Oxygen conserving devices
Brian L Tiep, MD
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INTRODUCTION — The functional disability experienced by patients with severe
pulmonary disease relates both to physiological impairment and to their ability to
adapt and maintain a nearly normal lifestyle. As patients develop a medical need for
supplemental oxygen, breathlessness and lack of energy are compounded by the
increased demand of carrying oxygen equipment.
Oxygen conserving devices have been introduced as a means of making oxygen
therapy more efficient, more portable, and less intrusive [1,2]. In addition,
hypoxemia can be prevented more readily in those patients with high oxygen flow
requirements with oxygen conserving devices, such as reservoir cannula and
transtracheal catheters. Escalating costs and reduced reimbursement have also
fostered the availability of these systems.
This review will compare traditional, continuous flow oxygen delivery by nasal
cannula with a variety of oxygen conserving devices. The indications for long-term
supplemental oxygen, the use of oxygen in hypercapnic patients, and issues
regarding oxygen therapy during air travel are discussed separately. (See "Long-term
supplemental oxygen therapy" and see "Use of oxygen in patients with hypercapnia" and see
"Traveling with oxygen aboard commercial air carriers").
CONTINUOUS FLOW NASAL CANNULA — Continuous flow nasal cannula oxygen
delivery via nasal prongs is the standard against which all developments or
improvements should be compared [3]. The nasal cannula is universal and generally
well accepted by patients and public alike, and its effectiveness in meeting the
oxygen needs of the stable chronic lung disease patient is essentially unquestioned.
However, it is inefficient, as only a small percentage of the oxygen delivered to the
nose actually reaches the alveoli.
Oxygen flowing through the standard nasal cannula is nearly 100 percent pure.
However, because the oxygen becomes entrained in a much larger volume of
inspired atmospheric air, containing 20.9 percent oxygen, the patient actually
receives a blend of pure oxygen and room air [4]. The resultant inspiratory oxygen
concentrations via usual flow settings are approximated in Figure 1 ( show figure 1).
Assuming the most effective oxygen delivery occurs in the first 200 msec of
inspiration, the fraction of inspired oxygen (FiO2) can be calculated and expressed as
a percentage. For example, the typical flow setting of 2 L per min would raise the
FiO2 to about 28 percent. This is a small oxygen enrichment, but it adequately
corrects hypoxemia in most patients during rest [5].
A model of inspiration is shown in Figure 2 (show figure 2) [4]. Assuming a tidal
volume of 450 mL, the first gas to reach the alveoli is the 150 mL of oxygen-poor
gas already occupying the airways; this is followed by the first 300 mL of inspired
gas. The final 150 mL of inspired gas is destined to fill the airways at the end of
inspiration (becoming the dead space gas), never reaching the alveoli. Ideally, all
oxygen delivery should occur prior to this last 150 mL. Figure 3 shows that airflow
slows at the end of inspiration, so that dead space inspiration occupies a
disproportionate 50 percent of inspiratory time (show figure 3). Consequently, the
ideal oxygen delivery should occur within the first 0.5 seconds of inspiration, as
oxygen delivered beyond this time is largely wasted. One study, for example,
demonstrated that nasal oxygen delivered incrementally earlier in inhalation
progressively increased oxygen saturation (show figure 4) [6].
TYPES OF OXYGEN CONSERVING DEVICES — There are three types of devices for
enhancing the efficacy of oxygen therapy:



Reservoir cannulas
Transtracheal catheters
Demand oxygen pulsing devices
Each device meets the goal of improving oxygen delivery, but each has unique
features that recommend for or against its use in the individual patient.
Reservoir cannulas — Reservoir cannulas function by storing oxygen during
exhalation, making that oxygen available as a bolus upon the onset of the next
inhalation. Oxygen is conserved because the patient can be adequately saturated on
a substantially reduced oxygen flow. These cannulas are available in two
configurations:

A moustache configuration (Oxymizer®), in which the reservoir is located
directly beneath the nose [7]. A thin, compliant membrane in the reservoir is pushed
forward during exhalation, creating a chamber between the membrane and the
posterior wall (show figure 5). This enables oxygen to be stored during most of
exhalation. When the patient is ready to inhale, he/she receives the stored oxygen
along with the continuously flowing supply oxygen.

