Comparison of three CPAP systems used in EMS

Comparison of three CPAP
systems used in EMS
Continuous positive airway pressure (CPAP) is used in emergency medical services (EMS)
with increasing frequency to treat a variety of respiratory conditions.1-8 There are several
devices available for clinicians to choose from when providing CPAP in pre-hospital care.
As with most products, variations in device performance exist and clinicians should
be aware of these differences in performance as they may impact desired outcomes.
Four commercially available EMS oxygen CPAP systems from three manufacturers were
bench-tested to determine performance characteristics, including pressure stability and
work of breathing. Each manufacturer's device performed differently at the same CPAP
setting on a mechanical test lung when given the same breathing patterns. It is important
for the clinician to understand how each CPAP device works as therapy may need to
be adjusted to achieve therapeutic objectives.
CPAP has been a widely accepted modality for treating
multiple respiratory conditions in the hospital since the
1970s. The ability to treat a patient noninvasively helps
to prevent the complications associated with intubation,
provide rapid improvement in oxygenation (SpO2), and
allow for easy initiation and discontinuation of therapy.
As in the hospital, CPAP – a type of noninvasive
positive pressure ventilation (NPPV) – has been gaining
popularity in EMS for nearly a decade. However, despite
numerous articles and literature supporting the efficacy
of CPAP in providing respiratory support, there are
still many EMS systems in the United States that have
not adopted NPPV as an option for treating respiratoryimpaired patients.
CPAP principles
CPAP is constant positive pressure applied throughout
the respiratory cycle to the spontaneously breathing
patient. With CPAP, the pressure in the lungs is always
positive and does not return to ambient pressure.
CPAP provides a specified amount of positive pressure
to the airways to act as a pneumatic splint in an effort
to open the alveoli, maintain airway patency and increase
functional residual capacity (FRC). This can accomplish
several objectives:
•Maintain airway patency and recruit alveoli that
may be full of fluid
• Improve oxygenation measured via pulse oximetry
•Decrease work of breathing (WOB) and
respiratory rate
•Decrease systolic blood pressure 2
CPAP applied noninvasively requires equipment capable
of generating sufficient flow to meet patient demand
and maintain stable pressure. CPAP is delivered through
a circuit and an appropriate mask interface.
In the pre-hospital environment, opinions vary on
when and how to apply CPAP. Each EMS system has its
own guidelines as to which respiratory conditions may
be treated with CPAP. In hospitals, oxygen for CPAP
is not a concern due to the availability of piped-in
oxygen systems. In EMS, however, oxygen supply is
limited and conserving oxygen is an issue. Other
pre-hospital clinical concerns relating to CPAP are
the patient’s WOB and the ability to maintain a stable
pressure throughout the respiratory cycle.
There are several commercially available CPAP
systems intended for EMS use. Each device has a unique
method of delivering oxygen CPAP therapy. The
Boussignac CPAP System uses the direct flow from
an oxygen cylinder to create pressure in the mask.
The PORTO2VENT ™ CPAPOS device features a demand
valve that delivers oxygen during inhalation only.
Therapy pressure for the WhisperFlow ® devices are
set by attaching a CPAP valve to the mask. A fixed
tension spring built into the valve regulates pressure
inside the mask. The objective of this testing was to
determine oxygen consumption times, patient work
of breathing, and pressure stability of these three
oxygen CPAP units commonly used in EMS.
Units tested
WhisperFlow, Respironics Inc.
•WhisperFlow Fixed CPAP Generator
(one unit tested)
•WhisperFlow Fixed Low Flow CPAP Generator
(one unit tested)
Boussignac CPAP System, Vitaid (two units tested)
Emergent Respiratory Products (two units tested)
To ensure impartial and accurate testing, each system
must be evaluated in the same way. On all of the
following tests, each device’s accompanying mask was
sealed to a thin sheet of clear plastic. The plastic sheet
was drilled to fit a pressure line adapter from which
pressure data was sampled. The pressure line was
also sealed to the plastic to prevent unintentional leak.
