RESPONSE AND CONTROL OF HEART RATE VIA POSTURE

AND MOVEMENT

Martin Wieser

1,2

, Rafael M Ruest

1,3

, Lilith Bütler

1,4

and Robert Riener

1,2

Sensory-Motor Systems Lab, Institute of Robotics and Intelligent Systems (IRIS)

ETH Zurich, Tannenstrasse 1, 8092 Zurich, Switzerland

Spinal Cord Injury Center, University Hospital Balgrist, Switzerland

Institute of Human Movement Sciences and Sport, ETH Zurich, Switzerland

HUMAINE Clinic Zihlschlacht, Switzerland

Keywords: Cardiovascular system, Mobilization, Tilt table, Heart rate, Control, Rehabilitation.

Abstract: After a few days of immobilization, patients need some form of treatment to stabilize their cardiovascular

system. It is known that mobilization has a major influence on the cardiovascular system and, therefore, is

an important component in neurorehabilitation. In this study a strategy is presented to control the heart rate

using two mechanical stimuli: body inclination angle and stepping frequency. First, we could show that the

heart rate of healthy subjects, as well as minimally conscious patients, shows a clear and repeatable

response to body tilting and stepping. Furthermore, first experiments demonstrated the feasibility to control

the heart rate of healthy subjects. Future experiments are required to optimize the control strategy with

healthy subjects and to present the feasibility of the controller for use with patients. The long term goal will

be to control heart rate, systolic and diastolic blood pressure, as well as respiration frequency, in order to

stabilize the patients’ cardiovascular system and improve their health state with a reduced amount of

pharmaceutical medication.

1 INTRODUCTION

An important therapeutic strategy in an early phase

of neurorehabilitation is mobilization by stepping of

the legs, and body tilting. Leg mobilization itself has

a major influence on the cardiovascular system.

Cardiovascular adaptation to an upright posture

depends on the proper interplay of the hemodynamic

system and the reflex mechanism that maintain

blood pressure homeostasis (Held, 2004; Hainsworth

and Al-Shamma, 1988).

First applications with the dynamic tilt table

Erigo showed a positive effect on blood circulation

in healthy adults (Czell et al., 2004) as well as in

patients (Luther et al., 2008). The Erigo enables two

sensory stimulation inputs to be applied. Firstly, the

subject can be tilted to different inclination angles

and secondly, the legs can be mobilized by a

stepping pattern. During tilting the stepping

mechanism supports blood circulation, providing a

significant reduction in the number of syncopes

observed by Czell (2004) and Luther (2008). As a

consequence, it was possible to integrate body tilting

more intensively into patient therapy and, hence,

improve the rehabilitation process.

In this project we investigated the relationship

between the two inputs provided by the Erigo, and

the cardiovascular output of the human. Many

research groups have previously performed classical

tilt table experiments to show a steady state response

of heart rate (HR) as well as blood pressure and the

occurrence of syncopes (Hainsworth and Al-

Shamma, 1988; Mukai and Hayano, 1995; Petersen

et al., 2000). However, an open question remains as

to how well the cardiovascular system reacts to

simultaneous leg mobilization and body tilting.

Thus, in our study, continuous data were measured

in order to observe also the behavior during the

transient state in the tilting phase, in addition to a

steady state.

The results presented in this paper show the

reaction of the HR in healthy subjects and minimally

conscious patients. Based on these results we

performed a first feasibility study to control the HR

of healthy subjects.

Our results form the basis to control the

Wieser M., Ruest R., Bütler L. and Riener R. (2010).

RESPONSE AND CONTROL OF HEART RATE VIA POSTURE AND MOVEMENT.

In Proceedings of the Third International Conference on Health Informatics, pages 77-81

DOI: 10.5220/0002743400770081

 SciTePress

cardiovascular output also for patients in order to

stabilize their clinical state with a reduced amount of

drugs.

2 METHODS

2.1 The Erigo System

The tilt table Erigo (Hocoma AG, Switzerland)

combines a continuously adjustable tilt table with an

integrated motor-driven stepping device (Figure 1).

The tilt angle α

tilt

can be adjusted between 0° and

76° (velocity: 3.4°/s), whereas, the stepping

frequency f

step

can be continuously adjusted up to 80

steps per minute (one leg: 40 steps per minute;

stepping frequency f

step

=0.67Hz). The duration of

extension and flexion phase is identical and the leg

elements move with a 180° phase shift to each other

at a constant speed.

Figure 1: Measurement setup with the tilt table Erigo and

the input variables tilt angle α

tilt

(normalized height

norm

=sin(α

tilt

)) and stepping frequency f

step

. The

physiological output signals are: Heart rate HR, systolic,

diastolic and mean arterial blood pressure p

sys

, p

dias

and

MAP, as well as respiration frequency f

resp

The trunk and hip are tightly fixed by a belt

system and the head is stabilized with a neck

cushion that is shapeable according to the individual

needs of the subjects. The feet are attached on two

separate mobile footplates and Velcro® strips fasten

the legs to an end-effector of the Erigo in order to

perform reproducible standardized movements. The

Erigo allows a highly synchronized movement of the

hip, knee and ankle of the left and right leg. In

contrast to real human gait, however, subjects move

their legs in only a vertical but not in horizontal

direction.

