Cases & Commentaries

Thin Air

Spotlight Case
Commentary By David M. Gaba, MD

Case Objectives

  • Identify the gas-specific
    non-interchangeable connectors used for bulk gas supply outlets in
  • Define forcing function, both in
    engineered safety devices and in human procedures.
  • Review strategies for prevention of
    delivery of wrong gas to hospitalized patients.

Case & Commentary: Part 1

A 73-year-old woman was admitted to the
hospital with fever and back pain, and was diagnosed with
pyelonephritis. The morning after admission, she became hypotensive
and short of breath. Her oxygen saturations were 70%. She was
placed on high-flow oxygen with little benefit. A chest x-ray
showed diffuse pulmonary infiltrates consistent with acute
respiratory distress syndrome (ARDS). The patient was intubated for
type I (hypoxemic) respiratory failure, and placed on high flow
oxygen. Shortly thereafter, the respiratory therapist arrived and
noticed that the patient was being treated with compressed
air—not oxygen.

In this case, it seems that both the high-flow
oxygen mask and the self-inflating bag were hooked up to an air
flowmeter, not an oxygen flowmeter. The hospital gas supply
delivers gas under pressure to outlets in clinical
settings.(1,2) In a ward
room, or bay in the Emergency Department or ICU, these are
typically on wall panels at the head of the bed. The working
pressure for such outlets for both air and oxygen are about 45-55
PSI, a pressure too high to deliver directly to masks and bags. A
flowmeter is connected to the outlet to meter gas at an appropriate
flow rate, typically 1-15 liters per minute, at only slightly
higher than ambient pressure. Such flowmeters are used to provide
gas to bags, masks, nebulizers, and other simple ventilatory
devices. More complex devices—like ventilators or anesthesia
machines—have hoses carrying compressed gas attached directly
to the compressed gas outlets.

The flowmeters used for air and oxygen are nearly
identical, differing to the casual observer, if at all, only in the
color-coding of the knobs or "Christmas Tree adapter," which
provides a place to securely attach delivery tubing and other
portions (Figure 1).
Green adapters are meant for oxygen and yellow adapters are meant
for air. Although the flowmeter itself may be physically designed
to insert into only its specific gas port (see below), the
threading for connection between the flowmeter and the Christmas
tree adapter is universal in size. Thus, it is possible for an air
flowmeter to have the green Christmas tree adapter attached instead
of a yellow one (Figure 2).
Someone attaching the delivery tubing might easily attach it to the
green Christmas tree adapter, expecting it to be connected to the
oxygen flowmeter and not noticing that it is actually an air
flowmeter plugged into an air outlet. Even with the appropriate
color Christmas tree adapter, it is quite possible to attach the
tubing to the wrong flowmeter, especially in a crowded space under
high stress and time pressure. Further, tubing from multiple airway
devices may be attached to the various flowmeters simultaneously
(eg, a self-inflating bag, a nebulizer, and a face mask), making
recognition difficult.

How frequently such swaps occur is not known, but
they do so often enough that on March 5, 2002 a Patient Safety
Advisory on this topic was issued by the Veterans Health
Administration Warning System. It is probably even more common for
oxygen tubing to become disconnected from a correct flowmeter
during a resuscitation. This issue is not easily recognized when a
facemask or self-inflating bag is used, because these devices
provide little feedback as to the gas flow rate.

Case & Commentary: Part 2

The patient was transferred to intensive care
and died the next day of overwhelming sepsis and systemic
inflammatory response syndrome (SIRS).

In this case, although errors occurred in
providing supplemental oxygen, the patient's underlying disease was
likely severe enough to cause death even in the absence of errors.
Nonetheless, a typical response to such an event would be to blame
clinicians for being ignorant, or lacking vigilance. However,
employing human
factors engineering concepts to this case would be more
productive. In addition to avoiding a culture of blame and striving
to identify root causes, the solutions offered by this approach
rely on improving the design of artifacts in the world rather than
relying on labels, instructions, training, or the usual admonition
to "be more careful."(3)

Much of the gas delivery system is designed to
avoid hooking up the wrong gas hose to a compressed gas port
through the use of physically non-interchangeable gas-specific
connectors. While there are several such systems, perhaps the
strongest is called the Diameter Index Safety System (DISS)
(Figure 3). It
is physically impossible to insert the oxygen hose or flowmeter
into any other port, or to attach something inappropriate into the
oxygen port. However, this safety net is incomplete. The threaded
output of the flowmeter is "one size fits all" and fits yellow,
green, and clear Christmas tree adapters. If some sort of
diameter-indexed system were extended also to the flowmeter output
port, or if the Christmas tree adapter was molded into the device,
no crossover of the adapters could occur. That would solve the
problem of the mismatched adapters, but it still would not prevent
attachment of oxygen tubing to a non-oxygen flowmeter. A
gas-specific non-interchangeable fitting for low-pressure oxygen
tubing would be possible to produce, but is not currently a
standard. Thus, the protection is not carried all the way to the

