Unipolar Ampliﬁer Enabling Measurement of Far-ﬁeld Intra-cardiac

Electromyogram for Blood Pump Control

Seraina Anne Dual

1,2,3 a

, Dominic Jacob

, Mirko Meboldt

1 b

and Marianne Schmid Daners

2 c

Product Development Group Zurich pd|z, ETH Zurich, Tannenstrasse 3, Z

urich, Switzerland

Radiology, Stanford University, 780 Welch Rd., Palo Alto, U.S.A.

Cardiovascular Institute, Stanford University, 780 Welch Rd., Palo Alto, U.S.A.

Keywords:

Electromyogram, Electrophysiology, Hemodynamic Monitoring, Unipolar Ampliﬁer, Brody Effect.

Abstract:

Heart pumps are implanted as an alternative to heart transplantation in patients with heart failure. Future de-

vices are expected to respond to the physiological need of each patient automatically. Physiological control

algorithms have shown to be robust if based on the measurement of end-diastolic volume (EDV); but real-time

measurements of EDV are not available. In theory, the EDV has been shown to correlate with the maxi-

mum depolarization amplitude (DA) of the intra-cardiac electromyogram (iEMG). In practice, this requires

the unipolar measurement of an electric signal, which has not been attempted inside the heart. We herein

present a custom-built unipolar ampliﬁer, which we connected to a heart pump cannula prototype with four

integrated off-the-shelf pacemaker electrodes. The recorded signals from the unipolar ampliﬁer showed excel-

lent agreement with the gold standard measurement of surface electrocardiogram (ECG) using a commercial

ECG simulator and in-vivo data acquired in four pigs. We present recordings of unipolar iEMG from the

cannula of a heart pump. The new unipolar ampliﬁer makes it possible to measure the DA of the iEMG and

therefore potentially provides a real-time EDV signal to heart pumps for physiological control in the future.

1 INTRODUCTION

Heart pumps are implanted in patients with heart fail-

ure as an alternative to heart transplantation (Kirklin

et al., 2018). The heart pump is implanted in paral-

lel to the heart and provides additional blood ﬂow to

supplement organ perfusion, where the level of sup-

port is determined by the pump speed. Inadequate

pump speed settings can lead to serious complica-

tions such as pulmonary congestion in the case of

under-pumping or suction of the myocardium in case

of over-pumping (Schima et al., 2008). Physiological

control algorithms based on the end-diastolic volume

(EDV) of the left ventricle (LV) have proven save and

robust in the experimental setting (Petrou et al., 2018;

Ochsner et al., 2014; Ochsner et al., 2017). How-

ever, their implementation is hindered by the lack of

real-time measurements of the EDV. A measurement

of EDV could further inform the clinical management

of patients with heart failure, as EDV is understood

to be a surrogate of ﬂuid status. Even if no physio-

https://orcid.org/0000-0001-6867-8270

https://orcid.org/0000-0001-5828-5406

https://orcid.org/0000-0002-6411-8871

logical control algorithm was implemented, real-time

measurement of EDV could help clinicians set an ap-

propriate pump speed or administer diuretics. How-

ever, real-time EDV measurements is unavailable to

date.

The LV EDV has been suggested to correlate with

the maximum electric depolarization amplitude (DA)

of the heart by Brody et al in 1956 (Brody, 1956). The

maximum electric signal (R-wave) coincides with the

end-diastolic time point. The so-called Brody effect

attenuates or increases a measured electric potential

due conduction inhomogeneity. The blood in the LV

has a higher conductivity then its surrounding and

thus constitutes an inhomogeneity. The electric po-

tential in a homogenous thorax Φ

hom

is different from

the electric potential if including the inhomogeneity

of the LV blood pool Φ

inhom

(Brody, 1956). The

Brody factor is calculated as the ratio between these

two electric potentials (β) (Eq. 1) and depends on the

size of the LV blood pool.

β =

inhom

hom

(1)

Hence, depending on the LV EDV, the measured elec-

tric potential is increased or decreased by the Brody

Dual, S., Jacob, D., Meboldt, M. and Schmid Daners, M.

Unipolar Ampliﬁer Enabling Measurement of Far-ﬁeld Intra-cardiac Electromyogram for Blood Pump Control.

