Online February 4, 2022
doi: 10.3400/avd.oa.21-00097

Ann Vasc Dis Vol. 15, No. 1; 2022; pp 14–21

Original Article

The Patency of Tibial/Peroneal Arteries Affects
the Increment of Regional Tissue Saturation of
Oxygen in Each Angiosome after Superﬁcial
Femoral Artery Revascularization
Naoki Unno, MD, PhD,1,2,3 Kazunori Inuzuka, MD, PhD,1,2 Naoto Yamamoto, MD, PhD,3
Masaki Sano, MD, PhD,1,2 Kazuto Katahashi, MD, PhD,1,2 Takafumi Kayama, MD,1,2
Tatsuro Yata, MD,3 Yuta Yamanaka, MD,1,2 Hajime Tsuyuki, MD,1,2 Yusuke Endo, MD,1,2,3
Nozomu Ishikawa, MD,1,2,3 Ena Naruse, CVT,1,2 Masatsugu Niwayama, PhD,4 andHiroya Takeuchi, MD, PhD2

Objective: The angiosome model is a controversial concept in the revascularization of patients with chronic limbthreatening ischemia (CLTI). The aim of this study was todemonstrate the importance of patency of the tibial/peroneal arteries for regional tissue oxygenation in each angiosome during endovascular therapy (EVT) of the superﬁcial
femoral artery (SFA).
Materials and Methods: We devised a novel near-infrared spectroscopy oximeter, “TOE-20,” for real-time monitoring of regional tissue oxygen saturation (rSO2). Using
TOE-20, we prospectively assessed rSO2 at each angiosome
in 23 CLTI patients who underwent successful revascularization of the SFA. During EVT, three sensor probes wereplaced at the dorsal foot, plantar foot, and outer ankle for
1 Division of Vascular Surgery, Hamamatsu University Schoolof Medicine, Hamamatsu, Shizuoka, Japan
Department of Surgery, Hamamatsu University
School of Medicine, Hamamatsu, Shizuoka, Japan
3 Division of Vascular Surgery, Hamamatsu Medical Center,
Hamamatsu, Shizuoka, Japan
4 Department of Electrical and Electronic Engineering, Shizuoka University, Hamamatsu Campus, Hamamatsu, Shizuoka, Japan
2 Second

Received: August 5, 2021; Accepted: December 1, 2021
Corresponding author: Naoki Unno, MD, PhD. Division of
Vascular Surgery, Hamamatsu University School of Medicine,
1-20-1 Handayama, Higashi-ku, Hamamatsu, Shizuoka 4313192, Japan
Tel: +81-53-435-2279, Fax: +81-53-435-2273
E-mail: unno@hmedc.or.jp©2022 The Editorial Committee of Annals of Vascular Diseases. This article is distributed under the terms of the Creative
Commons Attribution License, which permits use, distribution, and reproduction in any medium, provided the credit of the original work, a link to
the license, and indication of any change are properly given, and the original work is not used for commercial purposes. Remixed or transformed
contributions must be distributed under the same license as the original.

rSO2 monitoring.
Results: At the end of EVT, rSO2 at all angiosomes was signiﬁcantly elevated by SFA revascularization. The change inrSO2 in each angiosome was larger in patients with patent
relevant arteries than in those with occluded relevant arteries (i.e., anterior tibial artery patency, posterior tibial artery
patency, and peroneal artery patency).
Conclusion: The patency of the tibial/peroneal arteries
is important for regional tissue oxygenation in EVT. Using
TOE-20 and rSO2-based revascularization, it may possible toanticipate whether an ischemic ulcer will heal or not.
Keywords: ischemia, oxygen saturation, near-infrared spectroscopy, skin, TOE-20

Introduction
Growing incidence of diabetes and renal insufﬁciency has
increased the number of patients with chronic limb-threatening ischemia (CLTI) to more than 6 million globally.1)
Catheter-directed angiography and endovascular therapy(EVT) are regarded as playing an important role among
the treatment of CLTI, particularly in patients with a highsurgical risk.2)
Angiosome is an each arterial perfusion area. Recently, it has been proposed for the improvement of tissue perfusion in critial limb ischemia patientsʼ feet using
angiosome-guided EVT strategy.3) This original concept
was introduced by Taylor and Palmer in the planning ofskin ﬂaps based on the theory of arterial territories. The
concept was investigated using fresh cadavers.4) Both favorable and unfavorable results have been reported afterangiosome-guided EVT.5–7) Hence, the angiosome concept
has not yet been established as a recommendable strategy
for revascularization during EVT.
The discrepancy of the results after angiosome-guided