A physical arrangement that precludes doing the
wrong thing, like the gas-specific non-interchangeable connector,
is an example of an engineered safety device (ESD), and a forcing
function.(3) Other
forcing functions include "lockins," "lockouts," and
"interlocks."(4) Lockins
maintain a condition, and prevent easy exit from a sequence of
actions until the right conditions are met. Lockouts prevent easy
entrance to a dangerous set of actions or a segment of software,
without the proper conditions and access authority. Interlocks
enforce correct sequencing or isolate events in time; often they
are used to prevent one action from being taken while another is
already active. There are a variety of examples of these forcing
functions in health care.(5)

In addition to the indexing system for connectors
for bulk-supplied compressed gases, there is an indexing system for
medical gas cylinders, called the "Pin Index System."(1,2) Each
cylinder has a specific pattern of holes into which the matching
pattern of pins from the appropriate regulator must fit (Figure 4). This
helps to prevent the wrong regulator (in calibration and in
color-code) from being attached to a cylinder. For example, I
recently asked for an E-cylinder of oxygen to transport a
critically ill patient, and the nurse complained that the regulator
wouldn't fit on the tank properly. The nurse had inadvertently
brought a CO2 cylinder (gray in color) instead of an oxygen
cylinder. The Pin Index System prevented the oxygen regulator from
being attached to the CO2 cylinder, perhaps saving the patient's
life (since, once mounted under the bed, it would have been
difficult to detect this error). Ventilation with 100% CO2 would
rapidly cause a cardiac arrest in the patient, and the usual
resuscitative measures of promoting a clear airway, adequate
breathing, etc., would have only worsened matters (by promoting
more exchange of the wrong gas) if the gas swap were not

Engineered safety devices and forcing functions
are very common in anesthesiology, where experience has shown that
good intentions, warnings, or vigilance are not sufficient to
ensure patient safety. For example, the anesthesia machine itself
has flowmeters to control the flow of oxygen, nitrous oxide, and
air. In the heat of a crisis, or at the end of an anesthetic, the
anesthesiologist might reach back to turn off the nitrous oxide and
turn up the flow of oxygen, but inadvertently do exactly the
opposite. Modern anesthesia machines contain a mechanical,
pneumatic, or electronic oxygen proportion limiting control system
that physically prevents selecting an oxygen concentration of less
than 25%.(6-8) Another
forcing function is the mechanical "vaporizer interlock" that
prevents activating more than one vaporizer delivering a volatile
anesthetic gas (eg, Isoflurane) at a time.(6-8)

Engineered safety devices and mechanical forcing
functions are less common in the rest of medical practice. In
radiation therapy or X-ray units, interlocks may prevent activating
the radiation while the door to the unit is open. Most
computer-based systems have forcing functions in software that
double checks for irreversible actions (eg, "Do you really want to
empty the trash?"). However, frequent users sometimes click this by
habit even when they do not mean to approve the action.(4) This kind of
behavior was a factor in a set of catastrophic errors with
radiation therapy equipment.(9)

Despite effective forcing functions, near misses
and errors may still occur. For example, in another case I asked
for an oxygen cylinder, and was brought a CO2 cylinder with a CO2
regulator attached (these are used for certain purposes in cardiac
surgery). The Pin Index System played no role here, since it forces
the correct regulator on the matching tank and there was no
mismatch in this case. There is no engineered safety device to
ensure that the gas flowing is in fact oxygen. The anesthesia
machine has alarms for oxygen concentration, but these are not yet
used for in-hospital transport situations. Fortunately, I noticed
the gray color (color-coding is another safety feature) of the

There are also "procedural forcing functions" in
which the standard procedures call for personnel to verify certain
conditions before allowing actions to proceed. The requirement for
a two-person check of blood products before administration to the
patient is an example. The recently standardized "timeout" required
by JCAHO and the VA before surgical incision is a procedural
forcing function by which surgery cannot proceed unless all team
members have re-verified the patient's identity, the proposed
surgical procedure, and the exact site of surgery. Procedural
forcing functions like this can be valuable if implemented
systematically, and if the team is in fact empowered to act as an
interlock. However, procedural forcing functions are relatively
weak due to psychological factors such as haste, complacency, and a
behavior termed "social shirking." Social shirking occurs when
"members of an organization find it to their advantage to evade
their work responsibilities and trust that the efforts of others
are meet their goal."(10) When
shirking is present, checks that are independently redundant in
theory are neither independent nor redundant in practice.

