An anesthesia delivery system consists of the anesthesia workstation (anesthesia machine) and anesthetic breathing system (circuit), which permit delivery of known concentrations of inhaled anesthetics and oxygen to the patient, as well as removal of the patient’s carbon dioxide. Carbon dioxide can be removed either by washout (delivered gas flow greater than 5 L/min from the anesthesia machine) or by chemical neutralization.
Anesthesia Workstation
The anesthesia machine has evolved from a simple pneumatic device to a complex integrated computer-controlled multicomponent workstation ( Figs. 15.1 and 15.2 ). The components within the anesthesia workstation function in harmony to deliver known concentrations of inhaled anesthetics to the patient. The multiple components of the anesthesia workstation include what was previously recognized as the anesthesia machine (the pressure-regulating and gas-mixing components), vaporizers, anesthesia breathing circuit, ventilator, scavenging system, and respiratory and physiologic monitoring systems (electrocardiogram, arterial blood pressure, temperature, pulse oximeter, and inhaled and exhaled concentrations of oxygen, carbon dioxide, anesthetic gases, and vapors) ( Box 15.1 ). Alarm systems to signal apnea or disconnection of the anesthetic breathing system from the patient are included. The alarms present on the workstation including pulse oximeter and capnograph must be active and audible to the anesthesia provider. Most anesthesia machines are powered by both electric and pneumatic power.
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Inlet of hospital pipeline for compressed gases (oxygen, nitrous oxide, and air)
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Inlet of compressed gas cylinders
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Pressure regulators to reduce pipeline and cylinder pressure to safe and consistent levels
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Fail-safe device
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Flowmeters to control the amount of gases delivered to the breathing limb
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Vaporizers for adding volatile anesthetic gas to the carrier gas
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Common gas line through which compressed gases mixed with a volatile agent enter the breathing limb
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Breathing limb, including an oxygen analyzer, inspiratory one-way valve, circle system, gas sampling line, spirometer to measure the respiratory rate and volume, expiratory one-way valve, adjustable pressure-limiting valve, carbon dioxide absorbent, reservoir bag, mechanical ventilator, and scavenging system
The anesthesia workstation ultimately provides delivery of medical gases and the vapors of volatile anesthetics at known concentrations to the common gas outlet. These gases enter the anesthetic breathing system to be delivered to the patient by spontaneous or mechanical ventilation. Exhaled gases either exit the system via the scavenging system or are returned to the patient after passing through a CO 2 absorbent.
Fail-Safe Valve
Anesthesia machines are equipped with a fail-safe valve designed to prevent the delivery of hypoxic gas mixtures from the machine in the event of failure of the oxygen supply. This valve shuts off or proportionally decreases the flow of all gases when the pressure in the oxygen delivery line decreases to less than 30 psi. This safety measure is designed to protect against unrecognized exhaustion of oxygen delivery from a cylinder attached to the anesthesia machine or from a central source. This valve, however, does not prevent the delivery of 100% nitrous oxide when the oxygen flow is zero but gas pressure in the circuit of the anesthesia machine is maintained. In this situation, an oxygen analyzer is necessary to detect the delivery of a hypoxic gas mixture. Far superior to the fail-safe valve or an oxygen analyzer is the continuous presence of a vigilant anesthesia provider.
Compressed Gases
Gases used in the administration of anesthesia (oxygen, nitrous oxide, air) are most often delivered to the anesthesia machine from a central supply source located in the hospital ( Fig. 15.3 ). Hospital-supplied gases enter the operating room from a central source through pipelines to color-coded wall outlets (green for oxygen, blue for nitrous oxide, and yellow for air). Color-coded pressure hoses are connected to the wall outlets by fittings that are noninterchangeable (diameter index safety system [DISS] or quick connects), which are designed to prevent misconnections of pipeline gases. Oxygen or air from a central supply source may also be used to pneumatically drive the ventilator on the anesthesia machine.
Gas enters the anesthesia machine through pipeline inlet connections that are gas specific (threaded noninterchangeable connections) to minimize the possibility of a misconnection. The gas must be delivered from the central supply source at an appropriate pressure (about 50 psi) for the flowmeters on the anesthesia machine to function properly.
