Volume 5, No. 1 • Spring 1990

Features of Modern Anesthesia Machines

Charles Whitcher, M.D.

Problems with oxygenation account for a great number of the serious mishaps and complications seen in anesthesia practice. (1) Many more mishaps of all types are attributed to human errors than to equipment failures.(2) The avoidance of human errors leading to hypoxemia may be accomplished by two main approaches: 1) education and 2) the use of equipment that is designed to reduce the possibility of the development of hypoxia. Discussed here are selected means by which well-designed equipment and, in certain cases, modernized equipment, may help avoid hypoxia thereby contributing to safe anesthesia practice

Machine Replacement vs. Upgrading with Modern Safety Features

Most medical care facilities and anesthesia practitioners are not in a financial position to replace anesthesia machines frequently or all at once. As a result, many machines still now in regular use are not of current design and were not originally equipped with a full array of the most modem safety features. For purposes of discussion, such machines are identified as “vintage” machines. With a single exception, important safety features can be retrofitted to vintage machines. Some of the issues to consider in retrofitting are cost-effectiveness, reasonable service fife and the availability of funds for machine replacement vs. upgrading of existing machines.

Specific Safety Features Which May be Missing on Vintage Machines

Table I fists selected features of “modem” anesthesia machines. Any of these features may be missing from machines manufactured in the U.S. before 1984. Except for the oxygen/nitrous oxide ratio protector/controller, all listed freatures may be retrofitted to vintage machines. ‘The purpose of each of these features is the prevention of hypoxia.


Table 1. Safety Features of Modem Anesthesia Machines

1 Oxygen concentration monitor

2. Low oxygen pressure N20 cutoff (“fail safe7′)

3. Oxygen supply pressure failure alarm

4. Single oxygen flow control knob

5. Touch coded oxygen How control knob

6. Oxygen flow meter at extreme right (U.S.)

7. Central gas supply pressure gauges

8. Color coded flow meters & control knobs

9. Oxygen/nitrous oxide ratio monitor/controller

10. Locking common gas outlet

11. Pin index safety system(indexed cylinder yokes)

12. Diameter index saw system (indexed pipeline

inlet connectors)


1. Oxygen Concentration Monitor (With Low Concentration AL-mm)

The oxygen concentration monitor in the breathing system “oxygen analyzer” is the ubiquitous, standard-of-care monitor designed to measure oxygen concentration. In the circle breathing circuit, the most useful measurement site is within the breathing circuit, near the inhala6on check valve. In the modified Mapleson D (Bain) type system, the usual measurement site is within the fresh gas line.

Causes of inappropriate oxygen concentrations include: unrecognized nitrous oxide How (the bobbin of the N20 flow meter in pinned inconspicuously above full scale), a flow meter control knob is mistaken (e.g. air used instead of oxygen), the oxygen supply pressure fails, a disconnection from the oxygen source occurs, or the gas from the oxygen source is not pure oxygen.

Maintenance and Calibration

As with ad monitors, the “oxygen analyzer” has limitations. Most units are of the galvanic or polarographic type. To remain operable, consumable components of the sensors of these monitors must be regularly replaced (galvanic cell cartridge or membrane and electrolyte). Calibration is recommended at least daily. Three precautions about calibration procedures merit attention:

1. Allow for the normally long equilibration time of one-half to three minutes. Calibration is useful only when sufficient time is allowed for complete equilibration.

2. Test for failure of span, an important early failure mode. A fading monitor may calibrate only to 2 1 % oxygen or only to 100%. Inaccurate readings may be seen at concentrations other than those at the single calibrated value. Proper calibration requires two points, usually room air and 100% oxygen.

3. Assure that I 00% oxygen is actually present at the sensor during the span test. At the usual installation adjacent to the inhalation check valve of the circle absorber, oxygen may bypass the sensor site. Thie problem is usually preventable by the use of high flow rates of oxygen, e.g. ten liters per minute.

Even with appropriate calibration, data from the oxygen analyzer must be interpreted with caution. A reading of I 00’Y. oxygen does not guarantee that the patient is receiving 100% oxygen. In case of a disconnection between the Y-piece and the absorber, with the oxygen analyzer reading I 00’Yo, the patient may breath room air during spontaneous ventilation or not at all if dependent on controlled ventilation.

In order to minimize the inconvenience of calibration, the practitioner can begin the machine check out by detaching the oxygen concentration sensor and allowing stabilization in room air as other preparations for anesthesia are conducted. After about three minutes (varies among brands see your manual), the monitor should read 20-22% oxygen. failing this, the monitor must be adjusted to 2 1 % oxygen. Then the sensor is reattached to the breathing system with a high How rate of oxygen. After about three minutes, the monitor should read 97-103% oxygen. If not, span adjustment is attempted to bring the reading to 100% oxygen. If any adjustment is made at this stage, the room air test must be repeated. If the monitor then reads 20-22% without further adjustment, calibration is appropriate. Failing this, the analyzer should be replaced while being examined, tested and, if needed, serviced or even repaired.