What are potential specific solutions to the
problem of mismatched adapters on flowmeters? One solution is to
eliminate air flowmeters in most patient care settings. They appear
to be used for two main purposes: to administer nebulized
medications (eg, bronchodilators), and to provide humidified air to
patients with a tracheostomy. Yet, nebulized medications can be
delivered as safely using oxygen in all but a handful of cases. For
those rare cases, either an air flowmeter or an air cylinder and
flowmeter could be available. For patients with a tracheostomy, the
humidified gas is usually administered via large bore tubing from a
device that attaches directly to the flowmeter, without a Christmas
tree adapter (Figure 5).
Hence, many hospitals have banished air flowmeters with these
adapters. Other sites have found this hard to do, because they
argue that it requires more storage space and more effort to obtain
the air devices in the cases where they are truly needed. The VA
Advisory suggests converting color-coded Christmas tree adapters to
clear ones (Figure 6) so
that the adapter itself conveys no information as to the gas'
identity, forcing the user to look at the flowmeter itself. This
may not be a very effective way to alter the frequency of
connecting the tubing to the wrong flowmeter.

Ultimately, indexing the tubing and fittings
might be the better solution. In addition, following the lead of
the anesthesia machine, developing oxygen analyzer systems for
low-pressure oxygen delivery systems, allowing verification of
oxygen concentration, would reduce errors related to gas

Take-Home Points

  • Examine the use of gas flowmeters
    in your institution and consider the various strategies to reduce
    the likelihood of connecting oxygen tubing to the wrong gas:
           -    Eliminate
    air flowmeters;
           -    Permanently
    fix Christmas tree adapters to the correct flowmeter; and
           -    Use
    clear Christmas tree adapters rather than color-coded
  • Consider the possibility that the wrong gas (or no
    gas) is being administered when a patient does not respond to
    treatment with supplemental oxygen. Double check the flowmeter and
    tubing connections.
  • Consider the use of
    forcing functions in other portions of the hospital. Look for
    possible design solutions wherever possible rather than relying
    solely on training, warnings, or labels.

David M. Gaba, MD
Professor of Anesthesia
Stanford University School of Medicine
Staff Anesthesiologist
VA Palo Alto Health Care System

Disclosure: Dr. Gaba has declared that neither he, nor any
immediate member of his family, has a financial arrangement or
other relationship with the manufacturers of any commercial
products discussed in this continuing medical education activity.
In addition, his commentary does not include information regarding
investigational or off-label use of pharmaceutical products or
medical devices.


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Ehrenwerth J. Medical gases: storage and supply. In: Ehrenwerth J,
Eisenkraft J, eds. Anesthesia equipment. St. Louis, Mo: Mosby;

2. Dorsch J, Dorsch
S. Understanding anesthesia equipment. 4th ed. Philadelphia, PA:
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3. Norman D. The
psychology of everyday things. New York, NY: Basic Books;

4. Leveson N.
Safeware: system safety and computers. Reading, MA: Addison-Wesley
Publishing Company; 1995.

5. Gosbee J. Human
factors engineering and patient safety. Qual Saf Health Care.
2002;11:352-4.[ go to PubMed ]

6. Petty C. The
anesthesia machine. New York, NY: Churchill Livingstone Inc.;

7. Eisenkraft J: The
anesthesia machine. In: Ehrenwerth J, Eisenkraft J, eds. Anesthesia
equipment. St. Louis, Mo: Mosby;

8. Andrews J. Inhaled
anesthetic delivery systems. In: Miller RD, ed. Anesthesia. 5th ed.
New York, NY: Churchill Livingstone;

9. Leveson N. Medical
devices: the Therac-25 story. In: Leveson N, ed. Safeware: system
safety and computers. Reading, MA: Addison-Wesley Publishing
Company; 1995:515-553.

10. Heimann C.
Acceptable risks: politics, policy, and risky technologies. Ann
Arbor, MI: The University of Michigan Press;


Figure 1. The air
and oxygen flowmeters with the correct Christmas tree

Figure 2. The air
and oxygen flowmeters with the Christmas tree adapters

Figure 3. The
compressed gas outlets for oxygen and air illustrating the Diameter
Index Safety System. The threaded connector for the oxygen outlet
is much smaller than that needed for the air outlet, making it
physically impossible to attach the oxygen hose to the air

Figure 4. Pin
Index System on an oxygen cylinder. Photograph shows the pins on
the regulator which fit in matching holes on the cylinder. Other
gases have different pin positions.(1,2)

Figure 5. For
tracheostomy, humidified gas is administered via large bore tubing
from a device that attaches directly to the flowmeter, without a
Christmas tree adapter.

Figure 6. An
oxygen flowmeter with a clear Christmas tree adapter. If all
flowmeters use this arrangement, it forces the user to look at the
flowmeter itself or the wall outlet for color code