A novel ambulatory closed circuit breathing system for use during exercise.

Abstract

We describe a unique ambulatory closed circuit for delivering high fractions of inspired oxygen to an exercising user who does not require isolation from their environment. We describe the major components and their function and suggest potential applications for such a circuit. This circuit may benefit patients who are chronically dependant on oxygen, are unable to exercise due to hypoxia, or require oxygen supplementation at high altitude.

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ORIGINAL ARTICLE
A novel ambulatory closed circuit breathing system for use
during exercise
R. C. N. McMorrow,1,2J. S. Windsor,3,4M. G. Mythen,5,6M. P. W. Grocott7,8and
the Caudwell Xtreme Everest Research Group*
1 Research Fellow, 3 Research Fellow, 5 Professor, 7 Senior Lecturer, UCL Centre for Altitude Space and Extreme
Environment Medicine, UCL Institute of Child Health, University College London, London, UK
2 Consultant, Department of Anaesthesia, National Maternity Hospital & St Vincent’s University Hospital, Dublin,
Ireland
4 Specialist Registrar, Department of Anaesthesia, Barnet and Chase Farm Hospitals NHS Trust, Barnet, UK
6 Smiths Medical Professor of Anaesthesia and Critical Care, UCL ⁄ UCL Hospitals, National Institute of Health
Research Comprehensive Biomedical Research Centre, London, UK
8 Consultant, Department of Critical Care, Southampton University Hospitals NHS Trust, Southampton, UK
Summary
We describe a unique ambulatory closed circuit for delivering high fractions of inspired oxygen to
an exercising user who does not require isolation from their environment. We describe the major
components and their function and suggest potential applications for such a circuit. This circuit
may benefit patients who are chronically dependant on oxygen, are unable to exercise due to
hypoxia, or require oxygen supplementation at high altitude.
. .......................................................................................................
Correspondence to: Dr R. C. N. McMorrow
Email: mcmorrow.roger@gmail.com
*The members of the Caudwell Xtreme Everest Research Group are
listed in Appendix 1.
Accepted: 17 February 2011
Ambulatory closed circuits deliver high partial pressures
of inspired oxygen (PIO2) from a limited oxygen source
that is normally carried by the user. The circuits are
heavier and more complex than the open, semi-open
or semi-closed circuits described by Mapleson [1]. This
is largely due to the addition of a carbon dioxide (CO2)
absorber, and the associated tubing and valves that are
required to manage the flow of gases. The extra mass,
and the possibility of delivering a hypoxic mixture
during failure, have limited ambulatory closed circuits
use. Such circuits are generally used by trained
individuals, such as advanced divers, fire-fighters and
specialised mine rescuers who have no option but to be
isolated from their environment [2].
We describe below the components of, and rationale
behind, the design of a novel ambulatory closed circuit,
where the user does not have to be isolated from
his ⁄her environment. This may have applications for
patients who are chronically dependent on oxygen, are
unable to exercise due to hypoxia, or require oxygen
supplementation at high altitude.
Description of the closed circuit
The circuit plan is shown in Fig. 1 and a photograph of
the prototype is shown in Fig. 2. The major compo-
nents of the circuit are mounted on a back plate and
harness. The circuit is comprised of: a user interface
that includes the facemask; a CO2 absorber and
housing; a water trap; a reservoir limb; an oxygen
source; and an electronic control unit. These compo-
nents, that have a combined mass of approximately
10 kg, are described in detail below.
User interface ⁄⁄facemask
A T-shaped nasal and mouth two-way non-rebreathing
facemask (Hans Rudolph Shawnee, KS, USA, Part
Number 8932) is used. This is supplied with oxygen
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from one of two female Luer lock ports (Hans
Rudolph Part Number 171179) positioned on the
inspiratory port. A pair of 1-m long, internally smooth
bore, 35-mm internal diameter polyvinyl chloride
(PVC) externally corrugated tubes are connected to
the facemask. These tubes serve as the inspiratory and
expiratory limbs of the circuit.
