Shinshu M ed J, 61⑵ :65〜73, 2013

Functional Evaluation of Bubble CPAP for Neonates Using a Leak Model
Atsushi BABA , Ken SAIDA , Tomohiko NAKAMURA and Kenichi KOIKE
1） Department of Pediatrics, Shinshu University School of Medicine
2） Division of Neonatology, Nagano Childrenʼs HospitalObjective:We evaluated the inﬂuence of bias ﬂow magnitude and leak ﬂow on the provided CPAP level in thepresence or absence of a reservoir bag.
Methods:The 500 mL soft plastic reservoir bag was connected to the bubble CPAP system.The leak pathway
and a valve were prepared to simulate the function of the mouth. The pressure and ﬂow in the regions
corresponding to the lower airway (Pdistal) and the leak ﬂow were recorded simultaneously using a ﬂow
analyzer.
Results:At a bias ﬂow of 3 L/min,use of a reservoir bag resulted in a signiﬁcant decline of the pressure swing
magnitude at 60 and 90 breaths/min.A reservoir bag attenuated the rapid aspiration‑induced decreases of themean airwaypressure.When bubbling disappeared due to complete opening of the leak valve,the mean airway
pressure was maintained at approximately 2 cmH O. Additionally, the mean airway pressure increased withincreasing bias ﬂow even under 100% leak condition.
Conclusions:Even when a large leak occurs,a low but signiﬁcant airway pressure is loaded under moderatebias ﬂow. Inclusion of a reservoir bag in the bubble CPAP may oﬀer an eﬀective and inexpensive option forproviding respiratory support to preterm infants. Shinshu Med J 61 : 65 ―73, 2013
(Received for publication December 18, 2012;accepted in revised form January 8, 2013)
Key words:bubble CPAP, reservoir bag, ﬂow analyzer, breathing simulator, leak model

associated with a lower incidence of CLD . This

Backgroundevidence has led to an increase in the use of CPAP

Continuous positive airway pressure (CPAP) isoften used to provide noninvasive respiratory sup-as an alternative to intubation and mechanicalventilation.

port in neonates.CPAP allows better recruitment of

Bubble CPAP has been used since the 1970s .

alveoli, thereby increasing the functional residual

Because of its simple structure, handling of bubble

capacity, and improves oxygenation with less

CPAP is easier and it is less expensive than respira-imposed inspiratory work of breathing. The

tor‑derived CPAP.Therefore,several investigators

standard treatment for very preterm infants has

have recommended its use in developing coun-

been mechanical ventilation and surfactant since

tries

.

the 1980s . Nevertheless, the incidence of chronic

The pressure of bubble CPAP is determined large-

lung disease (CLD) has not decreased despite

ly by resistance of the expiratory circuit due to theimprovement of ventilation techniques.On the othersubmersion depth and ﬂow passing through an

hand,the early use of CPAP has been reported to beunderwater tube, and the leak ﬂow pathway. The
ﬂow passing through the underwater tube is deter-

Corresponding author:Atsushi Baba
Department of Pediatrics, Shinshu University School ofM edicine, 3‑1‑1 Asahi, M atsumoto, Nagano 390‑8621

mined by the bias ﬂow in the circuit and the leak
ﬂow. In a nasal CPAP system, leakage occurs

Japanthrough not only the junction with the nostril but

E‑mail:baba＠shinshu‑u.ac.jp

also the oral cavity. There have been several leakNo. 2, 201365

Baba.Saida.Nakamura et al.

