A PORTABLE ECG DEVICE IN A HOME CARE ENVIRONMENT

USING BURST TRANSMISSION

Emilio M. Bumachar, Rodrigo V. Andreão

Department of Electrical Engineering, Federal University of Espírito Santo (UFES)

Av. Fernando Ferrari, s/nº - Goiabeiras CEP 29060-900 – Vitória – ES – Brazil

José G. Pereira-Filho

Department of Informatics, Federal University of Espírito Santo (UFES)

Av. Fernando Ferrari, s/nº - Goiabeiras CEP 29060-900 – Vitória – ES – Brazil

Keywords: ECG, electrocardiogram, home care, home monitoring, microcontroller, telemedicine, wireless.

Abstract: This article presents a wireless microcontrolled system for ECG home monitoring. The main particularity of

the system is related to the data transmission strategy, which is intermittent to save battery power. In order

to evaluate the potential of the strategy proposed to diminish power consumption, a comparison of the

transmission time spent is made between a theoretical estimation and the experiments.

1 INTRODUCTION

Chronic heart patients need continuous monitoring

due to the probability of a new event occurring that

is harmful to their health. These patients could have

better chances of recovery, mainly for psychological

reasons, if they were able to be monitored at home

instead of in a hospital. Also, the number of patients

hospitalized could be reduced without affecting the

quality of the medical care.

In that context, it is important to create

conditions to allow for safe, efficient and reliable

home monitoring, reducing costs and restrictions to

the patient’s activity and comfort.

Home monitoring is viable in the current

technological environment. Even though

transmission failure must still be addressed, current

wireless technology allows for increasingly reliable

links. Moreover, size, cost and power consumption

of electronic components are lower than ever.

There are several applications for monitoring

systems with wearable medical sensors. (Reisner,

Shaltis and McCombie, 2004). Home patient

monitoring is one of them. Different approaches

have been proposed, such as: sending the data

through the phone network using mobile phones

(Istepanian, Woodward and Richards, 2001),

(Boquete, Bravo, Barea, Ascariz and Martín, 2005);

providing a terminal for the patient themselves to

view data and register events (Segura-Juaréz,

Cuesta-Frau, Samblas-Pena and Aboy, 2004);

transmitting data through the Web (Fensli,

Gunnarson and Gundersen, 2005); allowing direct

use by nearby medical staff (Paim, Correa and

Marques, 2004).

Existing works using wireless transmission keep

the transceiver on all the time. This practice does not

take advantage of the fact that the data being

transmitted is usually much less than the data

transmission rate.

This work presents a portable device able to

acquire and transmit electrocardiogram (ECG) signal

using a wireless link through a remote computer. It

has been conceived in the context of a project called

Telecardio, whose aim is to monitor cardiac patients

at home (Andreão, Pereira-Filho and Calvi, 2006).

Our main contribution is related to the data

transmission strategy. The data is stored in a

memory, and only then the transceiver is activated to

send all the data in a burst. This reduces battery

consumption, improving the critical features of

battery size, weight, and duration. These features are

critical because the patients will need to carry the

equipment (and the batteries) at all times, and

interrupt their routine to change batteries.

107

M. Bumachar E., V. Andreão R. and G. Pereira-Filho J. (2008).

A PORTABLE ECG DEVICE IN A HOME CARE ENVIRONMENT USING BURST TRANSMISSION.

In Proceedings of the First International Conference on Biomedical Electronics and Devices, pages 107-110

DOI: 10.5220/0001053901070110

 SciTePress

2 MONITORING SYSTEM

The proposed device for home ECG monitoring

(Bumachar, Andreão and Segatto, 2006) extends

battery life by transmitting in bursts. It consists in:

 A battery-powered, microcontrolled

acquisition device to be carried by the

patient at all times, which pre-amplifies then

samples their ECG, stores it and transmits

periodically;

 A receiving station that receives the data and

does some processing (such as filtering,

segmentation and automatic analysis), then

retransmits it through the internet;

 A telemonitoring server that can receive the

data of several patients, sort it, store it and

make it available to authorized parties.