Real-Time Monitoring of rSO2 at Each Angiosome

Fig. 1(A) TOE-20. (B) Schematic diagram of the TOE-20 NIRS system to measure skinrSO2. (C) Path length distribution superimposed on a typical MRI image of foot. (D)
0DJQL¿HGLPDJHRIWKHSDUWVXUURXQGHGE\DUHGFRORUHGVTXDUHLQC. The depth ofbone from the skin surface is 6.4 to 6.8 mm at the dorsum of the foot.
MRI: magnetic resonance imaging; NIRS: near-infrared spectroscopy; rSO2: regional
tissue oxygen saturation

EVT might be caused by the study design2); most studies
were performed in a retrospective manner with a limited number of patients. Furthermore, the outcomes werecompared by the rate of wound healing or limb salvage
between patients with direct (DR) and indirect (IR) revascularization, and evaluation of the results was performed 1
or 2 years after the procedure. None of the previous studies
compared tissue perfusion in real-time between IR and DR
during the EVT procedures, except one report, which measured skin perfusion pressure (SPP).8) Traditional diagnostic modalities such as ankle-brachial index (ABI),9) SPP,10)
and transcutaneous oxygen pressure11,12) are difﬁcult touse during EVT due to the limitations of the measurementsite and long measurement time. The lack of proper diagnostic modalities to facilitate real-time monitoring of tissue perfusion prevents daily use of intra-EVT assessment.
Previously, we introduced a ﬁnger-mounted tissue oximeter using the near-infrared spectroscopy (NIRS) technique (Toccare; Astem Co., Ltd., Kawasaki, Japan) as a
useful diagnostic device to assess the severity of ischemia
in peripheral artery disease (PAD) patients.13) In this study,
we further develop an NIRS device to simultaneously
facilitate the intra-EVT monitoring of tissue perfusion at
multiple sites. Using the device, we report our initial results of tissue oxygenation at each angiosome during EVT.

Materials and Methods
Study approval
This study was approved by the Ethical Committee ofthe Hamamatsu University School of Medicine (approval number: 16-057) and Hamamatsu Medical
Center (C014-2020). The study protocol was registeredat the UMIN Clinical Trials Registry (UMIN-CTR; ID:
UMIN000025021) and Japan Registry of Clinical Trials(CRB4180008). Written informed consent was obtained
from all participants.

TOE-20Near-infrared light can penetrate body tissues to somedepth. The new NIRS oximeter TOE-20 is manufactured
by Astem Co., Ltd. for the purpose of real-time monitoring of tissue perfusion and has been recently commercialized (Fig. 1A). The oximeter has three sensor probes tosimultaneously measure regional saturation of oxygen(rSO2) and can be connected to a tablet PC using Bluetooth. Each probe has near-infrared light emitting diodes(770 and 830 nm) and detectors (photodiodes). TOE-20
measures the amount of oxygenated and deoxygenatedhemoglobin in the microvascular blood ﬂow in both theepidermis and dermis tissues by determining light attenuation of the skin to which the sensor is attached,
and calculates the rSO2 in approximately 0.5 s (Fig. 1B,
Supplementary movie). Figure 1C shows the path length
distribution of the near-infrared light superimposed on a
typical MRI (magnetic resonance imaging) image of thedorsal foot, which was investigated by light propagation
using a simulation model consisting of skin, fat, and bonelayers.14–17) As the optical path length shows, the measurement depth of TOE-20 is between 0 mm and 5 mm depth

Unno N, et al.

tissue from the skin surface.