Fig. 1 A schematic of the experimental system
The bubble CPAP system was connected to a breathing simulator. An underwater bubbler was set at 5
cmH O immersion depth. The breathing simulator was set at a tidal volume of 20 mL, and the respiratory
rate was set at 30, 60, and 90 breaths/min. The leak valve was prepared to simulate the function of themouth.A 60 mL syringe was prepared near the test lung to simulate the movement of hiccups.A 500 mL soft
plastic reservoir bag was connected with the humidiﬁer outlet. The pressure and ﬂow in the regions
corresponding to the lower airway and the leak ﬂow were recorded simultaneously using a ﬂow analyzer.
Endotracheal tube.

studies in neonates, using conventional constantsystem (Fisher & Paykel Healthcare, Auckland,

ﬂow CPAP or constant pressure/variable ﬂow

New Zealand)to a breathing simulator. The distal

CPAP.Nevertheless,eﬀects of circuit leak andconnecting tube (internal diameter, 10 mm) wasa reservoir bag on breathing parameters remain

immersed under the water seal to a depth of 5 cm.

unclear in neonatal bubble CPAP.

Nine diﬀerent ﬂows were tested,ranging from 1 L/

The aim of this study is to evaluate the inﬂuence

min to 9 L/min. The humidiﬁer outlet was con-of bias ﬂow magnitude and leak ﬂow on the pro-nected via a T piece to the 500 mL soft plastic

vided CPAP level. We also investigate whether the

reservoir bag. The breathing simulator was driven

addition of a reservoir can attenuate the imposedby cyclic negative pressure. The compliance of theinspiratory work of breathing induced by a low ﬂow

test lung in the cylinder was set at 1.0 mL/cmH O,

and can reduce the pressure swing induced by rapid

and total airway resistance from the Y piece to theinspiration, such as hiccups and sighs.

test lung was adjusted to 85 cmH O/L/s by chang-

M ethods

ing the length of an endotracheal tube (Portex,
Smiths M edical Japan, Tokyo, Japan ; internal

A schematic of the experimental system is shown

diameter, 4.0 mm). The two values were conﬁrmed

in Fig.1. A Y piece connected the bubble CPAP

using a pneumotachograph (LFM ‑317 Aivision

66

Shinshu M ed J Vol. 61

Functional evaluation of bubble CPAP for neonates

Laminar Flow Meter, M etabo, Lausanne, Switzer-

whether the humidiﬁer chamber aﬀected the pres-land). Airway occlusion pressure was 20 cmH O.

sure of the circuit. There was no Pmean diﬀerence

Expiratory ﬂow was calculated as 0.235 L/minin the presence or absence of the humidiﬁer cham-according to the passive expiratory ﬂow‑volume

ber probably due to its very hard structure.

method. The breathing simulator was set at a tidal

Statistical Analysis

volume of 20 mL and the respiratory rate was set at30, 60 and 90 breaths/min to simulate the lung

In Table 1 and 2,ﬁve breaths located in the middle

volume and mechanics of a non‑intubated neonate.

of the record were selected from each experimental

The leak pathway and a valve simulated the func-

series, and values were expressed as meantion of the mouth. To simulate hiccups, a syringe(Pmean)± SD (pressure swing magnitude).Analysis

placed near the test lung in the breathing simulator

was performed using SPSS v.18 (IBM corporation,

circuit was aspirated rapidly.The pressure and ﬂow

Chicago,IL,USA).To determine the signiﬁcance ofin the regions corresponding to the lower airway

the diﬀerence between two independent groups, weand the leak ﬂow were recorded simultaneously

used the unpaired t‑test or Mann‑Whitney‑U test

using a ﬂow analyzer (PF‑300, ImtMedical, Buchs,

when the data were not normally distributed. To

Switzerland), and were calculated using built‑incompare the diﬀerence of SD between the two

software (FlowLab , ImtM edical, Buchs, Switzer-

samples, we used the F ‑test. The level ofland). Data were collected using a computerized

signiﬁcance was deﬁned as a P ‑value of less thandata accumulation system at a sampling rate of 200

0.05.

samples/sec.
In preliminary experiments, we examined
Table 1 Eﬀects of bias ﬂow magnitude on mean airway pressure under no‑leak conditionVʼ
bias (L/min)
Pdistal (cmH O)

mean
SD

9

8

7

6

5

4

3

2

1

5.95 #
1.41

5.81 #
1.28

5.76 #
1.24

5.65 #
1.1

5.55 #
0.95

5.5 #
0.83

5.44 #
0.63

4.98
0.76

3.42 #
2.01

Five breaths located in the middle ofthe record were selected from each experimental series,and values were expressed
as mean ± SD.
The values were compared with those obtained by 2 L/min of bias ﬂow.
#Signiﬁcant diﬀerence via unpaired t‑test.
Signiﬁcant diﬀerence via F‑test.

