Inhalational Anesthesia in Difficult Locations

Modern anesthesia workstations are complex and sophisticated pieces of medical equipment. They have evolved over many years to improve performance and reduce the risk of accidents and mishaps. The most recent workstations offer the ability to administer a wide range of volatile agents using a variety of breathing circuits. In addition, physiologic functions can be monitored and displayed, trends can be observed, and both audible and visual alarms can be set to predetermined limits to reduce risks to an absolute minimum.

Although these developments have led to improved standards of safety in wealthy countries, these advantages have not been mirrored in poorer countries. These sophisticated machines are unaffordable in many countries such that even when donated, major problems obstruct their successful deployment. These machines are designed to work in ideal conditions, and additional problems encountered in the most impoverished areas of the world often cannot be overcome. An anesthetic machines’ success in such an environment relies on its ability to function in the absence of oxygen, electricity, and regular servicing by skilled technicians.

It is important to adapt to adverse situations, and extensive technical support may not be available. There have been no overwhelming military or natural catastrophes in the United States in recent years; in addition, emergency medical crews are able to extricate victims from practically any accident without limb amputation, and patients in remote areas who require surgery can usually be transported to facilities equipped with standard anesthesia equipment.

This chapter reviews some of the equipment that can be used for anesthesia under mobile disaster circumstances and in isolated and economically deprived conditions, although techniques for providing anesthesia are not described in detail. Anesthesia equipment must be compact, portable, and robust. Surgery can be performed in a hazardous location but must be rapid and essential. In an emergency, evacuation of even mass casualties should be prompt. Cost is not a limiting factor, and anesthesia is normally administered by an expert anesthesia specialist using intravenous (IV) inductions followed by volatile inhaled agents if indicated. Regional anesthesia for surgery in conscious patients may not be as useful under these conditions as when it is used as an analgesic supplement or in more controlled conditions.

Draw-Over Breathing System

The oldest method of providing inhalational anesthesia involves breathing air, perhaps enriched with oxygen, drawn over a volatile anesthetic agent. This principle was first used when a handkerchief containing anesthetic was held to the face, with various Schimmelbusch types of masks, with the Flagg can, and with numerous hand-held or table-top inhalers. Few vaporizers were calibrated to deliver a known concentration of anesthetic agent. This knowledge was not considered essential for anesthesia, because more reliance was placed on clinical signs than on achieving a specified minimum alveolar concentration (MAC) in the end-tidal gas.

For a volatile agent to be vaporized, a carrier gas must pass through a vaporizing chamber. It can be driven through the chamber by positive pressure from a cylinder or central supply, as in continuous-flow anesthesia, or through the chamber by negative pressure generated by the patient`s inspiratory effort.

For use in isolated hospitals, oxygen cylinders must be transported over long distances on roads that may be impassable for prolonged periods, and supplies may run out altogether. In the absence of oxygen, the administration of inhalational anesthesia by continuous flow is impossible. Draw-over anesthesia offers an alternative that is used especially in military and disaster situations when no local facilities exist.

A draw-over system has the following basic components: 1) a reservoir tube, 2) a vaporizer, 3) a self-inflating bag, 4) a nonrebreathing valve, and 5) an optional reservoir tube ( Fig. 29-1 ).The reservoir generally consists of a section of corrugated tubing about 1 m in length, one end of which is attached to the vaporizer; the other end is open to the atmosphere. There is a side arm for the addition of supplementary oxygen, if it is available; while the patient is breathing out, the oxygen flows into the reservoir to be incorporated into the next breath. Plenum vaporizers are unsuitable for draw-over anesthesia, because the resistance to spontaneous breathing is too high. Vaporizers designed for draw-over anesthesia must have a sufficiently low resistance to enable spontaneous respiration to occur. The Epstein-Macintosh-Oxford (EMO) vaporizer is an example of an early draw-over vaporizer designed for use with ether ( Fig. 29-2 ). Later models include the Oxford miniature vaporizer (Penlon Inc., Minnetonka, MN; Fig. 29-3 ) and the DiaMedica (Minneapolis, MN) vaporizer, both of which can be used with halothane, isoflurane, and other volatile agents. The self-inflating bag for controlled or assisted ventilation is separated from the vaporizer by a one-way valve. When the bag is compressed, the valve ensures that the contents are directed toward the patient and that they cannot reenter the vaporizer. A nonrebreathing valve is situated as close to the patient as possible to minimize the dead space. The function of the valve is to ensure that only the anesthetic mixture is inhaled during inspiraction and that it is not diluted with atmospheric air. During exhalation, the valve directs the exhaled mixture into the atmosphere and prevents it from reentering the breathing system.

Components of a draw-over system.

Schematic of the Epstein-Macintosh-Oxford vaporizer. 1, Inlet port; 2, outlet port; 3, concentration control; 4, water jacket; 5, thermocompensator valve; 6, vaporizing chamber; 7, filling port for water; 8, filling port for anesthetic; 9, anesthetic level indicator.

(From Dobson MB: Anaesthesia at the district hospital. Geneva, 1988, World Health Organization. Reproduced by permission.)

Schematic of the Oxford miniature vaporizer (Penlon Inc., Minnetonka, MN). 1, Inlet port; 2, outlet port; 3, concentration control; 4, heat sink; 5, vaporizing chamber; 6, filling port for water; 7, filling port for anesthetic; 8, anesthetic level indicator.

(From Dobson MB: Anaesthesia at the district hospital. Geneva, 1988, World Health Organization. Reproduced by permission.)

Several types of nonrebreathing valves are on the market, including Laerdal Medical (Stavanger, Norway), Ambu (Ballerup, Denmark), and Ruben (Intersurgical Ltd., Workingham, UK) valves. A scavenging system can be connected to the expiratory port of the nonrebreathing valve. When using a face mask in draw-over anesthesia, an airtight seal is essential during inspiration to create the required negative pressure. In some circumstances, such as an inhalation induction of anesthesia in uncooperative children or in the presence of facial trauma, an airtight seal may be impossible to achieve. In these circumstances, the draw-over system can be converted to a continuous-flow system by occluding the open end of the reservoir. The same maneuver is required in small children (<10 kg) for whom the resistance to breathing through the vaporizer is excessive. In these patients the draw-over breathing circuit can be replaced by an Ayres T-piece system attached directly to the vaporizer.

Oxygen Concentrator

Cylinders of oxygen are expensive, hazardous to transport, and contain a limited volume. On the other hand, air costs nothing, does not require transport, and the supply does not run out. Therefore it is logical to use atmospheric air as the source of oxygen whenever possible, which can be done effectively by the use of an oxygen concentrator similar to the type used for domestic oxygen therapy ( Fig. 29-4 ).

Oxygen concentrator.