Anesthesia machines are also equipped with cylinders of oxygen and nitrous oxide for use should the central gas supply fail (see Fig. 15.3 ). Color-coded cylinders are attached to the anesthesia machine by a hanger yoke assembly, which consists of two metal pins that correspond to holes in the valve casing of the gas cylinder (pin indexed safety system [PISS]) ( Table 15.1 ). This design makes it impossible to attach an oxygen cylinder to any yoke on the anesthesia machine other than that designed for oxygen. Otherwise, a cylinder containing nitrous oxide could be attached to the oxygen yoke, which would result in the delivery of nitrous oxide when the oxygen flowmeter was activated. Color-coded pressure gauges (green for oxygen, blue for nitrous oxide) on the anesthesia machine indicate the pressure of the gas in the corresponding gas cylinder (see Table 15.1 ).
| Characteristic | Oxygen | Nitrous Oxide | Carbon Dioxide | Air |
|---|---|---|---|---|
| Cylinder color | Green a | Blue | Gray | Yellow a |
| Physical state in cylinder | Gas | Liquid and gas | Liquid and gas | Gas |
| Cylinder contents (L) | 625 | 1590 | 1590 | 625 |
| Cylinder weight empty (kg) | 5.90 | 5.90 | 5.90 | 5.90 |
| Cylinder weight full (kg) | 6.76 | 8.80 | 8.90 | |
| Cylinder pressure full (psi) | 2000 | 750 | 838 | 1800 |
a The World Health Organization specifies that cylinders containing oxygen for medical use be painted white, but manufacturers in the United States use green. Likewise, the international color for air is white and black, whereas cylinders in the United States are color-coded yellow.
Calculation of Cylinder Contents
The pressure in an oxygen cylinder is directly proportional to the volume of oxygen in the cylinder. For example, a full E size oxygen cylinder contains about 625 L of oxygen at a pressure of 2000 psi and half this volume when the pressure is 1000 psi. Therefore, how long a given flow rate of oxygen can be maintained before the cylinder is empty can be calculated. In contrast to oxygen, the pressure gauge for nitrous oxide does not indicate the amount of gas remaining in the cylinder because the pressure in the gas cylinder remains at 750 psi as long as any liquid nitrous oxide is present. When nitrous oxide leaves the cylinder as a vapor, additional liquid is vaporized to maintain an unchanging pressure in the cylinder. After all the liquid nitrous oxide is vaporized, the pressure begins to decrease, and it can be assumed that about 75% of the contents of the gas cylinder have been exhausted. Because a full nitrous oxide cylinder (E size) contains about 1590 L, approximately 400 L of nitrous oxide remains when the pressure gauge begins to decrease from its previously constant value of 750 psi. Vaporization of a liquefied gas (nitrous oxide), as well as expansion of a compressed gas (oxygen), absorbs heat, which is extracted from the metal cylinder and the surrounding atmosphere. For this reason, atmospheric water vapor often accumulates as frost on gas cylinders and in valves, particularly during high gas flow from these tanks. Internal icing does not occur because compressed gases are free of water vapor.
Flowmeters
Flowmeters on the anesthesia machine precisely control and measure gas flow to the common gas inlet (see Fig. 15.3 ). Measurement of the flow of gases is based on the principle that flow past a resistance is proportional to pressure. Typically, gas flow enters the bottom of a vertically positioned and tapered (the cross-sectional area increases upward from site of gas entry) glass flow tube. Gas flow into the flowmeter tube raises a bobbin or ball-shaped float. The float comes to rest when gravity is balanced by the decrease in pressure caused by the float. The upper end of the bobbin or the equator of the ball indicates the gas flow in milliliters or liters per minute. Proportionality between pressure and flow is determined by the shape of the tube (resistance) and the physical properties (density and viscosity) of the gas. The flowmeters are initially calibrated for the indicated gas at the factory. Because few gases have the same density and viscosity, flowmeters are not interchangeable with other gases. The scale accompanying an oxygen flowmeter is green, whereas the scale for the nitrous oxide flowmeter is blue.
Gas flow exits the flowmeters and passes into a manifold (mixing chamber) located at the top of the flowmeters (see Fig. 15.3 ). The oxygen flowmeter should be the last in the sequence of flowmeters, and thus oxygen should be the last gas added to the manifold. This arrangement reduces the possibility that leaks in the apparatus proximal to oxygen inflow can diminish the delivered oxygen concentration, whereas leaks distal to that point result in loss of volume without a qualitative change in the mixture. Nevertheless, an oxygen flowmeter tube leak can produce a hypoxic mixture regardless of the flowmeter tube arrangement ( Fig. 15.4 ). Indeed, flowmeter tube leaks are a hazard, reflecting the fragile construction of this component of the anesthesia machine. Subtle cracks may be overlooked and result in errors in delivered flow.
