Jpn J Extra-Corporeal Technology 51（2）：129 − 137, 2024

Investigation of a method for the electrical measurement ofvenous reservoir blood volume
─ secondary publication（complete translation）─

Atsushi Nakamura 1），Eiki Kogure 1），Masaru Inoue 2），Katsuyuki Kuwana 2）

Abstract
Blood volume in the venous reservoir（VR）is an important parameter that should be continuously monitored in patients with a cardiopulmonary bypass. In this study, we investigated the effects of measurement frequency, hematocrit（Ht）, and blood volume on the capacitance（Cs）of blood present in the venous reservoir by
utilizing electrical properties of polycarbonate, a material used in VR.
In Experiment 1, two electrodes were attached to the VR wall surface and the capacitance Cs［F］between
the electrodes at each liquid level was measured at frequencies from 100 kHz to 1 MHz. VR was filled withphysiological saline or bovine blood（Ht 20%, 25%, and 30%）. In Experiment 2, Cs values were measured at afrequency of 1 MHz by attaching an electrode to a cylindrical container and filling it with physiological saline.
The results of Experiment 1 showed that the Cs value increased with raising liquid level, with increases of
1.30 pF/cm（saline）, 1.26 pF/cm（Ht 20%）
, 1.28 pF/cm（Ht 25%）and 1.13 pF/cm（Ht 30%）at a frequency of 1
MHz. In Experiment 2, Cs increased by 1.41 pF/cm. The frequency range of 800 kHz to 1 MHz was found to bestable with minimal effects on the blood, suggesting that the frequency can be used for continuous measurement of the blood volume in VRs of various shapes.
Key words：cardiopulmonary bypass，venous reservoir，level sensor，reservoir volume，automation

Ⅰ．Introduction

trafiltration method（ECUM）, and increased by infu-

The venous reservoir（VR）used during car-sion, cardioplegia, suction, and other means.

diopulmonary bypass（CPB）serves as a receptacle

A level sensor is installed within the VR tofor venous blood drawn from the patient, facilitating

monitor decreases in blood volume, signaling poten-

adjustments to the circulating blood volume. Main-

tial issues with the venous cannula or the blood re-taining blood volume within the VR is achieved

moval circuit. This level sensor enhances safety bythrough a balance between the pump flow rate andtriggering an alarm when it detects a decline invenous drainage flow. In addition, blood volume canblood volume. Additionally, a fail-safe mechanism

be reduced by urine volume and extracorporeal ul-

regulates the pump flow rate. Nonetheless, the func-

1）Department of Clinical Engineering, Faculty of Health Sciences, Kyorin University
2）SENKO MEDICAL INSTRUMENT Mfg. CO., Ltd.
Corresponding author：‌Atsushi Nakamura
Department of Clinical Engineering, Faculty of Health Sciences, Kyorin University
5-4-1, Shimorenjaku, Mitaka, Tokyo, 181-8612, Japan［Accepted April 18, 2014］
footnote：‌This article is based on a study first reported in the “Nakamura A, Kogure E, Kuwana K, et al：Investigation of a method forthe electrical measurement of venous reservoir blood volume. Jpn J Extra-Corporeal Technology, 50（1）：11-18, 2023”.

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tionality of this mechanism depends on the blood

lizing hydrostatic pressure 1, 2）, the weight of thelevel reaching the sensor site, rendering it incapableVR 3）, and applying imaging techniques to the VR 4）.

of detecting early-stage declines in blood volume.