Theoretical analysis of NIRS
We investigated light propagation in a dorsal tissue model.
The simulation model consists of skin, fat, and bonelayers. The scattering coefﬁcient ȝ s and absorption coefﬁcient ȝ a were set to ȝ s_skin =26 mm−1, ȝ s_fat =24 mm−1,
ȝ s_bone =40 mm−1, ȝ a_skin =0.02 mm−1, ȝ a_fat =0.003 mm−1,
and ȝ a_bone =0.01 mm−1 based on literature data.14–16) The
anisotropic factors for each layer were 0.95.17) The movement of photons in the Monte Carlo simulation was basedon the radiative transport theory. The model was divided
into 0.5×0.5×0.5 mm cubes, and the optical path lengths
in each cube were determined by a weighted average using
the light intensity when the photon reached the detector
in order to examine measurement sensitivity. The optical
path lengths were calculated when the source-detector
distances were 6 and 8 mm, and it has been previously
shown that the difference between the two optical path
lengths is the measurement sensitivity in the spatially resolved method.18) As the difference in optical path length
in bone between the two states (source-detector distances:
6 or 8 mm) was minor and less than one-tenth of thetotal difference, it was shown that more than 90% of thetotal measurement sensitivity with the spatially resolved
method would be related to hemodynamics in the skinand subcutaneous adipose tissues, at a depth of 0–5 mm.19)
In the measurement using TOE-20, the hemoglobin concentrations of both the skin and adipose tissues were measured. We built a lookup table of the relationship between
the value of ȝ a and the spatial slope of light intensity. In
the measurement, the value of µa was calculated fromthe measured intensity slope using the lookup table. The
following equations were used to calculate the concentrations of oxyhemoglobin ([O2Hb]) and deoxyhemoglobin([HHb]):[O2Hb] =Ȝ 2 ȝ Ȝ1 − İ Ȝ1 ȝ Ȝ 2
İHb
a
Hb a
,
Ȝ1
Ȝ2
İHbO
İ Ȝ 2 − İHbO
İ Ȝ1
2 Hb
2 Hb

Ȝ1
İ Ȝ 2 2 ȝaȜ1 − İHbOȝaȜ 2
2
[HHb] = HbO
,
Ȝ1
Ȝ2
İHbO
İ Ȝ 2 − İHbO
İ Ȝ1
2 Hb
2 Hb

(1)

Fig. 2Completion angiography. (A) Case where both ATA andPTA are patent (ATA+, PTA+), but the PA is occluded
3$í  B) Case where both the ATA and PA are occluded$7$í 3$í  RQO\ WKH 37$ LV SDWHQW 37$  C) CaseZKHUHERWKWKH$7$DQG37$DUHRFFOXGHG $7$í37$í only the PA is patent (PA+). (D) Case where all three arterLHVDUHRFFOXGHG $7$í37$í3$í 
ATA: anterior tibial artery; PTA: posterior tibial artery; PA:
peroneal artery

Table 1 Demographics and clinical characteristics of thepatients
Patients, n

23$JH\HDUVPHGLDQLQWHUTXDUWLOHUDQJH

78, 62–92

Men, n (%)

11 (48)Hypertension, n (%)

13 (57)

Dyslipidemia, n (%)

10 (43)Diabetes mellitus, n (%)

11 (48)

History of smoking, n (%)

15 (65)End-stage renal disease, n (%)

15 (65)

Coronary artery disease, n (%)

5 (22)

Limbs, n23

Location of ulcer
Toe
Plantar footOuter ankle

20
2
1

Dorsal skin perfusion pressure, mmHg; mean±SD31.3±13.7

Plantar skin perfusion pressure, mmHg, mean±SD28.3±12.0

Dorsal foot tissue oxygen saturation, %, mean±SD

48.0±4.1

Plantar foot tissue oxygen saturation, %, mean±SD 49.3 ±2.9
Ankle-brachial index

0.57±0.33

SD: standard deviation(2)

where İ HHb Ȝ1,2 and İ O2Hb Ȝ1,2 are the extinction coefﬁcients of HHb and O2Hb, respectively, at the wavelengths Ȝ 1 and Ȝ 2.20) Regional tissue oxygen saturationrSO2 was calculated using the following equation:=[O2Hb]/([O2Hb]+[HHb]).

Application of TOE-20 to patients
Three sensor probes can be placed at the operatorʼs discretion to monitor tissue perfusion. Sensor probes wereplaced according to the angiosome model as follows: theﬁrst one on the dorsal aspect of the foot, the second one
on the outer ankle, and the last one on the plantar aspectof the foot.