DOI: 10.5220/0010253800400048

In Proceedings of the 14th International Joint Conference on Biomedical Engineering Systems and Technologies (BIOSTEC 2021) - Volume 1: BIODEVICES, pages 40-48

ISBN: 978-989-758-490-9

factor. Changes in the DA will correlate with changes

in the EDV.

Previous studies have investigated the Brody ef-

fect in the surface electrocardiogram (ECG) (Madias

et al., 2005; Amoore, 1985). The gold standard mea-

surement of ECG is performed by obtaining the elec-

tric potential difference between two points on the

body surface (DI, DII, DIII) according to the equa-

tions:

DI = φ

− φ

(2)

DII = φ

− φ

(3)

DIII = φ

− φ

(4)

It was shown that with respect to surface potentials,

the position of the heart in the thorax is likely to affect

the ECG to a much greater degree than the Brody ef-

fect (Amoore, 1985). The electric depolarization can

also be measured using the intra-cardiac electromyo-

gram (iEMG) inside the blood pool. Battler et al.

1980 showed that if measured in close proximity to

the heart the relationship between DA and EDV in-

verts from positive to negative, if measured endo- or

epicardially in conscious dogs (Battler et al., 1980).

Furthermore, the EDV and the DA were also corre-

lated when the iEMG was measured in the blood pool

using a pig tale catheter (Dual et al., 2016). Theoret-

ical analyses further concluded that the Brody effect

depends strongly on the choice of reference and can

only be measured with unipolar electrodes (van Oos-

terom, 2010), which has not been attempted so far.

We herein present a novel unipolar ampliﬁer to as-

sist in measuring the iEMG from electrodes deployed

on the cannula of a heart pump. First, the ampliﬁer

is tested against the gold standard surface ECG mea-

surement. Subsequently, we show unipolar measure-

ments of iEMG in the LV blood pool in four animals.

2 METHODS

2.1 Design of Cannula and Electrodes

A dummy cannula featuring four electrodes was de-

signed in order to facilitate positioning in the blood

pool using the typical implantation procedure for

heart pumps (see Figure 1). The cannula was 3D-

printed from polyamid 12 (PA 12) using a selec-

tive laser sinter process. The geometry of the can-

nula matched the HVAD (Medtronic, Minneapolis,

MN, USA) suture ring with an outer diameter of

20.6 mm. The cannula was retroﬁtted with two com-

mercially available pacemaker leads (25539254 IS-

1 BI, Biotronik, Germany) with two electrodes per

lead (B1-B4). The leads were attached and the cav-

ities sealed using silicone glue (Dow Corning 732).

Electrodes B3 and B4 were shielded from interac-

tion with the myocardium, while B1 and B2 were not.

The leads were connected to the data acquisition unit

(DAQ) using shielded cables.

2.2 In-vivo Trial

The cannula prototype was implanted and tested

in acute pig models (n = 4, female, Swiss large

white).The animal housing and all procedures and

protocols were approved by the Cantonal Veterinary

Ofﬁce (Zurich, Switzerland) under the license number

219/2016. Animal housing and all experimental pro-

cedures were in accordance with Swiss animal wel-

fare protection law, and conform to European Direc-

tive 2010/63/EU of the European Parliament and the

Council on the Protection of Animals used for Scien-

tiﬁc Purposes, and to the Guide for the Care and Use

of Laboratory Animals.

After loss of postural reﬂexes following premed-

ication with ketamine (15 mg/kg), midazolam (0.5

mg/kg) and atropine (0.1-0.2 mg/kg) anesthesia was

deepened by an intravenous bolus injection of propo-

fol (1-2 mg/kg body weight) and the animals were

intubated. General anesthesia was maintained with

propofol (2-5 mg/kg/h, i.v.) in combination with

isoﬂurane (1%-2.5%) by positive pressure ventila-

tion in an air-oxygen mixture (1:1, 4-6 L/min)with

an inspired oxygen fraction (FiO2) of 0.5, tidal vol-

ume of 6-8 mL/kg, a frequency of 10-15 breaths per

minute and a positive end expiratory pressure (PEEP)

of 5 cmH

O. For intra-operative analgesia, buprenor-

phine (0.01 mg/kg body weight) was administered

intravenously approximately 30 minutes before the

cut-down and was continued throughout anesthesia.