The system is illustrated in Figure 1.

In order to control all these tasks, an algorithm

has been developed to be run in the microcontroller,

as follows.

ANALOGICAL PRE-

AMPLIFICATION

EXTERNAL

MEMORY

ZIGBEE

TRANSCEIVER

PATIENT

COMPUTER

DOCTOR,

HOSPITAL,

DATABANK

INTERNET

3 channels

ZIGBEE

TRANSCEIVER

ACQUISITION

MICROCONTROLLER

A/D CONVERTER

Figure 1: Monitoring system.

2.1 Microcontroller Algorithm

The embedded microcontroller spends most of the

time idle, to save power. Every sample period, it is

interrupted by an internal timer and samples the

three A/D (analog-to-digital conversion) channels,

obtaining three 12-bit samples. This data is

organized in bytes and stored in internal memory,

and then there is a return to inactivity.

This cycle repeats until there are 42 samples, or

63 bytes. Then a 1-byte counter is added as a header,

completing a 64-bytes data vector, which is stored in

the external memory using the I2C serial interface,

freeing the internal memory for further samples.

The microcontroller counts these writing

operations to keep track of the amount of data and,

therefore, the amount of time stored. When 30

seconds of ECG are completed, the microcontroller

wakes the transceiver from its sleep mode, which

can take a few milliseconds. If a new data vector is

completed in that time, it is properly stored in the

external memory.

When the transceiver becomes active, the

microcontroller repeatedly attempts to contact the

receiving station until receiving an answer. The

station can send to the microcontroller a transmit

request or a repeat request.

After receiving a transmit request, the

microcontroller will read from the external memory

the oldest data packet and transmit it; after a repeat

request, it will retransmit the last packet. A data

packet is defined as 16 data vectors, or 1024 bytes.

While the microcontroller handles the

transmission, sampling continues through

interruptions, but the microcontroller no longer

becomes idle. The internal memory is monitored and

its content is transferred to the external memory,

unless the later is being read for transmission. To

avoid data loss in this situation, the internal memory

buffer size is defined with a margin. Similarly, the

external memory has a large margin to stand

eventual wireless transmission failures.

Finally, after several transmissions, there will be

no packets left in the external memory. Then, the

microcontroller will answer the next transmission

request with an ending request, and the receiving

station will send an ending confirmation. The

microcontroller will return the transceiver to sleep

mode and begin data accumulation again, reinitiating

the cycle.

2.2 Prototype

A prototype of the monitoring device was able to

sample, packet, store and transmit three channels of

pre-amplified signals.

The microcontroller, memory and transceiver

used were Analog Devices’ ADuC841, Microchip’s

24AA515 and MaxStream’s XBee respectively.

Despite the non-volatility of the memory being

unnecessary, a serial EEPROM memory was

selected instead of faster RAM memory because it

requires less power – only the EEPROM page being

written requires power, whereas RAM consumes

power all the time just to maintain data.

Figures 2, 3 and 4 show the prototype.

This prototype works with a single-channel pre-

amplifier to raise the very low biometric ECG signal

to voltage levels that could be accurately sampled by

the microcontroller’s A/D converter. Since two other

amplifiers are not yet available, the same amplified

signal is sampled by all three channels. The

amplifier contains lowpass filters and a Right-Drive-

Leg terminal (which prevents amplifier saturation).

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Figure 2: View of side one of the prototype.

Figure 3: View of side two of the prototype.

Figure 4: Single-channel amplifier.

3 TRANSMISSION TIME

The data storage time is the duration of each data

storage phase, in which the acquisition device does

not transmit. It has been fixed at 30 seconds.

The transmission phase lasts the time necessary

to transmit all the samples taken during the

transmission and storage phases.