A pendant configuration (Pendant®), in which oxygen is stored both in a
reservoir located on the anterior chest [8]. The reservoir membrane is pushed
forward during exhalation, creating a chamber (show figure 6). This enables oxygen to
be stored during exhalation in the reservoir. When the patient is ready to inhale,
he/she receives the stored oxygen along with the continuously flowing supply
oxygen.
Both reservoir cannulas are simple, reliable, inexpensive, and disposable. They
operate in response to the patient's nasal airflow. While the efficacy of both cannulas
is similar, there are differences in design that affect patient preference:

The Oxymizer tends to be more comfortable than the Pendant and, in some
cases, the standard nasal cannula. However, it is noticeable on the face, causing
some patients to refuse to wear it.

The Pendant is less noticeable. Previous pendant configurations utilized ear
loops and wide tubing that some found to be uncomfortable. The present
configuration has thinner tubing and no ear loops.
Both devices are partial rebreather systems; as they return some of the patient's
warmed moisture, they effectively increase the relative humidity of the oxygen.
Reservoir cannulas can be used by patients with high flow requirements because
they improve the efficacy of oxygen delivery [9]. In the inpatient setting, reservoir
cannulas have been used as a transition device from the Venturi mask, since they
are less intrusive and facilitate eating and communication [10]. In the home setting,
oxygen cylinders with limited storage and oxygen concentrators with limited flow
range present barriers for patients on long term oxygen, particularly as the flow
requirements reach 6 L/min. For these patients, a conserving device may provide the
only opportunity to live at home and maintain adequate oxygen saturation.
Transtracheal catheters — Delivery of oxygen by transtracheal catheter is discussed
elsewhere in detail. (See "Transtracheal oxygen therapy"). However, it is also mentioned
here for comparison with the other conserving devices. Transtracheal catheters
function by delivering oxygen directly into the trachea through a small opening in the
neck [11]. Continuously flowing oxygen is stored in the upper airways toward the
end of exhalation and is delivered during early inhalation along with supply oxygen,
bypassing the dead space of the upper airways.
Transtracheal catheters have been successfully utilized to oxygenate patients with
refractory hypoxemia [12]. This mode of delivery has the additional advantages of
high cosmetic acceptability and, when used with higher flows, the potential for
reduced work of breathing by lowering minute ventilation (show table 1) [12,13].
Demand oxygen pulsing devices — Demand devices deliver a pulse of oxygen to the
patient only during inhalation. Interposed between the pressurized oxygen source
and the nasal cannula, these devices consist of a sensor, logic circuitry, and a
solenoid valve (show figure 7). The patient's inspiratory flow is detected through the
nasal cannula or transtracheal catheter; the solenoid then opens and delivers a short
oxygen pulse. The aim is to discharge all oxygen during the earliest part of inhalation
[14].
Pulsing devices are available as stand alone modules or integrated into a liquid
oxygen system, compressed gas system, or portable oxygen concentrator. Because
they are high technology devices, a variety of protocols and formats may be included
in their design. Some devices are electronic with high precision settings while others
are pneumatically controlled and driven. Settings for oxygen delivery are adjusted in
some devices by varying the length of the oxygen pulse. Other devices deliver an
early pulse each time they discharge, avoiding lengthening of the pulse into the dead
space portion of inhalation. With this high efficiency of delivery, oxygen saturation
can be maintained by delivery of a pulse on every other breath, leading to lower flow
requirements [14].
Demand conservers vary in efficacy from 3:1 to 7:1 compared to continuous flow
delivery. The batteries may last from three hours to three weeks, depending on the
delivery protocol. Some devices have a feature in which they automatically revert to
continuous flow if they fail to detect a breath in 20 to 30 seconds, but this feature
uses more battery current. Pneumatically driven units do not require batteries. The
demand devices can be very efficient but are mechanically complex, so that failure is
possible. They can be coupled with transtracheal catheters to combine the
advantages of better inspiratory signal and cosmetic acceptability with high efficacy.