2.2 The Measurement System

HR and respiration frequency f

resp

were acquired

with a g.tec recording system from Guger

Technologies (Austria). For the ECG recording lead

I and II of the Einthoven’s triangle were used. A

flow sensor to monitor changes of temperature of

breathing (nose and mouth) captures the respiration

signal. All electrode and sensor signals are collected

via the g.tec system and the continuous blood

pressure signal is acquired noninvasively by a

CNAP Monitor 500 from CNSystems AG (Austria).

The monitor provides beat-to-beat values for

systolic, diastolic and mean blood pressure p

sys

, p

dias

and MAP that is fed into the g.tec system under

conditions of chronological synchronism.

2.3 Control Strategy

In this paper the focus is on a control strategy for the

HR signal. A feedforward – feedback structure is

used and the desired heart rate HR

des

is given by the

investigator (Figure 2).

Figure 2: Overview of the control strategy. HR = heart

rate; HR

des

= desired HR; h

stst

= corresponding normalized

height to a desired HR; h

= output of the controller; h

req

required normalized height; HR

mean

= mean of the

measured HR during 20s.

In the feedforward loop a steady state inverse

model describes the physiological reaction of the

subject to the change of the height of the heart in the

steady state. The normalized height h

norm

in all

subjects is determined by h

norm

=sin(α

tilt

). To describe

the dependence between the change of the

normalized height of the subject’s heart h

norm

and the

expected HR a 2

order polynomial function is

used. The expected normalized height for a desired

steady state h

stst

is the first input for the plant of the

controller.

This height together with the output of the PI-

controller h

determines the required height h

req

fed

to the Erigo. The PI-controller is given by the

structure

HEALTHINF 2010 - International Conference on Health Informatics

G(s) = K

= K

⎛

⎝

⎜

⎞

⎠

⎟

(1)

whereas, K

is the gain and K

-1

=1.3.

By means of a control signal to the Erigo the

subject is tilted to the required height h

req

and the

resulting HR is acquired by the g.tec system. An

adaptive threshold algorithm (Christov, 2004) is

applied to detect the actual HR, whereas, every 20s a

mean value is identified. To emphasise on the latest

detected HR values (during the period of 20s) a

linear weighting function is used. This mean value

mean

will be compared with the desired HR value

des

and fed into the PI-controller. The output h

only modified when the measured value HR

mean

differs more than 10% of the desired value HR

des

3 EVALUATION

3.1 Subjects

Eight healthy subjects (5 female and 3 male) with no

history of neurological, psychiatric or cardiovascular

disorder and an average age of 24.9 years (SD:

±2.23 years), weight of 60.9 kg (SD: ±6.85 kg) and

height of 174.9 cm (SD: ±7.61 cm) participated in

this study.

Further, 3 minimally conscious patients (1

female and 2 male) with an average age of 52.6

years (SD: ± 8.96 years) were included in the study

42, 4 and 2 months after the incident. Glasgow

Coma Scale values of the patients vary between 6

and 10.

3.2 Protocol

A baseline measurement (duration: 10 minutes) was

performed before and after the intervention phase. In

the intervention phase, subjects were tilted four

times to a normalized height of either sin(20°),

sin(40°), sin(60°) or sin(76°) and back to 0 for 3

minutes (healthy subjects) or 5 minutes (patients)

each time. The procedure was performed for three

different stepping frequencies: 0, 24 and 48 steps per

minute.

For the control strategy a initial phase of 6

minutes was needed in order to identify the minimal

HR during baseline condition (h

norm

=sin(0°) during

the first 3 minutes) and maximum condition

norm

=sin(76°) during the second 3 minutes). In the

following experiment the desired HR value HR

des

was exactly 50% of the evaluated difference

between HR at baseline and maximum condition.

The control strategy is shown in figure 2 and

after 10 minutes controlling the HR to 50%, the

desired HR went back to the value known from the

baseline condition.

3.3 Results

The evaluation of the steady state of the HR while

tilting a healthy subject and setting the stepping

frequency to 48 steps per minute is shown in figure

3. The corresponding HR values to the different

normalized heights are clearly distinguishable. In

general, the physiological system reacts with an

overshoot and turns back to a steady state after the

subject is tilted to a defined height (data not shown).

Figure 3: Steady state values (with standard deviation) of

the HR during different normalized heights for all subjects

(n=8). Stepping frequency was set to 48 steps per minute.

First, with stepping a steady state is reached

within the first minute, whereas, without stepping it

takes up to 3 minutes till a plateau is reached.

Second, without stepping the reached steady state is

up to 6 bpm higher.

