1. The pressure of a full E cylinder of oxygen (O₂) is 2200 psi (625 L). O₂ enters the anesthesia machine from the E cylinder at 45 psi, as regulated by the O₂ cylinder pressure regulator.

2. The pipeline O₂ supply pressure enters the machine at 50 psi. If both pipeline and cylinder are open, the pipeline is preferred.

3. The Pin Index Safety System is a safeguard introduced in the hanger yoke of cylinders to prevent cylinder interchanging and the possibility of accidentally placing the incorrect gas on a yoke designed to accommodate another gas. Two pins on the yoke are arranged so that they project into the cylinder valve. Each gas cylinder has a specific pin arrangement.

4. The check valve has three main functions:

5. If the O₂ supply suddenly falls to 20 psi, the O₂ flow will be maintained and 100% O₂ will be delivered, as the pressure sensor valve will shut off delivery of nitrous oxide (N₂O) and other gases.

2. The purpose of the second-stage regulator in the Ohmeda ventilator is to ensure that O₂ is the last gas flow to decrease if O₂ pressure fails. It is set at 12 to 19 psi. O₂ flowmeter output is constant when the O₂ supply pressure exceeds the set value.

3. In the Ohmeda machine, the fail-safe system is known as the pressure sensor shut-off valve. It acts as a threshold and is either open or closed. Once the O₂ supply pressure is below a certain threshold (i.e., 20 psi), the force of the valve return spring completely closes the valve → N₂O is shut off.

A hypoxic mixture may still be delivered when there is sufficient O₂ supply in the system, even though it may not pass the valve.

4. The Drager Narkomed machine fail-safe system is the Oxygen Failure Protection Device (OFPD). It interfaces the O₂ pressure with that of the other gases, such as N₂O, carbon dioxide (CO₂), or helium (He). It is based on a proportioning principle rather than on the threshold principle. The pressure of all gases controlled by the OFPD will decrease proportionally with the O₂ pressure.

A vulnerable O₂ supply pressure zone theoretically exists from 31 to 50 psi because flows can decrease by as much as 30% before the operator is alerted to an O₂ pressure problem.

5. A hypoxic gas mixture can be detected if the fail-safe system fails by measuring the inspired O₂ concentration on the inspiratory limb of the anesthesia circuit.

1. The flowmeter on the anesthesia machine is a variable orifice. It tapers with the inner diameter, increasing uniformly from bottom to top.

2. The Hagen-Poiseuille law states that volume flow increases in direct proportion to the pressure gradient. This law applies to laminar flow. Note that the flow is directly proportional to pressure change along the tube and one-fourth of the radius of the tube and is inversely proportional to the viscosity. Once flow becomes turbulent (a Reynold’s number >2100), the radius to fourth power law is invalid.

Laminar flow is found in a fixed-orifice flowmeter, upstream from the orifice; downstream, it is turbulent. Laminar flow may also be found at low flow rates in a variable-orifice flowmeter.

3. The calibration of the flowmeter depends on the viscosity and density at low flow rates, and only on density at higher flow rates. Flow through the annular space is tubular at low flow rates (Poiseuille’s law applies), and viscosity becomes dominant in determining flow rate. The annular space simulates an orifice at high flow rates, and gas flow rate primarily depends on density.

4. The flowmeters are NOT interchangeable. The scales are hand calibrated using the specific float to provide a high degree of accuracy.

5. The O₂ flowmeter is located downstream from all machine safety devices, except the O₂ analyzed. This arrangement is less likely to allow a hypoxic mixture of gases.

6. Proportioning systems can still deliver a hypoxic mixture under the following conditions:

7. The regulator/valve present in the O₂ flush line is the spring-loaded O₂ flush valve. If stuck open, it can cause barotrauma and patient awareness (dilution of the concentration of inhaled anesthetic gases).

1. Variable bypass refers to the method for regulating output concentration from the vaporizer. After the total gas flow enters the vaporizer’s inlet, the concentration control dial adjusts the amount of gas that goes to the bypass chamber and to the vaporizing chamber. The gas channeled to the vaporizing chamber flows over the liquid anesthetic agent and becomes saturated.