Gases mix in the manifold and flow to an outlet port on the anesthesia machine, where they are directed into either a vaporizer or an anesthetic breathing system (see Fig. 15.3 ). For emergency purposes, provision is made for delivery of a large volume of oxygen (35 to 75 L/ min) to the outlet port through an oxygen flush valve that bypasses the flowmeters and manifold. The oxygen flush valve allows direct communication between the oxygen high-pressure circuit and the low-pressure circuit (see Fig. 15.3 ). Activation of the oxygen flush valve during a mechanically delivered inspiration from the anesthesia machine ventilator permits the transmission of high airway pressure to the patient’s lungs, with the possibility of barotrauma.
Vaporizers
Volatile anesthetics are liquids at room temperature and atmospheric pressure. Vaporization, which is the conversion of a liquid to a vapor, takes place in a closed container, referred to as a vaporizer. The vapor concentration resulting from vaporization of a volatile liquid anesthetic must be delivered to the patient with the same accuracy and predictability as other gases (oxygen, nitrous oxide).
Physics of Vaporization
The molecules that make up a liquid are in constant random motion. In a vaporizer containing a volatile liquid anesthetic, there is an asymmetric arrangement of intermolecular forces applied to the molecules at the liquid-oxygen interface. The result of this asymmetric arrangement is a net attractive force pulling the surface molecules into the liquid phase. This force must be overcome if surface molecules are to enter the gas phase, where their relatively sparse density constitutes a vapor. The energy necessary for molecules to escape from the liquid is supplied as heat. The heat of vaporization of a liquid is the number of calories required at a specific temperature to convert 1 g of a liquid into a vapor. The heat of vaporization necessary for molecules to leave the liquid phase is greater when the temperature of the liquid decreases.
Vaporization in the closed confines of a vaporizer ceases when equilibrium is reached between the liquid and vapor phases such that the number of molecules leaving the liquid phase is the same as the number reentering. The molecules in the vapor phase collide with each other and the walls of the container, thereby creating pressure. This pressure is termed vapor pressure and is unique for each volatile anesthetic. Furthermore, vapor pressure is temperature dependent such that a decrease in the temperature of the liquid is associated with a lower vapor pressure and fewer molecules in the vapor phase. Cooling of the liquid anesthetic reflects a loss of heat (heat of vaporization) necessary to provide energy for vaporization. This cooling is undesirable because it lowers the vapor pressure and limits the attainable vapor concentration.
Vaporizer Classification and Design
Vaporizers are classified as agent-specific, variable-bypass, flow-over, temperature-compensated (equipped with an automatic temperature-compensating device that helps maintain a constant vaporizer output over a wide range of temperatures), and out of circuit ( Fig. 15.5 ). These contemporary vaporizers are unsuitable for the controlled vaporization of desflurane, which has a vapor pressure near 1 atm (664 mm Hg) at 20° C. For this reason, a desflurane vaporizer is electrically heated to 23° C to 25° C and pressurized with a backpressure regulator to 1500 mm Hg to create an environment in which the anesthetic has relatively lower, but predictable, volatility.
Variable bypass describes dividing (splitting) the total fresh gas flow through the vaporizer into two portions. The first portion of the fresh gas flow (20% or less) passes into the vaporizing chamber of the vaporizer, where it becomes saturated (flow-over) with the vapor of the liquid anesthetic. The second portion of the fresh gas flow passes through the bypass chamber of the vaporizer. Both portions of the fresh gas flow mix at the patient outlet side of the anesthesia machine. The proportion of fresh gas flow diverted through the vaporizing chamber, and thus the concentration of volatile anesthetic delivered to the patient, is determined by the concentration control dial. The scale on the concentration control dial is in volume percent for the specific anesthetic drug. A temperature-sensitive bimetallic strip or an expansion element influences proportioning of total gas flow between the vaporizing and bypass chambers as the vaporizer temperature changes (temperature compensated) (see Fig. 15.5 ). For example, as the temperature of the liquid anesthetic in the vaporizer chamber decreases, the temperature-sensing elements allow increased gas inflow into this chamber to offset the effect of decreased anesthetic liquid vapor pressure.
Vaporizers are often constructed of metals with high thermal conductivity (copper, bronze) to further minimize heat loss. As a result, vaporizer output is nearly linear between 20° C and 35° C. Designation of vaporizers as agent specific and out of circuit emphasizes that these devices are calibrated to accommodate a single volatile anesthetic and are isolated from the anesthetic breathing system.
