ADAPTIVE AURICULAR ELECTRICAL STIMULATION

CONTROLLED BY VITAL BIOSIGNALS

Transition from Fixed to Adaptive and Synchronized Electrical Stimulation

Controlled by Heart Rate Variability and Blood Perfusion

Eugenijus Kaniusas

Institute of Electrical Measurements and Circuit Design, Vienna University of Technology

Gusshausstrasse 27-29/E354, Vienna, Austria

Jozsef Constantin Szeles, Tilo Materna

Department of Surgery, University of Vienna, Vienna, Austria

Giedrius Varoneckas

Sleep Medicine Centre, Klaipeda University Hospital, Klaipeda, Lithuania

Keywords: Electrical stimulation, heart rate variability, physiological sensors, adaptive stimulation, ear.

Abstract: The auricular electrical punctual stimulation is usually applied for pain relief. The common application

involves fixed stimulation parameters, which makes the simulation insensitive to prevailing pain or stress

level and may lead to a disadvantageous over-stimulation. In order to address this issue, the given position

paper presents an experimental background leading to a conceptual design of an adaptive and synchronized

stimulation technique. Here parameters of the heart rate variability are used as stimulation biofeedback,

while the stimulating signal is synchronized with cardiac or respiratory activity to boost stimulation effects.

1 INTRODUCTION

The auricular electrical punctual stimulation (P-

Stim) is an electrical nerve stimulation technique,

newly introduced by Dr. Szeles (Szeles, 2001a). The

P-Stim is usually applied for acute and chronic pain

relief. A reduction of pain perception and pain-

relieving medications is attained (Szeles, 2001b;

Sator-Katzenschlager, 2006; Likar, 2007), even with

an induction of anaesthesia state (Litscher, 2007).

Furthermore, reduction of body mass index (BMI) in

obese patients (Szeles, 2001b), increase of blood

flow velocity and oxygenation (Szeles, 2004) were

reported during the P-Stim application. The

advantages of the electrical stimulation over

conventional (manual) acupuncture with respect to

pain relief, well-being and sleep quality were

documented in (Sator-Katzenschlager, 2004) for

extended periods of time up to 3 months.

Figure 1: a) Ear with indicated approximate auricula

branch of vagus nerve according to (Peuker, 2002; Gao,

2008). b) Electrical punctual stimulation of the auricula

vagus nerve (P-Stim).

Stimulating device

(= reference electrode)

Vagal stimulating point

Vagal nerve

branch

a) b)

304

Kaniusas E., Szeles J., Materna T. and Varoneckas G. (2009).

ADAPTIVE AURICULAR ELECTRICAL STIMULATION CONTROLLED BY VITAL BIOSIGNALS - Transition from Fixed to Adaptive and Synchronized

Electrical Stimulation Controlled by Heart Rate Variability and Blood Perf.

In Proceedings of the International Conference on Biomedical Electronics and Devices, pages 304-309

DOI: 10.5220/0001779703040309

 SciTePress

The particular beneficial effects of the P-Stim are

still under discussion, whereas a number of the

following mechanisms seem to be involved. The

electrical stimulation of the afferent nerve receptors

may influence gate mechanisms in the central

nervous system (CNS), preventing pain-related

action impulses from reaching the CNS and avoiding

the person’s perception of pain. Furthermore, an

indirect stimulation of pain receptors and activation

of inhibitory pain control systems may be involved,

as well as a stimulated release of neurotransmitters,

e.g., endorphins and other endogenous opioids.

Though the efficiency of the P-Stim was subject-

ively proved in many cases and the P-Stim is already

in clinical use, only recently some objective and

statistical evidence was established on the

stimulation effects. Given that an auricular branch of

vagal nerve (Fig. 1a) is electrically stimulated by the

P-Stim device (Fig. 1b), effects on the heart rate

variability (HRV) were assessed in the time and

spectral domain (Kaniusas, 2008; Gbaoui, 2008a)

and in the state space (Gbaoui, 2008b) by our group.

In addition, blood perfusion (BP) changes during

stimulation were investigated (Kaniusas, 2008). In

the latter studies optical plethysmography (OPG)

served as biofeedback to derive the HRV and BP.