Furthermore, there are documented efforts in the

Parameters to be documented during CPB in-literature 5-7） where attempts were made to imple-

clude pump flow rate, circuit pressure, perfusion

ment automatic control by affixing multiple photo-pressure, and blood temperature, all of which areelectric sensors to the reservoir, with the aim of reg-measured through sensors. While it is possible to au-

ulating the equilibrium between pump flow rate andtomate the recording of these readings by convert-drained blood flow. However, the practical applica-

ing them into digital data, contemporary CPB devic-

tion of these methods has not yet been realized.

es lack the capability to consistently measure andHence, our research focused on a novel approach forrecord blood volume within the VR. During CPB,

the continuous measurement of blood volume in themaintaining the blood volume within the VR is im-

VR. This method leverages the electrical properties

portant for balancing the pump flow rate and bloodinherent in polycarbonate, the material used to con-withdrawal volume, facilitating adjustments to cardi-

struct the VR.

ac preload, and influencing decisions related to fluid

The material used to construct the VR, poly-

removal and blood transfusion. Consequently, thecarbonate, is an insulator with low electrical conduc-continuous display and recording of blood volume

tivity. Thus, when positioned between two conduc-

during CPB procedures are essential. Moreover,tors, it results in the storage of an electric charge, aachieving continuous blood volume measurement

phenomenon known as capacitance. The method

within the VR holds the potential to significantly ad-

used in this study takes advantage of variations invance the development of stabilization control sys-

capacitance that occur between the electrodes andtems for VR levels and the automation of control

the blood within, achieved by affixing electrodes tomechanisms in CPB devices. Consequently, continu-

the exterior of the VR. When the electrodes areous measurement of blood volume in the VR is a

mounted on the outer surface of the VR, the two ca-

critical function, essential for enhancing the safety ofpacitors are electrically linked through the blood

CPB procedures and realizing the goal of automatedwithin（Fig.1）. The capacitance（C）can be consid-

control.

ered a linear function of the liquid level（L）, withPrevious studies have documented alternative

the width of the electrode being represented as W.

approaches for the continuous quantification of blood

However, it is important to note that, in practicalvolume in the VR. These methodologies include uti-

terms, capacitance（C）might be influenced by fac-

Electrode widthElectrode
Reservoir
wall

C=

d
Liquid
level

Fig.1

130

W・ L
d

ƐO：Permittivity of vacuumƐ ：Relative permittivity
d ：Reservoir wall thickness
W ：Electrode width
L ：Liquid level

Liquid level

C

・

C

The theoretical relationship between the liquid level and the capacitance value

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Jpn J Extra-Corporeal Technology 51（2），2024

tors such as the quantity of stored blood, the proper-

To conduct measurements, the liquid levelties of the blood, and the measurement frequency.

within the VR was systematically adjusted in 2 cm

These factors depend on the specific shape of theincrements, ranging from 2 cm to 22 cm. The Cs val-

VR. Consequently, in this study, we examined theue was subsequently recorded using an LCR metermeasurement frequency that ensures stable mea-

（NF Corporation ZM2376）over a range of measure-surements of blood volume in the VR, examining its

ment frequencies from 100 kHz to 1 MHz, at inter-

effects on the properties of the blood and the accura-vals of 50 kHz. It should be noted that a fresh VR

cy of volume measurements.

was used for each type of filling liquid, as illustrated
in Fig.2.

Ⅱ．Study methods2）Experiment 1-2

1. Measurement methods

Similar to Experiment 1-1, the filling liquid’s Ht1）Experiment 1-1

levels were adjusted to 20% and 30%. For each Ht

Two conductive cloth tapes（material: nickel

value, Cs values were measured at liquid levels of 2

copper plating, Seiwa Electric）, each 4 cm wide,

cm, 10 cm, and 18 cm, with measurement frequen-

were affixed to the wall of the VR（HVR-FHP4P:

cies ranging from 100 kHz to 1 MHz（50 kHz inter-

Senko Medical Instrument）on both the left andvals）. This allowed for an examination of the influ-

right sides of the VR’s blood volume scale. The ca-ence of hematocrit（Ht%）on the results, incorporating

pacitance Cs［F］between the electrodes was thenthe findings from Experiment 1-1, which was con-measured. To assess the impact of the properties ofducted with an Ht level of 25%, as shown in Fig.2.
3）Experiment 2

blood and determine the optimal measurement frequency, the experimental circuit was filled with a

In Experiment 2, to investigate the impact of

combination of physiological saline and bovine blood

VR shape, two segments of conductive cloth tape［hematocrit（Ht）: 25%］.

