The Anesthesia Machine
No piece of equipment is more intimately associated with the practice of anesthesiology than the anesthe-sia machine (Figure 4–1). On the most basic level, the anesthesiologist uses the anesthesia machine to con-trol the patient’s ventilation and oxygen delivery and to administer inhalation anesthetics. Proper func-tioning of the machine is crucial for patient safety. Modern anesthesia machines have become very sophisticated, incorporating many built-in safety features and devices, monitors, and multiple micro-processors that can integrate and monitor all compo-nents. Additional monitors can be added externally and often still be fully integrated. Moreover, modu-lar machine designs allow a wide variety of configu-rations and features within the same product line. The term anesthesia workstation is therefore often used for modern anesthesia machines. There are two major manufacturers of anesthesia machines in the United States, Datex-Ohmeda (GE Healthcare) and Dräger Medical. Other manufacturers (eg, Mindray) produce anesthesia delivery systems. Anesthesia providers should carefully review the operations manuals of the machines present in their clinical practice.
Much progress has been made in reducing the number of adverse outcomes arising from anesthetic gas delivery equipment, through redesign of equipment and education. Misuse of anesthesia gas delivery systems is three times more likelythan failure of the device to cause equipment-related adverse outcomes. Equipment misuse is character-ized as errors in preparation, maintenance, or deployment of a device. Preventable anesthetic mis-haps are frequently traced to an operator’s lack of familiarity with the equipment or a failure to check machine function, or both. These mishaps account for only about 2% of cases in the American Society of Anesthesiologists’ (ASA) Closed Claims Project database. The breathing circuit was the most common single source of injury (39%); nearly all damaging events were related to misconnects or disconnects. A misconnect was defined as a
nonfunctional and unconventional configuration of breathing circuit components or attachments. In decreasing frequency, other causes involved vapor-izers (21%), ventilators (17%), and oxygen supply (11%). Other more basic components of the anes-thesia machine (eg, valves) were responsible in only 7% of cases. All malpractice claims in the database that involved the anesthesia machine, oxygen supply tanks or lines, or ventilators occurred before 1990; since then claims involving breathing circuits and vaporizers have continued to occur.
The American National Standards Institute and subsequently the ASTM International (formerly the American Society for Testing and Materials, F1850–published standard specifications for anesthesia machines and their components. Table 4–1 lists essential features of a modern anesthesia work-station. Changes in equipment design have been directed at minimizing the probability of breath-ing circuit misconnects and disconnects and auto-mating machine checks. Because of the durability and functional longevity of anesthesia machines, the ASA has developed guidelines for determining anesthesia machine obsolescence (Table 4–2).
In its most basic form, the anesthesia machine receives medical gases from a gas supply, controls the flow and reduces the pressure of desired gases to a safe level, vaporizes volatile anesthetics into the final gas mixture, and delivers the gases to a breathing circuit connected to the patient’s airway (Figures 4–2 and 4–3). A mechanical ventilator attaches to the breathing circuit but can be excluded with a switch during spontaneous or manual (bag) ventilation. An auxiliary oxygen supply and suction regulator are also usually built into the workstation. In addition to standard safety features (Table 4–1) top-of-the-line anesthesia machines have additional safety features, enhancements, and built-in com-puter processors that integrate and monitor all com-ponents, perform automated machine checkouts,
and provide options such as automated record-keeping and networking external monitors and hospital information systems (Figure 4–4). Some machines are designed specifically for mobility, magnetic resonance imaging (MRI) compatibility or compactness.
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