Here the suitability of the HRV and BP analysis

is given by the fact that the stimulated afferent vagal

nerve goes to the nucleus solitarius in the CNS,

whereas the sinus node of the heart is controlled by

the efferent vagus nerve from the nucleus ambiguous

in the CNS. The node initiates heart contractions

with particular rate dynamic and ejection strength,

thus the HRV and BP being the appropriate

parameters to register the stimulation effects.

The given position paper is intended to introduce

a novel technology for an adaptive and synchronous

P-Stim controlled by the HRV and BP. As a starting

point, technical data and new experimental results

concerning parasympathetic/sympathetic power in

the HRV from the standard P-Stim are presented,

which yield a substantial basis and arguments for the

introduction of the adaptive stimulation.

2 ESTABLISHED STIMULATION

2.1 Methodology

The P-Stim was applied in supine position of three

healthy volunteers: two men aged 41/29 with BMI

25/23 kg/m

and one female aged 19 with BMI of

20 kg/m

. A precise positioning of the needle in the

vicinity of the vagal nerve (Fig. 1) was facilitated by

local conductivity measurements, for the local

conductivity increases in the region of the nerve and

its supporting blood vessels.

As demonstrated in Fig. 2a, the voltage U of the

electrical stimulation comprises monophasic

impulses with changing polarity, stimulation

(=repetition) rate f

of 1 Hz, amplitude A of 4 V and

impulse duration of about 1 ms.

The duration of the recordings was about 15 min

before, during, and after the stimulation,

respectively. At least two recordings were performed

per volunteer with a time-lag in-between of more

Figure 2: Stimulation waveforms of a fixed (a) and (b) adaptive electrical punctual stimulation.

1 /f

Pause Stimulation Stimulation

1 /f

U, I

1 /f

Pause

1 /f

(≠ 1 /f

)

ADAPTIVE AURICULAR ELECTRICAL STIMULATION CONTROLLED BY VITAL BIOSIGNALS - Transition from

Fixed to Adaptive and Synchronized Electrical Stimulation Controlled by Heart Rate Variability and Blood Perfusion

305

than 10 days. It should be noted that the needles for

stimulation were inserted about 5 min before the

recording to avoid needle’s positioning effects, i.e.,

to avoid temporal effects of manual acupuncture.

In parallel, the OPG signal s

OPG

from the finger

was assessed as biofeedback. Here the relatively

high sampling rate of 2 kHz is needed for an

accurate HRV analysis (Guidelines, 1996). A typical

course of s

OPG

is depicted in Fig. 3a.

The instantaneous heart rate f

for the HRV

analysis was estimated from s

OPG

, as demonstrated

in Fig. 3a, with artefacts and noisy segments being

manually removed. The prominent minima in s

OPG

which correspond to the onset of the systole or blood

ejection, were detected as fiducial points for the

calculation of the instantaneous f

The investigation of the resulting f

sequence in

the spectral domain comprised power in the

established frequency ranges (Guidelines, 1996):

low frequency range 0.04-0.15 Hz corresponding to

sympathetic power P

SYM

and high frequency range

0.15-0.4 Hz corresponding to parasympathetic

power P

PAR

. Both P

SYM

and P

PAR

were estimated for

sequence windows of 300 s with 50 % overlap. It

should be noted that there are controversial

indications that P

PAR

is also present in the low

frequency range.

The BP is given by the course of s

OPG

(Fig. 3a).

In particular, the amplitude deflection of s

OPG

within

a single heart cycle corresponds approximately to

both amount of blood ejected (=left ventricular

stroke volume) and vesicular compliance.

The respiration reference s

(Fig. 3b) was

established by a skin curvature sensor on the chest,

as described in (Pfützner, 2006; Kaniusas, 2004).

2.2 Results

2.2.1 Heart Rate Variability

Fig. 4b and Fig. 5b demonstrate a temporal increase

of P

PAR

during stimulation, which temporal

activation is given in Fig. 4a and Fig. 5a. The

relative increase of P

PAR

among volunteers was

about 20 %, which was observed in all sessions but

one, probably because of a relatively high initial

value of P

PAR

. A temporal dip of P

PAR

was often

observed during the stimulation.