Table 2 Eﬀects of a reservoir bag on pressure swing magnitude at 1 to 7 L/min of bias ﬂow rate under no‑leakcondition
Respiratory rate
(breaths/min)
30
60
90

Reservoir bag
(＋)
(−)
(＋)
(−)
(＋)
(−)

Vʼ
bias (L/min)
753

5.5±1.31
5.54±1.43
5.49 ±1.32
5.51±1.43
5.57±1.48
5.60±1.61

5.34±0.93
5.35±1.10
5.31±1.03
5.31±1.125.17±1.31
5.43±1.55

4.96±0.67 #5.25±0.7
4.55±1.15
4.55±1.39
4.41±1.35
4.35±1.97

12.80±1.37 #3.45±1.73
3.02±1.65 #2.78±2.34
3.25±1.82 #2.27±2.74

The Pdistal data (cmH O)of ﬁve breaths are expressed as mean ±SD.
#Signiﬁcant diﬀerence via unpaired t‑test.
Signiﬁcant diﬀerence via F‑test.

No. 2, 201367

Baba.Saida.Nakamura et al.

Results
A Eﬀects of bias ﬂow magnitude on mean airway

reservoir bag, rapid aspiration‑induced decrease of
the mean airway pressure was attenuated (3.03
cmH O). When the bias ﬂow was 1 L/min, rapid

pressure under no‑leak condition

aspiration caused negative mean airway pressure in

When the bias ﬂow was between 9 L/min and 2 L/

the absence of a reservoir bag (‑1.43 cmH O),

min, the mean delivered pressure was 5 cmH O or

whereas the mean airway pressure was 2.01 cmH O

higher (Fig. 2 and Table 1). It should be noted thatin the presence of a reservoir bag. On the otherthe diﬀerence between the mean delivered pressure

hand,a reservoir bag‑mediated decline of the mean

and the intended pressure was onlyapproximately1

airway pressure was inferior at a bias ﬂow of 5 L/

cmH O at a bias ﬂow of 9 L/min. When the biasmin (data not shown).

ﬂow was 1 L/min, the mean delivered pressure

DInﬂuence of leak ﬂow on the mean airway

decreased to 3.42 cmH O. In addition, bubblingpressure

disappeared and the aqueous column in the under-

We examined the eﬀects of leak on the meanwater tube ﬂowed back in the inspiratory phase.

airway pressure at 5 L/min of bias ﬂow. When the

B Eﬀects of a reservoir bag on pressure swing

leak valve was completely opened, bubbling

magnitude under no‑leak condition

disappeared,and the mean Vʼ
leak revealed approxi-

Under no‑leak condition, the Pmean and SD (asmately 5 L/min, as shown in Fig. 4. According topressure swing magnitude) of 5 breaths located in

the formula reported by Fischer et al. ,we deﬁned

the middle of the record were statistically analyzed

it as 100% of a leak. The airway pressure swing

in the presence or absence of a 500 mL soft reser-

decreased with increasing leak ﬂow.During moder-voir bag. The results are presented in Table 2.

ate leakage,intermittent bubbling and a suboptimal

There was no substantial diﬀerence in Pmean in themean airway pressure were observed. When bub-presence or absence of a reservoir bag at a bias ﬂow

bling disappeared due to complete opening of theof 3 L/min and a respiratory rate of 60 or 90

leak valve, the mean airway pressure wasbreaths/min.However,use of a reservoir bag result-maintained at approximately 2 cmH O.

