Ambulatory Devices Measuring Cardiorespiratory Activity with Motion

Marcel Mły

nczak

, Marek

Zyli

nski

, Wiktor Niewiadomski

and Gerard Cybulski

1,2

Institute of Metrology and Biomedical Engineering, Faculty of Mechatronics, Warsaw University of Technology,

Boboli 8, 02-525, Warsaw, Poland

Department of Applied Physiology, Mossakowski Medical Research Centre, Polish Academy of Sciences,

Pawinskiego 5, 02-106 Warsaw, Poland

Keywords:

Ambulatory Monitoring, Impedance Pneumography, Cardiorespiratory Activity, Motion, Pulse Oximetry.

Abstract:

Holter-type devices with sets of sensors, enabling long-term measurement of quantitative respiratory parame-

ters, were designed and constructed. Pneumonitor 2 was intended for physiologic and athletic applications, and

Pneumonitor 3 for sleep studies. Both allow simultaneous, comfortable, ambulatory monitoring of cardiorespi-

ratory activity, such as ECG, impedance pneumography (IP), and motion; the second device also allows pulse

oximetry and uses improved setting with combined receiving ECG and IP electrodes. Preliminary results

showed that our prototypes provide signals reliable to monitor heart and breathing activity quantitatively. We

tested the devices in different conditions, including walking, stair-climbing, cycle ergometer training, natural

daily activity, and sleep. They can quantitatively measure respiratory ﬂows, volumes, and minute ventilation

using IP after calibration. They are also able to estimate tachogram from ECG. They allow the detection of

subject activity and body position via accelerometer and gyroscope, which is helpful during IP calibration

and interpretation. Pneumonitor 3 also enables measurement of blood saturation with a pulse wave (pulse

oximetry).

1 INTRODUCTION

1.1 Traditional Respiratory Monitoring

Respiratory monitoring is mainly based on airﬂow

measurement using spirometry and longer record-

ing of ﬂows and volumes with pneumotachometry

(PNT ), allow one to assess obstruction, restriction,

and other issues in patients with asthma and chronic

obstructive pulmonary disease (Miller et al., 2005).

These methods measure ﬂow values directly and are

the most accurate. However, they are hard to use in

ambulatory conditions and cannot be performed in an

outpatient setting. In clinical practice, respiratory pa-

rameters are considered only for conditions which al-

low connection to a tube, not for natural daily and

nightly functioning or athletics outside a gym.

The ability to record respiratory effort and quan-

titative ﬂow- and volume-related parameters (as well

as inspiratory and expiratory phases, and respiratory

rate) in new settings could be of real beneﬁt and im-

pact both for physiologists and for sport medicine ex-

perts.

1.2 Ambulatory Respiratory

Monitoring

Traditional examination captures a single point in

time. From a clinical point of view, additional testing

carried out under more natural conditions and taking

into account activity, circadian rhythms, etc., could

expand early diagnosis.

Another factor is sleep; respiratory activity is

known to weaken during the night (McNicholas,

1997). Basic analysis commonly performed during

polysomnography consists of detection of snoring and

central or obstructive sleep apnea, e.g., in connection

with blood saturation and heart activity (Hoyer et al.,

2001; Roebuck et al., 2013). The breathing patterns

are usually measured indirectly by a belt and a can-

nula, which make sleep less natural and comfortable.

There are some methods, which could be consid-

ered as an alternative to PNT , impedance pneumog-

raphy, respiratory plethysmography, or acoustical ap-

proach. Based on the context of continuous studies,

breathing could be precisely described by simple pa-

rameters: minute ventilation, tidal volume, and res-

piratory rate. All of these could be determined with

MÅ

CyÅ

Dczak M., Å

zyliÅ

Dski M., Niewiadomski W. and Cybulski G.

Ambulatory Devices Measuring Cardiorespiratory Activity with Motion.

DOI: 10.5220/0006111700910097

In Proceedings of the 10th International Joint Conference on Biomedical Engineer ing Systems and Technologies (BIOSTEC 2017), pages 91-97

ISBN: 978-989-758-216-5

impedance pneumography (IP), which seems to be

the most accurate in terms of shape of the volume-

related signals. It is known that this method can be

used to measure ventilation in ambulatory settings

(Seppa et al., 2010; Mlynczak et al., 2015). IP could

be performed during sleep along with Holter ECG

monitoring.

1.3 Motion and Heart Activity

It has been noted that the calibration coefﬁcients con-

verting impedance to volume are dependent mainly

upon subjects and body positions (breathing depth

and rate were of less impact) (Mlynczak et al., 2015).