No unique tendencies were registered in the

behaviour of P

SYM

, as demonstrated in Fig. 4c and

Fig. 5c. However, stress relaxation effects could be

observed in some cases even in healthy volunteers.

In Fig. 4b,c and Fig. 5b,c dashed ellipses mark the

corresponding time intervals, where P

PAR

increases

and P

SYM

concurrently decreases. In general, such

changes of P

PAR

and P

SYM

tend to indicate ongoing

restorative effects.

The stimulation effects on P

PAR

were discussed

in a wider context in (Kaniusas, 2008; Gbaoui,

2008a), considering additionally parameters in the

Figure 3: a) Optical plethysmography signal s

OPG

with an estimated cardiac rate f

from indicated systolic onset points (*)

and an estimated respiratory rate f

from the envelope. b) The corresponding respiration signal s

from the chest ski

curvature sensor.

OPG

(rel.units)

t (s)

Cardiac components

Envelope

1/ f

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306

Figure 4: Effects on heart rate variability in the female

subject. a) Temporal activation of the electrical

stimulation (P-Stim OFF or P-Stim ON). b) The

corresponding parasympathetic power P

PAR

. c) The

corresponding sympathetic power P

SYM

time domain and state space. Aforementioned

tendencies of P

PAR

were also found in (Haker, 2000),

even during non-electrical auricular stimulation by

acupuncture needle.

In contrast to P

PAR

, none of the mentioned

studies indicate clear tendencies of P

SYM

. This is

likely to be attributed to the study enrolment of only

healthy unstressed pain-free individuals in resting

state, where potential changes or improvements of

SYM

are strongly restricted.

2.2.2 Blood Perfusion

The BP is given by the course of s

OPG

, as show in

Fig. 3a. It is important to observe that not only the

instantaneous cardiac activity but also the respiration

can be derived from s

OPG

In particular, the systolic onset points, as marked

by asterisks in Figure 3a, give a useful reference to

heart excitation. These points are delayed by about

200 ms from the actual excitation of the heart

ventricles (= R peaks in electrocardiography (ECG))

with the delay being nearly constant.

Figure 5: Effects on heart rate variability in a male subject.

a) Temporal activation of the electrical stimulation (P-

Stim OFF or P-Stim ON). b) The corresponding

parasympathetic power P

PAR

. c) The corresponding

sympathetic power P

SYM

The respiratory cycle can be derived from s

OPG

as indicated by the envelope in Fig. 3a. Here the

amplitude modulation of s

OPG

results from the

respiratory induced modulation of the left ventricu-

lar stroke volume which temporally increases during

expiration. The simultaneously recorded respiration

reference s

(Fig. 3b) proves the respiratory related

modulation of the s

OPG

deflection.

3 PROPOSED STIMULATION

3.1 Rationale

Since the spectral HRV parameters are specifically

influenced by the standard P-Stim application and

the instant cardio-respiratory data can be derived

from the BP, as shown above, a novel adaptive and

synchronized P-Stim could be established.

A targeted control of the stimulation waveform

(compare Fig. 2) is highly reasonable for avoiding

over-stimulation and realising stimulation on-

ADAPTIVE AURICULAR ELECTRICAL STIMULATION CONTROLLED BY VITAL BIOSIGNALS - Transition from

Fixed to Adaptive and Synchronized Electrical Stimulation Controlled by Heart Rate Variability and Blood Perfusion

307

Figure 7: Establishment of biofeedback for controlling and synchronization purposes with IG as the impulse generator.

demand controlled by HRV parameters. In other

words, if pain perception is already reduced, as

detected by e.g., reduced stress and diminished

SYM

, then A, f

(Fig. 2b) could be reduced as well.

In addition, efficient energy use in the stimulation

would be facilitated.

