EXTENDED HEALTH VISIBILITY IN THE HOSPITAL
ENVIRONMENT
H. Fernández López, J. A. Afonso, J. H. Correia
1
Industrial Electronics Engineering Department, University of Minho, Campus de Azurém, 4800-058, Guimarães, Portugal
Ricardo Simões
2
Technology School, Cávado and Ave Polytechnic Institute, Urbanização das Calçada
Edifício Galo - SN R/C, 4750-117, Arcozelo, Barcelos, Portugal
3
Institute of Polymers and Composites, University of Minho, Campus de Azurém, 4800-058 Guimarães, Portugal
Keywords: Vital signals monitoring, Health monitoring device, Wireless sensor networks, ZigBee.
Abstract: Wireless sensor networks can help healthcare providers enhance patient monitoring and communication
capabilities. This paper describes the present state of the development of a vital signal monitoring network
applied to the hospital environment. The proposed network is based on non-obstructive sensors able to
communicate through a low power wireless sensor network based on the ZigBee protocol. This network
enables continuous patient monitoring, creating entirely new mechanisms for providing healthcare under a
plethora of cases (e.g. post-op, continuous care, and chronic diseases). The main advantages of this system
include increased patient mobility, faster detection of potential problems, real-time feedback to caregivers
and patients, and faster response to emergency situations.
1 INTRODUCTION
We have specified a system that facilitates vital
signals gathering in a hospital environment that
resembles the basic configuration analyzed in
(Cypher, Chevrollier, Montavont, & Golmie, 2006)
with some improvements. We have added a
mechanism to send messages back from the
hospital’s monitoring system to the patient. With
this system we want to introduce the concept of
“extended health visibility”, where non-critical
patients can be continuously and unobtrusively
monitored while simultaneously being informed
about their condition and receive messages from the
hospital caregivers (e.g., a doctor appointment, a
reminder to take their medicine, or a visitor arrival
notice). Through this system, instead of collecting
vital signals only a few times along the day,
hospitals will be able to maintain continuous patient
records which will provide better emergency
condition detection and response, enhanced
diagnosis capabilities, and promote, among the
patients, a better appreciation of the care being
provided.
Despite the evident benefits that can result from
the adoption of wireless technologies, there are still
many concerns that limit the widespread application
and challenge researchers to devise potential
solutions. It is essential to satisfy demanding
requirements in terms of quality of service such as
sustainable throughput, bounded delay and reliable
packet delivery. At the same time, it is necessary to
guarantee that the power consumption of the sensor
nodes is small, since they are powered by batteries,
in order to increase their autonomy. Another
difficulty arises from the fact that some sensors must
be sampled quite often, generating a large amount of
data and, consequently, requiring the network to
operate under high load, which is not common in
typical wireless sensor network scenarios. Several
strategies are been used to overcome those problems
and they include the use of techniques to compact
data and the efficient use of the communications
channel.
In order to adequately introduce and describe our
solution, we present a quick review of related work.
The scenario envisioned in (O'Donoughue, Kulkarni,
& Marzella, 2006) differs from ours because the
422
López H., Afonso J., Correia J. and Simões R. (2009).
EXTENDED HEALTH VISIBILITY IN THE HOSPITAL ENVIRONMENT.
In Proceedings of the International Conference on Biomedical Electronics and Devices, pages 422-425
DOI: 10.5220/0001551304220425
Copyright
c
SciTePress
developed system aims at transmitting vital signals
from in-patients in a very restricted area, an
intensive care unit. In our case, patient mobility
poses another challenge. It will be necessary to
provide means of efficiently collecting the signals
throughout the hospital’s targeted areas without
losing information.
Two different scenarios are also considered in
other related works. In case of mass casualty,
overwhelming quantities of patients need to be
monitored and triaged. AID-N is a real-time patient
monitoring system that integrates vital signs and
location sensors, ad-hoc networking, electronic
patient records, and Web portal technology to allow
remote monitoring of patient status (Tia, Greenspan,
Welsh, Juang, & Alm, 2005). CodeBlue also
considers critical care environments (Lorincz et al.,
2004). It is a common protocol and software
framework which allows wireless monitoring and
tracking of patients and healthcare professionals.
