Br. J. Anaesth. (1989), 62, 340-347

EVALUATION OF THE AMBU CPAP SYSTEM

P. J. H. VENN

Continuous positive airways pressure (CPAP) isan established technique for improving arterialoxygenation when there is a mismatch of pul-monary ventilation and perfusion. This benefit isachieved largely by an increase in funtionalresidual capacity (FRC) [1-3]. However, there isrecent evidence to suggest that patients withventilatory failure may benefit from the decreasein inspiratory work associated with the use ofCPAP [1,4-6], and that CPAP may be a morephysiological form of management than mech-anical ventilation [1,7]. In patients with res-piratory failure, atelectasis may reduce FRC,thereby decreasing compliance. Restoration ofFRC towards normal increases compliance anddecreases airways resistance and inspiratory work[6]. In order to keep the work of ventilation to aminimum, little or no variation in airway pressureshould occur during breathing [1,5]. This is bestachieved with a high gas flow in the patient circuit[8], or by using a lower flow with a weightedbellows or a large volume-high compliance re-servoir bag [9-11]. These maintain the pressure inthe system by buffering the changes in flowduring inspiration and expiration.

DESCRIPTION OF APPARATUS

Ambu International Ltd of Denmark has in-troduced a system combining the advantages of ahigh flow in the patient circuit with a lowconsumption of fresh oxygen (fig. 1). The gas flowis achieved by a Venturi driven by oxygen at apressure of 3-4 bar, entraining air into a large (5-litre) compliant reservoir bag. This mixture maybe enriched further by additional oxygen throughan inlet on the bag mount. The gas mixture passesthrough a 1-m length of disposable corrugated

PETER J. H. VENN, M.B., B.S., F.F.A.R.C.S., The NuffieldDepartment of Anaesthetics, The Radcliffe Infirmary,Oxford OX2 6HE. Accepted for Publication: August 8, 1988.

SUMMARY

We have assessed a Venturi driven device fordelivering continuous positive airway pressure(CPAP) using a reservoir bag and expiratoryvalve under conditions of continuous flow andsimulated spontaneous breathing. The systemperformed well and was economical, consumingonly 3.5 litre min~' of fresh gas. One Venturi waspartially blocked and performed inadequately,but the function of a second one was close to themanufacturer's specification (inspired oxygen33%, flow 20 litre min~1 against end-expiratorypressures of 0-1.8 kPa). Compliance curves fortwo reservoir bags (new and old) were defined;these showed that compliance increased as thepressure in the circuit increased. The charac-teristics of the expiratory valve approached thoseof a threshold resistor. Small fluctuations inairway pressure occurred at all settings of CPAPand decreased with the increasing compliance ofthe circuit at higher values of CPAP. The methodprovided to monitor the airway pressure wasinaccurate and overestimated the true pressureby 20% at pressures greater than 1 kPa.

tubing (and humidifier if desired) to a patientY-piece. The expiratory limb of the Y-piece isconnected to an adjustable spring loaded valvewhich provides the pressure difference betweenthe circuit and the atmosphere. The valve consistsof a circular plate pressed against a plastic seatingby two springs. The mechanism is encased in aplastic housing. The pressure of gas in the systemlifts the plate against the springs and allows gas toescape through the orifice. As the flow across thevalve increases, the orifice increases also so that arelatively constant pressure is maintained in thecircuit. In the steady state the pressure created bythe compliance of the bag is sufficient to drive gas

AMBU CPAP SYSTEM 341

Patient tubingY- piece

A Auxiliary fresh gas inletB Buffer reservoirC Pressure scaleD One-way valve

G Mouthpiece (face mask)H PEEP valveI Venturi inlet

FIG. 1. Diagram showing the general arrangement of theAmbu CPAP system.

FIG. 2. The Ambu CPAP buffer reservoir bag showing thetransparent strap provided to measure the pressure.

at 20 litre min"1 across the valve to atmosphere.The CPAP is gauged by measuring the distensionof the bag using a transparent strap with agraduated scale of numbers which hangs infront of a single horizontal line drawn on the bag(fig. 2).

The manufacturers state that the system sup-plies fresh gas at a flow of 20 litre min"1 to thepatient circuit. It is capable of delivering CPAPbetween 0 and 1.8 kPa, with an FIO J of 0.33 (whenthe Venturi is driven by oxygen but with noauxiliary oxygen). The airway pressure shouldvary less than 0.2 kPa during quiet breathing. TheVenturi consumes oxygen at 3.5 litre min"1.

