Examiner’s finger-mounted fetal
tissue oximetry
Naohiro Kanayama
Masatsugu Niwayama

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Journal of Biomedical Optics 19(6), 067008 (June 2014)

Examiner’s finger-mounted fetal tissue oximetry
Naohiro Kanayamaa,* and Masatsugu Niwayamab,caHamamatsu University School of Medicine, Department of Obstetrics and Gynecology, Hamamatsu 431-3192 Japan
Shizuoka University, Department of Electrical and Electronics Engineering, Hamamatsu 431-3192 Japanc
Shizuoka University, Research Institute of Electronics, Hamamatsu 431-3192 Japanb

Abstract. The best way to assess fetal condition is to observe the oxygen status of the fetus (as well as toassess the condition of infants, children, and adults). Previously, several fetal oximeters have been developed;
however, no instrument has been utilized in clinical practice because of the low-capturing rate of the fetal oxygen
saturation. To overcome the problem, we developed a doctor’s finger-mounted fetal tissue oximeter, whose
sensor volume is one hundredth of the conventional one. Additionally, we prepared transparent gloves. The
calculation algorithm of the hemoglobin concentration was derived from the light propagation analysis basedon the transport theory. We measured neonatal and fetal oxygen saturation (StO2 ) with the new tissue oximeter.
Neonatal StO2 was measured at any position of the head regardless of amount of hair. Neonatal StO2 was foundto be around 77%. Fetal StO2 was detected in every position of the fetal head during labor regardless of thepresence of labor pain. Fetal StO2 without labor pain was around 70% in the first stage of labor and around 60%
in the second stage of labor. We concluded that our new concept of fetal tissue oximetry would be useful for
detecting fetal StO2 in any condition of the fetus. © The Authors. Published by SPIE under a Creative Commons Attribution 3.0
Unported License. Distribution or reproduction of this work in whole or in part requires full attribution of the original publication, including its DOI.
[DOI: 10.1117/1.JBO.19.6.067008]

Keywords: fetal tissue oximetry; finger pulp; examiner; near-infrared spectroscopy; parturition.
Paper 130788RR received Nov. 5, 2013; revised manuscript received May 13, 2014; accepted for publication May 16, 2014; published
online Jun. 24, 2014.

1

Introduction

Fetal heart rate (FHR) monitoring is a popular worldwide
method to evaluate the condition of the fetus. The best approachto diagnose the fetal condition is to measure the fetal bloodpressure and oxygen status, which at the moment, cannot be
achieved noninvasively. The FHR is the gold standard in evaluating the fetal condition, however, the FHR monitoring has
shown low false-negative and high-false positive rates. Thus,
obstetricians have attempted to measure the fetal oxygen status
instead of or in addition to the FHR monitoring. As a result,
there have been many reports on the development and clinical
trials of the fetal pulse oximetry.
The pulse oximetry enables us to determine the degree ofoxygen saturation both noninvasively and continuously bymeasuring pulsating wave absorbance in the peripheral tissues,
such as fingertips. Some researchers have reported that the oxygen dynamics can be measured over the course of delivery using apulse oximetry probe applied transvaginally to the fetal head.1–3
Moreover, the intravaginal optical probe for tissue oximetry using
the near-infrared spectroscopy (NIRS) has been developed.4,5
However, the sensor volume of 4 cm3 (diameter 1 cm, length
5 cm) was too large to apply during delivery. The dimensionsof the commercially available small optical probes for the brain(NIRO-200NX, Hamamatsu Photonics, Hamamatsu, Japan)
and the muscle (Hb-14, ASTEM, Kawasaki, Japan) were1 × 3 × 0.5 cm (width, length, and thickness). The sensors’ volume of 1.5 cm3 was too large for monitoring the fetus directly.
Additionally, the significantly low data capturing rate is one of*Address all correspondence to: Naohiro Kanayama, E-mail kanayama@hama-med.ac.jp

