Craniofacial tissue oxygen saturation
is associated with blood pH using anexaminer’s finger-mounted tissue
oximetry in mice
Toshiyuki Uchida
Naohiro Kanayama
Kenta Kawai
Masatsugu Niwayama

Toshiyuki Uchida, Naohiro Kanayama, Kenta Kawai, Masatsugu Niwayama, “Craniofacial tissue oxygen
saturation is associated with blood pH using an examiner’s finger-mounted tissue oximetry inmice,” J. Biomed. Opt. 21(4), 040502 (2016), doi: 10.1117/1.JBO.21.4.040502.

JBO Letters

Craniofacial tissue
oxygen saturation isassociated with blood pH
using an examiner’sfinger-mounted tissue
oximetry in mice
Toshiyuki Uchida,a,* Naohiro Kanayama,aKenta Kawai,a and Masatsugu Niwayamab

aHamamatsu University School of Medicine, Department of Obstetricsand Gynecology, 1-20-1 Handayama, Higashiku, Hamamatsu,Shizuoka 431-3192, JapanbShizuoka University, Department of Electrical and Electronic
Engineering, 3-5-1 Johoku, Nakaku, Hamamatsu 432-8561, JapanAbstract. Although fetal scalp blood sampling is an examination to assess fetal acidosis during the intrapartum
period, it has not been widely used by obstetriciansbecause of its invasiveness. We have developed asmall, portable oximetry with a sensor attached to theexaminer’s finger. Our previous report using this oximetry
concluded that fetal head tissue oxygen saturation (StO2 )correlated with umbilical cord artery blood pH. We investigated whether the association between StO2 and blood
pH in mice could be validated using this oximetry.
Eleven the Institute for Cancer Research (ICR) mice
were measured using a near-infrared spectroscopy
probe at the craniofacial site in a closed polyethylene
bag while changing the oxygen concentration. A total ofnine blood samples were collected and analyzed for pH.
The StO2 and tissue blood pH showed a strong positive
correlation (r ¼ 0.90 and P ¼ 0.0009). The StO2 andtotal hemoglobin index also showed a positive correlation(r ¼ 0.84 and P ¼ 0.0049). Thus, the results of the present
study support those of our previous report on clinical cases
and allow examiners to easily check the status of fetal
acidosis. Fetal management using this oximetry might
gain popularity with obstetricians in the near future. © TheAuthors. Published by SPIE under a Creative Commons Attribution 3.0
Unported License. Distribution or reproduction of this work in whole or inpart requires full attribution of the original publication, including its DOI.
[DOI: 10.1117/1.JBO.21.4.040502]

Keywords: near-infrared spectroscopy; fetal scalp blood sampling;fetal acidosis; portable oximetry; tissue oxygen saturation.
Paper 160092LR received Feb. 16, 2016; accepted for publication
Mar. 22, 2016; published online Apr. 12, 2016.

There are only a few methods for fetal assessment during intrapartum, including fetal heart rate (FHR) monitoring, ultrasound,and fetal scalp blood sampling (FSBS). FHR monitoring is thegold standard to evaluate fetal well-being because this*Address all correspondence to: Toshiyuki Uchida, E-mail: utty@hama-med.ac.jp