One potential problem with some pulsing devices is their inability to adequately
oxygenate some patients during exertion. This limitation can be prevented by
modifying the device to deliver a larger pulse during the earliest part of inhalation or
use a higher setting [15,16]. It is always advisable to test each patient during rest
and exertion to assure adequate saturation on the device being prescribed.
Oxygen conserving technology has spawned the development of newer oxygen
demand delivery devices that therapeutically adapt to the patient's physiological
requirements [15,17]. One such device has an activity sensor that automatically
adjusts to the exercise setting and remains at that setting for 50 seconds following
the discontinuation of activity [17]. Such devices not only conserve oxygen, but also
respond to meet the higher energy requirements during exertion. Consequently,
these devices may be therapeutic beyond simple oxygen supplementation.
EFFICACY OF VARIOUS DEVICES — The efficacy of these systems is expressed as the
ratio of oxygen flow required by continuous flow delivery to the flow required by the
conserver to achieve equivalent oxygen saturation.
Efficacy = Continuous flow ÷ flow to conserving device (at equivalent saturation)
The data presented in this section represent statistical means, and the actual
equivalency may vary between patients. Thus, it is advisable to draw a blood gas or
measure oximetry to determine the actual prescription.
Reservoir systems — The efficacy of reservoir cannulas compared to continuous flow
delivery is shown in Figure 8 (show figure 8) [7,8,18-21]. Oxygen supplied at 0.5 L/min
via the reservoir cannulas achieves equivalent saturation to continuous flow at 2
L/min. Similarly, 1 and 2 L/min reservoir settings are equivalent to 3 and 4 L/min
continuous flow, respectively. Thus, the efficacy of reservoir cannulas ranges
between 2:1 and 4:1.
For higher flows, reservoir cannulas add 2 L per min to continuous flow settings. For
example, a reservoir cannula setting of 4 L/min is equivalent to 6 L/min by
continuous flow. Studies performed during exercise and sleep generally yield results
consistent with resting values, albeit with some variation [20-22].
Transtracheal systems — The efficacy of transtracheal oxygen delivery ranges
between 2:1 and 3:1 over continuous flow delivery. While the efficacy is reduced
during exercise [13,23], patients have greater exercise tolerance and reduced work
of breathing via transtracheal delivery [23,24]. (See "Transtracheal oxygen therapy").
Pulsed oxygen devices — The efficacy of pulsed demand oxygen delivery varies
between 3:1 and 7:1 (show figure 9) [14,25-28]. In general, this efficacy is maintained
during exercise and sleep [15,26-28]. Pulsing oxygen through a transtracheal
catheter maximizes the efficiency of oxygen delivery at 7:1 over continuous flow
[6,29].
Small lightweight compressed gas cylinders — The total weight of an oxygen system
consists of the cylinder, regulator and the oxygen conserving device. Composite
cylinders are manufactured from aluminum liners and strengthened by carbon fiber
in an epoxy resin matrix. These cylinders weight less than 50 percent of standard
aluminum cylinders. They accept higher pressures up to 3000 psi and thus improve
the portability of the portable oxygen system.
CLINICAL USE — Not all patients require an oxygen conserving device under all
conditions. In general, an oxygen conserving device should be considered for
patients who would benefit from a lower flow of oxygen or a lighter weight system,
or those who have refractory hypoxemia. This would include patients who are
ambulatory and wish to maintain an active lifestyle. Also, patients who live a long
distance from their HME supplier should have a wider variety of options, as they
must endure the practical limitation of less frequent home deliveries. Figure 13
demonstrates the effect of oxygen conserving devices on extending cylinder life
(show figure 10).
Each conserving device has inherent advantages and drawbacks which should be
weighed by the physician and patient. The choice between systems often requires
compromise. Table 2 compares the strengths and weakness of each device ( show
table 2).

Reservoir cannulas are the simplest, least expensive, and easiest for patients
to convert from standard cannulas, but they may be obtrusive for some patients.

Transtracheal oxygen delivery excels in cosmetics and may reduce the work
of breathing, but it entails a minor surgical procedure, the catheter may form
mucous balls at its tip, and it requires significant patient training.