The results of the feasibility study about

controlling HR via posture are shown in figure 4. In

the first 6 minutes the baseline and maximum

condition is evaluated. In baseline condition the

mean value for HR is 53bpm (0%) and after body

tilting to h

norm

=sin(76°) the value rises to 72 bpm

(100%). As a consequence, for the following 10

minutes the input for the controller is 62.5 bpm

(50%) and after this period the desired value

decreases back to 53 bpm (0%). The measured HR

shows a natural variability, nevertheless, the mean

value of the HR during the controlled period is 61.7

RESPONSE AND CONTROL OF HEART RATE VIA POSTURE AND MOVEMENT

bpm (desired value: 62.5 bpm) and 52.8 bpm

(desired value: 53 bpm), respectively.

Figure 4: Controlled HR (measured HR) with the desired

HR (upper panel) and the corresponding normalized

height (lower panel).

Figure 5: Mean HR during different normalized heights

for all patients (n=3) during a stepping frequency of 48

steps per minute. The standard deviation for α

tilt

= 20°,

40°, 60° and 76° was 2.23 bpm, 3.45 bpm, 3.06 bpm and

5.50 bpm, respectively.

The patients’ data show a very similar behaviour to

different normalized heights while the stepping

frequency was also fixed to 48 steps per minute

(Figure 5). Generally, during the tilting phase the

variability is higher in patients compared to healthy

subjects. No clear steady state can be seen in the

data. With this preliminary data set, differences

regarding stepping and normalized heights are not

statistically significant.

3.4 Discussion

A general issue is the subjects inter- and intra-

variability between different measurements.

However, a relationship between mean HR and

different normalized heights can be clearly

demonstrated. Figure 3 shows mean values

composed of 8 healthy subjects and consequently,

most of the short-term variability is cancelled out in

that illustration.

Establishing a control strategy for cardiovascular

signals requires working with signals from

individual subjects. As shown in figure 4, even with

no control of the HR signal in the first 6 minutes the

natural short-term variability can be relatively high

(±5 bpm). For that reason it was necessary to

implement strategies that can cope with this

variability. Two issues are implemented to deal with

such a high variability: First, the normalized height

was only changed in the interval of 20s and, second,

the height was only modified when the measured

value differed more than 10% of the desired value.

Using such mechanisms it was not possible to

eliminate the variability, but during the two control

phases (2 times of 10 minutes) it could be shown

that the mean values (61.7 bpm and 52.8 bpm)

differed less than 1 bpm from the desired values

(62.5 bpm and 53 bpm). For a clinical application

the control of cardiovascular signals in longer time

periods is more important than short term effects.

Heart rate variability (HRV) is a sign of healthiness

and, therefore, it is worth to sustain it or perhaps to

support it. In contrast, in the long term it is

important for the cardiovascular system of patients

to be maintained within well defined bounds. Thus,

short term effects like the variability are not

essential for our control strategy and control over

longer periods is satisfactory.

The patients’ data also show a repeatable

response to different normalized heights.

Nevertheless, during the tilting phase no clear steady

state can be observed in the data (Figure 5). This

may be due to the small sample size (3 patients),

whereas, the data of healthy subjects is based on 8

persons. On the other hand minimally conscious

patients are clinically not stable and, therefore, it

could be that these patients don’t show a steady state

because of the medication or the individual injury.

The study is still in progress and more patients will

be assessed before drawing clear conclusions based

on the acquired data of patients.

4 CONCLUSIONS & OUTLOOK

We conclude that the HR of healthy subjects as well

as of patients in minimally conscious state shows a

clear response to different normalized heights. With

HEALTHINF 2010 - International Conference on Health Informatics

the presented control strategy it is possible to control

the HR of healthy subjects.

Future work will be done to arrange more

experiments with healthy subjects in order to

optimize the control strategy. Additionally, more

patients will be included into the study to get refined

results about their cardiovascular response. To

understand the patients’ behaviour in a better way

will give us the opportunity to adjust the control

strategy for needs in the rehabilitation process.

The long term goal will be to find robust

behaviours of cardiovascular signals to investigate

control strategies based on HR, systolic and diastolic

blood pressure as well as respiration frequency. This

would establish a basis to influence the

cardiovascular system of patients in order to

stabilize their clinical state without additional drugs.

ACKNOWLEDGEMENTS

This work was supported by the National Center of

Competence in Research (NCCR) on Neural

Plasticity and Repair funded by the Swiss National

Foundation (SNF).

Special thanks to Heike Vallery, Gery Colombo,

Jan Lichtenberg, Marc Lawrence, Claudia Fischer

and Khanh Nguyen for their technical support as

well as Jenny Haefeli, Ximena Omlin, Esther Keller,

Clemens Gutknecht, Josef Ludwig Schönberger and

Daniel Zutter for their clinical support during the

study.

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RESPONSE AND CONTROL OF HEART RATE VIA POSTURE AND MOVEMENT