Therefore, the reliable calibration procedure should

concern measurements performed in various body po-

sitions, and it would be worthwhile to track the cur-

rent position during IP measurements to apply spe-

ciﬁc calibration coefﬁcient.

Motion tracking seems also very important with

regards to sleep studies, e.g., for hypnogram estima-

tion. The motion-associated artifacts in IP signal

could be adaptively removed, smoothed, or marked

using a motion signal synchronized with the respira-

tory one, without any cooperation from the subject.

Based on the present guidelines, each sleep study

device should be equipped with pulse oximetry and

heart activity registration unit (Collop et al., 2007).

Furthermore, the simultaneous analysis of heart activ-

ity along with respiratory one could allow assessment

of the autonomic nervous system operation (e.g., au-

tonomic heart regulation investigated from heart rate

variability). Such experiments are rarely carried out

under natural conditions. Grossman et al. (Grossman

and Taylor, 2007) suggested that the depth and the

frequency of breathing affect heart activity in differ-

ent way and presented different physiological mech-

anisms of control between heart rate and heart rate

variability.

1.4 Objective

The aim of the study was to prepare portable de-

vices which would register respiratory activity (using

impedance pneumography) together with ECG, mo-

tion, and/or pulse oximetry (saturation, pulse wave)

for physiologic and sport applications, and sleep stud-

ies.

Figure 1: Pneumonitor 2 measurement device.

2 METHODS

2.1 The Devices

Pneumonitor 2 was made as a modiﬁcation of the ﬁrst

version (Mlynczak et al., 2014), extending the device

sensors (IP, ECG, and motion) and power manage-

ment. It is presented in Fig. 1.

Pneumonitor 2 is 14.2cm x 6.9cm x 2.3cm and has

a weight of 160g. It is based on the tetrapolar

impedance measurement method with a sinusoidal

application current with an amplitude adjustable up

to 1mA, and 100kHz frequency.

We improved power management by replacing the

elements that consumed the most energy, and by us-

ing a rechargeable battery, similar to those found in

mobile phones, with 900mAh capacity. This allowed

measurement for at least 12h and SD card recording,

with a 250Hz sampling frequency. The sampling fre-

quency is chosen as a compromise between the output

data size, the possible jitter in the estimation of the R-

wave ﬁducial point, and the accuracy sufﬁcient from

sleep studies perspective (Task Force, 1996).

ECG ampliﬁer has the gain of 100V /V , the band-

width up to 100Hz, 10nV /sqrt(Hz) noise and 10bit

resolution). InvenSense’s MPU-6050 (accelerometer

and gyroscope unit, available commercially) was em-

ployed to estimate motion.

Wireless communication was omitted since no

clinical necessity for online analysis of results was

noted. A very small OLED screen was added to show

signal waveforms. A simple, native PC application

capable of exploratory data analysis and recording of

samples in a database was also prepared.

Pneumonitor 3 had the same IP, ECG, and mo-

tion components. It differed due to the addition of a

wireless pulse oximetry module (Contec CMS50EW,

commercially available), connected to the main de-

BIODEVICES 2017 - 10th International Conference on Biomedical Electronics and Devices

Figure 2: Pneumonitor 3 device with pulse oximeter.

vice via Bluetooth communication. It had a custom-

made housing, which was more solid due to the ex-

pected operating conditions (e.g., regarding the possi-

bility to connect the device to A/D converter via BNC

connectors). It uses improved setting of electrodes de-

scribed in the Conﬁguration section. Pneumonitor 3

prototype is 16.7cm x 10.1cm x 3.5cm with a weight of

330g. It is presented in Fig. 2.

2.2 Conﬁguration

Pneumonitor 2 has 7 leads, intended to be attached

using standard spot, disposable ECG electrodes: 4 for

IP and 3 for single-lead ECG. Pneumonitor 3 has

5 leads, 2 receiving ECG electrode were combined

with IP inputs - both signals could be easily separated

based on different frequency spectra.

The devices provide two preset levels of internal

ampliﬁcation in the impedance receiving chain, in or-

der to adjust the measuring range depending on the

electrode conﬁguration.

Pneumonitor 2 and Pneumonitor 3 are intended

to be used with one of two electrode conﬁgurations.

First, the one proposed by Seppa et al. likely pro-

vides the best linearity between impedance and vol-

ume changes; in this conﬁguration, the receiving elec-

trodes were placed on the midaxillary line at about

5th- and 6th-rib level and the application electrodes

were mounted on the proximal side of the arm at the

level of the receiving ones (Seppa et al., 2013). The

second conﬁguration is a ”classical” one, where both

application and receiving electrodes are positioned at

about 5th- and 6th-rib level. It is considered worse in

terms of transition linearity, yet most likely optimal in

terms of motion artifacts.