The synchronization of the stimulation waveform

with the cardio-respiratory activity would allow a

constructive interference of the stimulated pain-

relieving effects and the residual body attempts. In

particular, the cardiac synchronization would allow a

timely activation of the gate mechanisms in the CNS

or a timely and indirect stimulation of receptors

(e.g., blood pressure), regulating vital body

functions. The respiratory synchronization would

help to interfere with body phenomena like

respiratory sinus arrhythmia, yielding a forced

increase of P

PAR

in the expiration phase.

3.2 Realization

The proposed set-up is shown in Fig. 6. The input

parameters A, f

and the activation rate f

of the

impulse generator are adaptively adjusted according

to the HRV parameters via a control loop. The

cardio-respiratory synchronization signal for the

impulse generator is also derived from s

OPG

In particular, Fig. 7 suggests the difference P

PAR

- P

SYM

as a possible realization of the stimulation

feedback, while the targeted value could be the pain

intensity to be reduced. That is, the higher P

PAR

and

the lower P

SYM

get in the course of the stimulation,

the more strongly the pain has already been reduced.

Similar behaviour of P

PAR

and P

SYM

during

stimulation was already observed in Fig. 4 and Fig.

5. Obviously the ratio P

PAR

/ P

SYM

could be used

instead of the difference.

According to Fig. 7 an adaptive control of A and

is established, assuming that these parameters are

directly interrelated with the stimulation strength. In

an analogous way, a composition of bursts by

controlling of f

could be attained (compare Fig. 2).

Here a proportional-integral controller or integral

controller could be applied, for the human (Fig. 6)

can be roughly approximated as a proportional

control process with a single time constant (compare

Fig. 4b). The time delay in Fig. 7 may be needed for

synchronizing the stimulation pulses with a

particular time instant in the heart cycle.

Fig. 2b exemplifies a possible adaptive control-

ling of the stimulation curve, while more efficient

biphasic impulses are used (compare Fig. 2a). In

addition, constant current stimulation would be

preferred over voltage application, for the skin

impedance is relatively low with electrode needles

inserted and thus the risk of local tissue damage

Figure 6: Control loop of the adaptive auricular stimulation with OPG as the optical plethysmography.

Impulse generator Ear Body

Stimulation electrode

OPG

Human (= control process)

Parameters

of OPG

Stimulation

parameters: A, f

, f

Synchronization

(heart or respiration)

Controller

Targeted

values

Feedback (= actual

HRV parameters)

OPG

Finger

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though locally increased current density is low.

4 DISCUSSION

It is worth to note that the HRV is usually derived

from the ECG (Guidelines, 1996). However, the P-

Stim induced very strong artefacts in the ECG since

the stimulation and the ECG have the same electrical

origin. In contrast, the OPG with optical origin

serves as a reliable biosignal, being independent of

the P-Stim activation. However, the OPG conveys

mechanical information on the systole-diastole cycle

rather than electrical on the heart excitation (= origin

for the HRV). In addition, the OPG exhibits

relatively slow changes if compared to the ECG, for

the pulse waves are much more inert than electrical

heart excitation. The use of the OPG may have

reduced an effective time resolution of f

The time delay of about 200 ms between the

systolic onset in the OPG and the R peak in the ECG

depends on the speed of the heart excitation and

mechanical vessel properties. Nevertheless, the

delay can be assumed to be constant, if the respirato-

ry induced blood pressure changes and thus arterial

distension and stiffness changes can be neglected.

Lastly, the limitations of the presented

experimental results should be mentioned. The

observed effects, especially concerning P

SYM

, are

restricted by the fact that all volunteers were young

pain-free healthy persons. Furthermore, the

stimulation duration was relatively short: 15 min

versus 4 hours (with 4 hours pause in-between) over

at least seven days, as clinically applied and

subjectively verified for being effective. The initial

state of the volunteers, as their possible excitation at

the beginning of the recording, and their mental

activity changes during the investigation - both

influencing the HRV - may have limited the range of

potential changes or improvements of HRV

parameters during the stimulation.

However, the provided experimental background

leads to a comprehensible design of an adaptive and

synchronized stimulation technique. This would

allow a pain sensitive adjustment of the stimulating

parameters avoiding over-stimulation and

comforting the patients.

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