ed in a signiﬁcant decline of the SD at 60 breaths/

E Eﬀects of bias ﬂow magnitude on the mean

min and 90 breaths/min. A reservoir bag‑mediatedairway pressure under 100% leak condition

decrease in the SD was larger at a bias ﬂow of 1 L/

We examined whether an increase in the bias ﬂow

min, as compared with the values at a bias ﬂow of

could secure the intended CPAP level of 5 cmH O

3 L/min.On the other hand,SD as well as the meaneven under 100% leak condition. As shown in Fig.

in the presence of a reservoir bag were similar to5,the mean airway pressure increased with increas-

the values in the absence of a reservoir bag at a bias

ing bias ﬂow. When the bias ﬂow was 9 L/min,

ﬂow of 7 L/min and a respiratoryrate of 30,60 or 90

approximately 5 cmH O mean airway pressure wasbreaths/min.

obtained.

C A reservoir bag attenuates rapid aspiration‑

Discussion

induced decrease of the mean pressure at a low
bias ﬂow under no‑leak condition

Oxygenation is tightly linked to the mean airway

To simulate hiccups,using a syringe,60 mL of airpressure applied to the lung. On the basis of thewas aspirated rapidly and synchronously with theresults reported by Kahn et al. , the mean airway

inspiration phase,and was returned to the circuit inpressure in bubble CPAP is generated mainly bythe

the expiratory phase. When the bias ﬂow was 3 L/

submersion depth of the CPAP generator and themin, rapid aspiration decreased the mean airway

bias ﬂow.They reported,using a static lung model,pressure to 1.21 cmH O in the absence of the reser-that the pressure delivered in a bubble CPAP sys-

voir bag, as shown in Fig. 3. In the CPAP with atem was68

2 cmH O higher than the immersion
Shinshu M ed J Vol. 61

Functional evaluation of bubble CPAP for neonates

Fig. 2 Eﬀects of bias ﬂow magnitude on mean lower airway pressure under no‑leak condition
Eﬀects of bias ﬂow rates ranging from 9 L/min to 1 L/min were examined under the condition of no leak.
Simulator on ;# Start CPAP ; Stop CPAP ; Simulator oﬀ.

Fig. 3 A reservoir bag attenuates rapid aspiration‑induced decrease of the mean airway pressure
To simulate hiccups,60 mL of air was aspirated rapidly and returned to the circuit synchronously with theinspiration phase and expiratory phase, respectively, using a syringe. The bias ﬂow was set at 3 L/min.

No. 2, 201369

Baba.Saida.Nakamura et al.

Fig. 4 Inﬂuence of a leak ﬂow on the mean airway pressure
We examined the eﬀects of leakage on the mean airway pressure. The leak was changed from 0% to100% using a leak valve.When bubbling disappeared,we deﬁned it as representing 100% of leak.The bias
ﬂow was set at 5 L/min.
continuous bubbling ; intermittent bubbling ; no bubbling.

Fig. 5 Eﬀects of bias ﬂow magnitude on the mean airway pressure under 100% leak condition
We examined whether an increase in the bias ﬂow could get close to the intended CPAP level of 5 cmH
O under 100% leak condition. The bias ﬂow was changed from 9 L/min to 1 L/min.

70

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Functional evaluation of bubble CPAP for neonates

depth of the expiratory tubing with an inner diame-

mediated decline of the mean airway pressure waster of 10 mm when the bias ﬂow was 8 L/min undersmaller at a bias ﬂow of 5 L/min. Accordingly, ano‑leak condition. The pressure overshoot became

bias ﬂow of 6‑9 L/min is generally used for

greater as the ﬂow magnitude increased.This pres-neonates with respiratory distress in the developed

sure overshoot was also observed in pretermcountries. Given the present results, use of a reser-

infants . In the present study, the diﬀerence

voir bag plus the bubble CPAP system may be

between the mean delivered pressure and theclinically anticipated to improve the inspiratory

intended pressure (5 cmH O) at a bias ﬂow of 9 L/

work of breathing imposed at a low ﬂow,particular-

min was only approximately 1 cmH O.M estriner etly in neonates with tachypnea. Additionally, it isal.