2. Factors influencing vapor output are gas flow rate, temperature, intermittent back pressure associated with positive-pressure ventilation or with O₂ flushing (can cause a higher vaporizer concentration than that set on the dial), and carrier gas flow composition.

3. The output of all variable-bypass vaporizers is less than the dial setting at low flow rates (<250 mL/min) because of the relatively high specific gravity of volatile anesthetics. Insufficient pressure is generated at low flow rates in the vaporizing chamber to advance the molecules upward. At extremely high flow rates (>15 L/min), the output of most variable-bypass vaporizers is less than the dial setting because of incomplete mixing and saturation in the vaporizing chamber. In addition, the resistance characteristics of the bypass chamber and the vaporizing chamber can vary as flow increases (may result in decreased output concentration).

4. The vaporizers on Ohmeda and Drager Narkomed machines are temperature compensated from 20 to 35°C. The output is almost linear over a wide range of temperatures (Vapor pressure is a function of temperature). The bypass chamber has an automatic temperaturecompensating mechanism, and wicks are placed in direct contact with the metal wall of the vaporizer to help replace the heat that is used for vaporization.

5. Vaporizers should not be tilted because this can cause the liquid agent to enter the bypass chamber and can result in a high-output concentration. If tipped, it should not be used until it has been flushed for 20 to 30 minutes at high flow rates with the vaporizer set at a low concentration.

1.

2. The Mapleson A circuit is best suited for spontaneous ventilation. The Mapleson D circuit is best suited for controlled ventilation.

3. The Bain circuit is a modification of the Mapleson D circuit.

Advantages: Low flow required to maintain normocarbia (200 to 300 mL/kg spontaneous ventilation; 70 mL/kg during controlled ventilation); light-weight, convenient, easily sterilized, and reusable. Scavenging is facilitated because the expiratory valve is located away from the patient. Exhaled gases in the outer reservoir tubing add warmth and humidity to inspired fresh gases.

Disadvantages: Unrecognized disconnection/kinking of the inner fresh-gas hose → hypercarbia from inadequate gas flow or increased resistance.

4. Jackson-Rees (Mapleson F) is a modification of the Mapleson D circuit. It is a commonly used T-piece system with a reservoir bag. The adjustable unidirectional expiratory valve is incorporated into the reservoir bag, and the fresh gas flow (FGF) inlet is located close to the patient.

Advantages: Ease of instituting assisted/controlled ventilation and monitoring ventilation by movement of the reservoir bag during spontaneous breathing; simple and inexpensive.

Disadvantages: Lack of humidification; need for high FGF; easily occluded relief valve → increased airway pressure → barotrauma. The degree of rebreathing is affected by the management of venting and ventilation.

1. The type of circle system depends on the amount of fresh gas inflow:

2. Components of the circle system:

3. The most efficient location of the unidirectional valves in a circle system allows the highest conservation of fresh gases—the valves near the patient and the pop-off valve just downstream from the expiratory valve. The arrangement commonly found on the anesthesia machine is more practical but less efficient because it allows mixing of alveolar and dead space gas before venting.

4. To prevent rebreathing in the circle system:

1. Two types of CO₂ absorbers are commonly used today: soda lime and Baralyme (BaOH lime).

The absorption of CO₂ by the absorbers is a chemical process, not a physical one. The CO₂ reacts with water to form carbonic acid, which reacts with the hydroxides to form sodium (or potassium) carbonate, water, and heat.

2. The smaller the granules, the more surface area is available for absorption. However, air flow resistance increases. The granular size of soda lime and Baralyme in anesthesia practice is between four and eight mesh, for which the resistance to air flow is negligible.

3. Ethyl violet is added to soda lime and Baralyme to assess functional integrity of absorbent. As the absorbent becomes exhausted, the pH decreases to <10.3; ethyl violet changes to its violet form through alcohol dehydration. Maximum CO₂ absorbed is 26 L/100 g absorbent. Fluorescent lights can deactivate the dye.

4. Channeling is the flow of exhaled gases through a fixed pattern of loose granules in the CO₂ absorber.

5. Sevoflurane (Ultane) is somewhat unstable in soda lime, but this does NOT produce toxic effects. It is not clinically significant. Degradation is a direct function of temperature.