Journal of Biomedical Optics

the biggest drawbacks to these procedures. East et al. havereported that during the active phase of labor, every method
has a significant rate of signal loss and the loss of sensor contact
occurred up to 64% of time with oximetry.6 Another drawbackof this transvaginal procedure is that the probe often moves
away from the fetal head during delivery, which may lead toan intrauterine infection caused by rupture of the gestational
sac. Furthermore, these methods are essentially invasive,as the probe is inserted into the uterine cervix only after the
rupture of the gestational sac. In addition to these problems,many patients feel discomforted by the intravaginal sensor.
Obstetricians also feel that there is interference with the cervicalexamination by the sensor. Although many clinical trials of the
fetal pulse oximetry have been performed, its clinical usefulness
has been controversial. Thus, there are no established methods
for detecting the fetal hypoxia during parturition yet. At present,
many obstetricians look forward to further improvement ofoximetry and a better understanding of clinical management.
We changed the conventional placement of sensors to overcome those problems. We proposed an idea of moving the sensor attachment from the fetus to the examiner. Namely, we
attached an NIRS probe to the doctor’s finger, instead of the
fetal head or cheek. The obstetricians and midwives need toperform a cervical examination to understand the progress ofdelivery. They need to touch the fetal head to detect any cervicaldilatation and the fetal position during delivery. Therefore, wehypothesized that if an NIRS probe was placed on the examiner’s finger pulp and the appropriate calculation algorithms on
the near field of the light source were developed, we could
obtain the fetal oxygen status noninvasively. With this concept,
we succeeded in attaching an NIRS probe to the examiner’s
finger pulp. Here, we report our intruments and preliminary
clinical data.

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June 2014 • Vol. 19(6)Kanayama and Niwayama: Examiner’s finger-mounted fetal tissue oximetry

2

Instrumentation

An NIRS probe consists of near-infrared light emitting diodes(LEDs) and photodiodes. The source-detector distance was
4.0 cm in accordance with the Nelcore’s fetal pulse oximetry(Nellcor N-400 system), which has been used in many clinical
trials. We considered that if the distance could be shortened toaround 1 cm, it would be possible to place the sensor on theexaminer’s finger. Our previous study showed that the strong
signals of the arteries in the fetal scalp and brain surface
were obtained at the point of 10 mm of the source-detector separation.7 Therefore, we thought that the miniaturized sensor
would detect a fetal oxygenation signal by (1) deriving the calculation algorithm of the hemoglobin concentration satisfied in
the near field of the light source, (2) blocking light propagation
to the examiner’s finger to prevent it from mixing with theunwanted finger oxygenation, and (3) manufacturing an ultrathin optical probe which fits to the finger pulp. Fortunately,
we successfully developed a new small volume sensor witha polyimide-based flexible substrate. The wavelengths of thelight sources were 770 and 810 nm, and sensitivity of the photodiodes (PD2501, Kyoto, Epitex) was high in the near-infrared
band. The bare chips of the LEDs and photodiodes were
mounted on the substrate with a wire bonding. The detectors
were located 6 and 8 mm away from the LEDs in order to determine an absolute value of the hemoglobin concentration using
the spatially resolved NIRS [Fig 1(a)], which was created basedon a laboratory-made low noise instrument.
We placed a 1-mm thick black rubber sheet between the optical probe and finger pulp to block the light scattered throughthe examiner’s finger. Then, the probe was fixed with an adhesive sheet on the black-colored finger sac. The optical elements
integrated with a flexible cable (80-μm thick, 100-mm long)
were connected to a soft shield cable on the examiner’spalm. A 1.5-m long shield line was connected to the microcomputer unit in the examiner’s breast pocket. In order to reduce
external noise, the unit was battery powered and had a
2.4 GHz wireless module. The measured data were wirelesslytransmitted into the computer. The trend of the hemoglobin
concentration processed8 and calculated by the computer was
shown on the screen in real time.
We prepared transparent vinyl gloves in collaboration with
Utsunomiya Seisaku Co., Ltd. (Higashi Osaka-shi, Japan). The
same amount of scattered light as without gloves was detectedwith the gloves. Figure 1(b) shows the examiner’s finger withthe developed optical probe.