The demand devices are more efficient for both nasal and transtracheal
delivery. These devices have enabled the development of integrative systems that
maximize oxygen delivery. However, they make audible pulses that may be
distracting and mechanical failure, albeit rare, is possible. Some pulsing devices fail
to oxygenate patients adequately during exertion [16].
Economic considerations — The cost of long term oxygen therapy is significant and
includes oxygen, equipment, and service to the home. In the United States, the
Center of Medicare and Medicaid Services (CMS) has reported paying $2 billion for
long term oxygen therapy. In 1989, CMS instituted a system of fixed reimbursement
based on liter flow prescription for home oxygen. Since then CMS has instituted
several significant reductions in reimbursement for home oxygen. The profit margin
for the home medical equipment (HME) suppliers can be maintained only by reducing
their overhead. As a result, the HME supplier is economically driven to provide the
least expensive system to purchase and maintain and one that requires less frequent
home service calls. However, in localities that have several competitive HME
suppliers, those who supply patients with a more portable and ambulatory system
may have a competitive advantage.
Liquid oxygen, which offers the highest efficiency in oxygen storage with safe and
easy patient transfilling to his/her ambulatory unit, is unfortunately the most
expensive system to purchase and maintain, and it requires more frequent home
deliveries. Consequently, there is a financial disincentive to offer liquid oxygen to
patients.
Two integrated systems of oxygen concentrators that refill oxygen cylinders, Venture
Home Fill® (Invacare Inc, Elyria, OH), and Total O2® (CHAD Therapeutics Inc,
Chatsworth CA), have become available. They obviate the necessity for home
deliveries, which is the most costly aspect in providing domiciliary oxygen.
The most commonly dispensed system is an oxygen concentrator with a backup E
cylinder designated to serve in the event of power failure or for infrequent portable
use. Concentrators contain a synthetic aluminum silicate that traps and removes
nitrogen molecules and thus yields a gas mixture of approximately 95 percent
oxygen and five percent argon [3].
Given their economic disincentive to supply liquid oxygen, some HME suppliers are
considering alternatives. Oxygen conserving devices have expanded the delivery life
of the smallest oxygen cylinders from one hour of continuous flow to seven hours of
pulsed flow. Some of these compressed oxygen systems weigh less than 1.8 kg,
comparing favorably to standard 4.3 kg liquid systems supplying a similar delivery
life. Newer liquid oxygen systems have also been developed that are lighter and
more portable, owing to the integration of oxygen conserving devices. The Helios
weighs 1.5 kg and lasts 10 hours between refills, thus providing the dual advantages
of portability and transfillability.
When considering the cost savings of oxygen conserving methodologies, the cost of
the oxygen conserving device must be included. This actually happens in the
marketplace, since the HME supplier must decide which equipment it will make
available to patients. In addition, patients who require higher oxygen flows or have
refractory hypoxemia would derive clinical benefit beyond convenience and
portability, since the alternative may require the patient to tolerate hypoxemia.
The Medicare system of reimbursement also has some financial disincentives to using
oxygen conserving devices. If the patient, as a result of requiring less oxygen, falls
into a lower reimbursement category based on a lower prescribed liter flow, the HME
supplier would receive a smaller payment. With a device that is more expensive, this
incentive seems backwards. Reservoir cannulas and transtracheal catheters are
particularly affected by lower liter-flow settings, as opposed to the pulsing devices,
whose settings (in absolute numbers) correspond to continuous flow settings.
SUMMARY — Oxygen conserving devices offer greater versatility in portability,
reduced costs, and feasibility of using oxygen systems previously deemed
impractical. They all involve a series of tradeoffs. Each patient who is considered for
an oxygen conserving device should have arterial blood gases or oximetry tested
using that device during wakeful rest, usual exercise, and perhaps during sleep to
ensure an adequate prescription. The development of oxygen conserving devices has
spawned further developments such as a portable oxygen concentrator, oxygen
concentrators that refill portable cylinders, and activity adaptive oxygen delivery
devices.
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