Journal of Biomedical Optics

examination is noninvasive for the fetus and pregnant
woman. However, FHR monitoring has been shown to havea high number of false positive readings.1 This increases therate of cesarean sections and instrumental labor, despite thefact that positive FHR monitoring may not be associatedwith fetal hypoxemia. To cover the drawbacks of FHR monitoring, obstetricians have performed FSBS during the intrapartum
period. In 1962, Saling2 reported a fetal blood gas analysis used
for assisting the evaluation of fetal well-being intrapartumally. A
fetal pH value of >7.25 is regarded as normoxemic status.3 A
pH value of <7.15 in the second stage of labor means hypoxemia, which requires an emergency intervention for rescuingfrom neonatal asphyxia.3 A meta-analysis reported that both
FSBS and FHR monitoring during the intrapartum period
resulted in avoiding unnecessary operative deliveries.4 The
National Institute for Health and Care Excellence recommends
FSBS to assess fetal status when FHR monitoring yields a positive result.5 Although FSBS is a good test for the managementof labor, it is infrequently performed in clinical settings.
Jørgensen and Weber4 revealed that the FSBS procedure wasperformed for 3.8% and 6% of cases in 2005 and 2011, respectively, for all full- and post-term deliveries in Denmark. Usually,
FSBS for pH measurement requires 30 to 50 μL of blood.
Because FSBS is invasive for the fetus, many cases of complications have been reported.6–9 According to a recent case report,
one serious complication, cerebrospinal fluid leakage from theincision site, occurred during FSBS.9Kanayama and Niwayama10 have developed a smaller oximetry with the sensor attached to the examiner’s finger (KN-15,ASTEM, Kwasaki, Japan). This medical instrument consists ofa probe with near-infrared spectroscopy (NIRS) and a numeral
display device [Figs. 1(a) and 1(b)]. The size of the sensor in theprobe is <1 cm2 (Fig. 2). This probe is attached to the examiner’s finger [Fig. 1(a)]. Examiners can attempt to measure anysites of the human body, even the organs within a body cavity
during surgery, because the device is small in size and portable.
This oximetry can measure tissue oxygen saturation (StO2 )and the absolute value of hemoglobin concentration. We havepreviously investigated the characteristics of this oximetry technique and clinical data obtained during labor.11 We concluded
that the mean fetal StO2 for the fetal head during labor significantly correlated with umbilical cord arterial pH immediately
after delivery. This result could potentially indicate that themeasurement of fetal head StO2 can be substituted for an
FSBS pH assessment. Therefore, using this noninvasive oximetry might dramatically change the management of labor. In thepresent study, we further investigated whether craniofacial StO2
using the oximetry with the sensor attached to the examiner’s
finger is associated with blood pH in an animal model.
The wavelengths of the light sources were 770 and 810 nm,
and the sensitivity of the photodiodes (PD2501, Epitex Co.,Ltd., Kyoto, Japan) was high in the near-infrared band. The
bare chips of the light-emitting diodes (LEDs) and photodiodes
were mounted on the substrate with a wire bonding. The detectors were located 6 and 8 mm away from the LEDs to determine
the absolute value of hemoglobin concentration using the laboratory-made spatially resolved NIRS (Fig. 2).
This study has been conducted in compliance with theAnimal Research Committee, Hamamatsu University Schoolof Medicine, and the principles of the Guide for the Careand Use of Laboratory Animals: 8-week-old ICR female
mice, weighing 29 to 31 g, were obtained from Japan SLC040502-1April 2016 • Vol. 21(4)JBO LettersFig. 1 Examiner’s finger-mounted tissue oximetry: (a) the measurement of the newborn and (b) the device displays StO2 in the upperarea and T-HbI in the lower area.

correlation coefficient. A value of P < 0.05 was considered statistically significant.
A total of 11 mice underwent measurement using the oximetry and their blood was collected. Eleven mice were measured
for the StO2 and T-HbI under the changing oxygen concentrations. A total of nine blood samples were collected and their pH
was analyzed; two samples could not be measured because ofinsufficient blood volume. The range of the oxygen gas concentration in the polyethylene bag was 16.7% to 21%, detected in
the blood of the analyzed mice using an oxygen concentration
meter (n ¼ 9). The oxygen concentration and StO2 were positively correlated (Fig. 3; r ¼ 0.864, P ¼ 0.0027, n ¼ 9). A correlation was strongly exhibited between the StO2 and pH inmice (Fig. 4; r ¼ 0.90, P ¼ 0.0009, n ¼ 9). The StO2 andT-HbI also had a strong positive correlation (Fig. 5; r ¼ 0.837,P < 0.0049, n ¼ 9).
The present study is the first report of an animal experimentusing this small, portable device with NIRS. The existing oximeters for human are attached to the head or muscle and arerelatively large in size. This new oximeter can measure considerably small sites, such as the forehead and face in mice if theyremain immobile. After examiners with the sensor probe had
touched the targets, the numerical data on the monitor screen
stabilized within 10 s. Because the power source of this device

Fig. 2 Flexible substrate mounted with two LEDs, and two photodiodes adhered to a black rubber sheet.

Inc. (Hamamatsu, Japan) for this study. They were housed individually in a specific pathogen-free facility. The hair on the forehead of the mice was cut short to decrease the influence of hairin oximetry measurement. At first, every mouse was measured at
the craniofacial site by the oximetry at 21% of oxygen concentration (indoor oxygen content) and in the range of24.0°C to 24.5°C. One mouse was placed in a cage (CL0113-4; width 136 mm, depth 208 mm, height 115 mm,CLEA Japan Inc., Tokyo, Japan), with some blocks of dry
ice (solidified carbon dioxide), and an oxygen concentration
meter (XP-3180E, New Cosmos Electric Co., Ltd., Osaka,Japan) was enclosed in a polyethylene bag (width 65 cm anddepth 80 cm). Examiners detected the decrease in the oxygen
concentration and measured the StO2 and total hemoglobin
index (T-HbI) at the craniofacial site in the polyethylene bag,preventing the entry of any outside air as much as possible.
After a mouse was measured one to four times in the polyethylene bag, examiners immediately cut the facial vessel using
a lancet and collected the blood using a capillary tube (atleast three drops). Immediately after blood collection, a sample
for pH measurement was evaluated using the handheld
i-STAT®1 Analyzer (Abbot Laboratories, East Windsor, NewJersey) and EG6+ cartridge (Abbott Laboratories, EastWindsor, New Jersey). Examiners did not investigate theStO2 and T-HbI using oximetry again once the blood sample
was collected. The value of the StO2 and T-HbI was investigatedthree times until the numeric data stabilized. A median value
from all the measurement data was selected as a numericvalue for the data under the conditions of oxygen concentration.
Statistical significance was calculated using a Spearman’s
Journal of Biomedical Optics

