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Anesthesia Machine

Editor: Danielle B. Horn Updated: 11/17/2022 2:56:17 PM


The modern anesthesia machine is a complex operating room instrument that incorporates a ventilator to optimize the delivery of inhaled anesthetics. The anesthesia machine has gradually evolved from simply a means to anesthetize and oxygenate a patient to an anesthesia workstation incorporating increasingly complex ventilator modes, end-tidal CO2 monitors, end-tidal anesthetic concentrations, minimal alveolar concentration estimators, and a means of monitoring vital signs.[1] Despite all these innovations and new instruments added to the anesthesia machine, an understanding of the anesthesia machine is still a core component of the practice of anesthesiology.


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The modern anesthesia machine seamlessly incorporates a ventilator to allow for optimal anesthetic delivery.  

There are four main functions of the modern anesthesia machine: 

  1. Oxygenation
  2. The accurate mixture of anesthetic vapors
  3. Appropriate ventilation
  4. Reduce exposure of anesthetic vapors to personelle[2]

The anesthesia machine can be divided into three basic areas: (1) a high-pressure system, (2) an intermediate pressure system, and (3) a low-pressure system. The high and intermediate pressure systems have pressure measured in pounds per square inch (psi) or kilopascals (kPa), while the low-pressure system has pressure measured in centimeters of water (cmH20). The high-pressure system receives its supply of oxygen (2200 psi), air, and nitrous oxide (745 psi) from an E-cylinder ('E' refers to the size of the cylinder) which is attached to the back of the anesthesia machine. Subsequently, a pressure regulator sets the pressure to 45 psi. The high-pressure system is primarily utilized when the pipeline supply fails or is not available for connection, such as in a remote anesthesia location. The intermediate pressure system receives its supply of gases from the hospital pipeline supply, which is set to 50 psi. In modern hospitals, anesthesia machines rely on the pipeline supply as the primary source of gas. The intermediate pressure system feeds gas into the flowmeters.  Downstream from the flowmeters, the low-pressure system exists, which provides fresh gas flow (oxygen [14 psi] and/or nitrous oxide [26 psi]) to vaporizers to allow for a source of volatile anesthetics. The patient inspires and expires in the low-pressure system of the anesthesia machine.[2]

Gas Flow Through the Anesthesia Machine

The following describes the generalized flow of gas through the modern circle system anesthesia machine:

  1. Gas (oxygen, air, nitrous oxide) enters the anesthesia machine through a pipeline supply and cylinder gas supply.  The primary source of gas is typically the pipeline supply, which functions at 45 to 60 psi (the intermediate pressure system).  When the pipeline supply fails, the E-cylinder (high-pressure system) takes precedence.  The E-Cylinder has a much higher and variable pressure and thus requires the assistance of a pressure regulator to bring the pressure down to about 45 to 60 psi.  
  2. Once the gas is in the intermediate pressure system, gas travels to the flowmeter, where the anesthesiologist can titrate the flow of fresh gas to the patient.  Downstream from the flowmeters, the gas enters a low-pressure system where pressures are less than 1 psi and measured in cm H20 (1 psi is about 70 cm H2O).  
  3. Gas may subsequently interact with a variable bypass or measured flow vaporizer to attain its volatile anesthetic.
  4. The gas then passes a unidirectional inspiratory valve and may be inhaled by the patient via the inspiratory limb of the circle system.
  5. Expired gas from the patient then travels down the expiratory limb of the circuit and passes through the expiratory unidirectional valve.
  6. Presuming the ventilator is "switched off," or on a manual/spontaneous setting, excess pressure in the system can be released by the adjustable pressure-limiting (APL) valve (this released gas is suctioned by the scavenger system)
  7. Gas that remains in the circuit then travels through a carbon dioxide (CO2) absorber, where the CO2 is removed from the expired gas.
  8. Beyond the CO2 absorber, the expired gas is then reunited with fresh gas flow from the pipeline or E-cylinders, where it can be recycled (recycling of gas allows for low flow anesthesia and minimizes the waste of volatile anesthetic).[3][4][5]