Alarms and Adjustments

On the one hand, anesthesia care involves multiple simultaneous responsibilities and several competitors for attention. Since the anethesiologist physically cannot focus full attention on all of the component elements at once, there is the possibility of distraction. On the other hand, a well designed alarm has a single specific purpose and cannot be distracted. This is to the patient’s advantage.

Alarms should be designed so that they can be kept on throughout an anesthetic, without imposing Use al-mm or causing other inappropriate distractions. It is good practice to keep alarms thresholds set at levels sufficient to sustain a reasonable margin of safety appropriate for the specific clinical situation. If a minimum of 30% oxygen is desired, the alarm might be set for 29% oxygen. Between cases, the monitor should remain on and distracting alarms can be prevented by maintaining a sufficient oxygen flow rate. The minimum flow rate of a modern anesthesia machine is usually sufficient.

Oxygen analyzers of the modem paramagnetic, Raman, and mass spectrographic types may require less frequent calibration and service than the usual galvanic and polarographic types. These newer analyzers are capable of rapid measurements. By sampling continuously at the Y-piece or endotracheal tube, both inhaled and end-tidal oxygen concentrations may be measured. Adequate inspired to endtidal oxygen concentration difference (e.g. three percent to five percent oxygen) suggest that the patient has actually received a safe concentration of oxygen, distributed oxygen to the tissues, employed oxygen in metabolism and exhaled the excess oxygen.

2. Low Oxygen Pressure Shut Off (“Fail Safe system”)

The low oxygen pressure shut off device promptly terminates the flow of nitrous oxide if the oxygen supply pressure fads. further delivery of nitrous oxide and volatile agent is prevented. With gas supplies cut off, the rebreathing bag or bellows empties quickly. This draws attention to the problem. Causes include: empty oxygen link; interrupted oxygen flow from the high pressure oxygen source; and a compressed, kinked, or disconnected high pressure oxygen hose to the machine.

It is important to recognize that the low oxygen pressure shut off feature does not cut off the flow of gases other than nitrous oxide. Thus, anesthesia machines equipped with helium may deliver 100% helium a hazard often overlooked. Protection against ” potential cause of a major hypoxic event is provided both by constant vigilance and the continuous monitoring of oxygen in the breathing system with a concentration monitor with an audible alarm as discussed above.

Test Procedure

When a central oxygen supply system is used, shut off the oxygen cylinder(s) on the anesthesia machine and connect the high pressure oxygen supply hose to its source After providing for the scavenging of nitrous oxide (see Caveat 1. below), establish flow rates of nitrous oxide and oxygen and then disconnect the high pressure oxygen hose at the wall or ceiling connector. After a few moments, both oxygen and nitrous oxide flow rates should fall to zero. If oxygen is supplied only from cylinders, establish flow rates of nitrous oxide and oxygen as above, then turn off the oxygen cylinder. Again, after a few moments, the flows should fag to zero.

Caveat 1. Without due care this test causes occupational exposure to nitrous oxide. Minimal exposure by venting the common gas line to suction. Disconnect the common (“fresh”) gas fine connector at the machine end. When nitrous oxide is flowing during the test, place the suction line up to or near the common gas outlet. After the test, reattach the common gas connector.

3. Oxygen Supply Pressure Failure Alarm

The oxygen supply pressure failure alarm sounds an audible alarm in case of oxygen supply pressure loss. Usually this feature is combined with the low oxygen pressure cutoff feature, so that the alarm sounds and the flow of gases stop at about the same tune. With the alarm calling attention to low oxygen pressure, the shut off of all gases is likely to be noticed.

Test Procedures

Conduct this test along with the abuse shut off test. Hear alarm signal as the flow of nitrous oxide oxygen falls to zero.

4. Single Oxygen flow Control Knob

A single flow control knob for oxygen reduces the possibility of selecting a wrong knob for adjusting the oxygen How rate. Certain vintage machines have separate knobs for administering high raw and low oxygen flow meters. This is inherently dangerous and should be considered for replacement. failing this, extra attention to the oxygen concentration monitor is mandatory.

5 . Touch-coded Oxygen flow Control Knob

The modem oxygen flow control knob is distinguished from all other flow control knobs by its larger diameter, and fluted design. These features reduce the possibility of delivering an inappropriate gas mixture by facilitating instant tactile recognition of the oxygen flow control knob, Regardless of color blindness and poor ambient fighting conditions.

Inspect your anesthesia machine for the presence of a modem fluted, large diameter oxygen flow control knob. If not present, it should be installed.