3

Theoretical Analysis

We examined the light propagation in the fetal head model. The
simulation model consists of the fetal scalp, skull, cerebrospinal
fluid (CSF), brain, optical block, and examiner’s finger. Table 1
shows the scattering coefficient, absorption coefficient, andthickness, which were set based on the literature data.9–11
The anisotropic factors for each layer were 0.95.12 The
propagation of photons in the simulation was based on the transport theory with the Monte Carlo method. The model was
divided into small cubes, and the optical path lengths in eachcube were calculated to examine the measurement sensitivity.
Figure 2 presents the sensitivity distribution obtained fromthe simulation results.
The results suggest that the light propagated to the finger
does not contribute to the detected light intensity. In the simulation, the absorption coefficient of the optical block attached to
the finger was 10 mm−1 . The optical absorber of the actual
probe was easily realized because a typical colored adhesive
tape has an absorption coefficient from 1 to 20 mm−1 . Thechanges in spatial intensity slope were examined to reveal
the measurement sensitivity for the brain and the scalp. The
slope values of S0 , Sbrain , and Sscalp represent an initial state(μa brain ¼ 0.02 mm−1 , μa scalp ¼ 0.02 mm−1 ), a state where
only the brain has changed (μa brain ¼ 0.025 mm−1 , μa scalp ¼
0.02 mm−1 ), and a state where only the scalp has changed(μa brain ¼ 0.02 mm−1 , μa scalp ¼ 0.025 mm−1 ), respectively.
Table 1 Optical properties and thickness of each layer for Monte
Carlo analysis.

Fig. 1 Scheme (a) and photograph (b) of the oximeter probe attachedto the examiner’s finger pulp. Flexible substrate mounted two LEDs
and two photodiodes are adhered to a black rubber sheet.

Journal of Biomedical Optics

TissueScattering
coefficientμs (mm−1 )Absorption
coefficientμa (mm−1 )Thickness
(mm)

Scalp

26

0.00–0.050

1.5

Skull

40

0.010

1.5

Cerebrospinal
fluid

8

0.002

0.4

Brain

32

0.00–0.050

100.0

Finger

26

0.020

10.0

Optical block

20

10.0

0.4

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June 2014 • Vol. 19(6)Kanayama and Niwayama: Examiner’s finger-mounted fetal tissue oximetry
λ1
λ1 λ2
ελ2
Hb μa − εHb μa
;
λ1
λ2
λ1
εHbO2 εHb − ελ2
HbO2 εHb

(1)

λ1
λ1
λ2
ελ2
HbO2 μa − εHbO2 μa;½HHb ¼ − λ1λ2
λ2λ1εHbO2 εHb − εHbO2 εHb(2)½O2 Hb ¼

λ1;2
λ1;2
where εHHb
and εO2Hb
are the extinction coefficients of HHb
and O2 Hb, respectively, at the wavelengths λ1 and λ2.13
Tissue oxygen saturation StO2 was calculated by ½O2 Hb∕
ð½O2 Hb þ ½HHbÞ.

Fig. 2 Model for the Monte Carlo analysis and the sensitivity distribution obtained by the simulation.

The changes in slope were calculated as follows: ΔSbrain ¼
Sbrain − S0 and ΔSscalp ¼ Sscalp − S0 . The sensitivity ratio wasdefined as ΔSbrain ∕ΔSscalp . The ratio became high due to anincrease in the source-detector distance, as shown in Fig. 3.
When the detectors were located 6 and 8 mm from the lightsource, the sensitivities for the brain and the scalp were comparable. If the hemoglobin concentration of the brain is higher
than that of the scalp, the values of the oxygen saturation
and blood volume would contain more information about thebrain than the scalp. Although the long source-detector distance
was preferable in order to focus on measuring the cerebral
tissue, we adopted the length of the 6 and 8 mm pair, considering convenience due to the size reduction.
In this study, the hemoglobin concentrations of both brain
and scalp were measured. Therefore, we simultaneously varied
the values of μa brain and μa scalp from 0.00 to 0.05 mm−1 . Then,
we made a lookup table of the relationship between the value of
μa and the spatial slope. In the measurement, the value of μa wascalculated from the spatial slope of the measured light intensity
using the lookup table. The following equations were used to
calculate the concentrations of oxyhemoglobin [O2 Hb] anddeoxyhemoglobin [HHb]:

Fig. 3 Relationship between the brain-scalp sensitivity ratio andthe pair of source-detector distances.

Journal of Biomedical Optics

4

Clinical Experiments

To determine whether our tissue oximetry obtained the fetal
oxygenation, we measured StO2 of three just delivered newborns at first. All the neonates were born in spontaneous vaginal
delivery from 39 to 40 weeks of gestation. The FHR monitored
during labor showed a reassuring fetal status pattern in all cases.
The Apgar scores of the neonates were 9, 9, 10 at 1 min and 10,
10, 10 at 5 min. We attached the probe on the front hair, parietal,
and occiput of head just after delivery. Each measurement lasted
at least 10 s.
Second, we measured fetal StO2 during labor, as shown in
Fig. 4. We measured StO2 for one parturient at the first andthe second stages of labor after the rupture of membranes.
The parturient was in her 39th week of pregnancy. The courseof delivery was normal. The probe was attached to the fetus head
during delivery. We examined the results for 2 min during thefirst and second stages of labor. We continued the measurementfor 10 min after delivery. Informed consent was obtained fromthe subjects, and all measurements were approved by the ethicalcommittee of Hamamatsu University School of Medicine.
The StO2 values of the parietal region in the three newborns
were 78.5  2.6%, 78.4  3.2%, and 75.8  2.3%, respectively.
The values of front and occiput were similar to those of the parietal region in each newborn (data not shown). We could obtain
the StO2 easily in any condition of the neonate head, such as
wetness or color of the hair.
The intrapartum fetal StO2 was detected through the whole
delivery (Fig. 5).
The fetal StO2 was detected even during labor pains and thebearing down effort. We obtained the StO2 data smoothly andquickly. The doctor got the fetal StO2 as well as the cervicalfindings at the same time. Furthermore, the patient felt neither
pain nor discomfort during the measurement. The fetal StO2 in

Fig. 4 Outlook of the measurement with an oximeter attached tothe examiner’s finger during labor.

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June 2014 • Vol. 19(6)Kanayama and Niwayama: Examiner’s finger-mounted fetal tissue oximetry

Fig. 5 The actual chart of the fetal StO2 during labor. Left and rightparts correspond to the first and the second stages of labor, respectively. UC denotes uterine contraction.

the first stage of labor was about 70% without labor pains andabout 60% in a presence of labor pains. In the second stage oflabor, the fetal SpO2 was 60% without labor pains, and it
decreased during labor pains and the bearing down effort.
The lowest level of the fetal StO2 was around 30%. After delivery, the neonate StO2 increased rapidly up to 70%.