Two light emitting diodes (LEDs), easily visible to
the user, are mounted on the superior aspect of the
facemask. These are connected to the electronic
control unit to convey information about the status
of the electronics and the current PIO2.
A mechanical flow sensor, comprised of a sprung
bobbin held in a perspex cylinder, is also mounted on
the superior aspect of the facemask (Poisk, St-Peters-
burg, Russia). It is connected in series with the oxygen
tubing to indicate the presence or absence of the flow
of oxygen to the mask. When oxygen flow is present
the bobbin appears; however, when the supply ceases,
the bobbin disappears.
CO2absorber and housing
A plastic housing made from Delrin (DuPont,
Wilmington, DE, USA) was used to hold a non-
granular ExtendAirTMCO2absorber (Micropore Inc.,
Newark, DE, USA) containing 2 kg of calcium
hydroxide (CaOH) (Fig. 3). Two bespoke T-shaped
connectors (CRDM Silicone sealed SLS Nylon-12)
were used to connect the 35-mm inspiratory and
Figure 2 The prototype circuit highlighting the major
components mounted on a back plate and harness: (1) user
interface comprising mask, valves, two light-emitting diode
(LED)indicators,aflowindicator,oxygenportandinspiratory
and expiratorylimbs;(2)CO2absorberand housing. Inlet(IL)
and outlet (OL) ports are indicated; (3) expiratory limb water
trap; (4) reservoir limb; (5) oxygen source; (6) electronic
control, display and three oxygen sensors.
Figure 3 The Solid State ExtendAirTMcalcium hydroxide
CO2 absorber. The inset shows the castellation channels
incorporated to aid the flow of gas through the absorber.
1
2
3
4
6
5
IL
OL
Figure 1 A circuit diagram for the breathing system: (1) user
interface comprising mask, valves, two light-emitting diode
(LED) indicators, a flow indicator, oxygen port and inspira-
tory and expiratory limbs; (2) CO2absorber and housing.
Inlet (IL) and outlet (OL) ports are indicated; (3) expiratory
limb water trap; (4) reservoir limb; (5) oxygen source; (6)
electronic control, display and three oxygen sensors.
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expiratory limbs to the absorber outlet and inlet,
respectively. The combined weight of the complete
housing and absorber was 3.5 kg.
Water trap
A 30-cm long section of internally smooth bore, 35-
mm internal diameter PVC externally corrugated
tubing was capped with a removable plug and
connected to the T-shaped absorber inlet connector
to serve as a water trap.
Reservoir limb
This is comprised of six 1-m long sections, of
internally smooth bore, 35-mm internal diameter
PVC externallycorrugated
together in series with bespoke connectors (CRDM
Silicone sealed SLS Nylon-12). This was designed to
collect exhaled gases following CO2scrubbing and to
provide gas for subsequent inspiration. It was con-
nected at its proximal end to the T-shaped connector
on the CO2absorber’s expiratory port. Its distal end
was open to, and in continuous fluid movement with,
the atmosphere. The chosen reservoir volume is a
compromise between having a sufficiently large
volume with low resistance to flow and the practi-
calities of carrying sufficient tubing that doesn’t
generate excessive resistance to flow. Ideally, the
reservoir limb should at least exceed the user’s vital
capacity so that all of their exhaled gas is always
recovered. In this embodiment the reservoir limb’s
volume was approximately 6.25 l.
tubing,connected
Oxygen source
A four-litre oxygen bottle was filled to 300 bar at
24 ?C, and connected to a flow regulator and low-
pressure hose to deliver oxygen to the facemask via the
electronic control unit (Poisk). The cylinder, regulator
and hose weighed approximately 2.5 kg.
