Anesthetic Gas Machines

Introduction:

The modern anesthesia machine is a complex operating room instrument that incorporates a ventilator to optimize the delivery of inhaled anesthetics. The machine enables elective surgeries that would be impossible without anesthesia. In addition, this device has a relatively broad range of effects on the course of the operation, resulting in optimal and safe working environments for surgeons. The anesthesia machine has gradually evolved from a simple means of anesthetizing and oxygenating a patient to an anesthesia workstation with increasingly complex ventilator modes, end-tidal CO2 monitors, end-tidal anesthetic concentrations, minimal alveolar concentration estimators, and a process of monitoring vital signs.

What Is the Function of the Anesthetic Gas Machines?

• Modern anesthesia machines control several vital functions, such as the exact mixture of anesthetic vapor, which should expose heart rate, airway pressure, oxygen concentration, flow, automatic ventilation, and the staff to fewer anesthetic vapors.

What Are the Types of Anesthetic Gas Machines?

• There are three types of anesthesia machines:

  1. High-pressure systems.

  2. Intermediate pressure systems.

  3. Low-pressure systems.

• The high and intermediate pressure systems hold pressure measured in pounds per square inch (psi) or kilopascals (kPa). In contrast, pressure in the low-pressure system is measured in centimeters of water (cmH20).

• An E-cylinder attached to the back of the anesthesia machine supplies oxygen (2200 psi), air, and nitrous oxide (745 psi) to the high-pressure system.

• Subsequently, a pressure regulator sets the pressure to 45 psi. The high-pressure system is primarily utilized when the pipeline supply fails or is unavailable for connection, such as in a remote anesthesia location.

• The intermediate pressure system receives its 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 gas source. In addition, the intermediate pressure system feeds gas into the flowmeters.

• The low-pressure system exists downward from the flowmeters. It provides a fresh flow of gases oxygen at 14 psi or nitrous oxide at 26 psi to vaporizers to serve as a source of volatile anesthetics. The patient breathes in and out of the anesthesia machine's low-pressure system.

How is the Flow of the Gas in the Anesthetic Machine?

• A pipeline and cylinder gas supply carry gas (oxygen, air, nitrous oxide) into the anesthesia machine. The pipeline supply is usually the primary gas source, operating at 45 to 60 psi (intermediate pressure system). When the pipeline supply falls, the E-cylinder takes over. The E-cylinder has an elevated and changeable pressure, necessitating the installation of a pressure regulator to reduce the stress to around 45 to 60 psi.

• Once the gas reaches the intermediate pressure system, the gas is directed to the flowmeter, where the anesthetist can titrate the flow of fresh gas to the patient. The gas then passes downwards to a low-pressure system, where pressure is less than one psi and measured in cm H20 (one psi is about seventy centimeters H2O).

• The gas may communicate with a variable bypass or measured flow vaporizer to achieve its volatile anesthetic.

• The gas is then passed through a unidirectional inspiratory valve and is aspirated by the patient through the circle system's inspiratory limb.

• Exhaled gas from the patient then passes down the circuit's expiratory limb and through the expiratory single-direction nozzle.

• The device can release excess pressure by the adjustable pressure-limiting (APL) nozzle if the ventilator is "switched off" or in a manual or spontaneous setting.

• The gas that persists in the circuit is then passed through a carbon dioxide (CO2) absorber, which eliminates the CO2 from the expired gas.

What Are the Important Components of the Anesthetic Machine?

1. Vaporizer:

• The two broad classifications of anesthesia machine vaporizers are variable bypass and measured flow vaporizers. Variable bypass vaporizers operate by adjusting the "splitting ratio" on a dial that controls the vaporizer.

• The splitting ratio is the ratio of fresh gas flow entering the vaporization tank compared to fresh gas flow trying to bypass the vapor chamber. As a result, the gas that reaches the vapor chamber becomes anesthetic-saturated before rejoining the fresh gas flow to deliver a precisely calculated dosage of volatile anesthetic.

• The variable bypass vaporizer instantly adjusts for a broad range of operating room temperatures to ensure a consistent anesthetic output at a specified atmospheric pressure. In addition, each vaporizer is customized for a particular volatile anesthetic, such as Halothane, Isoflurane, Enflurane, and Sevoflurane.