Our work shares the basic concerns of systems
planned to operate in those scenarios, namely device
discovery, naming, routing and security.
Nevertheless, we are not concerned with data
prioritization but in assuring the reliability of the
data transmission.
Telediagnosis and teleconsulting are considered
in the AMBULANCE project (Pavlopoulos,
Kyriacou, Berler, & Koutsouris, 1998). A portable
device, carried inside an ambulance, allows
telediagnosis, long distance support and
teleconsultation by specialized physicians. It allows
the transmission of vital signals and still images of
the patient from the incident place to the hospital.
While AMBULANCE uses GSM, a system
primarily designed to handle voice communications,
we have chosen to use ZigBee (ZigBee.Alliance,
2007), an emerging wireless sensor network (WSN)
protocol primarily conceived to be applied in low
traffic scenarios.
In the next section, we present some tests results
considering the electrocardiogram data acquisition
and its wireless transmission. The conclusions and
future work will be presented in the last section.
2 EXTENDED HEALTH
VISIBILITY
Gathering current patient medical data promptly and
accurately is vital to proper health care. Continuous
monitoring offers the capability of identifying and
responding to events as they occur, possibly
preventing a dangerous condition, instead of simply
allowing its diagnosis after the danger has taken
place.
2.1 The Envisioned Solution
The envisioned system is not intended to eliminate
current existing devices or routines but to improve
patients monitoring capabilities. Under this
assumption, it is not intended, for instance, to
replace wire-based continuous monitoring devices
from intensive care units. Urgency, observation,
ward and recovery from procedures that require
local anesthesia or sedation are possible scenarios.
The system in development is based on a ZigBee
ad-hoc wireless sensor network with routers and
gateways distributed all over the hospital spaces
intended for monitored patients. This network will
be responsible for receiving data from sensor units
and transmitting this information to the main
application running in the nurses’ station through a
ZigBee to Wi-Fi gateway. The data sent to the main
application will be processed and will become
available, through a web portal, to registered
healthcare providers carrying a portable device (e.g.,
a Personal Digital Assistant, PDA), as depicted in
Figure 1, where only one ZigBee network is shown.
Figure 1: Envisioned system architecture.
Five vital signals will be monitored: cardiac rate,
electrocardiogram (ECG), arterial pressure, pulse
oximetry and temperature. Medical sensors will be
designed to be minimally obtrusive.
The main application will analyze patient data
and, based on thresholds defined by the physician,
generate alerts to healthcare providers if a critical
condition occurs. Furthermore, it will be possible to
alter threshold values individually. Additionally,
portable devices will allow healthcare providers to
remotely access patient information and change
monitoring configurations at any time. In the patient
perspective, the system will be able to provide a
mechanism for communication between patients and
EXTENDED HEALTH VISIBILITY IN THE HOSPITAL ENVIRONMENT
423
the nurses’ station. Nurses will be able to send
messages to patients and to receive assistance
requests.
2.2 Current Project Status
ZigBee is a wireless network standard which
resulted from the collaborative efforts of a
consortium of companies known as the ZigBee
Alliance. IEEE 802.15.4 (IEEE, 2003) is a standard
defined by the IEEE for low-rate, wireless personal
area networks which specifies the physical (PHY)
and the medium access control (MAC) layer used by
ZigBee. The specification for the physical layer
defines a low-power spread spectrum radio
operating at 2.4 GHz with a bit rate of 250 kbps.
There are also PHY specifications for 915 MHz and
868 MHz that operate at lower data rates.
Even though ECG data rates are relatively high
compared with data rates generated by other sensors,
they are well below the bit rate of a ZigBee network
operating at 2.4 GHz, so it is expected that ZigBee
will be able to carry the generated traffic using its
CSMA/CA protocol, provided that high traffic loads
are avoided, in order to limit the percentage of
packets lost due to collisions. ZigBee was chosen
because it allows the construction of networks
composed by a large number of nodes that can cover
large areas through the use of multihop
communication, in contrast, for example, with
Bluetooth, which supports only seven end nodes per
piconet in a short-range star topology. Additionally,
ZigBee is a standard based protocol that operates on
a license-free ISM band and provides low-cost, low-
power and small form factor modules.