Electronic control unit
A bespoke electronic control unit was designed
specifically for the circuit (Owen Drumm, Dublin,
Ireland). It was used to monitor, record and display the
PIO2in the circuit on a digital LED screen. Three
oxygensensors (Part Number
Analytical Industries Inc., Pomona, CA, USA) were
mounted on the expiratory end cap of the CO2
absorber housing. Three sensors were used to provide
redundancy should one or two fail. A solenoid valve in
the control unit was used to regulate the flow of
oxygen. When powered, the valve closes, thus stop-
ping the flow of oxygen to the facemask. Intermittent
operation of this valve is used to restrict the flow of
oxygen to the circuit to maintain the desired PIO2.
Four AA lithium ion batteries powered the electronics.
The mask mounted LEDs were used to indicate the
status of the circuit to the user (Table 1).
PSR-11-39-MD;
Circuit operation
To operate the circuit, the flow from the oxygen
source is first set to 3 l.min)1. This value is assumed to
be greater than a user’s typical maximal oxygen
Table 1 Table outlining the implication of the facemask mounted light-emitting diodes (LEDs).
LED statusIndicatingExplanation
No lightNo power ⁄ power failure ⁄
firmware fault
This indicates that the electronics have not been switched on, that the power
has failed, or that the electronics have detected a firmware fault and switched
themselves off. There will be no restriction to the flow of oxygen in this state
Flashing dim green Normal operationThis indicates that the firmware is working correctly and that the PIO2is above
the set value*
Solid bright green Valve closedThe user may notice from the mechanical flow sensor that there is no oxygen flow.
This indicates that this is a deliberate action by the electronics rather than a
failure of oxygen flow for another reason
Flashing bright
amber
PIO2lower than set value*This indicates that there is an unknown problem in maintaining the PIO2at or
above the set value*, or that atmospheric air has entered the circuit
Flashing bright redPIO2is 20 kPa or less This indicates that there is a problem with the delivery of oxygen to the circuit and
the user should immediately convert the circuit to an open circuit or remove
the facemask entirely until the problem is identified and corrected
*The set value is user definable.
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consumption. Assuming a correct seal has been made
with the facemask, the user is able to begin breathing
from the circuit. The volume for the user’s first breath
from the circuit will come from the oxygen enriched
air in the inspiratory limb. As this volume is removed
from the inspiratory limb an exact matching volume of
atmospheric air enters the reservoir limb to compensate
for this volume change. If the oxygen sensors register a
fall in PIO2 due to atmospheric air that enters the
circuit, the electronic control unit will increase the
flow of oxygen by allowing it to flow unrestricted into
the circuit, until the PIO2rises to the set value, again.
As the user exhales, the CO2-laden, oxygen-enriched
air travels down the expiratory limb and the volume of
atmospheric air that was originally entrained is once
again expelled. Carbon dioxide is absorbed on its first
pass through the absorber. The next breath the user
takes will be the scrubbed oxygen-enriched air with an
appropriate portion of fresh oxygen added, determined
by the electronic control unit. The user can never
directly inhale the atmospheric air, as the inhaled
volume comes from the exhaled scrubbed gas. By
regulating the flow of oxygen in a closed loop fashion
over time, the supply of oxygen will match the user’s
oxygen consumption plus the portion that will leak
from the circuit.
When the minute volume is constant, atmospheric
air will enter the distal end of the reservoir limb and
oscillate in keeping with the user’s tidal volume. As the
reservoir limb is open to atmosphere, rapid changes in
tidal volume can be easily accommodated and water is
free to drain from the circuit.
Discussion
This circuit demonstrates a potential solution for users
requiring a high PIO2and minimal airway resistance, or
for individuals who wish to rebreathe an additional gas
such as helium or xenon. These performance charac-
teristics are achieved by combining the classical
circular, closed circuit, with the unique open reservoir
limb that is in free fluid communication with the
atmosphere.
There are a number of advantages to this open
reservoir system which are as follows. The open limb
allows for a large flux in tidal volume without the risk
of the distressing sensation where the user attempts to
inspire against a collapsed reservoir bag. The sensation
is known as ‘bottoming out’ among ambulatory closed
circuit users. In addition, an open limb minimises the
expiratory resistance by allowing the user always to
exhale freely without the need for a pressure release
valve. The open limb also allows nitrogen gas to exit
the circuit as the PIO2rises, without the user formally
expelling it using high flows of oxygen before use. This
is a practice that divers and fire-fighters must currently
endure [3]. Conversely the open limb allows nitrogen
to enter the circuit allowing the user to deliver a set
PIO2without the need to carry a second bottled buffer
gas.