The instructions for use indicate that the

system, with the exception of the Venturi andreservoir bag, can be cleaned by any conventionalheat, chemical or gas sterilizing method. Thereservoir bag should not be autoclaved, and theVenturi does not require cleaning.

MATERIALS AND METHODS

The Venturi, reservoir bag and expiratory valvewere studied separately.

Analysis of the Venturi entrainment system

The device was assessed to see how the amountof CPAP would affect the total gas flow deliveredto the patient circuit. The Venturi was connectedto the hospital oxygen pipeline system at 4 bar.The patient Y-piece was replaced by a connectionwith a side arm attached to a spirit-filled mano-meter calibrated in mbar (0.1 kPa). Because of thedifficulty in collecting gas issuing from the CPAPvalve, the valve was replaced by a length of silastictubing with an adjustable gate-clip. This wasconnected to a dry gas meter (Singer DTM-115)to measure the total flow through the system atambient pressure. The time for 100 litres of gas topass at constant flow through the system atpressures of 0-1.8 kPa was recorded and the flowsexpressed in litre min"1. At each value of CPAPthree flow measurements were made; in addition,the mean concentration of oxygen was calculatedfrom three gas samples which were withdrawnfrom the system into a 100-ml oiled ground-glasssyringe and analysed using a paramagnetic oxygenanalyser (Servomex OA 101 Mkll).

Analysis of the compliance characteristics of thereservoir bag and system

To investigate the compliance characteristics ofthe whole system, the pressure in the system wasmeasured with the manometer as 100-ml in-crements of air were added from the ground-glasssyringe through a three-way tap. The Y-piece inthe patient circuit, the additional oxygen inlet tothe bag and the CPAP valve at the end of theexpiratory limb were sealed with rubber bungs(fig. 3). There is a one-way valve at the outlet fromthe bag mount directed into the patient circuitwhich prevents free communication between allparts of the system; this outlet was also sealedwith a bung and the circuit connected to the bagmount via the Venturi inlet port and a double sidearm for the attachment of the syringe andmanometer. The whole system was tested for

342 BRITISH JOURNAL OF ANAESTHESIA

normal patient Venturicircuit outlet ..inlet

Spirit filledmanometer

100-ml syringe/ ,

Rubber bung/

Rubber bung

FIG. 3. Diagram showing the method used to measure the compliance of the reservoir bag and system(see text).

leaks using soap solution before the pressuremeasurements were made. To see if the com-pliance of the bag changed with age and use, a newbag and a bag which had been pressurized duringuse for several hours were tested. The readingtaken from the transparent strap on the side of thebag was compared with the manometer pressure.

Analysis of the Ambu CPAP valveThe pressure across the valve was measured at

different flows to see whether the valve functionedas a flow or as a threshold resistor. The manometerand CPAP valve were placed in series with acalibrated flow meter delivering air up to 100 litremin"1. The valve was adjusted to give a pressure of0.5 kPa at 20 litre min"1 flow and at that setting aseries of pressure measurements was made as theflow was increased from 10 to 70 litre min"1 in 10-litre min"1 steps. Another set of readings wastaken as the flow was reduced in stepwise fashionfrom 70 to 10 litre min"1. The process wasrepeated for pressure settings of 1.0, 1.5 and2.0 kPa.

In addition, measurements were made to dem-onstrate the effect of gravity on the charac-teristics of the valve. First, the valve was set togive a pressure of 1.0 kPa at a flow of 20 litremin"1 whilst held with its axis horizontal. Thepressures were measured as the flow was changedstepwise between 10 and 70 litre min"1 as above,

but with the distal end of the valve pointingupwards, horizontally or downwards.