Important Components of the Anesthesia Machine


There are essentially two broad categories of anesthesia machine vaporizers: variable bypass vaporizers and measured flow vaporizers.  Variable bypass vaporizers work by setting a "splitting ratio" on a dial controlling the vaporizer.  The splitting ratio describes the ratio of fresh gas flow that enters the vaporizing chamber compared to the fresh gas flow that bypasses the vapor chamber. The gas that enters the vapor chamber becomes saturated with anesthetic and then reunites with the fresh gas flow to deliver a carefully calculated dose of volatile anesthetic. The variable bypass vaporizer automatically compensates for a wide range of temperatures in the operating room to ensure a steady output of anesthetic at a given atmospheric pressure.  Each vaporizer is specific for a certain volatile anesthetic such as halothane, isoflurane, enflurane, and sevoflurane.[5]

Measured flow vaporizers function differently. The most common example of a measured flow vaporized is the desflurane vaporizer. Due to desflurane's low boiling point and proclivity to volatility, it is heated to a constant temperature of 39 degrees Celsius, and the vaporizer's circuit begins in the vaporizer itself as opposed to fresh gas flowing over the volatile anesthetic. There are two independent circuits (fresh gas flow and inhalational anesthetic flow) which are arranged in parallel and do not mix until they are downstream from the vaporizer, just before entering the inspiratory limb.[5]

Adjustable Pressure Limiting Valve (APL Valve)

As expired gases travel back to the anesthesia machine from the patient, the APL Valve, otherwise known as the "pop-off valve," sits between the expiratory unidirectional valve and the carbon dioxide absorber. The APL valve serves as a pressure relief valve to prevent excessive pressures in the breathing circuit when tubing may become obstructed.  Excessive pressures may result in barotrauma to the patient or damage to flowmeters or vaporizers. As the name suggests, the APL valve can be adjusted during different phases of anesthesia to best suit the patient. During spontaneous ventilation, the valve remains open to facilitate easier breathing. After induction, when positive pressure ventilation is required, the valve can be closed partially (typically to less than 20 cm H20) to allow for positive pressure ventilation by squeezing the reservoir bag. Any gas released from the APL valve to limit pressure is routed to the scavenger system to minimize operating room pollution.[2][3]

Oxygen Flush Button

The intermediate pressure system has a button referred to as the "oxygen flush" button, which, when pressed, allows for the opening of a direct connection between the pipeline oxygen and the oxygen pressure regulator to deliver 35 to 70 liters per minute of pure oxygen at a pressure of 45 to 60 psi to the patient. Its most common use is during mask ventilation when an inadequate mask seal cannot be obtained due to various reasons such as a patient's beard, operator error, and patients with difficult airways. The anesthesia provider needs to be cognizant that while pushing the oxygen flush button, only oxygen and not any volatile anesthetic or nitrous oxide is being administered to the patient even if the volatile anesthetic or nitrous oxide is turned on. Use of the oxygen flush can result in periods of awareness during anesthesia and barotrauma to the patient's lungs due to the flow of gas at higher pressures (45 to 60 psi) than the typical low-pressure system of the anesthesia machine.

Carbon Dioxide Absorbent

An essential component of the circle system breathing circuit is the carbon dioxide (CO2) absorbent. CO2 absorbent contains various mixtures of calcium hydroxide, sodium hydroxide, potassium hydroxide, and barium hydroxide to prevent the introduction of carbon dioxide into the inspiratory limb of the anesthesia machine.[6] Expired gases pass through the filter where carbon dioxide undergoes chemical reactions with these respective bases to be trapped in the filter to allow for safer rebreathing of the expired air. The filtered expired air allows for expired gases to be recycled, allowing for low-flow anesthesia (gas flows less than alveolar ventilation to minimize costs of providing anesthesia). CO2 absorbents used in anesthesia machines typically have chemical indicators that change color as the filter becomes saturated. When the filter is two-thirds saturated, the filter should be changed to prevent the re-breathing of carbon dioxide. Modern anesthesia machine monitors will often indicate when carbon dioxide is detected in the inspiratory limb, further alerting the anesthesiologist that the soda lime filter needs to be exchanged. Carbon dioxide becomes present in the inspiratory limb when the carbon dioxide absorbent fails to absorb the carbon dioxide in the expired gases present in the expiratory limb.[1][7]

Types of Anesthesia Circuits

Different types of anesthesia circuits exist. The most commonly used system in the modern-day anesthesia machine is the circle system, but other examples of anesthesia circuits include the Mapleson A, B, C, D (and Bain modification), E, and F (Jackson-Rees) systems. There is a T piece close to the patient for Mapleson D, E, and F systems.[8]