During surgery, animals received a continuous intra-

venous infusion of crystalloids (5-7 mL/kg/h).

For maximum control of hemodynamics, a mod-

iﬁed cardiopulmonary bypass (CPB) was installed

without oxygenator, via a single femoral vein access

realized by inserting a venous CPB-cannula in the ani-

mal’s femoral vein. The surface electrodes on the left

arm (LA) and leg (LL) as well as on the right arm

(RA) were attached using sub-dermal steel needle-

electrodes.

After hemi-sternotomy was performed for access,

the cannula prototype was implanted into the LV at

the apex, using an HVAD suture ring, and thus mim-

icking a heart pump implant (see Figure 1). Af-

ter completion of both surgical steps, the thoracic

spreader was removed, and the suprasternal tissue

closed using sutures, while the pericardium was left

Unipolar Ampliﬁer Enabling Measurement of Far-ﬁeld Intra-cardiac Electromyogram for Blood Pump Control

cannula

HVAD

suture

ring

electrode

shield

2 cm

1 cm

lead

passages

1 cm

Figure 1: Cannula featuring four electrodes from pacemaker leads.

open. The thorax was re-opened only if necessary, to

stabilize or resuscitate the animal.

2.3 Unipolar Ampliﬁer

The design of the unipolar ampliﬁer is based on the

approach by Garguilo et al. (Gargiulo et al., 2013;

Gargiulo, 2015). The ampliﬁer was modiﬁed to use

only one single common pseudo-inﬁnite reference for

all other channels (see Figure 2). This means, that

also the reference electrode is referenced to itself and

allows for signal recording. The signal is further am-

pliﬁed across two ampliﬁer stages.

The goal of the reference signal was to create

a pseudo-inﬁnite potential by low-pass ﬁltering the

signal from the reference electrode. The reference

electrode is ﬁltered using a 2

-order Sallen-Key low

pass ﬁlter with a cutoff frequency of 0.034 Hz (47

µF(10%) capacitors and 100 kΩ (1%) resistors). The

resulting high input impedance minimizes crosstalk

with the surface ECG. Both ampliﬁer stages are

grounded through the DAQ, which is also wired to

the right leg surface electrode of the animal.

The ﬁrst ampliﬁer stage consists of an INA116 in-

strumentation ampliﬁer with a ﬁxed gain, optimized

to provide sufﬁcient common-mode rejection ratio,

while avoiding saturation due to DC offset voltages.

The inverting input is wired to the buffered output

of the pseudo-inﬁnite reference. The measuring elec-

trode connects to a 100 kΩ coupling resistor and then

buffered and forwarded to the non-inverting input of

the INA116. The guard terminal of the non-inverting

input is connected to the shield of the measuring elec-

trode. Of note, the ﬁrst differential ampliﬁer stage

does not remove high frequency, as the two input sig-

nals are not identical in their noise levels. High fre-

quency noise is reduced by band pass ﬁlter.

Between the two ampliﬁer stages we implemented

a high pass followed by a low pass ﬁlter. The high

pass attenuates offsets and low frequency drift to

increase to achievable gain in the second ampliﬁer

stage. The high pass was implemented as a ﬁrst or-

der active high pass with a cutoff frequency of 0.015

Hz (47 µF(10%) capacitors and 220 kΩ(1%) resis-

tors). The low pass was implemented as a 2

-order

Sallen-Key ﬁlter 590 Hz to suppress high frequency

noise and attenuate signals close to and above the

Nyquist Frequency (1 kHz)(27 kΩ(1%) resistors and

10 nF(5%) capacitors).

The second ampliﬁer stage (INA128) features

an adjustable gain to adapt to variable electrode

impedances and the unknown value of the iEMG am-

plitude. The inverting input of the second ampliﬁer

stage is wired to ground. The reference terminal is

connected to the inverting input of the signal con-

ditioner, allowing the ampliﬁer to supply signals to

DAQs ﬂoating at different levels. If the signal condi-

tioner is a differential type, the inverting DAQ input

is connected to the DAQ ground by the jumper to pro-

vide a reference level. If the signal conditioner is a

single ended type, the jumper is left open, as the sin-

gle ended inverting input provides a ground reference

itself.

The design illustrated in Figure 2 can be scaled to

any number of channels by replicating the circuitry in

the grey box for each channel. The rest of the circuit is

needed once, independent of the number of channels.