Fig. 3 There is a significant correlation between oxygen concentration and StO2 in the closed polyethylene bag.

Fig. 4 There is a significant relationship between StO2 and blood pH
at the craniofacial site in mice.

040502-2April 2016 • Vol. 21(4)JBO Lettershave managed to take care of the fetus with a few limited examinations. This oximetry would be a breakthrough device for theearly detection of fetal asphyxia. Obstetricians hesitate to perform FSBS in most cases because FSBS is an invasive procedure. If this oximeter is used during labor, obstetricians
can attempt to check the status of fetal acidosis at several
time intervals. Because the present study evaluated only asmall number of animal cases, the validity of these conclusions
is another limitation. However, we demonstrate that craniofacial
StO2 is associated with blood pH using an examiner’s fingermounted tissue oximetry in mice. Larger clinical trials using
this oximetry are required to confirm our findings. We are confident that our small oximetry has the potential to become a new
fetal monitoring approach in a clinical setting.
Fig. 5 The StO2 and T-HbI have a strong positive correlation in mice
when they are immobile.

is supplied by two dry batteries, wiring with an external
power source is unnecessary. Using this oximeter, we have previously reported that our fetal StO2 reflects the contributions ofapproximately two-thirds of the fetal brain and one-third of thescalp in the measurement of the fetus during labor.7 This source–
detector distance provides a depth of 3.0 to 4.0 mm. Because it isrelatively easy to obtain blood samples from facial vessels in amouse, we attempted to attach the probe to the mouse’s facialsite in the present study. We believed that this source–detector
distance would provide the thickness of the mouse’s facial skin(0.5 mm) and subcutaneous tissue (2.5 mm). Craniofacial tissue
consists of skin, muscles (masseters and temporalizes), brain
with skull, and connective tissues. Because the length of the cutting edge in the lancet that was used in this study was 3.0 mm,
we thought that the depth of tissue measured using this oximetry
and the depth of tissue at which blood was collected with a lancet were similar. The mouse has a very narrow face, and the dataobtained from our oximetry might provide mixed information
regarding facial tissue and cerebral tissue. Because the valuesof StO2 and T-HbI are slightly reduced without cutting thehair, we cut the hair on the foreheads of the mice in the presentstudy. One limitation of this study was that we could not analyze
two blood samples. One blood sample was thick and viscous;
this abnormal blood coagulation might have occurred becauseof 12.4% oxygen concentration. A technical mistake occurred
while collecting the other blood sample.
No matter how excellent the examinations for fetal evaluations are, invasive tests such as FSBS are not extensively popular worldwide.4 The results of this study demonstrate that theStO2 and tissue blood pH in mice exhibit a strong positive correlation. Thus, the results of the present study support the conclusion of the previous report that StO2 during labor isassociated with umbilical arterial blood pH. We assume thatthe correlation between the StO2 and tissue blood pH can be
explained by the Bohr effect (i.e., hemoglobin easily releases
oxygen when the blood pH decreases).12 Furthermore, T-HbI
is useful for determining tissue ischemia and congestion,13and correlated with StO2 in the present study. Under low oxygen
concentrations in mice, the absolute volume of the blood might
decrease in the craniofacial section because blood perfusion
decreased due to the hypometabolic state. Obstetricians

Journal of Biomedical Optics

Acknowledgments
The authors thank the following for their excellent technicalsupport: Mr. Hikaru Suzuki, Mr. Yutaka Yokoyama, Yukio
Chitose, and Ms. Aya Takiguchi (ASTEM Co., Ltd.,
Kawasaki, Japan) and Ms. Miuta Sawai and Ms. Urmi Jeenat
Ferdous (Department of Obstetrics and Gynecology,
Hamamatsu University School of Medicine). We acknowledge
the financial support provided by MEXT KAKENHI, Grant
No. 15K10667.

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