If the circuit is ever compromised by a breach or an
undetected leak on the expiratory limb, the user will
exhale proportionately more gas to the atmosphere and
hence lose volume from the circuit. A compensatory
volume of air automatically enters via the open limb,
thereby preserving the volume in the circuit. If the leak
is on the inspiratory limb, the user will entrain more
atmospheric air. The circuit pressure will not rise as
proportionately more gas is then expelled via the open
reservoir. Any fall in PIO2 will be detected by the
electronic control unit, which will increase the flow of
oxygen to compensate. In essence, the circuit behaves
more and more like an open circuit as an undetected
leak increases. This failure mode ensures that without
any user intervention the circuit automatically behaves
as an open circuit, unlike classic closed circuits which
require an operator to intervene either to compensate
for the loss of volume with fresh gas or to fix the leak.
The user may also convert the circuit to an open circuit
by deliberately disconnecting the expiratory limb,
ensuring no exhaled gas re-enters the circuit. The
rigid reservoir also means that the user cannot
accidently mechanically collapse the reservoir. Finally,
an open reservoir allows water to drain without
accumulating.
The disadvantage of an open reservoir is that oxygen
will inevitably be lost via the open limb. The rate of loss
will be determined by the exact physical characteristics
of the limb, the user’s breathing pattern and the PIO2
that the user is trying to maintain within the circuit.
The sensation of ‘bottoming out’, while unpleasant, is
nevertheless a physical warning that is lost in the current
system. If no oxygen flowed into this circuit and the
user was unaware of or ignored the low oxygen
warnings, they would be able to breathe a potentially
lethal hypoxic mixture. While positive end-expiratory
pressure may be added easily, this circuit is not suitable
for positive pressure ventilation.
During exercise, individuals increase both their
minute volume and oxygen demand. If they are using
an open circuit with a fixed flow of oxygen, their PIO2
will fall proportionately with the increase in minute
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volume due to the dilutional effect of atmospheric air.
This has a paradoxical effect; as the user’s demand for
oxygen increases, their inspired PIO2actually falls [4].
By regulating the PIO2, this circuit demonstrates a
closed loop system whereby oxygen delivery is
matched to the user’s demand for oxygen. This has
the dual effect of conserving a limited oxygen supply
and delivering an effective amount of oxygen during
exercise. Similar closed loop control of oxygen
delivery is described by Johannigman et al. [5]. We
opted for a solid state ExtendAirTMCO2 absorber
due to its predictable performance, low resistance to
air flow and the lack of a requirement for dust filters
[6]. Unlike granules, this absorber doesn’t shift with
position, which can cause channelling and early
absorber failure, nor does it produce dust, which
despite filters is known to cause lung injury to closed
circuit users [7, 8]. This circuit combines the
advantages ofclassic ambulatory
(particularly that of oxygen conservation by using
low flows to maintain a high PIO2) with a design that
can accommodate easily the high minute volumes and
flux in those volumes that are found during exercise.
There are a number of non-diving-related portable
closed circuit systems commercially available, such as
the BioPak (BioMarime, Exton, PA, USA). While this
system is used by professional law enforcement,
military, fire and underground rescue services, it has
also been used successfully on a caving expedition to
Northern Thailand where the atmosphere was intol-
erable to humans due to microbial activity’s raising the
CO2concentration to over 5% [9]. This system is,
however, fully closed and its performance characteris-
tics during exercise are not published. The North
American Diver Alert Network (http://www.divers-
alertnetwork.org) manufactured a surface Remote
Emergency Medical Oxygen (REMOTM; DAN,
Durham, NC, USA) system that was a portable
rebreather designed to help nitrogen washout during
an emergency. It demonstrated excellent conservation
of oxygen supply while maintaining a high PIO2. The
system had problems with leakage of nitrogen into the
system and it was not tested with high minute volumes
[6]. Interestingly, both systems also opted to use the
ExtendAir solid state CO2absorber.