2.4 Data Acquisition

All data was acquired using the ACQ 7700 (Data Sci-

ence International, St. Paul, MN, USA). A Windows

machine running the Ponemah software logged the

data at a sampling frequency of 2000 Hz. The in-

BIODEVICES 2021 - 14th International Conference on Biomedical Electronics and Devices

output

signal

Ba ery - GND

low pass

f = 0.034 Hz

high pass

f = 0.015 Hz

low pass

f = 590 Hz

ampli

electrode

Data

acquisi

reference signal

for other channels

bu er

control unit

INA116

INA128

Figure 2: Schematic of the unipolar ampliﬁer circuit.

put range of the Universal XE Signal Conditioner and

the Advanced Basic DC4 Signal Conditioners were

set to ±5 V. The Universal XE operated as a differen-

tial signal conditioner and the Advanced Basic DC4

operated in single ended mode. The unipolar ampli-

ﬁer was connected to the Advanced Basic DC4 Signal

conditioner. As Gold Standard surface ECG we used

a Multi-Lead Pod connected to the digital Communi-

cation Module.

2.5 Experimental Protocol

The unipolar ampliﬁer was tested against the gold

standard using (1) an ECG generator (Prosim 8, Fluke

Biomedical, Washington, USA) and (2) in-vivo mea-

surements of surface ECG in four pigs.

The Gold standard records the differentials be-

tween leads denoted as DI, DII and DIII. The unipo-

lar ampliﬁer measures single potentials denoted as left

arm (φ

), left leg (φ

) and right leg (φ

). The dif-

ferential leads (DI, DII, DIII) were reconstructed from

the unipolar ECGs for comparison according to equa-

tions 2,3 and 4.

DII

DIII

Unipolar

ampli

DII

DIII

Gold

standard

ECG

simulator

Figure 3: Schematic of the experimental setup.

The ﬁrst step consisted of three conﬁgurations

(see Figure 3): (1A) Unipolar ampliﬁer - ECG simula-

tor; (1B) Gold standard - ECG simulator; (1C) Unipo-

lar ampliﬁer and Gold standard - ECG simulator. In

the second step, the Unipolar ampliﬁer and the Gold

standard ECG ampliﬁer were connected to the surface

electrodes on the LA, LL and RA of each of the pigs

(2C). In the last step, the unipolar ampliﬁer was con-

nected to the four electrodes on the cannula instead

(see Figure 4). Each protocol was repeated four times

and each measurement consisted of 15s of recorded

signal length.

DAQ

Unipolar

ampli

Cannula

Figure 4: Connections cannula and unipolar ampliﬁer.

2.6 Data Processing

A median ﬁlter of length three samples was used to

smooth the signal. A high pass ﬁlter eliminated re-

maining low frequency drifts. The high pass ﬁlter

was realized with a minimum order inﬁnite impulse

response ﬁlter with a passband frequency of 0.05 Hz

and a stopband frequency of 0.01 Hz. Finally, a 6

or-

der notch ﬁlter with a center frequency of 50 Hz and

a -3dB bandwidth of 0.5 Hz was implemented and

a low pass ﬁlter removed additional high frequency

noise with a cutoff frequency of 250 Hz.

Unipolar Ampliﬁer Enabling Measurement of Far-ﬁeld Intra-cardiac Electromyogram for Blood Pump Control

2.7 Data Analysis

2.7.1 Crosstalk

Crosstalk can occur during simultaneous measure-

ments of surface ECG leads and the iEMG using the

same ampliﬁer. Crosstalk was assessed by comparing

measurements of each ampliﬁer alone (1A, 1B) and

together (1C). Only differential measurements were

compared, computed using equations 2, 3, and 4.

The ECG without crosstalk is denoted as ECG and

the one with possible crosstalk is denoted as

]

ECG.

If no crosstalk is present and noise neglected, then

ECG =

]

ECG holds. The normalized root mean square

error (NRMSE), the DA and the ECG’s signal power

were used as performance indicator (

•). The NRMSE

is computed as follows, using the root mean square

(RMS):

NRMSE =

RMS(

]

ECG −ECG)

RMS(ECG)

(5)

Furthermore, we compute the error in detecting DA

calculated as relative number according to (equation

6).