Subjects
This prospective study included 23 CLTI patients who
underwent successful revascularization of the superﬁcial
femoral artery (SFA) without intervention of the anterior
tibial artery (ATA), posterior tibial artery (PTA), and peroneal artery (PA) in two vascular centers between January

Real-Time Monitoring of rSO2 at Each Angiosome

2020 and September 2020 (Table 1). All patients werecategorized as Rutherford classiﬁcation 5 with intractablefoot ulcers and underwent measurement of the ABI, SPP(SensiLase PAD 3000; Vasamed, Inc., Eden Prairie, MN,
USA), and rSO2 to evaluate the severity of the ischemia
in the outpatient clinic. All patients also underwent preprocedural imaging to conﬁrm the diagnosis of chronicarterial occlusive disease (computed tomography arteriography and digital subtraction angiography). Successful
revascularization of the SFA was deﬁned as less than 30%
residual stenosis of the target lesion on the completion
angiogram without peripheral emboli.
Fig. 3

Continuous rSO2 measurements during endovascular revascularization
Before the start of EVT, three sensor probes at the dorsal foot, plantar foot, and lateral ankle were placed on
the patientsʼ skin surface. The rSO2 was measured inreal-time from the beginning of the procedure to theend of EVT. rSO2 is the percentage of oxyhemoglobin[rSO2(%)=100×HbO2/(HbO2 +Hb)]. rSO2 values were
measured within 0.5 seconds at each region. AlthoughrSO2 was monitored continuously, we waited 5 min to see
the effect of the revascularization after each procedure
because it took a few minutes for the value to stabilize. We
recorded the rSO2 values of the three sensor probes placedat the dorsal foot, plantar foot, and the outer ankle at theend of EVT.

Statistical analysis
Results are expressed as mean± standard deviation.
Paired t-tests were used to compare rSO2 values between
pre- and post-revascularization at the same sites. Wilcoxon signed-rank test was used to compare rSO2 values
between the relevant artery-patent and artery-occluded
groups. P-values <0.05 were considered statistically signiﬁcant. Statistical analyses were performed with SPSS
version 24.0 software (IBM Corp., Armonk, NY, USA).

Results
Patientsʼ demographics and rSO2 monitoring during EVT procedures
The baseline clinical characteristics of the study subjects
are shown in Table 1. The numbers of ulcers were 20 at
the toe, 2 at the plantar foot, and 1 at the outer ankle
among the patients. SFA revascularizations were successfully performed by endovascular interventions withballoon angioplasty in 15 patients, Viabahn stentgraft (W.
L. Gore & Associates, Flagstaff, AZ, USA) in 5, and stentplacement in 3.
From the beginning to the end the EVT, monitoringof rSO2 at the dorsal foot, plantar foot, and the lateralrSO2 before and after SFA revascularization. * indicatesp<0.01 in comparison with pre-EVT.
rSO2 WLVVXH R[\JHQ VDWXUDWLRQ 6)$ VXSHU¿FLDO IHPRUDO
artery; EVT: endovascular therapy

ankle was successful in all patients. In most cases, rSO2
decreased mildly to be lower than that at the starting
point during the intervention procedures, such as duringballoon inﬂation and deﬂation. However, after SFA revascularization, the sensor probes gradually responded to theincreased blood ﬂow to the relevant area and increasedrSO2 to a stable value.

Interpretation of the completion angiography
We interpreted the completion angiography after the ﬁnal
EVT procedures according to the patency of the ATA,
PTA, and PA. Figure 2 shows the typical patterns of arterial patency.

rSO2 after SFA revascularization

At the end of EVT, measurement of rSO2 with TOE-20
identiﬁed that all angiosome rSO2 values were signiﬁcantly elevated by SFA revascularization in comparison
with those at the beginning of EVT (48.0%±4.0% vs.
53.2%±5.6% at the dorsal foot, p<0.01; 49.3%±2.8%
vs. 52.7%±4.3% at the plantar foot, p<0.01;
51.8%±4.5% vs. 55.6%±5.1% at the outer ankle,
p<0.01) (Fig. 3). According to the angiosome model,
the relevant artery in the dorsal foot is the ATA, that in
the plantar foot is the PTA, and that in the outer ankle
is the PA.4) We further analyzed all values at the threeareas, whether the relevant arteries were patent or not,at completion angiography by measuring the amountof the variation (ǻ) in rSO2 at the end of EVT (Fig. 4).
The amounts of ǻrSO2 were signiﬁcantly larger when
the relevant arteries were patent at all areas (i.e., dorsal
foot and ATA+, plantar foot and PTA+, and outer ankle
and PA+, compared to cases such as the dorsal foot and
ATA−, plantar foot and PTA−, and outer ankle and PA−,
respectively), while the ǻrSO2 was not signiﬁcant eventhough the irrelevant arteries were patent.