  • The circle system: depending upon the settings of the fresh gas inflow, circle systems can be used as a closed system (fresh gas flow [FGF] = oxygen and anesthetic update), semi-closed system (high FGF, gas exits through the expiratory valve), or semi-open system. They have the benefit of conserving airway moisture and heat while limiting the leakage of anesthetic gases out of the system and into the environment. Disadvantages are discussed below (see "Issues of Concern").
  • The Mapleson D (Bain) system is a modification of the Mapleson D, which has a tube supplying fresh gas to the patient inside of the corrugated expiratory tube (in a "coaxial" formation). This offers the benefit of maintaining moisture and warming the fresh gas as it flows to the patient.[8]
  • The Mapleson F (Jackson-Rees) system is a modified version of the Mapleson E. It has a reservoir bag attached to the expiratory limb and may also have an adjustable overflow bag. It has minimal dead space and imposes little resistance to spontaneous ventilation so that it may be used for pediatric patients. However, this is an inefficient system because it requires high FGF to prevent rebreathing.[8]

For the remainder of this article, we will focus on the circle system.

Issues of Concern

Pitfalls of the Circle System

Although highly functional, the circle circuit breathing system commonly used in modern anesthesia is very complex and prone to leaks that can inhibit its optimal use. To minimize leaks, many of the components are internalized to prevent disruption related to human error. The beauty of its nature is that it allows for low-flow delivery of anesthesia which minimizes the costs associated with expensive inhalational anesthetics and minimizes pollution of the operating room. Low fresh gas flows utilize less volatile anesthetics in the vaporizers, and volatile anesthetics are more effectively recycled. Low-flow anesthesia is defined as the delivery of anesthesia utilizing a flow of gases that is less than alveolar ventilation.[1][4] It is advisable to perform a leak test/machine test before the anesthesia machine's use to minimize the chances of a leak inhibiting proper ventilation and anesthetic delivery.[9] The following are common sources of leaks that can inhibit an anesthesia machine's function:

  1. Poor mask seal 
  2. Dislodged endotracheal tube
  3. Circuit discontinuity - i.e., Inspiratory/expiratory limb disconnection, open filling tank on a variable bypass vaporizer, etc
  4. A poorly attached/detached carbon dioxide absorbent canister

When a leak occurs, prompt recognition and tracing of the leak are essential to prevent respiratory decompensation due to ventilator failure or loss of patient anesthesia, creating awareness. Common methods of tracing the leak include listening for a "hissing" sound and smelling for the source of volatile anesthetic. If the patient begins to decompensate (such as with desaturation), it is essential to call for assistance early to aid in identifying the leak to restore proper anesthesia machine function. It is advisable to utilize an alternate means of ventilation such as with a bag valve mask and an alternate means of anesthesia such as with IV anesthetics until the anesthesia machine can be restored to proper functioning when machine failure occurs.

The circle breathing circuit is efficient but still may leak volatile anesthetics into the operating room environment. Thus a scavenging system was created to minimize exposure to volatile anesthetics leaking into the operating room.  Exposure of volatile anesthetics to operating room personnel is a potential occupational health hazard. In addition to potential nephrotoxicity, hepatoxicity, and neurotoxicity, long-term exposure to volatile anesthetics has been correlated with genotoxic and mutagenic events such as chromosomal aberration and sister chromatid exchange.[10][11] Female anesthesiologists have been found to have a higher rate of first-trimester abortions compared to the general population.[12]

Prior to using the anesthesia machine, a thorough checklist to ensure proper working of all the machine components helps prevent equipment failure resulting in awareness during anesthesia, hypoxic gas mixtures, excessive concentrations of inhalational anesthetics, and inability to ventilate the patient after administering anesthesia.  Modern machines typically incorporate an automatic machine check that is performed at least once per day.  Automated leakage tests have also been incorporated to allow for quicker checks of circuit discontinuity between cases during the day.  However, the anesthesia provider must be aware that although the automated tests are designed to reduce incidences of user error when preparing the machine, a manual test of all the components can be performed, albeit more labor-intensive. Routine servicing and checking of machines help to prevent clinically adverse events.  When a proper machine check does not identify equipment malfunction, rapid early detection of issues and prompt addressing of concerns helps to minimize possible adverse events.[7][13]