（width: 4 cm, height: 21 cm）were affixed to a cylin-

LCR meter
22 cm（≈3,600 mL）
18 cm（≈2,500 mL）

10 cm（≈650 mL）

2 cm（≈110 mL）
0 cm（≈40 mL）

Artiﬁcial lung

Experiment 1-1
Liquid : saline solution, Bovine blood（Ht 25%）
Measurement point : 2 cm ～ 22 cm（every 2 cm of liquid level）

Roller pumpExperiment 1-2
Liquid : Bovine blood Ht 20% and Ht 30%
Measurement point : 2 cm, 10 cm, 18 cm
Fig.2

Schematic layout of Experiment 1

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drical container（polycarbonate, outer diameter: 76

nificance level of P＜0.05 was adopted.

mm, thickness: 3 mm）. The Cs value was measured

Ⅲ．Results

at a frequency of 1 MHz, with the liquid level adjusted from 2 cm to 16 cm in 2 cm increments, as il-

1. Experiment 1

lustrated in Fig.3. The experimental circuit was1）Experiment 1-1
Fig.4-a shows the results of Cs values when

filled with physiological saline.
2. Date analysis

physiological saline was used as the filling fluid in

Under each condition, measurements were con-the VR. At consistent liquid levels, minimal fluctua-

ducted three times using the LCR meter, with thetions were observed in the Cs values across frequen-

voltage set at 5 V and the roller pump halted. Statis-

cies ranging from 100 kHz to 1 MHz. Moreover, atical analysis was carried out using SPSS Statistics

positive correlation was noted, with higher liquid

version 21（IBM）, using the Tukey HSD method forlevels corresponding to increased Cs values. At acomparisons between different Ht（%）levels. A sig-

frequency of 1 MHz, the Cs value increased from 2

LCR meter
Liquid Level
16 cm
14 cm
12 cm
10 cm
8 cm
6 cm
4 cm
2 cm
0 cm
Roller pump
Fig.3

Schematic layout of Experiment 2

a : Saline solution
40

2 cm

4 cm

6 cm

8 cm

10 cm

14 cm

16 cm

18 cm

20 cm

22 cm

40

12 cm

35

30

30

25

25

Cs［pF］

Cs［pF］

35b : Bovine blood Ht 25%

20
15

5

5
200

400

600

800

0

1,000

6 cm

8 cm

10 cm

16 cm

18 cm

20 cm

22 cm

0

Frequency［kHz］
Fig.4

132

200

400

600

Frequency［kHz］

Capacitance values at each liquid level（Experiment 1-1）

体外循環技術Vol.51

12 cm

15
10

0

4 cm

14 cm

20

10

0

2 cm

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1,000

Jpn J Extra-Corporeal Technology 51（2），2024

Table 1a：Ht 20%‌Venous reservoir measurement outcomes（Experiment 1-1）
Blood（Ht 25%）