Pressure variation during the ventilatory cycle

In order to examine the pressure fluctuationswhich occur whilst breathing at different values ofCPAP, the system was connected to a lung modelwhich simulates spontaneous ventilation. Flow,tidal volume and pressure in the bag and at the Y-piece were measured during the ventilatory cycle.A mechanically operated lung simulator wasconstructed to ensure a similar pattern of in-spiratory and expiratory flow at all values ofCPAP. It consisted of a pair of spring loadedbellows mounted in parallel on a rigid frame, andtherefore linked mechanically. One bellows wasdriven by a ventilator (Penlon Nuffield Series200), whilst the other bellows acted as the lungmodel for spontaneous ventilation. A resistance of2.0 kPa litre"1 s"1 was placed in series with thelung bellows (fig. 4). A flow approximating to asquare wave was generated during inspiration,with a decreasing flow in expiration. The Y-pieceof the CPAP circuit was attached to the lungmodel through a Fleisch No. 1 pneumotachographand the pressure across it was measured with adifferential pressure transducer (Validyne) toderive flow. Electrical integration of the flowsignal (Morgan Respiration Integrator Mkll)gave tidal volume. The pressure in the circuit was

AMBU CPAP SYSTEM

— Pressure

343

<J

Pressure.

Vol.r

Return springs

Penlon NuffieldSeries 200 Ventilator

.Fresh gas supply

FIG. 4. Flow diagram showing how the system was connected to the simulated lung and monitoring(see text).

measured with a pressure transducer (Gould)connected between the Y-piece and the pneumo-tachograph, whilst the pressure in the bag wasmeasured simultaneously through the auxiliaryfresh gas port. The four signals were displayed ona chart recorder (Electromed MX 19).

Calibration of the pressure and volume signalswas carried out with the manometer and a 1-litresyringe, and the flow signal was calibrated with airfrom a flowmeter. The lung simulator wasadjusted to give tidal volumes of 500 ml at a rateof 12 b.p.m. with an inspiratory: expiratory timeratio of 1:2. Inspiratory flow was approximately15 litre min"1 with a peak expiratory flow ofapproximately 40 litre min"1 increasing to 50 litremin"1 at greater values of CPAP as the bellowscompliance decreased. At each 0.2-kPa incrementbetween 0 and 1.6 kPa, the CPAP valve wasadjusted to give the required airway pressure at aconstant flow with the lung simulator "switchedoff". During simulated spontaneous breathing,recordings were made of the fluctuation ofpressure in the bag and at the Y-piece, with theflow and volume changes.

RESULTS

Characteristics of the Venturi and bag

The first Venturi studied (A) delivered a lowerthan expected flow, because of a partial blockagein the jet. A second Venturi (B) was testedsubsequently. When CPAP was increased from 0

to 1.8 kPa using Venturi A, there was a meanreduction of gas flow through the patient circuit of7.20 litre min"1 (from 18.69 litre min"1 to 11.49litre min"1) and the inspired oxygen concentrationincreased from 31.0% to 34.7%. For Venturi B,the flow was 20.33 litre min"1 at 0 kPa, decreasingto 18.69 litre min"1 at 1.8 kPa. The inspiredoxygen concentration increased from 35.1% to36.2%.

As the pressure increased, the strap readingoverestimated the actual pressure (table I).

As the volume of the two bags increased,compliance increased progressively and there wasvirtually no difference in compliance between thenew and used bags (fig. 5).

1 2Pressure (kPa)

FIG. 5. Compliance of the system. •—• • = worn bag.

- • = New

344 BRITISH JOURNAL OF ANAESTHESIA

TABLE I. The characteristics of the Ambu Venturi with increasing pressure in the airway. Two Venturiswere assessed after Venturi A was found to have a partially blocked orifice causing the air entrainment tobe lower than expected. The strap on the side of the reservoir bag was set intially to zero when the mano-

meter reading was zero. Each value shown is the mean of three readings

Strap reading

Venturi AFlow in circuit (litre min"1)Oxygen delivered (%)

Venturi BFlow in circuit (litre min"1)Oxygen in circuit (%)

0

0

18.6931.00

20.3335.10

Manometer reading (kPa)

0.5

0.5

17.3431.75

19.8635.25

1.0

1.2

13.7632.50

19.4235.50

1.5

1.7

12.2632.85

18.8635.93

1.8

off scale

11.4934.70

18.6936.20

TABLE II. The pressure decrease (kPa) measured across the Ambu CPAP valve. The valve was adjustedto each setting with a flow of 20 litre min'1, and the flow was then varied between 10 and 70 litre min'1

while the actual pressure change was measured with the manometer. Measurements were made as flow wasincreased to 70 litre min'1 (up), and again as flow was decreased to 10 litre min'1 (down)

(litre min"1)

10203040506070

Up

0.480.500.540.580.610.610.64

]

0.5

Down

Pressure set at 20

1

Up

0.48 0.990.500.540.550.570.60

1.0.03.07.07.08.06

0

Down

0.980.991.011.041.051.05

litre min"

1.