Unno N, et al.

Fig. 4 &RPSDULVRQRIWKHDPRXQWRIWKHLQFUHDVH ǻ LQU622 after SFA revascularization at
WKHGRUVDOIRRWSODQWDUIRRWDQGRXWHUDQNOHEHWZHHQǻU622 where the relevant arteries are patent or occluded. * indicates p<0.01 in comparison with pre-EVT; n.s., not
VLJQL¿FDQWATA: anterior tibial artery; PTA: posterior tibial artery; PA: peroneal artery; rSO2: tissue
R[\JHQVDWXUDWLRQ6)$VXSHU¿FLDOIHPRUDODUWHU\(97HQGRYDVFXODUWKHUDS\

Discussion
We have previously reported that our NIRS technique witha ﬁnger-mounted oximeter can diagnose the severity of
PAD in a manner similar to that of traditional modalities
such as ABI, SPP, and transcutaneous oxygen pressure.13)
We have also shown that TOE-20 demonstrated the utility to enable the monitoring of simultaneous perfusion inthree angiosomes with three sensor probes during EVT.21)
The measurement depth of the previous NIRS oximeters
was 10–20 mm under the skin surface to focus on brain or

muscle oxygen levels22); however, the skin/subcutaneous
tissue of the toe might be too thin to measure because thebones or tendons are present at depths less than 10 mmfrom the skin surface. On the other hand, TOE-20 is speciﬁcally designed to measure tissue oxygen saturation focusing only on the skin and subcutaneous tissue. For that
purpose, a simulation model consisting of skin, fat, andbone layers was created for algorithm development. The
superimposed foot MRI image revealed that the rSO2 of
TOE-20 reﬂects tissue oxygen levels up to 5 mm under theskin surface and measures rSO2 of the skin and subcutane-

Real-Time Monitoring of rSO2 at Each Angiosome

ous tissue without the inﬂuence of bone. This characteristic is quite unique compared to other NIRS devices, whichmainly measure cerebral oxygen levels using an algorithm
that diminishes the contribution of the skin and scalp.23)
Because high and low rSO2 areas are seen even in the sameangiosome, the peripheral tissue perfusion in the CLTI
patientʼs foot, especially diabetic patients, is determinedby the peripheral microvascular blood ﬂow in the skin andsubcutaneous tissue.24) Therefore, the use of TOE-20 may
be applicable not only for foot tissue monitoring during
EVT but also for assessment of free skin graft perfusion
during plastic surgery.
Using TOE-20, we successfully monitored the changes
of rSO2 at each angiosome simultaneously during EVT.
The results showed that successful SFA revascularization
signiﬁcantly increased the oxygen levels at all angiosome
areas irrespective of tibial artery patency. However, theamount of ǻrSO2 in each angiosome depended on relevant tibial artery patency. It is believed that the development of collateral blood ﬂow is individually different
in PAD patients, so that the angiosome model may notnecessarily be applied to PAD patients with diseased tibial
and peroneal arteries. Indeed, Kawarada et al. reported
no differences between DR and IR of the relevant ATA
or PTA in the amount of ǻSPP in the area of the dorsal
and plantar foot after revascularization.8) In this report,
however, the results of the amount of ǻrSO2 after SFA
revascularization showed that ǻrSO2 was signiﬁcantly
larger in angiosomes with patent relevant tibial/peroneal
arteries than in those with occluded arteries, suggestingthat even in patients with diseased tibial/peroneal arteries, the conventional angiosome model works. Therefore,
when we perform revascularization of the SFA, improved
oxygenation in the region where the relevant artery ispatent can be anticipated. Moreover, in combination withtibial/PA interventions, the angiosome concept may be
associated with improved perfusion at the region where
ulcers are located.
While the angiosome-targeted EVT is planned byreference to pre/intra-EVT angiography, the completion
angiogram at the end of EVT reﬂects increased perfusion
because the intra-EVT angiogram may be affected bycatheter/sheath insertion, causing perturbed blood ﬂow.
Because of this, we and others previously reported theusefulness of indocyanine green (ICG) ﬂuorescence angiography at completion angiography for the assessment oftechnical success of the below-the-knee revascularizationin either bypass surgery or EVT.25–27) ICG ﬂuorescenceangiography allows visualization of the perfusion areawith ﬂuorescence signals obtained 5–8 mm deep to theskin surface, so that the effect of revascularization can beassessed in real-time; however, ICG ﬂuorescence angiography has several limitations. First, the ICG injection cannot