Saline solution

35

Cs［pF］

Level［cm］

Cs［pF］

30

2

4.77 ± 0.04

2

8.38 ± 1.32

25

4

7.06 ± 0.04

4

10.03 ± 0.34

69.51 ± 0.13

612.80 ± 1.87

8

12.13 ± 0.12

8

16.23 ± 0.42

1014.80 ± 0.171017.53 ± 0.49

12

16.67 ± 0.12

12

20.13 ± 0.50

1419.27 ± 0.06

14

22.23 ± 0.35

16

22.13 ± 0.06

16

25.27 ± 0.55

18

25.13 ± 0.25

18

28.33 ± 0.40

2028.10 ± 0.10

2031.67 ± 0.47

2230.87 ± 0.15

2233.67 ± 0.67

20
15
10
5
0

0

200

400

600

800

1,000

Frequency［kHz］
b：Ht 25%

2 cm
10 cm
18 cm

40
35
30
Cs［pF］

 Measurement frequency 1MHzCs［pF］

Level［cm］

 Measurement frequency 1MHz

2 cm
10 cm
18 cm

40

25
20
15
10
5
0

40.0
y = 1.28x + 5.17
R² = 0.995

30.0

400

600

800

35

y = 1.30x + 1.72
R² = 0.998

15.0

Salin solution
Blood（Ht 25%）

10.0
5.0

1,000

2 cm
10 cm
18 cm

40

20.0

30
25
20
15
10

0

5

10

15

20

5

25

0

Liquid level（cm）
Fig.5

200

c：Ht 30%

25.0

0.0

0

Frequency［kHz］

Cs［pF］

Capacitance（pF）

35.0

0

200

400

600

800

1,000

Frequency［kHz］‌The relationship between the liquid level and capacitance values at 1MHz measurement frequency（Experiment 1）

Fig.6

cm: 4.77±0.04 pF to 22 cm: 30.87±0.15 pF（Table 1）.

Capacitance values at various liquid levels（Experiment 1-2）

2）Experiment 1-2

Conversely, when using blood（Ht 25%）
, a slight dif-Fig.6 presents the measurement outcomes ofference in Cs values among various liquid levels was

Cs values at various frequencies for Ht levels of 20%

observed at lower frequencies. Nevertheless, beyond

and 30%, alongside the results obtained in Experi-

800 kHz, a linear increase in Cs values was evident,

ment 1-1 at liquid levels of 2 cm, 10 cm, and 18 cm

aligning with rising liquid levels. At 1 MHz, the val-

when Ht was 25%. Consequently, similar to the find-ues rose from 2 cm（8.38±1.32 pF）to 22 cm（33.67

ings in Experiment 1-1, the impact of Ht on Cs val-

±0.67 pF）, yielding outcomes comparable to thoseues decreased with increasing frequency. At a fre-observed with physiological saline（Fig.4-b, Table

quency of 1 MHz, the Cs values were 8.55±1.07 pF

1）. At a frequency of 1 MHz, the increment in Cs

at 2 cm, 17.50±0.35 pF at 10 cm, and 28.67±0.15 pF

value（pF/cm）was 1.30 pF/cm for physiological sa-

at 18 cm for Ht 20%. At Ht 30%, the obtained valuesline and 1.28 pF/cm for blood（Ht 25%）
. Similar

were 2 cm: 8.60±1.15 pF, 10 cm: 17.33±0.46, and 18

slopes were observed, as depicted in Fig.5.

cm: 26.70±0.29 pF（Table 2）. Significant differences

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35.0
Ht 20%

Ht 25%

P＜0.01

Ht 30%

n.s.

30.0

28.67 28.33

25.0
Capacitance (pF）

P＜0.01

26.70

n.s.
n.s.

20.0
15.0
10.0
5.0

17.50 17.53 17.33

n.s.
n.s.
8.55

0.0

n.s.

n.s.
8.60

8.38

2cm

10cm

18cm

Liquid level
Fig.7

Table 2

Capacitance values for each hematocrit level‌Venous reservoir measurement outcomes at the frequency of 1MHz（Ht 20, 25 or 30%）

Ht 20％

Ht 25％

Ht 30％

Table 3

Level［cm］

Cs［pF］

Level［cm］

Cs［pF］

2
10
18

8.55 ± 1.07
17.50 ± 0.35
28.67 ± 0.15

25.38 ± 0.19

Level［cm］

Cs［pF］

2
10
18

8.38 ± 1.32
17.53 ± 0.49
28.33 ± 0.40

Level［cm］

Cs［pF］

21018

8.6 ± 1.15
17.33 ± 0.46
26.70 ± 0.29

4

7.86 ± 0.32

610.50 ± 0.44

813.37 ± 0.47

10

16.17 ± 0.47

1219.10 ± 0.53

14

22.10 ± 0.53

16

24.97 ± 0.57

saline solution（1MHz）

Measurement frequency 1MHz

30.0

（P＜0.01）were observed at a liquid level of 18 cm

25.0

25% and Ht 30%. However, no notable difference wasobserved between each Ht level at liquid levels of 2
cm and 10 cm（Fig.7）.
2. Experiment 2
Table 3 shows the Cs values when the cylin-