There are three interfaces to be considered that

can influence the transmission time: the I2C serial

interface between the microcontroller and the

memory, the UART serial interface between the

microcontroller and the transceiver, and the ZigBee

rf link between the transceiver and another

transceiver at the receiving station.

3.1 Theoretical Minimum

The bottleneck of the transmission, theoretically, is

the UART serial interface, with a rate of 115,2 kbps.

Disregarding transmission overhead,

acknowledgements, and processing delays (such as

servicing interrupts), the transmission time will be

given by the following equation:

61,2

123256115200

12325630

××−

(1)

where 30 is the data storage time in seconds, T is the

desired transmission time, 256 is the sampling

frequency in Hertz, 3 is the number of A/D channels

being sampled, 12 is the number of bits per channel,

and 115200 is the bottleneck rate in bits per second.

As a result, the desired transmission time T is 2,61

seconds, which corresponds to the transmission

phase occurring 8% of the time, as shown in the

following equation:

61,230

61,2

(%) =

=TP

(2)

3.2 Experimental Value

The I2C interface was implemented in software on

the microcontroller’s (master’s) side. At first, was

assumed that it was not the bottleneck, but it was so

inefficient that it became the actual bottleneck,

determining the transmission time of 19 seconds.

Consequently, I2C interface implementation was

improved, by having a timer do some counting

previously done by executing useless processor

instructions. However, after the I2C improvement,

the device stopped functioning properly. Most data

A PORTABLE ECG DEVICE IN A HOME CARE ENVIRONMENT USING BURST TRANSMISSION

109

would get through to the receiving station, but not

all, at a distance too short for link failure.

The device worked properly when an arbitrary

delay was inserted between transfers of each byte

from memory to transceiver. This delay was then

calibrated to its approximate minimum value that

would still allow for proper functioning.

The resulting transmission time was 5,5 seconds.

It is more than double the theoretical, ideal value. It

corresponds to the transmission occurring 15,5% of

the time.

3.3 Discussion

The acquisition device was not able to function with

transmission time lower that 5,5 seconds, that is, it

did not function properly without deliberate delays

that increased the transmission time to 5,5 seconds.

The microcontroller code keeps the I2C interface

from getting ahead of the UART interface, that is,

the next byte is read from the memory only after the

last byte is forwarded to the transceiver.

The UART interface is very much slower than

the rf link, and the former shouldn’t overwhelm the

latter. However, the transceiver was configured not

to wait for any number of bytes to form an rf packet,

that is, as soon as a byte is received by the UART, it

is put in an rf packet with significant overhead. (This

configuration is necessary to transmit one-byte

control messages used for reliability.) Therefore, if

assembling the packet takes too long, perhaps the

transceiver’s transmission buffer overflows.

At the receiving station’s side, the rf link, being

faster then the UART interface, could overwhelm it

This shouldn’t happen since all the data coming

through the rf link has passed by the UART interface

at the acquisition device. However, non-uniform

delays due to rf packet assembling and

disassembling could cause the receiving buffer to

overflow.

4 CONCLUSIONS

This work presented a wireless microcontrolled

solution for an ECG home monitoring system

application. It was based in the concept of battery

power saving through intermittent data transmission.

It was observed that in practice the transmission

time is much higher than the ideal theoretical

minimum. Further experimentation is necessary to

discover whether it can be reduced.

The use of intermittent data transmission has

opened another strategy to reduce battery

consumption by setting the power-down modes of

the microcontroller and transceiver, but that wasn’t

achieved with this prototype. For example, if the

storage phase power consumption can be ten percent

of the transmission phase power consumption, then

the total consumption as a percentage of the

transmit-all-the-time consumption will be 24%.

Therefore, it is useful to use intermittent

transmission instead of transmitting all the time.

ACKNOWLEDGEMENTS

This work is supported by FAPES – Fundação de

Apoio à Ciência e Tecnologia do Espírito Santo

(30899583/2005), and partially by CNPq.

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