preventive strategies that are equivalent and often surpass all the protective strategies of the machines that we use.