The voltage-to-impedance transition function was

established for two settings of internal input ampli-

ﬁcation. We conﬁrmed that the function is linearly

Figure 3: The scheme for positioning of electrodes for

impedance pneumography and ECG, and of sensors for mo-

tion tracking. #1 - proposed in (Seppa et al., 2013); #2 -

”classical” one; positions of IP inputs are the same for both

conﬁgurations; for Pneumonitor 3 they are combined with

receiving ECG electrodes (ECG1 and ECG2).

dependent on the ampliﬁcation setting.

ECG could be registered by Pneumonitor 2 using

various electrode conﬁgurations. For basic measure-

ments, we used differential electrodes placed below

the right clavicle on the chest and below the left 12th

rib. A reference electrode was then placed below the

right 12th rib, roughly symmetric to the second elec-

trode across the sagittal plane.

Different strategies could be applied for motion

measurement using accelerometer and gyroscope.

Fig. 3 shows a full schematic for placement of elec-

trodes and proposed placement of motion sensors.

Saturation level and pulse wave were measured

with the ﬁnger sensor.

2.3 Cardiac Component in Impedance

Signal

We observed that electrical heart activity is a com-

ponent of the raw signal registered using impedance

pneumography. Despite having the option to extract

the ECG-like signal (cardiac IP component), we did

not remove the one-lead ECG unit, because:

• the ratio of amplitudes of cardiac and respiratory

IP components in raw impedance signals changes

during breathing and differs depending on the

subject,

• the cardiac IP component is smoothed compared

to the real ECG signal, particularly during the

QRS complex,

• the cardiac IP component is registered with a spe-

ciﬁc electrode conﬁguration optimized for respi-

ratory, not cardiac recordings, and

• the one-lead E CG signal could be recorded for

various conﬁgurations and synchronized with the

Ambulatory Devices Measuring Cardiorespiratory Activity with Motion

distorted IP signal, allowing its adaptive ﬁltration.

2.4 Evaluation

The described devices are intended to be used in ex-

tensive study taking into account physiology-, sport-

and sleep-related protocols. To validate the relia-

bility of the respiratory- and E CG-related parts of

the devices we performed the calibration procedure

(free 30-second-lasting breathing in supine, sitting

and standing), and the test procedure consisting of

6 normal breaths and then 6 deep breaths (with the

subjective difference), for three breathing rates (6, 10

and 15 BPM) and for the same three body positions

as during calibration (Mlynczak et al., 2014).

Based on the reference PNT (from Pneumota-

chometer M909, by Medikro Oy, Finland, and Simp-

son’s quadrature for integration of ﬂow values into

volumes), we ﬁtted the best linear model without in-

tercept (after mean value removal) between PNT and

respiratory IP component (after subtraction of the

smoothed (respiratory-related) signal from the raw

one). The calibration coefﬁcient was used to convert

the impedance signal into volumes. We evaluated the

accuracy of tidal volume estimation by analysis of the

difference between maximum and minimum value of

volume for each breathing cycle, between reference

PNT signal and respiratory IP component after ap-

plying the calibration coefﬁcient.

We also estimated and compared heart rates from

ECG and cardiac IP component signals using adap-

tive amplitude thresholding method (after drift re-

moval). The RR intervals were calculated as a

difference between two consecutive R points, and

tachogram were transformed to beats per minute

(BPM) and interpolated to 250Hz in order to have the

same number of samples as original IP or ECG.

The preliminary pilot testing was carried out on a

group of 10 participants (all males). MATLAB soft-

ware was used to review and analyze the results.

In order to evaluate qualitatively the acquired sig-

nals and the usability from subjects’ perspective, we

also asked to carry out testing during more ”natural”

conditions: walking, climbing stairs, exercising on a

cycle ergometer for 90 seconds with the increasing

load (from 50W to 200W). The activity and changing

positions on a bed were tested during sleep.

3 RESULTS

Simultaneous ECG, IP, and PNT recordings during

the static test and cycle-ergometer exercise showed

Figure 4: Sample impedance pneumography signal with-

out removal of the ECG component and pneumotachometry

signals, obtained during exercising on a cycle ergometer.

that the quality of the signals allows one to calculate

the tachogram, respiratory rate, and tidal volume.