Each device and its accompanying mask was connected
to a Hans Rudolph Series 1101 breathing simulator.
Each device was tested under the same breathing
pattern, lung condition, and CPAP pressure to enable
direct performance comparisons. Devices were
compared on several performance characteristics
including oxygen consumption, pressure delivery
characteristics (deviation, stability), and patient WOB.
Oxygen run-out time on a full D-size oxygen cylinder was calculated by
operating each device at a common therapeutic pressure of 10 cmH2O.
The breathing simulator represented the patient.
Average time to run-out on a D-size oxygen cylinder
WhisperFlow Fixed
WhisperFlow Low Flow
Time (mm:ss)
Figure 1: Using a D-size oxygen cylinder, the Boussignac CPAP System had the shortest run time (13:11 minutes)
while the WhisperFlow Low Flow System had the longest run time.
Work of breathing (WOB)
In the context of this evaluation, WOB is a measure of
the energy a patient would expend while breathing on a
CPAP device that is not delivering ideal CPAP therapy.
Ideal CPAP is defined as delivering a fixed, stable
pressure throughout the breathing cycle. When pressure
is below set therapy pressure during inhalation or
above therapy pressure during active exhalation, these
conditions are considered work for the patient because
patient energy is required to overcome the differential.
The goal is to reduce WOB as much as possible for
the patient. Figure 2 illustrates this principle.
WOB values are reported in Joules per liter. For this
test, both pressures below baseline (during inhalation)
and above baseline (during exhalation) were used for
the calculation of the WOB value.
Figure 2: Imposed WOB is defined by the pressure time product
(PTP). PTP is made up of both P T and D TOT, where P T is the
pressure drop from baseline and D TOT is the total time spent
The same breathing patterns used in, and the resulting
data collected from, the pressure delivery characteristics
test (see page 5) were used for the WOB calculations.
below baseline. Anything that increases P T or D TOT increases
Average work of breathing per breath (Joules/liter)
Boussignac 1
Therapy pressure 5 cmH20
30 l/min
60 l/min
Therapy pressure 7.5 cmH2O
30 l/min
60 l/min
Therapy pressure 10 cmH2O
30 l/min
60 l/min
Boussignac 2
Low Flow
Figure 3: The WhisperFlow Low Flow and WhisperFlow Fixed devices had the lowest WOB values when compared to the Boussignac
CPAP System and the CPAPOS . The WhisperFlow device’s stable pressure delivery is the main contributing factor to the differences in
WOB values when compared to the other devices in this evaluation.
Pressure delivery characteristics
Each device in this evaluation was tested at CPAP therapy pressures of 5.0, 7.5 and
10.0 cmH2O. The parameters in the chart below were entered into the breathing simulator.
30 l/min
60 l/min
The next section presents the data collected for all
devices at a CPAP therapy pressure of 7.5 cmH2O,
60 l/min peak inspiratory flow and with no intentional
mask leak. Only 7.5 cmH2O of CPAP and 60 l/min are
displayed in the graphs below because substantially
equivalent results were seen at CPAP pressures of
5 cmH2O and 10 cmH2O, and at 30 l/min. The
numerical and graphical data presented below note
the following characteristics:
% Inhale
•Pressure deviation – the total peak-to-peak pressure
swing during a given breath
•Peak inspiratory flow – the peak flow value
during inhalation
•Total tidal volume (VT) – the total volume inhaled
•Input flow (patient demand) vs. actual flow –
Input Flow is the flow profile used by the breathing
simulator for each breath;
•Actual flow is the flow profile that is recorded while
the device is connected to the breathing simulator
•Pressure delivery profile – the pressures recorded
throughout a given breath
•Ideal CPAP – the ideal pressure delivery profile for
a given CPAP device (as marked by the red line in
each of the graphs on page 7 and 8)
Boussignac: 7.5 cmH2O therpay pressure
60 l/min peak inspiratory flow/no leak
Pressure deviation
BN 1: 4.5 cmH2O
BN 2: 3.9 cmH2O
Peak inspiratory flow
BN 1: 50.0 l/min
BN 2: 50.0 l/min
Total tidal volume
BN 1: 620 ml
BN 2: 626 ml
Figure 4: The actual flow (solid gray line) does not meet the input flow representing patient demand (dotted line). Pressure
is below 7.5 cmH2O throughout inspiration (A). During exhalation target pressure is above 7.5 cmH2O for the active part of
exhalation (B). Both the Boussignac 1 and the Boussignac 2 devices had identical performance.