Up

1.481.531.551.571.601.611.59

(kPa)

5

Down

1.481.521.531.551.581.58

Up

1.952.002.012.012.042.082.09

2.0

Down

1.941.992.002.002.032.07

Characteristics of the Ambu CPAP valveThere was a slight increase in pressure as flow

across the valve increased. This was least whenthe valve was set at 1 kPa (table II). There wasalso a hysteresis between pressures measuredwhilst flow was increased and decreased, thepressure being slightly lower during the de-creasing phase. The pressure was consistently0.16 and 0.18 kPa greater at all flows when thevalve was held outlet-upwards, compared withoutlet-downwards (table III).

Pressure variation during the ventilatory cycle

At a constant flow, the pressure in the bag (PB)was 0.1 kPa greater than at the Y-piece (PY) at allvalues of CPAP (table IV). When 500-ml tidalvolumes were inspired by the simulated lung, PYand PB fluctuated, decreasing to less than their setvalues during inspiration and increasing abovethem during expiration. As CPAP increasedbetween 0.2 kPa and 1.6 kPa, the fluctuation of

PY declined from 0.4 kPa to 0.16 kPa. Thefluctuation in PB decreased from 0.16 kPa to0.07 kPa between 0.2 kPa and 1.6 kPa CPAP. At1.6 kPa CPAP, the increase in PB during ex-piration disappeared, and the fluctuation resultedonly from the decrease in pressure during in-spiration (fig. 6).

DISCUSSION

VenturiAlthough large compliant bags and adjustable

valves have been used to deliver CPAP, theyrequired a supply of 20-30 litre min"1 of fresh gasto buffer the effects of the ventilatory cycle. Incontrast, this Venturi driven system uses only 3.5litre min"1 of oxygen, but still supplies 20 litremin"1 of gas to the patient circuit. One of its majorfeatures is that it is more economical than manyother devices. Increasing the CPAP decreased thepressure change across the Venturis and this

AMBU CPAP SYSTEM 345

TABLE III . Variation in pressure decrease measured across the Ambu CPAP valve with the valve in the hori-zontal, distal end upwards and distal end downwards positions. The valve was set to 1.0 kPa with a flow20 litre min'1 whilst in the horizontal position. Measurements of actual pressure change were made using

the manometer at flows between 10 and 70 litre min~l

Pressure decrease across CPAP valve (kPa)Gas flow

(litre min"1) Upwards Horizontal Downwards Max. diff.

203040506070

1.111.131.141.161.151.16

1.001.001.031.041.071.09

0.940.950.960.980.981.00

0.170.180.180.180.170.16

TABLE IV. Fluctuation of pressure in the bag (PB) and at thepatient Y-piece (PY) recorded throughout the ventilatory cycleas CPAP was increased from 0.2 kPa to 1.6 kPa. PV includes

the initial transient "opening" pressure of the valve

PY(kPa)PB (kPa)

0.2

0.400.16

CPAP

0.6

0.230.10

(kPa)

1.2

0.300.08

1.6

0.160.07

0.5—

1

4-

1

\

h

i\j

K

I Q .

» •

-

-

-

-

1•+-

1•

•V

-

v

V

1 1

-

• -

• -

- _

^ _

- -

FIG. 6. Traces obtained during simulated spontaneous breath-ing at 0.2, 0.6, 1.2 and 1.6 kPa CPAP. The pressure wasset at the Y-piece. The zero was offset on the PY and PBtraces to record pressures at 1.2 kPa and again at 1.6 kPa.

Inspiratory flow is negative on the trace.

reduced air entrainment and total gas flow in thecircuit. Thus the oxygen: air ratio increased withincreasing CPAP. Venturi B exceeded the manu-facturer's claim of 33 % oxygen in the circuit at allvalues of CPAP. The flows obtained from VenturiA were much less than expected because of apartial blockage of the Venturi orifice, causing alow oxygen flow and reduced air entrainment. Wepresume that this was caused by dirt from thehospital pipeline, but it is possible that it was amanufacturing fault. In clinical use, reduced flowswould remain unnoticed. Although the entrainedair passes through a sintered brass filter, there isno filter before the Venturi jet to prevent silting.As the instructions state, the Venturi cannot bedisassembled for cleaning. In addition, the Ven-turis were rather noisy.