Patients who are critically dependant on oxygen,
such as those with acute or chronic cardiorespiratory
conditions, or healthy individuals requiring oxygen
supplementation at high altitude, may benefit from a
circuit that matches their demand for oxygen with an
adequate oxygen supply in a system that has highly
closedcircuits
desirable breathing characteristics. This may enable
individuals to exercise where hypoxaemia would
otherwise be a limiting factor with respect to exercise
capacity.
This circuit may also be useful for the delivery and
recovery of expensive gases such as helium and xenon
that would otherwise be lost to the atmosphere.
Reducing inspired gas density has been shown to
double endurance time for constant work rate ergom-
eter exercise tests in chronic obstructive airway patients
[10, 11].
Empirical measurement of the actual circuit perfor-
mance with regard to heat production and loss by the
user and CO2absorber, water management, breathing
resistances during typical peak flows, and the charac-
teristics of the ideal design of the reservoir limb, have
yet to be determined. Oxygen will inevitably leak via
the reservoir but the rates have yet to be established.
This novel closed circuit may provide advantages in
the ambulatory delivery of oxygen-enriched air or for
the recovery of expensive gases for an exercising user.
Trials in the laboratory setting are being undertaken
and are planned in healthy volunteers; the results will
be made public in due course.
Acknowledgements
ThisprojectwassupportedbyMrJohnCaudwell,BOC
Medical (now part of Linde Gas Therapeutics), Eli Lilly,
the London Clinic, Smiths Medical, Micropore Inc,
Analytical Industries Inc, Deltex Medical, and the
Rolex Foundation (unrestricted grants), the Association
of Anaesthetists of Great Britain and Ireland, the United
Kingdom Intensive Care Foundation, and the Sir
Halley Stewart Trust. Some of this work was under-
taken at University College London Hospital–Univer-
sity College London Comprehensive Biomedical
Research Centre, which received a proportion of
funding from the United Kingdom Department of
Health’s National Institute for Health Research Bio-
medical Research Centres funding scheme. Caudwell
Xtreme Everest is a research project coordinated by the
Centre for Altitude, Space, and Extreme Environment
Medicine, University College London. Membership,
roles, and responsibilities of the Caudwell Xtreme
Everest Research Group can be found at http://
www.caudwell-xtreme-everest.co.uk/team.
Competing interests
RCNM holds a patent on the new circuit design.
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Appendix 1: Membership of the Caudwell
Xtreme Everest Research Group
Investigators: V. Ahuja, G. Aref-Adib, R. Burnham,
A. Chisholm, K. Clarke, D. Coates, M. Coates,
D. Cook, M. Cox, S. Dhillon, C. Dougall, P. Doyle,
P.Duncan,M.Edsell,L.Edwards,L.Evans,P.Gardiner,
M. Grocott, P. Gunning, N. Hart, J. Harrington,
J. Harvey, C. Holloway, D. Howard, D. Hurlbut,
C. Imray, C. Ince, M. Jonas, J. van der Kaaij,
M. Khosravi, N. Kolfschoten, D. Levett, H. Luery,
A. Luks, D. Martin, R. McMorrow, P. Meale,
K. Mitchell, H. Montgomery, G. Morgan, J. Morgan,
A. Murray, M. Mythen, S. Newman, M. O’Dwyer,
J. Pate, T. Plant, M. Pun, P. Richards, A. Richardson,
G.Rodway,J.Simpson,C.Stroud,M.Stroud,J.Stygal,
B. Symons, P. Szawarski, A. Van Tulleken, C. Van
Tulleken, A. Vercueil, L. Wandrag, M. Wilson,
J. Windsor.
Scientific Advisory Group: B. Basnyat, C. Clarke,
T. Horn-Bein, J. Milledge, J. West.
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