5Discussion

All previous oximeter sensors were attached to the fetus. We
have recognized the accidental possibility of the movementof the sensor and its slip during labor. Poor data capture wasobserved in some pregnant women due to the detachment of
the sensor caused by frequent changes of the body position
during labor.
Our oximetry system is able to detect the fetal StO2 in thefirst and the second stages of labor very precisely regardless ofthe presence of labor pains. As the examiner can touch suitablesites during the internal examination, it is possible to measure
the fetal StO2 very naturally. Our fetal StO2 measurements canbe performed at the same time and in the same manner as obstetrical examinations during delivery. We recognized the fetal head
promptly and obtained the fetal StO2 precisely, because the pressure for an ultrathin probe propagated to the finger and the finger
pulp was very sensitive. Furthermore, our sensor never touches
the fetus directly because the sensor is inside the transparent
sterilized gloves. We think that our measurement system issafer and more noninvasive than the previous one. Moreover,the parturient does not feel discomfort during the measurement.
As for the clinical analysis, the fetal StO2 in the first stage oflabor was about 70% without labor pains and about 60% in thepresence of labor pains. The fetal oxygen saturation SpO2 with apulse oximeter is 59  10% in the first stage of labor, and 53 
10% in the second stage.2 The measurements of low saturation
were estimated at 33% as two standard deviations below themean. Since the decrease in arterial oxygenation rapidly resultsin low tissue oxygenation, the value of StO2 would be correlated
to SpO2 strongly. Chipchase et al. have reported that the cerebral
StO2 for a fetus was 59  12% using an optical fiber-based
NIRS instrument.14 The oxygen saturation values in thisstudy were the same or slightly higher compared to the previous
studies. One of the reasons would be that the value of StO2 was amixed measurement of the scalp and the brain. If the sensitivityof the scalp was suppressed by using a long separation probe,
the values may be close to the cerebral oxygen saturation values
Journal of Biomedical Optics

previously reported. The comparison of results suggests that our
new concept of oximetry could be applicable to clinical practice.
In this study, there was no caput succedaneum in the fetal
head. When the thickness of the caput succedaneum layer isthick, the cerebral sensitivity (the ratio of ΔSbrain ∕ΔSscalp )would decrease. In the future, we may solve the problem of
the swelling layer by squeezing the subcutaneous fluid andby measuring the surface layer thickness with an ultrasoundapparatus.
As for neonatal monitoring, the values of cerebral StO2 for achild and a neonate have been reported to range from 65% to85%15 and from 67% to 80%,16,17 respectively. The oxygen saturation values in this study (75%–78%) were similar to thatreported in the literature.
Fouzas et al. described that SpO2 with a pulse oximeter
might not be measured or might be inaccurate in low-perfusion
states, such as low cardiac output, shock, hypothermia, vasoconstriction, and arterial occlusion.18 They also stated that a continuous oxygen saturation monitoring is essentially needed
for the safety of the newborn. Rapid measurements of several
sites of StO2 are needed in the case of neonatal asphyxia.
But the commercially available sensor is set on one site of
the skin of the neonate. Our sensor can be set on any site of
the newborn where doctors want to measure. In addition, thespatially resolved NIRS system can detect the tissue oxygenation in a low-perfusion condition, because the StO2 measurements require only the spatial intensity slope and do not
require the weak pulsation signals on a pulse oximetry. Our
system could be useful in practice for such cases. To assure
the usefulness of this oximetry, we must accumulate moredata. At the moment, we are planning a multicenter study toconfirm the usefulness of our system.
In conclusion, we developed an oximeter with the sensors
attached to the fingers of an obstetrician or a midwife, in contrast
to conventional oximeters whose sensors were attached to thefetus. We successfully detected the fetal oxygenation smoothly
and certainly. We are sure that our system could become a new
fetal monitoring approach.

Acknowledgments
We would like to thank Mr. Hikaru Suzuki, CEO of ASTEM
Co., Ltd. (Kawasaki, Japan) for general advice in developmentof the instruments.

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Naohiro Kanayama received his PhD degree from Hamamatsu
University School of Medicine in 1980 and he has been a professor(chairman) of the Department of Obstetrics and Gynecology inHamamatsu University School of Medicine since 1999. His major
research interests include obstetrical disseminated intravascular
coagulation (DIC), preterm delivery, mechanism of cervical maturation, infrared spectroscopy for fetal monitoring, and photodynamic
therapy.
Masatsugu Niwayama received the PhD degree in biomedical engineering from Hokkaido University, Sapporo, Japan, in 2001. He is currently an associate professor in the Department of Electrical andElectronic Engineering, Shizuoka University, Hamamatsu, Japan.
His current research interests include development of accurate andconvenient NIRS systems.

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