• Vaporizers that measure the flow operate differently. The Desflurane vaporizer is an example of a measured flow vaporizer. Desflurane is heated to a fixed temperature of 39 degrees Celsius because it has a low boiling point and inclination to volatility. As a result, the vaporizer's circuit starts in the vaporizer itself rather than fresh gas flowing over the volatile anesthetic.

• Two independent circuits (fresh gas flow and inhalational anesthetic flow) are parallelized. They do not combine until they are down from the vaporizer, just before entering the inspiratory limb.

1. Adjustable Pressure Limiting Valve (APL Valve):

• As expired gases return to the patient's anesthesia machine, the APL valve, also referred to as the "pop-off valve," stays between the unidirectional valve of the expiratory and the carbon dioxide absorber.

• When tubing becomes obstructed, the APL valve functions as a pressure release valve, preventing excessive pressure in the breathing circuit. However, too much stress can cause barotrauma in the patient and damage flowmeters or vaporizers.

• As the term indicates, can adjust the APL valve during different stages of anesthesia to match the patient perfectly. For example, the valve remains open during spontaneous ventilation to allow for easier breathing.

• When positive pressure ventilation is needed after induction, the valve can be partially closed (generally to less than 20 cm H20) by pressing the reservoir bag to permit positive pressure ventilation.

• Any gas ejected by the APL valve to control pressure to reduce operating room pollution is transferred to the scavenger system.

1. Oxygen Flush Button:

• The intermediate pressure system has an "oxygen flush" button that, when pressed, begins a direct link between both the pipeline oxygen and the oxygen pressure regulator, allowing 35 to 70 liters per minute of pure oxygen at a pressure of 45 to 60 psi to be delivered to the patient.

• Its most common application is during mask ventilation, when one cannot obtain an insufficient mask seal for several reasons, including a patient's mustache and beard, operational errors, and patients with difficult airways.

• Because of the flow of gas at higher pressures (45 to 60 psi) than the typical low-pressure system of the anesthesia device, the utilization of the oxygen flush can contribute to periods of awareness during anesthesia and barotrauma to the patient's lungs.

1. Carbon Dioxide Absorbent:

• The carbon dioxide (CO2) absorbent is integral to the circle system breathing circuit. CO2 absorbent is a mixture of calcium hydroxide, sodium hydroxide, potassium hydroxide, and barium hydroxide that inhibits carbon dioxide from entering the anesthesia machine's inspiratory limb.

• Exhaled gases transmit through the filter, where carbon dioxide reacts chemically with these bases to become trapped in the filter, allowing for safer re-inhalation of the exhaled air.

• In addition, filtered expiratory air permits the recycling of expired gases, enabling low-flow anesthesia (gas flows less than alveolar ventilation to reduce anesthesia costs). Chemical indicators in CO2 absorbents used in anesthesia machines usually change color as the filter becomes saturated.

• It should be replaced whenever the filter is two-thirds saturated to avoid re-breathing carbon dioxide. When carbon dioxide is identified in the inspiratory limb, modern anesthesia machine monitors will notify the anesthesiologist to replace the soda lime filter with a new filter.

What Are the Various Types of Anesthetic Circuits?

There are various types of anesthesia circuits. The circle is the most frequently used system in modern anesthesia machines. Still, other anesthesia circuits would include Mapleson A, B, C, D (and Bain modification), E, and F (Jackson-Rees) devices. For Mapleson D, E, and F devices, a T piece is nearer to the patient.

• The Circle System: Circle systems can be employed as a closed system based on the fresh gas inflow settings.

• The Mapleson D (Bain) System is a variation of the Mapleson D that includes a tube inside the corrugated exhalation tube that delivers fresh gas to the patient. This can preserve moisture and warm the fresh gas as it flows to the patient.

• The Mapleson F (Jackson-Rees) System is a variation of the Mapleson E. It has a reservoir bag connected to the expiratory limb and an adjustable overflow bag. It has a small dead space and offers little resistance to spontaneous ventilation, making it suitable for pediatric patients.

Conclusion:

Modern anesthesia machines have become more complex, with various safety mechanisms, alarms, and display messages to improve user-friendliness. But besides these advancements, the device is still susceptible to operator error, necessitating the attention and focus of everyone in the operating room. In addition, the modern anesthesia machine, which uses a closed-circuit system, also allows for recycling inhaled gasses, significantly reducing environmental pollution. Finally, although knowledge of the intricate workings of the device is not required of all operating room staff, knowledge and understanding of machine components may assist in providing a safer anesthetic to the patient.