NDAE =

DA − DA

(6)

2.7.2 Comparison Unipolar Ampliﬁer vs. Gold

Standard

The comparison between the unipolar and the gold

standard ampliﬁer was done using the simultaneously

acquired data of both the ECG simulator (1C) and

the in-vivo data (2C). The signal of the gold standard

ampliﬁer served as reference, while the reconstructed

differential leads from the unipolar ampliﬁer ECGs

were compared to it. The same performance indica-

tors as in the previous section were computed, with

minor adjustments as displayed in equations 6 and 7.

NRMSE =

RMS(ECG|

− ECG|

)

RMS(ECG|

)

(7)

DA =

DA|

(8)

3 RESULTS

3.1 Crosstalk Quantiﬁcation

The crosstalk between the unipolar ampliﬁer and the

gold standard ampliﬁer were tested on the ECG sim-

ulator. The values of the performance indicators are

shown in Table 1 for the gold standard ampliﬁer and

in Table 2 for the unipolar ampliﬁer. The

NRMSE re-

mained below 2% in all three surface leads. The error

of the estimated DA also remained below 2%. The

sum of the power from 0.05 Hz to 250 Hz remained

constant over the crosstalk test with an error of less

than 5%. We conclude that our ampliﬁer measures

the surface and intra-cardiac potentials independently

from each other. A simultaneous measurement is ad-

vantageous, because we can then report the effect of

changes in LV volume for both the surface ECG as

well as the intra-cardiac iEMG. The surface ECG may

be confounded by a change in distance between the

heart and the chest wall. The unipolar iEMG, how-

ever, should allow for measurement of the Brody ef-

fect.

Table 1: Crosstalk of gold standard.

Gold Standard

NRMSE

NDAE

Unit [%] [%]

A1-DI 0,37 -0,10

A1-DII 0,67 0,00

A1-DIII 0,52 0,00

A2-DI 0,45 0,15

A2-DII 0,34 0,31

A2-DIII 0,30 -0,10

A3-DI 0,34 0,61

A3-DII 0,41 0,58

A3-DIII 0,45 -0,50

A4-DI 0,43 -0,34

A4-DII 0,31 -0,66

A4-DIII 0,30 0,50

Table 2: Crosstalk of unipolar ampliﬁer.

Unipolar Ampliﬁer

NRMSE

NDAE

Unit [%] [%]

A1-DI 0,37 0,00

A1-DII 0,34 -0,17

A1-DIII 0,32 1,09

A2-DI 0,72 -0,31

A2-DII 1,54 -1,01

A2-DIII 1,21 0,00

A3-DI 0,39 -0,24

A3-DII 0,34 0,51

A3-DIII 0,33 -1,43

A4-DI 0,35 0,00

A4-DII 0,34 0,00

A4-DIII 0,32 -0,41

BIODEVICES 2021 - 14th International Conference on Biomedical Electronics and Devices

3.2 Performance of 50 Hz Notch Filter

The 50 Hz notch ﬁlter was assessed using a single

surface electrode (φ

) measured against the pseudo-

inﬁnite potential (Figure 5). In the top panel, the am-

pliﬁer was wired to the ECG simulator (1C) and in

the bottom panel to the animal (2C). The signal was

ﬁltered with a 6th order notch ﬁlter at 50 Hz. The root

mean square error was calculated under the assump-

tion that the ﬁltered signal was the true signal. The re-

sulting RMSE for the signal obtained from the ECG

simulator is 0.04 mV. The error for the in-vivo data

was 0.24 mV, and increased with respect to the value

obtained in the ECG simulator. The 50 Hz notch ﬁlter

was critical in obtaining an undisturbed signal. The

large amount of digital equipment in the laboratory

facilities introduced a high amount of noise. We ex-

pect this to be true to a similar extent if such a device

was implemented in the clinical context.

0.0 0.1 0.30.2 0.70.5 0.60.4

[mV]

ECG Simulator

0.0 0.1 0.30.2 0.70.5 0.60.4 0.8

[mV]

In-vivo data

Figure 5: Reduction of 50 Hz noise using notch ﬁlter.