Capacitance（pF）

between Ht 20% and Ht 30%, as well as between Ht

15.0
10.0
5.0

The Cs values exhibited an increase from 2 cm（5.38

0.0±0.19 pF）to 16 cm（24.97±0.57 pF）. As the liquid
level increased, the Cs values linearly increased,reaching 1.41 pF/cm, as shown in Fig.8.

体外循環技術y = 1.41x + 2.24
R² = 0.999

20.0

drical container was filled with physiological saline.

134‌Polycarbonate cylinder measurement outcomes（Experiment 2）

0

5

10

15

20

Liquid level（cm）
Fig.8

Vol.51

No.2‌Relationship between the liquid level and capacitancevalue（Experiment 2: saline solution）

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Jpn J Extra-Corporeal Technology 51（2），2024

Ⅳ．Discussion

an operational pump is imperative. Particularly at

1. Regarding measurement principles and results

lower levels, fluctuations in the Cs value may occur

The affixing of two conductive tape segments

because of variations in the liquid level caused byonto the front wall of the VR resulted in the align-

pump pulsations. Given that the primary objective ofment of two polycarbonate insulators, as depicted inthis research was to closely examine the intricate

Fig.1. This setup essentially forms a circuit compris-

relationship between liquid level and Cs values, fu-

ing two capacitors. In this scenario, the liquid level isture efforts in developing a specialized measuring

interpreted as being directly proportional to capaci-

device should include data averaging and measures

tance. The experimental findings demonstrated a re-to mitigate electrical noise.

lationship between the liquid level and capacitance

2. Regarding the effects of blood cell components

value of 1.41 pF/cm（physiological saline）in the cy-

Blood can be characterized according to threelindrical container（Experiment 2）. In the VR（Ex-factors: cell membrane capacitance（Cm）, which cor-

periment 1）, the capacitance values were 1.30 pF/

responds to the cell membrane of blood cells; intra-

cm（physiological saline）, 1.26 pF/cm（Ht 20%）, 1.28

cellular fluid resistance（Ri）; and extracellular fluid

pF/cm（Ht 25%）
, and 1.13 pF/cm（Ht 30%）
（Tableresistance（Re）. The cell membrane capacitance4）. The findings of this study indicate a direct pro-（Cm）acts as a capacitor, allowing current to flow

portionality between the liquid level and the Cs val-through the extracellular fluid in the DC to low-fre-ue, suggesting the practicality of determining thequency range. Conversely, at a high-frequency cur-blood volume within the VR by measuring the ca-

rent, it can traverse the cell membrane capacitancepacitance value using the described method.

（Cm）,allowing it to pass not only through the extra-

In the conducted experiment, measurements

cellular fluid but also through the cell itself, as illus-

were obtained with the roller pump in a stationary

trated in Fig.9-a. Due to the electrical properties ofstate. However, for a more realistic simulation ofblood, its impact is minimal within measurement fre-clinical application, performing measurements with

quencies ranging from 800 kHz to 1 MHz. Further

Table 4

Relationship between capacitance（Y）and liquid level（X）

container

solutiona（pF/cm）b

R2

cylindersaline

1.41

2.24

0.99

venous reservoir

saline

1.30

1.72

0.99

Ht 20%

1.26

5.66

0.99

Ht 25%

1.28

5.17

0.99

Ht 30%

1.13

6.24

0.99
Y＝a X + b
Y：capacitance（pF）
X：Liquid level（cm）

a）

Cm

Ri

Cw

Cw

Re
b）

Cw

Cb

Cw

Fig.9

Cw ：Reservoir wall capacitance
Cm：Cell membrane capacitanceRi ：Extracellular ﬂuid resistance
Re ：Intracellular ﬂuid resistance
Cb ：Blood capacitance