1. Ventilators may be classified by the following:

2. The two classes of bellows are ascending and descending. The ascending bellows will not fill if a disconnection occurs in the system (safer), but the descending bellows will continue its up-and-down movement.

3. The ventilator pressure relief valve prevents anesthetic gas from escaping into the scavenging system during inspiration. A weighted ball is incorporated into the base of the ventilator relief valve. This ball produces 2 to 3 cm H₂O back pressure; therefore scavenging occurs only after the bellows fills completely and the pressure inside the bellows exceeds the pressure threshold. Scavenging occurs only during expiration, as the ventilator relief valve is open only during expiration.

4. O₂ flushing during inspiration can result in barotrauma because excess volume cannot be vented (the vent relief valve is closed and the adjustable pressure limiting valve is either out of circuit or closed). In addition, the risk of patient awareness is increased by diluting the concentration of anesthetic agents delivered.

5. The delivered tidal volume may increase when you increase gas flow rate despite not changing the ventilator settings. Usually, this increase is lost to the compliance of the breathing circuit.

6. A hole in the bellows can lead to hyperinflation and possible barotrauma in some ventilators. The high-pressure driving gas can enter the patient circuit. It will also increase the inspired fraction of O₂.

1. The components of the scavenging system are as follows:

2. An open interface contains no valves and is open to the atmosphere, allowing both positiveand negative-pressure relief. Such systems require a reservoir because waste gases are intermittently discharged in surges, whereas flow to the active disposal system is continuous.

3. Closed systems communicate with the atmosphere through valves. All must have a positive-pressure relief valve to vent excess system pressure if obstruction occurs downstream from the interface. A negative-pressure relief valve is mandatory to protect the breathing system from subatmospheric pressure if an active disposal system is used.

4. There are two different types of gas disposal assembly systems:

1. The O₂ analyzer is the only machine monitor/safety device that evaluates the integrity of the low-pressure circuit. It is also the only monitor that detects problems downstream from the flow-control valves.

2. The low-pressure leak test evaluates the integrity of the anesthesia machine from the flow-control valves to the common outlet. It evaluates the portion of the machine that is downstream from all safety devices except the O₂ analyzer.

3. The leak test is performed through the following steps:

A leak in the low-pressure circuit will be reflected by a decline in the value on the airway pressure gauge.

4. A limitation of this leak test is its lack of sensitivity as compared with leak tests using special devices (i.e., squeeze bulb).

5. In Ohmeda machines, the low-pressure leak test may not detect leaks in the flowmeters and vaporizers. These can be detected with a negative-pressure leak test using a suction bulb device.

6. The mucus escalator is a blanket of mucus produced by the epithelial cells of the respiratory tract. The mucous blanket is kept in a constant streaming motion toward the larynx. The production and transport of mucus/ciliary motion is optimal between a pH of 6.8 and 7.2 and a temperature of 28 to 33°C. The ciliary action is depressed by opiates and nicotine directly, and inhibited by atropine sulfate by increasing the viscosity of the mucous blanket. Inhaled anesthetics at concentrations that produce general anesthesia will depress ciliary function.

7. Delivery of gases at temperatures >40°C at the endotracheal tube (ETT) may produce hemorrhagic, bronchospastic tracheobronchitis. The minimum recommended humidity for anesthesia is 60% or 12 mg/L.

1. The American Society of Anesthesiologists (ASA) standard I requires qualified personnel to be present in the operating room (OR) for the continuous monitoring of patients throughout the conduct of all general or regional anesthesia and during monitored anesthesia care.

2. The ASA standard II focuses on continually evaluating a patient’s oxygenation, ventilation, circulation, and temperature. The following are specifically mandated:

3. The O₂ analyzer will detect a disconnection in FGF when placed in the inspiratory limb. Only in the expiratory limb will a decreased O₂ show up by identification of an ETT disconnection. Placing the O₂ analyzer in the inspired limb will ensure that a hypoxic mixture is not delivered to the patient but does not guarantee the adequacy of arterial oxygenation.

4. Placement of the O₂ analyzer in the expired limb will most closely approximate alveolar O₂ concentration.