Tipping of vaporizers can cause liquid anesthetic to spill from the vaporizing chamber into the bypass chamber, with a resultant increased vapor concentration exiting from the vaporizer. Nevertheless, the likelihood of tipping is minimized because vaporizers are secured to the anesthesia machine and there is little need to move them. Leaks associated with vaporizers are most often due to a loose filler cap.
Commonly, two to three anesthetic-specific vaporizers are present on the anesthesia machine. A safety interlock mechanism ensures that only one vaporizer at a time can be turned on. Turning on a vaporizer requires depression of a release button on the concentration dial, followed by counterclockwise rotation of the dial. This prevents accidental movement of the dial from the off to the on position. The location of the filler port on the lower portion of the vaporizer minimizes the likelihood of overfilling of the vaporizing chamber (>125 mL) with liquid anesthetic. A window near the filler port permits visual verification of the level of liquid anesthetic in the vaporizing chamber. Use of an anesthetic-specific keyed filler device prevents placement of a liquid anesthetic into the vaporizing chamber that is different from the anesthetic for which the vaporizer was calibrated. This is uniquely important for desflurane because its vapor pressure is near 1 atm and accidental placement of desflurane in a contemporary vaporizer could result in an anesthetic overdose. As with anesthesia machines, periodic maintenance (usually every 12 months) is recommended by the manufacturers of vaporizers.
Anesthetic Breathing Systems
The function of anesthetic breathing systems is to deliver oxygen and anesthetic gases to the patient and to eliminate carbon dioxide. Conceptually, the anesthetic breathing system is a tubular extension of the patient’s upper airway. Anesthetic breathing systems can add considerable resistance to inhalation because peak flows as high as 60 L/min are reached during spontaneous inspiration. This resistance is influenced by unidirectional valves and connectors. The components of the breathing system, particularly the tracheal tube connector, should have the largest possible lumen to minimize this resistance to breathing. Right-angle connectors should be replaced with curved connectors to minimize resistance. Substituting controlled ventilation of the patient’s lungs for spontaneous breathing can offset the increased resistance to inhalation imparted by anesthetic breathing systems.
Anesthetic breathing systems are classified as open, semiopen, semiclosed, and closed according to the presence or absence of (1) a gas reservoir bag in the circuit, (2) rebreathing of exhaled gases, (3) means to chemically neutralize exhaled carbon dioxide, and (4) unidirectional valves ( Table 15.2 ). The most commonly used anesthetic breathing systems are the (1) Mapleson F (Jackson-Rees) system, (2) Bain circuit, and (3) circle system.
| System | Gas Reservoir Bag | Rebreathing of Exhaled Gases | Chemical Neutralization of Carbon Dioxide | Unidirectional Valves | Fresh Gas Inflow Rate a |
|---|---|---|---|---|---|
| Open Insufflation Open drop | No No | No No | No No | None None | Unknown Unknown |
| Semiopen Mapleson A, B, C, D Bain Mapleson E Mapleson F (Jackson-Rees) | Yes Yes No Yes | No b No b No b No b | No No No No | One One None One | High High High High |
| Semiclosed circle | Yes | Partial | Yes | Three | Moderate |
| Closed circle | Yes | Total | Yes | Three | Low |
a High, greater than 6 L/min; moderate, 3 to 6 L/min; low, 0.3 to 0.5 L/min.
b No rebreathing of exhaled gases only when fresh gas inflow is adequate.
Mapleson Breathing Systems
In 1954, Mapleson analyzed and described five different arrangements of fresh gas inflow tubing, reservoir tubing, face mask, reservoir bag, and an expiratory valve to administer anesthetic gases ( Fig. 15.6 ). These five different semiopen anesthetic breathing systems are designated Mapleson A to E. The Mapleson F system, which is a Jackson-Rees modification of the Mapleson D system, was added later. The Bain circuit is a modification of the Mapleson D system ( Fig. 15.7 ).
Flow Characteristics
The Mapleson systems are characterized by the absence of valves to direct gases to or from the patient and the absence of chemical carbon dioxide neutralization. Because of no clear separation of inspired and expired gases, rebreathing occurs when inspiratory flow exceeds the fresh gas flow. The composition of the inspired mixture depends on how much rebreathing takes place. The amount of rebreathing associated with each system is highly dependent on the fresh gas flow rate. The optimal fresh gas flow may be difficult to determine. The fresh gas flow should be adjusted when changing from spontaneous and controlled ventilation. Monitoring end-tidal CO 2 is the best method to determine the optimal fresh gas flow. The performance of these circuits is best understood by studying the gas disposition at end exhalation during spontaneous and controlled ventilation ( Fig. 15.8 ).