3.3 Comparison of Unipolar Ampliﬁer

vs. Gold Standard

We compare the surface lead ECGs measured by the

unipolar and the gold standard ampliﬁer for both sim-

ulated ECGs (see Table 3) and data obtained in-vivo

(see Table 4). A representative example of a time se-

ries data is shown in Figure 6. While connected to the

ECG simulator, the respective errors are within the

range of what was observed during the crosstalk ex-

periments (Section 3.1). The

NRMSE was below 1%

during all tests. The values of the performance indi-

cators

DA remained below 1.5%. The unipolar am-

pliﬁer measured the same potentials as the gold stan-

dard, despite relying on a pseudo-inﬁnite potential as

a reference.

Table 3: ECG Simulator Unipolar Ampliﬁer vs. Gold Stan-

dard.

ECGSimulator

NRMSE

NDAE

Unit: [%] [%]

A1-DI 0,39 0,50

A1-DII 0,52 0,90

A1-DIII 0,43 -0,67

A2-DI 0,40 0,70

A2-DII 0,83 0,12

A2-DIII 0,72 1,44

A3-DI 0,39 0,51

A3-DII 0,35 0,18

A3-DIII 0,32 1,03

A4-DI 0,35 0,70

A4-DII 0,32 0,63

A4-DIII 0,29 0,71

In-vivo, the errors increase as more environmental

disturbances and noise are picked up by the ampliﬁers

(see Figure 5). The

NRMSE of the signal remained

below 5% for 10 out of 12 measurements. The devi-

ations were especially high in Animal 2 (A2), which

also showed a variable heart rhythm. Deviations in

the detection of the DA (

DA) were found to be less

than 5% in 9 out of 10 evaluated ECGs. The very

high value for A3-DII originated from an error in the

signal processing of the maximum DA peak detection.

The errors in differential lead DII were mostly higher,

when compared to the DI and DIII lead. It is impor-

tant to note that we report relative error values, such

that the low signal strength of DII results in unfavor-

able error values.

An ideal ampliﬁer has minimal low frequency

drift. In Figure 5 on the right we can see a deviation

of the red signal from the iso-electric line. The sig-

nal measured by the gold standard ampliﬁer is con-

founded by a low frequency drift. In this particu-

lar example, the unipolar ampliﬁer outperformed the

gold standard measurement. Any reported error val-

ues might thus be confounded by low frequency drift

in the gold standard ampliﬁer.

In summary, the accuracy of measuring unipolar

surface leads is accurate enough and the concept of

using a unipolar measurement works.

Unipolar Ampliﬁer Enabling Measurement of Far-ﬁeld Intra-cardiac Electromyogram for Blood Pump Control

ECG Simulator

In-vivo data

Figure 6: Comparison between Gold Standard and Unipolar Ampliﬁer.

Table 4: In-Vivo Unipolar Ampliﬁer vs. Gold Standard.

In-vivo

NRMSE

NDAE

Unit: [%] [%]

A1-DI - -

A1-DII 0,83 -1,18

A1-DIII - -

A2-DI 5,17 3,46

A2-DII 2,10 2,75

A2-DIII 6,17 1,15

A3-DI 1,81 -0,50

A3-DII 2,59 -92,57

A3-DIII 2,62 2,83

A4-DI 1,31 0,01

A4-DII 1,91 1,93

A4-DIII 0,85 -0,73

3.4 Effect of Post-processing of

Depolarization Amplitude

The effect of post-process ﬁltering is displayed in Fig-

ure 7. We estimated the amount of 50 Hz noise, by

applying an additional 50 Hz ﬁlter and calculating the

error between the ﬁltered and the original signal. The

ECG lead II was used for this analysis. In the sur-

face ECG, a 50 Hz noise with amplitude of 0.018 mV

was present with a DA of 0.61 mV (3%). In the iEMG

leads, the 50Hz noise amplitude equals approximately

5% (0.63 mV) of the DA. The noise reduction and the

distortion of the ECG signal and DA due to the ﬁl-

ters is shown in Figure 7. Post-processing of the ac-

quired signals using an additional 50 Notch ﬁlter and

a lowpass ﬁlter was found to be necessary and feasible

without losing any signal amplitude of the DA.