The mean overall accuracy of tidal volume estima-

tion for the static test was 86.5%. Heart rate estima-

tion based on cardiac IP component, for supine body

position, reached mean 97.3% agreement in compari-

son with ECG.

No artifacts that might preclude analysis (with-

out the one at the beginning of some motions) were

shown during pilot evaluation in natural situations,

e.g., changing positions on a bed or walking.

Fig. 4 provides sample raw IP signal measured

while exercising on a cycle ergometer (with pneumo-

tachometry ﬂow and volume signal). Sample record-

ings of IP and ECG with breathing phases and HRV

analysis were presented in Fig. 5. Finally, Fig.

6 presents sample smoothed blood saturation data,

along with pulse wave, the IP signal distorted with

motion artifact, the 3-axis accelerometry signal and

ECG, registered during sleep.

4 DISCUSSION

The ﬁrst ambulatory system was described by Vuorela

at al. (Vuorela et al., 2010). In contrary to their con-

struction, we decided to remove most of the analog

blocks for signal conditioning and processing and ex-

pand the analysis performed simultaneously after sig-

nal acquisition. The key novelties are adding the abil-

ity to measure blood saturation and pulse wave, and

reducing the number of electrodes in Pneumonitor 3,

from 7 to 5, which could be particularly useful for

subjects’ comforts during sleep studies.

Our prototypes will allow further research to con-

ﬁrm whether the use of IP as a clinically relevant

method is possible in ambulatory conditions (in par-

BIODEVICES 2017 - 10th International Conference on Biomedical Electronics and Devices

Figure 5: Sample ECG and IP signals and correspondingly calculated respiration phases and tachogram; recorded using

Pneumonitor 2.

Figure 6: Sample ECG, smoothed impedance pneumography, pulse wave, smoothed blood saturation and accelerometry

signals; with motion artifacts connected to the body position changes around 142 and 143 minutes of measurement; recorded

during sleep using Pneumonitor 3.

Ambulatory Devices Measuring Cardiorespiratory Activity with Motion

ticular, whether it is feasible to measure quantitative

parameters, particularly tidal volume and minute ven-

tilation, as well as breathing phase timing, under ”dy-

namic” conditions).

In our previous work, we conﬁrmed that a cali-

bration procedure is needed (particularly for various

body positions) to measure quantitative parameters

with accuracy comparable to the reference. However,

we provided those results and consideration for static,

”clear” conditions. They were free of possibly dis-

torting maneuvers, such as irregular, shallow, inter-

mittent breathing; quick movements; changes in elec-

trode attachment (pressure of contact with the skin);

or non-breathing-associated changes in the volume of

the thorax, etc. These could inﬂuence the calibration

coefﬁcient in a way that would only allow one to state

qualitative information about the depth of breathing.

One-lead ECG appears to provide results that do

not allow clinical investigation and diagnosis, but do

allow screening-like conclusions with greater com-

fort. Occurrences of the cardiac IP component may

lead to the concept of removing the classic ECG mea-

surement, however it seems, that cardiac IP compo-

nent is not fully visible in each participant. In our

opinion, if we are able to use the same electrodes to

measure impedance and ECG signal, it will be worth

having the redundant measurements for veriﬁcation

purposes.

Removing separated ECG input for combined one

is prepared in order to improve the usability of Pneu-

monitor 3. It is also related with motion tracking. We

decided to limit ourselves to a single inertial unit (in-

cluding both accelerometer and gyroscope sensors)

to maintain comfort and accuracy of detecting basic

body positions and activity levels (Bouten et al., 1997;

Ermes et al., 2008).

5 CONCLUSIONS

Prototypes of two devices intended for cardiorespi-

ratory measurements in ambulatory conditions (with

motion tracking, including activity and body position

changes) were prepared and preliminarily evaluated.

The devices utilize impedance pneumography

to calculate respiratory parameters (tidal volume,

minute ventilation and respiratory rate) after calibra-

tion, ECG to record heart activity, and a combina-

tion of 3-axis accelerometer and gyroscope to track

motion. Pneumonitor 3 also includes pulse oximetry

providing saturation and pulse wave, and uses 5 elec-

trodes setting with 2 ECG electrodes combined with

receiving I P ones.

Pneumonitor 2 is designed for the environment

physiology analyses (registering ventilation and car-

diac functioning in subjects with obesity or nervous-

muscle-related illnesses) and sports medicine (for am-

bulatory diagnostics, monitoring training, and deter-

mining exercise capacity). Pneumonitor 3 is intended

mainly for sleep studies to monitor breathing disor-

ders and the treatment progress.

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