The ZigBee network developed is based on
JN5139 modules manufactured by Jennic (Jennic,
2008). Basic network functionalities have already
been tested using several end devices that
continuously send simulated ECG data in a star
network topology.
2.2.1 The Wireless Network for ECG
Monitoring
The ECG application was chosen as the first one to
be assessed because it is the most demanding one.
The network operates in the 2.4 GHz band and the
access to the radio channel is gained using the
unslotted CSMA/CA. The chosen sample rate is the
same used by commercial monitoring ECG
equipments, 120 Hz (Fulford-Jones, Gu-Yeon, &
Welsh, 2004).
2.2.2 Power Consumption Estimation and
Measurement
Power consumption is an important issue. It is
desirable that end devices consume the least possible
power to allow for continuous operation for long
periods without batteries replacement. To estimate
the battery lifetime, we must consider that the CPU
is always active because frequent ADC
measurements are required. During these periods,
the module consumption is equal to 9.21 mA. In
reception and transmission modes the current
consumption reaches 37mA (Jennic, 2007).
The module reads the ECG sensor thirty times,
once every T
S
= 8.33 ms (which is the inverse of the
sampling rate, 120 Hz) using an internal 12-bit
resolution ADC, and then, every 250 ms, sends a
data packet with the measurements. One module
cycle is depicted in Figure 2, where current
consumption values are shown for each activity:
ADC reading (ADC), backoff (BP), clear channel
access (CCA), data transmission (T
DF
),
acknowledgment waiting (t
ack
) and acknowledgment
reception (T
ACK
).
Figure 2: Module current estimation.
Taking those operation parameters into
consideration, it is possible to determine that the
average current over the 250 ms period is equal to
9.84 mA. The total operating time considering the
module is powered by two AAA 1200 mAh batteries
is approximately 122 hours or 5 days.
The current drained by the module as a function
of time was measured to confirm the power
consumption estimation using a data acquisition
board driven by a MATLAB program. In Figure 3 it
is possible to observe the current variation over a
period of approximately 300 ms where ADC sensor
readings and backoff, CCA, data frame transmission,
t
ACK
and acknowledge reception are shown.
BIODEVICES 2009 - International Conference on Biomedical Electronics and Devices
424
Figure 3: Module current measurement.
2.2.3 Network Operation Tests
We have tested the packet error rate by considering
just one end device transmitting a 2-byte payload
data frame to the coordinator. Five runs were
executed involving the transmission of a total of
40,000 packets at different intervals. No packet was
lost in any run but we have observed packets
retransmissions during the longer tests probably due
to interference from Wi-Fi traffic. The number of
packets that required retransmission in each run is
shown on Table 1. The results confirm the
satisfactory coexistence between ZigBee and Wi-Fi
networks.
Table 1: Transmission test results.
Interval between
packets
Number of retransmitted packets in
each run
1 2 3 4 5
100 ms 6 5 6 0 0
20 ms 1 0 0 0 0
10 ms 0 0 0 0 0
5 ms 0 0 0 0 0
A second test was performed using five ZigBee
modules to simulate a small ZigBee star network.
Four modules were used as end devices to gather
and transmit ECG signals using the unslotted
CSMA/CA mode. Every 250 ms a 45-byte payload
data packet was sent to the fifth module, which was
programmed to perform as the network coordinator.
It also checks if any data packet from any end device
is lost and reports it. Ten runs involving the
transmission of at least 2400 packets by all four end
devices were executed. No packet was lost in any
run, which can be considered a promising result.
3 CONCLUSIONS
Wireless sensor networks promise ubiquitous vital
signal monitoring that might completely transform
the way health care is provided.
We have proposed a system that facilitates vital
signal gathering in a hospital environment for what
we call extended health visibility. Instead of
collecting vital signals only a few times along the
day, healthcare professionals will have access to
continuous patient records which will provide
several improvements compared to current
capabilities. Power consumption estimation and
measurements, as well as network performance test
results were discussed. These preliminary results
seem to validate the potential applicability of the
proposed system for the intended application.
ACKNOWLEDGEMENTS
This work has been supported by the Portuguese
Foundation for Science and Technology and the
POCTI and FEDER programs.
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