3.5 Intra-cardiac Electromyogram

All four intra-cardiac electromyograms could be ac-

quired synchronously with one intra-cardiac electrode

chosen as reference. An example of the far-ﬁeld intra-

cardiac signals obtained in Animal 2 is shown in Fig-

ure 8. The surface ECG shows a normal rhythm in all

three leads DI, DII, and DIII. The recordings between

the four unipolar intra-cardiac electrodes are highly

similar. However, the shape of the iEMG waveform

(B1-B4) is distinctly different compared to the surface

leads (DI-DIII). The R-wave in the surface ECG coin-

cides with the intra-cardiac depolarization amplitude.

Still, the intra-cardiac potentials show another distinct

peak originating from the re-polarization at the end of

the cardiac cycle. The re-polarization is less distinct

in the surface recordings.

3.6 Implications for Blood Pump

Control

The proposed technology constitutes and important

step towards integrated real-time measurement of

EDV for blood pump control. The real-time measure-

ment of LV volume has been attempted previously

using alternative measurement principles. Four ring-

shaped electrodes were integrated into the cannula of

a heart pump to allow for a measurement of electric

impedance (Cysyk et al., 2018). The outer two elec-

trodes span an electric ﬁeld, while the inner electrodes

record changes in resistance. The technology showed

limited sensitivity, mainly because the measurement

BIODEVICES 2021 - 14th International Conference on Biomedical Electronics and Devices

One heart beat

Depolarization

Depolarization amplitude

Figure 7: Effect of ﬁltering on the measurement of the depolarization amplitude.

-0.5

0.5

Amplitude [mV]

DII

DIII

0 5 10 15

-15

-10

-5

Amplitude [mv]

Time [s]

Surface Electrocardiogram

Intra-cardiac Electromyogram

Figure 8: Surface (top) and intra-cardiac electromyogram (bottom).

was limited to the blood pool immediately surround-

ing the cannula. The motivation for developing this

unipolar ampliﬁer was to allow for a far-ﬁeld electric

measurement of the entire LV blood pool. Another

promising approach makes use of electric impedance

measurement across implantable deﬁbrillator leads

(Haines et al., 2017). As most patients with heart fail-

ure have a pacemaker, this could be feasible but would

require communication across devices. Alternatively,

ultrasonic concepts have been proposed and studied

but their applicability remains limited considering the

complexity of integrating such technology in an im-

plantable device (Dual et al., 2019).

Blood pump control requires an accuracy in EDV

of 20% for robust control. As a next step, the unipo-

lar ampliﬁer will be used to study how accurate we

can estimate the EDV from the proposed unipolar

iEMG measurements. Furthermore, the inﬂuence of

the hematocrit needs to be carefully assessed. Pro-

vided positive results, the iEMG could enable blood

pump control based on the EDV using established

electrode technology.

4 CONCLUSIONS

We herein present a method to measure the unipo-

lar intra-cardiac electro-myogram using a novel am-

pliﬁer. The design enables accurate detection of the

depolarization amplitude in the intra-cardiac blood

pool and will allow us to investigate the relationship

between iEMG and the EDV (Brody-effect) in the

future. The unipolar ampliﬁer reproduces the gold

standard differential ECG measurements with mini-

mal errors. The unipolar ampliﬁer is thus capable of

measuring electric potentials in a robust and accurate

way. Furthermore, the post-processing methodology

enables the preservation of the peak depolarisation

amplitude, despite signiﬁcant noise. This technical

work will enable future investigations of the Brody

Unipolar Ampliﬁer Enabling Measurement of Far-ﬁeld Intra-cardiac Electromyogram for Blood Pump Control

effect during changes in left ventricular volume. The

current design is limited by the use of the right leg

of the animal as ground for the data acquisition soft-

ware. Furthermore, the current design of the unipolar

ampliﬁer does not actively isolate the subject from the

DAQ. The performance of a fully portable system will

need to be re-evaluated using a similar experimental

setup as proposed in this paper.

ACKNOWLEDGEMENTS

The authors have no conﬂict of interest relevant to this

publication. The authors thankfully acknowledge the

ﬁnancial support by the Georg und Bertha Schwyzer-

Winiker Foundation, the IMG Foundation, as well as

the ETH Zurich Foundation. This work is part of the

Zurich Heart project under the umbrella of Univer-

sity Medicine Zurich. Furthermore,the authors thank

Simon Suendermann, Christoph Starck, Nikola Ce-

sarovic, Mareike Kron and Marko Canic for their sup-

port with the animal study and Sara Mettler for the

electrical engineering support.

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