be repeated during EVT procedures because the ﬂuorescence signals remain for a while due to ICG binding to thetissue proteins. Second, the cut-off values of the ischemic
region for wound healing remain to be clariﬁed. Third, patient cardiac function may affect the ﬂuorescence signals.
Several additional limitations such as the difference of infrared cameras, patient physique, and the area of the footprevent the technique from being the standard modality
for real-time assessment of perfusion. On the other hand,real-time monitoring of rSO2 using TOE-20 can overcome
such problems. TOE-20 can repeatedly measure rSO2 at
the site of probe placement after each EVT procedure. A
previous study identiﬁed that an rSO2 of more than 50%
may be a cut-off value for wound healing.28) Patient cardiac function, physique, and skin color do not inﬂuence
the rSO2 value. Therefore, TOE-20 may become an ideal
diagnostic device to monitor tissue perfusion/oxygenationin real-time.
The deviceʼs name, TOE, is an abbreviation for target
region oxygenation-based endovascular treatment, whichhas previously been proposed as a new EVT strategy.29) In
TOE, we aim to adequately perfuse the target region up
to rSO2 >50%. For that purpose, the multi-channel oximeter, TOE-20, was designed to monitor tissue oxygenation
because CLTI patients often possess multiple ulcers in
different angiosomes. Using this method, we may be able
to anticipate whether the ischemic ulcer will heal or not.

Study LimitationsFirst, the number of patients who were investigated for theutility of TOE-20 was limited. Second, we did not show
the follow-up data of the rate of patient wound healing
as well as limb salvage in relation to the rSO2 values after
EVT. Third, we did not include cases with tibial/PA revascularization but only those with SFA revascularization
to observe changes in the ǻrSO2 values. Finally, some of
the authors hold a patent associated with TOE-20, whichmight cause potential bias during result evaluation. Considering the abovementioned limitations, this study is still
preliminary and further studies are needed with longer
follow-up periods.

Conclusion
We devised a novel oximeter, TOE-20, for real-timemonitoring of tissue oxygenation during EVT for CLTI
patients. TOE-20 demonstrated usefulness in the measurement of rSO2 values and identiﬁed the increase of
perfusion after SFA revascularization in all angiosomesof the foot, irrespective of relevant tibial/PA patency. The
amount of ǻrSO2 was signiﬁcantly larger in the angiosomes with patent relevant tibial/peroneal arteries than inUnno N, et al.

those with occluded arteries, which suggests that the angiosome model may be important, even in CLTI patientswith diseased infra-popliteal arteries.

Acknowledgments
We would like to thank Mr. Hikaru Suzuki, CEO of Astem
Co., Ltd. (Kawasaki, Japan) for general advice in the development of TOE-20.

Funding
This work was supported by the project “Developmentof Medical Devices and Systems for Advanced Medical
Services” from the Division of Medical Device Researchin the Japan Agency for Medical Research and Development (AMED), Japan (ID: 19he1602005h0003 awarded
to N.U.) and JSPS KAKENHI (ID: JP18K12053 awarded
to M.N.).

Disclosure Statement
N.U. and M.N. hold patents associated with TOE-20(PCT/JP2020/039614; WO 2007/139192). The other authors report no conﬂict.

Author Contributions
Study conception: NU
Data collection: NU, KI, MS, KK, TK, TY, YY, HT, EN,
NI, MN
Analysis: NU, KI, MN, EN
Writing: NU, MN
Funding acquisition: NU, MN
Critical review and revision: all authors
Final approval of the article: all authors
Accountability for all aspects of the work: all authors

Supplementary Information
Supplementary movie is available at the online articleʼssites on J-STAGE and PMC.

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