Clinical Significance

The modern anesthesia machine implementing a closed-circuit system allows for recycling inhaled gases (i.e., oxygen, nitrous oxide, isoflurane, sevoflurane, etc.), resulting in a significant reduction in environmental pollution.  Advances in carbon dioxide absorbents have allowed carbon dioxide to be absorbed and filtered out from exhaled gases so that oxygen and volatile anesthetics can be re-used.  The effectiveness of recycling gases can be compounded by utilizing the practice of low-flow anesthesia (gas flows less than alveolar ventilation).  Anesthesia circuits without a closed-circuit system may be used to administer anesthesia; however, at a significant financial cost to administer anesthetic and significant environmental impact when implemented widely.[4]

Other Issues

Safety Mechanisms

Pin Index Safety System

The Pin Index Safety System (PISS) exists to prevent the incorrect E-Cylinder from being attached to the yoke of the anesthesia machine at the wrong site. The PISS consists of two index pins specific to the corresponding hole of only one type of E-cylinder (i.e., air, oxygen, nitrous oxide). This prevents misconnections and the delivery of incorrect gases to the patient. The pins on the yoke of the anesthesia machine should never be altered or exchanged to prevent erroneous delivery of anesthetics.[2]

Diameter Index Safety System

Much like the PISS, the Diameter Index Safety System (DISS) prevents inappropriate connections that would cause the incorrect delivery of gases to the patient. The DISS has non-interchangeable pipeline connections of varying diameters, so there is only one unique connection for each pipeline gas.  [3]

The Sequence of Flowmeters

The flowmeters allow for titration of gases from the pipeline or E-Cylinder and consist of a tapered tube with the smallest diameter at the bottom.  A bobbin sits at the bottom of the meter and gradually is elevated towards the top of the flowmeter as the flow of the gas being titrated increases. The sequence of the flowmeters is of paramount importance to prevent the creation of a hypoxic gas mixture. The oxygen should always be situated downstream of other gases to prevent a hypoxic gas mixture. In a scenario where the oxygen is stationed upstream from other gases, a leak in the system between the entry of the oxygen and another gas would divert oxygen away from the patient resulting in a hypoxic gas mixture. With the oxygen situated downstream from other gases, a leak near the flowmeters may result in a lighter degree of anesthesia but a significantly lessened risk of a hypoxic gas mixture.[2]

Gas Scavenging System

The long-term effects of exposure to volatile anesthetics are a potential occupational hazard. Some studies have demonstrated an increased risk of spontaneous abortions, increased infertility, and having children with congenital anomalies. Long-term exposure to volatile anesthetics possibly predisposes anesthesiologists to multiple organ system dysfunctions. The modern anesthesia machine incorporates a gas scavenging system to capture volatile anesthetics to minimize exposure to volatile anesthetics. The scavenging system gathers exhaled volatile gases from the anesthesia equipment. It transports them away from the operating room, likely into the atmosphere and outside the building, housing the anesthesia machine. Most anesthesia machines utilize an open system that sits over the exhaust port and adjustable pressure limiting valve of the anesthesia machine. The scavenging system gathers gases into a reservoir then utilizes the pipeline suction to transport the reservoir gases outside of the operating room. Positive and negative pressure relief valves are utilized to prevent the transmission of scavenged gases into the breathing circuit.[7][3]

Pressure Regulator

An anesthesia machine requires consistent, constant flow of gas at a suitable low pressure to prevent barotrauma to the patient's lungs.  A pressure regulator is situated between the variable high-pressure E-cylinders and the intermediate pressure system of the anesthesia machine to provide consistent lower pressure. The pressure of an E-cylinder of oxygen can reach over 2000 psi or 13,700 kPa, whereas the intermediate pressure system typically relying on the pipeline supply operates at 45 to 60 psi (475 to 413 kPa). Pressure regulators help provide smooth and consistent gas flow despite fluctuations in pipeline pressures due to peaks/troughs with demands of the pipeline supply throughout the day and with varying pressures of E-cylinders.[3]