pointed out that the inner diameters of both

expected that a reservoir bag attenuates momen-

expiratory tube and air‑escape oriﬁce are impor-tary and sequential decreases of the mean airway

tant to obtain an exact airway pressure:the tubingpressure induced by hiccups at a low ﬂow.

must be 8 mm or larger in inner diameter, and the

Neonatal vital capacity is almost 40 mL/kg.In a

air‑escape oriﬁce should be 8 mm or larger. The

neonate with 3 kg of body weight, 120 mL of medi-

CPAP generator used in our study had an irrigation

cal gas was necessary in the appearance of a sudden

funnel on the cap, which might function as a

hiccup.Since several hiccups occur sequentially,we

suﬃcient air‑escape oriﬁce. Additionally, the innerset 500 mL as the capacity of the reservoir bag indiameter of the expiratory tube of the CPAP circuit

this study. In the case of an infant, a larger reser-

was 10 mm. Therefore, these two factors may

voir bag may be required.

account for the smaller diﬀerence between the mean

Unlike test lungs or intubated neonates,leaks aredelivered pressure and the intended pressure.When

clinically problematic for the employment of nasal

the bias ﬂow is set between 2 L/min and 9 L/min,

CPAP. In addition to leakage through the junction

the Fisher & Paykel Healthcare bubble CPAP sys-

between the device and the nostril, mouth opening

tem may safely support the breathing of neonatescauses a markedly high level of leakage . A chinwith closed mouths at rest.

strap has been used to close the mouth, but long‑

A reservoir bag is a traditional device to stabilize

term ﬁxation of the lower jaw is unfavorable forthe pressure in a respiratorycircuit .Nevertheless,neonates. The latest model of the CPAP generator

it remains unclear whether the addition of a reser-

can cope with varying leaks and adjust the bias ﬂow

voir can clinicallyattenuate the imposed inspiratory

to maintain the desired pressure for neonates using

work of breathing and reduce the pressure swing

a computer‑controlled feedback system.In develop-

induced by hiccups or sighs. In the present study,

ing countries,a low ﬂow setting is desirable to savethere was no substantial diﬀerence in Pmean in the

medical gas. In the present study,we examined thepresence or absence of a reservoir bag at a bias ﬂow

eﬀects on the airway pressure of the leak ﬂow

of 3 L/min and a respiratory rate of 60 or 90

produced by opening a leak valve. Although thebreaths/min.However,use of a reservoir bag result-airway pressure swing decreased with increasinged in a signiﬁcant decline of the SD at both 60

leak ﬂow, intermittent bubbling and a suboptimal

breaths/min and 90 breaths/min.On the other hand,mean airway pressure were observed during moder-

the mean and SD in the presence of a reservoir bag

ate leakage at a bias ﬂow of 5 L/min.M ean airway

were similar to the data in the absence of a reser-pressure of approximately2 cmH O was retained atvoir bag at a bias ﬂow of 7 L/min and a respiratory

a bias ﬂow of 5 L/min when the leak valve wasrate of 30, 60 or 90 breaths/min. In the CPAP withcompletely opened. This ﬁnding may be model‑

a reservoir bag,rapid aspiration‑induced decreases

dependent, because the possibility of the restingof the mean airway pressure were attenuated at apressure caused by the ﬂow resistance of the valve

bias ﬂow of 3 L/min, whereas a reservoir bag‑and the pneumotachograph can not be excluded.

No. 2, 201371

Baba.Saida.Nakamura et al.

Our ﬁndings maybe related to evidence that 5 cmH

without the reservoir bag suggests that the reser-

O of the set pressure at the prongs dropped tovoir bag does not inﬂuence leak volume.

approximately 2‑3 cmH O at the pharynx when

In conclusion, inclusion of a reservoir bag in thepreterm infants opened their mouths . Leakbubble CPAP may oﬀer an eﬀective and inexpen-

appears to be generated by a diﬀerence between

sive option for providing respiratory support toairway pressure and atmospheric pressure. The

preterm infants.

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