C=

2

1

+

1

……… ①

Electrical equivalent circuit

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analysis suggests that 1 MHz is an optimal frequen-

the VR, which is directly connected to the humancy for accurate measurements.

body via a blood circuit, to measure the liquid level

Furthermore, if we adopt the simplified as-within the VR. Therefore, assessing the measured

sumption that the capacitance of the liquid insidecurrent on the patient’s side is essential for ensuring

the VR is represented as Cb and equate it, as shown

electrical safety.

in Fig.9-b, two capacitance components （Cw）

To effectively evaluate the current experienced

emerge, attributed to the polycarbonate that makesby the patient, the current value is determined uti-

up the VR wall, and with the Cb of the filling liquid

lizing a leakage current measurement device（MD）.

positioned between them. The combined capacitanceThe circuit within this device simulates the phenom-

in this situation is shown in Fig.9-Equation ① . Con-

enon that the threshold for electric shock at highsidering the influence of different filling liquids

frequencies, specifically at 1 kHz or higher, increases（physiological saline and blood）based on their rela-with frequency. This allows for assessments thattive dielectric constant（εr）, the εr of polycarbonate,consider the frequency characteristics of patients inthe material of the VR, ranges from 2.9 to 3.0, where-

response to electrical shock. Using this MD circuit toas the εr of blood remains constant within the range

theoretically find the measured current on the pa-of 1 kHz to 1 MHz and is significantly high, ranging

tient side, when the input voltage is an effective val-from 10 to 10. Furthermore, εr is higher in physi-ue of 5 V（1 MHz）and the measured value Cs is 30

ological saline（50）and pure water（80）than inpF, the measured current on the patient side is 0.80

polycarbonate . Consequently, Cb is significantly

μA. The permissible value for patient leakage cur-higher than Cw, leading to the conclusion that therent for cardiac floating devices is set at 10 μA or

measured Cs value is primarily determined by Cw

below. The measured value obtained in this study

components. This study’s findings confirm that thewas less than 1/10 of the allowable limit, thereby

impact of blood is negligible.

falling within the standard safety range. Based on

3. Regarding the shape of the VR

the above, we believe that the method used in this

3

4 8）

9）

The correlation between the rise in liquid level
and the Cs value was 1.41 pF/cm in the cylindrical

study is electrically safe.
5. Regarding the usefulness of this study

container（Experiment 2）and averaged 1.24±0.07

Continuous monitoring of liquid levels in the

pF/cm within the VR（Experiment 1）. Although

VR has the potential to enable early detection of athere was a difference in the correlation between

reduction in blood volume, thereby enhancing safety

the increase in liquid level and capacitance for theby enabling prompt intervention at an early stage.

VR compared to cylindrical containers, the observed

We also suggest that this technique could be utilized

difference amounted to 0.17 pF/cm（Table 4）. Thisto stabilize blood levels and enable automatic con-implies that the measured Cs value can serve as atrol. Moreover, the method has applications beyond

reliable indicator for liquid level measurement, inde-measuring blood volume in VRs. For example, it canpendent of blood volume or the specific shape of thebe used to quantify the amount of cardioplegia ad-

VR. It suggests that this method can be applied con-ministered, monitor the volume of fluid removed bysistently across VRs with varying configurations.

ECUM systems, assess urine output, and continuous-

4. Regarding electrical safety

ly monitor water balance during CPB.

When integrating this method into CPB equip-

6. Future challenges

ment, adherence to safety standard JIS T 0601-1 is

The experiments detailed in this study demon-

imperative. This standard outlines acceptable values

strate that this method, being non-invasive and not

for leakage current and patient leakage current inin direct contact with blood, can detect even subtle

terms of electrical safety. In this study, a high-fre-changes in liquid level. Nevertheless, in this study,

quency current is applied through the outer wall of

the electrode width was set at 4 cm to enhance reso-136

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lution and achieve a substantial change in Cs values

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