Reservoir bag

The Ambu CPAP bag is constructed from latexand its compliance increases with increasingCPAP. Whilst the bags appear to withstand thepressures at which they would be used clinicallywithout changing their compliance charac-teristics, extrapolation of the compliance curve(fig. 5) indicates that at approximately 2.3 kPaprogressive distension and bursting of the bagwould occur without any further increase inpressure. This makes it impossible to generatedangerously high pressures in the patient's airwayif the outlet from the circuit becomes obstructed.

CPAP valveThe Ambu CPAP valve acted as a reasonably

good threshold resistor, because the resistancedecreased as the flow across the valve increased.The Ambu valve performed better than someother designs [12,13] as the pressure decreaseacross the valve increased only a little as the flow

346 BRITISH JOURNAL OF ANAESTHESIA

increased. However, the pressure was greaterwhen it was held with its distal end upwards thandownwards. This is because gravity acts with thespring in the former position, but against thespring when it is held end downwards. Toovercome this, there is a plastic loop on top of thebag-mount to hold the valve in the horizontalposition. The pressure change across the valve ateach level of flow was higher when increasing theflow in incremental steps than when decreasing it.This hysteresis is characteristic of stress de-formation [14], but is seen more often withsynthetic materials than with metals.

Airway pressure variation during the ventilatorycycle

In any system where there is airflow resistancein the breathing circuit, the airway pressuredecreases during inspiration and increases duringexpiration. However, if there is a continuous flowin the circuit greater than the patient's inspiratoryflow, the resistance sensed by the patient duringinspiration is negligible [15] and the charac-teristics of the CPAP valve govern the pressurechange. If the valve acts as a pure thresholdresistor, airway pressure during inspirationshould not decrease until inspiratory flow exceedsthe continuous flow in the circuit. When thisoccurs, the pressure in the circuit is maintained bythe elastic properties of the bag, and decreases asgas in withdrawn from the bag according to itscompliance characteristics. Further, during ex-piration, airway pressure should not increaseabove the value of CPAP set by the valve if it hasthreshold characteristics.

PY and PB deviated from their settings eventhough inspiratory flow did not exceed thecontinuous flow from the Venturi. The valve istherefore not a true threshold resistor, and the bagis necessary to buffer the change in pressure bysupplementing the inspiratory flow. Because ofthe shape of the compliance curve of the reservoirbag, this effect might be expected to decrease asCPAP increases [11] (fig. 5), and the studyconfirmed this (table IV, fig. 6). PY decreasedleast during inspiration when the CPAP valve wasset at approximately 1 kPa (fig. 6), when thethreshold properties of the valve are most efficient(table II).

During expiration, the characteristics of the bagare not relevant because, as soon as the pressure inthe circuit exceeds that in the bag, the one-wayvalve closes and diverts all the gas from the

Venturi into the bag, and all the expired gas outthrough the CPAP valve. Following a hightransient, expiratory pressure increases to thatexpected during peak expiratory flow (table II).The initial high transient appears to be a result ofinertia in the valve. Expiratory work is related tothe area under the expiratory pressure curve. Theadditional work required to overcome this open-ing pressure is therefore the area enclosed by thistransient peak, and is probably not significantclinically compared with the total work ofexpiration.

The pressure measured in the bag was con-sistently 0.1 kPa greater than that measured at theY-piece, because of the resistance of the one-wayvalve at the connection of the circuit to the bagmount and that of the circuit (approximately 0.3kPa litre"1 s"1). Although the system was assessedwithout a humidifier, this would affect thepressure fluctuations during quiet breathing byadding further resistance to the circuit.

In conclusion, this system is well designed andperforms adequately. Its economical consumptionof fresh gas makes it suitable particularly for long-term use in the intensive therapy unit.

ACKNOWLEDGEMENTS

I wish to thank Dr L. Loh and Professor M. K. Sykes for theiradvice, and Dr G. Abbondati, Mondial Assistance U.K., forhis contribution.

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AMBU CPAP SYSTEM 347

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