Prevention of Hypoxia

Modern anesthesia machines typically have a minimal pre-determined, mandatory oxygen flow once the anesthesia machine is turned on to prevent the lack of oxygen delivery to the patient. Anesthesia machines also dissuade users from selecting a gas mixture below that of atmospheric oxygen concentration (21%) to prevent hypoxic gas mixtures. There have been documented incidents of hypoxic gas mixtures while utilizing nitrous oxide without an appropriately complementing oxygen concentration. To protect against hypoxic gas mixtures with nitrous oxide, a mechanical or electronic linkage exists between the two flow control valves to allow for adequate proportioning of the nitrous oxide: oxygen ratios in modern machines. Likewise, interlocks on the vaporizers prevent more than one vaporizer from being used at the same time to prevent excessively high concentrations of volatile anesthetics.[3][5]

Enhancing Healthcare Team Outcomes

The operating room can be a stressful environment requiring synchrony of motion between all members of the operating room staff: anesthesiologists, surgeons, nursing, and even operating room aides. At the center of the operating room is the patient, the surgeon performing a procedure, and an anesthesia provider often manipulating an anesthesia machine to ensure appropriate patient comfort, oxygenation, and ventilation. Anesthesia machines have become complex pieces of medical equipment incorporating a ventilator and various forms of monitoring devices to ensure patient safety requiring an anesthesia provider's vigilant attention to ensure proper functioning. Despite the advances made in anesthesia delivery, the anesthesia machine is still prone to human error inhibiting proper function. Teamwork and education about the anesthesia machine amongst all operating room personnel can help create awareness about its importance, areas of weakness and solutions to common problems to reduce morbidity and mortality in the operating room.  For example, a nurse noting an anesthesia provider having a difficult time mask ventilating a patient with a beard can squeeze the reservoir bag of an anesthesia machine while an anesthesia provider uses two hands to create a better mask seal to facilitate anesthesia machine function.  

Technology related to anesthesia machines has advanced extensively to make anesthesia safer to administer. Human error appears to be the dominant factor related to critical anesthesia incidents and negative outcomes related to the administration of anesthesia. In a cohort study of critical incidents related to the administration of anesthesia, only 4% of substantive negative outcomes were related to equipment failure. [Level 3] Greater education amongst healthcare providers about the functions/workings of the anesthesia machine and better interprofessional communication can allow for more rapid identification of errors and likely reduce the morbidity and mortality associated with anesthesia.[14]  

Operating room staff must be educated about the occupational hazards associated with improper use of anesthesia machines. Exposure of volatile anesthetics to rats has been associated with a range of medical complications, including nephrotoxicity from Compound A associated with sevoflurane and impairments in memory.[15][16] However, a meta-analysis of a plethora of randomized control studies involving the relationship between compromised renal function in a healthy patient failed to find any significant relationship.[17] [Level 1] Despite this, operating room staff, particularly female anesthesiologists, have been shown to have higher rates of spontaneous abortions and infertility compared to the general population.[7] Thus, volatile anesthetics from anesthesia machines must have a properly functioning gas scavenging apparatus and proper education on its use to reduce the risk of long-term exposure of volatile anesthetics to operating room staff.

Nursing, Allied Health, and Interprofessional Team Interventions

Modern anesthesia machines have become more complex over the decades, incorporating an array of safety features, alarms, and displayed messages enhancing user-friendliness.  Despite these advances, the machine is still prone to user error and thus requires the attention and focus of all operating room personnel in anticipation of a dreaded scenario of failure to oxygenation/ventilate or anesthesia awareness. Although an understanding of the intricate workings of the anesthesia machine is not a requirement of all operating room staff, familiarity and awareness of components of the machine may help provide a safer anesthetic to the patient. 

Healthcare team awareness of the location of a bag-valve-mask or self-inflating bag to hand off to the anesthesia provider can provide a temporary means of oxygenation and ventilation until anesthesia machine issues are resolved if the machine fails.  Familiarity with the flaws of a low-pressure, circle-circuit anesthesia machine that is complex and prone to leaks may allow for greater confidence in nursing and allied health professionals to point out when a tube is disconnected so the anesthesia provider can quickly resolve issues to prevent unnecessary adverse outcomes.  With the emphasis on faster turn-over times between cases, it can be easy for operating room staff to forget that the anesthesia machine is the most important piece of equipment in the operating room. Failure of its proper function can lead to hypoxia and death or anoxic brain injury in a matter of minutes.  Although routine, anesthesia providers must have the time they need to check the machine and ensure its proper functioning before the arrival of a patient to the operating room so concerns and issues can be addressed safely and effectively.



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