CPAPOS : 7.5 cmH2O therapy pressure
60 l/min peak inspiratory flow/no leak
Pressure deviation
CPAPOS 1: 8.6 cmH2O
CPAPOS 2: 8.6 cmH2O
Peak inspiratory flow
CPAPOS 1: 55.1 l/min
CPAPOS 2: 64.7 l/min
Total tidal volume
CPAPOS 1: 564 ml
CPAPOS 2: 609 ml
Figure 5: The simulated patient needed to pull a negative 6 cmH2O to trigger oxygen delivery (A). There was a delay in
the onset of flow from the CPAPOS device: the arrow (B) indicates when the patient initiated the breath and the arrow
(C) indicates when flow from the CPAPOS device begins. Pressure remains below baseline for all of inspiration and above
baseline for the initial phase of exhalation. The CPAPOS 1 and CPAPOS 2 devices did not have identical performance.
WhisperFlow: 7.5 cmH2O therpay pressure
60 l/min peak inspiratory flow/no leak
Pressure deviation
WFL: 2.2 cmH2O
WFF: 2.5 cmH2O
Peak inspiratory flow
WFL: 59.2 l/min
WFF: 64.1 l/min
Total tidal volume
WFL: 724 ml
WFF: 704 ml
Figure 6: Pressure remained reasonably stable throughout the breath cycle (A). Actual flow and input flow (patient's
demand) are equal (B). The WhisperFlow Fixed and WhisperFlow Fixed Low Flow had identical performance.
This testing showed performance differences in three
CPAP devices that are currently used in EMS. Each
device had unique methods of delivering CPAP therapy.
When each device was tested, with the same simulated
patient characteristics, there were varying results.
In EMS, oxygen run-out time may be a consideration
in choosing a device. The faster a device depletes an
oxygen cylinder, the more cylinders will be required to
sustain a patient while CPAP is delivered. Depending
on transport time, the ambulance’s oxygen system may
be an alternative to compressed gas, however, this is
not always an option. EMS caregivers using the devices
tested in this evaluation should be aware of the oxygen
run-out times associated with use of each of these
The goals of a CPAP generator are to provide a
pressure curve that is as flat as possible. Achieving a
flat pressure curve means the device is maintaining
therapeutic pressure throughout the breath cycle and
reducing the patient’s work of breathing. Therapeutic
pressure and WOB are clinical issues that may have an
impact on patient care. These need to be known and
understood when selecting and using any CPAP device.
This evaluation observed notable differences in the
pressure delivery characteristics between the three
manufacturers’ devices tested. Persons administering
oxygen CPAP using any of these devices need to select
the device that best fits their patient’s therapeutic
CPAP is a valuable tool for providing respiratory
assistance to patients in the EMS environment.
The clinician needs to understand the application,
capabilities and limitations of the devices.
Understanding the CPAP devices’ performance
capabilities is also important because performance
of individual units can vary. This testing documented
the variability in static therapeutic pressure and work
of breathing between four commercially available
EMS CPAP systems from three manufacturers. More
research is necessary to determine if this variability.
Written by
Robert W. McCoy, BS, RRT, FAARC;
Ryan Diesem, BA
Valley Inspired Products, Apple Valley, MN
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