6. Oxygen flow Meters on the Right (U.S.)

The U.S. standard location for oxygen flowmeters is at the extreme right side of the How meter control panel. This standardized location helps prevent the adjustment of the wrong flow meter. This feature reinforces the effectiveness of points four and five above

Inspect the front panel of your anesthesia machine to check that the oxygen flow meter is positioned to the extreme right.

7. Pipeline Pressure Gauges

Pipeline pressure gauges measure gas pressure in the high pressure lines of oxygen, nitrous oxide and air. Each pipeline pressure gauge is normally located adjacent to the corresponding cylinder pressure gauge, within easy view of the practitioner. Gauges verify that hoses are connected and pressurized and assist in localizing any failure of the gas supply.

Inspect the pressure gauges, viewed from the front of anesthesia machine to two gauges for each gas. Pipeline gauges are distinguished from cylinder gauges by the label and pressure range. Pipeline gauges have typical range of 0-100 psig; cylinder gauges have much higher maxima.

8. Color Coded Flow Meters and Flow Control Knobs

Flow meter tubes, or the background material behind each, and flow meter control knobs, are uniquely colored for each gas represented: in the U.S., green for oxygen, blue for nitrous oxide and yellow for air. Color coding is intended to reduce the possibility of mishap due to selection of a wrong flow tube or control knob. Many other countries use different color coding: beware both when working outside the U.S. and when using imported anesthesia machines that have not been retrofitted.

9. Oxygen-Nitrous Oxide Ratio Monitor and/or Controller

The oxygen-nitrous oxide ratio monitor senses the ratio of flow meter settings for nitrous oxide and oxygen and issues an alarm when the ratio is unsafe. A ratio controller assures that flows cannot be adjusted outside a specific range of ratios. Specifically, pure nitrous oxide cannot be administered.

Test Procedure

Set a usual flow rate of oxygen (e-g. 2 liters/min), then increase the nitrous oxide How from zero up to a rate above the acceptable ratio. Results vary with the make of anesthesia machine. In the NA Drager version, once the specified maximum ratio has been reached, no further increase of nitrous oxide How rate is possible. In the Ohmeda version, once the specified maximum ratio has been achieved, a further increase of the nitrous oxide flow rate automatically increases the flow rate of oxygen (thus preventing & minimum proportion of oxygen at any total flow rate).


Other pertinent safety features aiding in the prevention of hypoxic gas mixtures, considered in a previous article by Beverly Nichols, C.R.N.A. in this publication, are listed here for completeness:

10. Pin index safety system (indeed cylinder yokes-).

11. Locking common gas outlet.

12. Diameters index safety system (indexed pipeline inlet connectors).

Safe Practices and the Prevention of Hypoxia

All of the test and calibration procedures mentioned should be considered saw practices to aid in hypoxia prevention. An additional saw issue not for the machine itself, but applicable to all anesthesia machines, both vintage and modem, is suspension of all gas and scavenging hoses off the floor.

Hoses on the floor may be occluded by the wheels of any heavy piece of equipment such as an anesthesia machine In the case of an oxygen hose,

the oxygen supply may W. Collapse resistant oxygen hoses are available and offer some protection. In case of a scavenging hose, occlusion may cause back pressure, preventing exhalation in a breathing system. Intravenous poles may assist in the safe suspension of all hoses. As a supplement to the practice of keeping scavenging km off the floor, relief vanes and collapse proof hose offer protection.


Anesthesia-related mishaps will continue to occur because anesthesia is administered by humans. At least half of major mishaps may be considered preventable. Vigilance is a critical factor, but, realistically, one which is subject to only limited improvement because human factors are traditionally difficult to control. On the other hand, safety features and monitoring devices have only a single function to perform. Given well designed alarms, monitors can help compensate for fatigue, lapses of vigilance, and distractions. Although anesthesia machines themselves only rarely appear to play a primary role in substantive mishaps, it is intuitively logical that machines with appropriate safety features can help reduce both the number and severity of mishaps. Anesthesia equipment is a relatively controllable modality compared for human vigilance.

Due attention should be paid to safe practices such as the thorough regular machine checkout procedure the use of appropriate monitors and their calibrations keeping alarm on and keeping hoses suspended when possible. Likewise, careful attention must be given to the relative merits of the upgrading of vintage anesthesia machines with safety features that can be retrofitted versus the timely replacement of those vintage machines.

Dr. Witcher, Stanford University Department of Anesthesia, is a member of the APSF Education Committee.


1. Cheney FW Potential Risks and Causes of Incidents. In: Gravenstein JS and Holzer JF (eds) Safety and Cost Containment in Anesthesia. Boston: Bufferworth’s 1988, 11-20.

2. Cooper JR Newbower RS, Kitz RJ. An analysis of major errors and equipment failures in anesthesia management: considerations for prevention and detection . Anesthesiology 1984; 60:34-42.