Residency · Residency · Anesthesiology

The Anesthesia Machine: Structure, Function, and Checkout

Introduction

The anesthesia workstation is the primary life-support device in the operating room. Every anesthesiology resident must understand its components, gas flow pathways, safety features, and how to perform a thorough checkout procedure. Equipment failure, though rare, can be catastrophic, and systematic machine checks prevent the majority of preventable adverse events.

Major Components of the Anesthesia Machine

Gas Supply System

The pipeline supply delivers oxygen, nitrous oxide, and air from wall-mounted outlets at 50 to 55 psi via the diameter index safety system (DISS), which prevents incorrect connections. The cylinder supply consists of backup E-cylinders mounted on the machine yoke via the pin index safety system (PISS). An oxygen cylinder reads 1900 psi when full and holds approximately 660 L. A nitrous oxide cylinder reads 745 psi when full (reflecting its liquid phase) and holds approximately 1590 L. Notably, the N2O cylinder pressure remains constant until the liquid phase is exhausted, at which point approximately 75% of the contents have been used.

Pressure regulators reduce cylinder pressure to approximately 45 psi for use in the machine. Pipeline pressure gauges and cylinder pressure gauges monitor supply pressures.

Flowmeters and Flow Control

Needle valves provide fine control of gas flow rates. In older machines, rotameter tubes serve as variable-orifice flowmeters with bobbin or ball indicators. The oxygen flowmeter is positioned downstream (closest to the common gas outlet) to prevent hypoxic mixtures from leaks. Modern machines from GE and Drager use electronic flow sensors with digital displays.

The minimum oxygen ratio device ensures the FiO2 cannot fall below 21 to 25% by mechanically or electronically linking the oxygen and nitrous oxide flow controls.

Vaporizers

Variable bypass vaporizers (Tec-type) divert a portion of fresh gas flow through the vaporizing chamber. They are agent-specific, with keyed filling systems (AFSS) that prevent filling with the wrong agent. Temperature compensation via a bimetallic strip or electronic mechanism maintains consistent output despite cooling from vaporization.

The desflurane vaporizer (Tec 6) is heated and pressurized because desflurane's boiling point of 22.8 degrees Celsius is near room temperature, requiring electrical power. An interlock mechanism prevents simultaneous use of two vaporizers. Vaporizer output is affected by altitude: it delivers the same partial pressure but a higher percentage at altitude.

The Breathing Circuit

Circle System

The circle system is the most commonly used breathing system in modern anesthesia. Its components include inspiratory and expiratory one-way valves, a CO2 absorber, an adjustable pressure-limiting (APL) valve, a reservoir bag, a fresh gas inlet, and a Y-piece. It allows rebreathing of exhaled gases after CO2 removal, conserving volatile agent and moisture.

The CO2 absorbent is typically soda lime or calcium hydroxide lime (Amsorb), with a color change indicating exhaustion. Desiccated absorbent combined with sevoflurane can produce Compound A, which is nephrotoxic in rats but clinically insignificant at recommended flows. Desiccated absorbent combined with desflurane can produce carbon monoxide.

Fresh Gas Flow (FGF)

High-flow anesthesia (FGF of 4 L/min or greater) allows rapid changes in delivered concentration with minimal rebreathing. Low-flow anesthesia (FGF of 0.5 to 1 L/min) is economical and environmentally friendly but requires equilibration time. During low-flow anesthesia, the delivered concentration increasingly differs from the dialed concentration, and the clinician must rely on end-tidal agent monitoring.

Manual vs. Mechanical Ventilation

In manual (bag) mode, the reservoir bag and APL valve are in the circuit, allowing spontaneous or manually assisted ventilation. In mechanical ventilation mode, the ventilator bellows replaces the reservoir bag, and the APL valve is excluded from the circuit. Modern ventilators use ascending bellows (bellows that rise during expiration); a descending bellows would not alert to a circuit disconnect.

Safety Features

Fail-Safe Mechanisms

The oxygen failure protection device (fail-safe valve) reduces or interrupts N2O flow when oxygen supply pressure falls below 20 psi. Importantly, it does not prevent delivery of a hypoxic mixture if the oxygen flowmeter is set too low with adequate pressure. The oxygen flush valve delivers 100% oxygen at 35 to 75 L/min directly to the common gas outlet, bypassing vaporizers and flowmeters. A low oxygen pressure alarm sounds an audible alarm when oxygen pipeline pressure drops.

Alarms and Monitors

The oxygen analyzer (paramagnetic or galvanic cell) continuously measures FiO2 in the inspiratory limb. The airway pressure monitor detects disconnection (low pressure), obstruction (high pressure), and sustained pressure. The volume monitor measures expired tidal volume and minute ventilation. The agent analyzer uses infrared spectroscopy to identify and quantify volatile agents and CO2.

Anesthesia Machine Checkout Procedure

2008 ASA Recommendations

The ASA published checkout recommendations that should be performed daily and between cases.

The daily checkout (before the first case) includes verifying that the auxiliary oxygen supply and self-inflating manual ventilation device are available, verifying pipeline gas pressures (oxygen at 50 psi or greater), verifying that cylinder supplies are adequate and turned off after the check, verifying that the CO2 absorbent is not exhausted (color indicator), performing a breathing system pressure test (leak test) by closing the APL valve, occluding the Y-piece, pressurizing to 30 cmH2O, and verifying less than 1 cmH2O drop in 10 seconds, verifying that vaporizers are filled and seated properly with a vaporizer leak test, verifying that flowmeters function through their full range, testing ventilator function by setting parameters and confirming delivery using a test lung, calibrating the oxygen analyzer to 21% and 100%, verifying that the scavenging system is connected and functioning, and performing a complete alarm system check.

The between-case checkout includes verifying breathing circuit integrity and absence of leaks, confirming vaporizer settings and agent levels, replacing CO2 absorbent if indicated, and running a ventilator self-test if the machine offers one.

Negative Pressure Leak Test

The negative pressure leak test is specific to the vaporizer and machine low-pressure system. A suction bulb is attached to the common gas outlet, collapsed, and verified to remain collapsed for 10 seconds or more. This test detects leaks in the vaporizer, flowmeter, and low-pressure circuit that the positive pressure test may miss.

Clinical Pearls

An N2O cylinder at 745 psi can be nearly empty or nearly full because the pressure gauge is unreliable until all liquid N2O has evaporated; the cylinder should be weighed for an accurate assessment. The fail-safe valve prevents flow of N2O without oxygen pressure but does not guarantee a non-hypoxic mixture; only the oxygen analyzer provides that assurance. A desiccated CO2 absorbent should never be used with volatile agents because this is a known source of carbon monoxide production that has caused patient harm. The pre-use checkout is a medico-legal standard of care, and its completion should be documented before every anesthetic.

References

1. Dorsch JA, Dorsch SE. Understanding Anesthesia Equipment. 5th ed. Lippincott Williams & Wilkins; 2008.
2. ASA Committee on Equipment and Facilities. Recommendations for Pre-Anesthesia Checkout Procedures. American Society of Anesthesiologists; 2008.
3. Butterworth JF, Mackey DC, Wasnick JD. Morgan & Mikhail's Clinical Anesthesiology. 7th ed. McGraw-Hill; 2022. Chapter 4: The Anesthesia Workstation.
4. Brockwell RC, Andrews JJ. Inhaled anesthetic delivery systems. In: Gropper MA, ed. Miller's Anesthesia. 9th ed. Elsevier; 2020. Chapter 14.

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