Lifeguard

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382

IEEE TRANSACTIONS ON INFORMATION TECHNOLOGY IN BIOMEDICINE, VOL. 9, NO. 3, SEPTEMBER 2005

A Multiparameter Wearable Physiologic Monitoring System for Space and Terrestrial ApplicationsCarsten W. Mundt, Kevin N. Montgomery, Usen E. Udoh, Valerie N. Barker, Guillaume C. Thonier, Arnaud M. Tellier, Robert D. Ricks, Robert B. Darling, Senior Member, IEEE, Yvonne D. Cagle, Nathalie A. Cabrol, Stephen J. Ruoss, Judith L. Swain, John W. Hines, and Gregory T. A. Kovacs, Member, IEEE

AbstractA novel, unobtrusive and wearable, multiparameter ambulatory physiologic monitoring system for space and terrestrial applications, termed LifeGuard, is presented. The core element is a wearable monitor, the crew physiologic observation device (CPOD), that provides the capability to continuously record two standard electrocardiogram leads, respiration rate via impedance plethysmography, heart rate, hemoglobin oxygen saturation, ambient or body temperature, three axes of acceleration, and blood pressure. These parameters can be digitally recorded with high delity over a 9-h period with precise time stamps and user-dened event markers. Data can be continuously streamed to a base station using a built-in Bluetooth RF link or stored in 32 MB of on-board ash memory and downloaded to a personal computer using a serial port. The device is powered by two AAA batteries. The design, laboratory, and eld testing of the wearable monitors are described. Index TermsAmbulatory physiologic monitoring, Bluetooth, crew physiologic observation device (CPOD), electrocardiogram (ECG), high altitude, LifeGuard, respiration, vital-signs, wearable.

I. INTRODUCTION

T

HERE ARE A number of situations in which noninvasive and continuous monitoring of physiologic and acceleration parameters is extremely useful in an ambulatory or stationary setting. For space applications, these include extravehicular activities (EVA, or spacewalks), launch and deorbit, exercise in microgravity, physiologic research, and unanticipated medical events [1]. There are also a number of terrestrial settings in which such capabilities are likely benecial, including monitoring of patients with cardiovascular disease to aid in diagnosis and to evaluate therapies, assessing gait stability, activity level, the quality/quantity of sleep, and monitoring of rst responders and accident victims [2][5].Manuscript received October 11, 2004; revised April 13, 2005. This was supported in part by NASA Contracts NCC-1010 and NNA-04CC32A. Human subject testing was carried out under Stanford University Human Use Protocol numbers 78 527 (in-lab testing), 79 640 (Licancabur Expedition), and 79 825 (KC-135 ight). C. W. Mundt, K. N. Montgomery, U. E. Udoh, and G. T. A. Kovacs are with Stanford University, Stanford, CA 94304 USA and also with the NASA Ames Research Center, Moffett Field, CA 94035 USA (e-mail: cmundt@mail.arc.nasa.gov; kovacs@cis.stanford.edu). V. N. Barker, Y. D. Cagle, N. A. Cabrol, and J. W. Hines are with the NASA Ames Research Center, Moffett Field, CA 94035 USA. G. C. Thonier, A. M. Tellier, R. D. Ricks, S. J. Ruoss, and J. L. Swain are with Stanford University, Stanford, CA 94304 USA. R. B. Darling is with the University of Washington, Seattle, WA 98195-2500 USA. Digital Object Identier 10.1109/TITB.2005.854509

Current technology for ambulatory physiologic monitoring includes portable patient monitors for bedside and transport monitoring, and wearable devices for recording electrocardiographic data such as Holter monitors [6] and event monitors that are used for storing electrocardiographic data for subsequent analysis. Holter monitors record heart rate and/or electrocardiogram (ECG) continuously for several hours or days, while event monitors record these data for brief periods, and only upon activation by the user. Commercially available vital signs monitors include a number of portable and wearable devices. The Micropaq from Welch Allyn (Beaverton, OR), and the ApexPro from GE Medical Systems (Waukesha, WI) are two of the most advanced ambulatory patient monitors available today for wireless portable bedside monitoring and transport monitoring. Both devices measure and wirelessly transmit multiple physiologic parameters. The Micropaq acquires heart rate, SpO (an estimate of arterial oxygen saturation SaO measured by pulse oximetry), and several channels of ECG, and transmits these to a Welch Allyn FlexNet 802.11 wireless access point from where the data can be distributed for patient monitoring. The ApexPro has similar capabilities, but can also be interfaced to an external blood pressure device (Accutracker DX, Suntech Medical, Morrisville, NC). Both devices rely on access to a wireless infrastructure and do not record data internally. This limits their use to well-equipped, relatively benign environments, such as hospitals. Commercial Holter monitors, event recorders, and transtelephonic monitors do record data, but only heart rate and one or more channels of ECG. They do not stream these parameters wirelessly in real-time. Other commercial devices for monitoring vital signs include PCMCIA-Cards for measuring ECG, SpO , blood pressure, and carrying out spirometry (QRS Diagnostics, Plymouth, MN), sleep study devices [Nellcor Puritan Bennett (Melville) Ltd., Kanata, ON, and Itamar Medical Inc., Boston, MA] and shirt-type devices with attached or integrated physiologic sensors (Vivometrics, Ventura, CA, and Sensatex, New York). A exible vital signs monitoring system for sports and military applications was developed by FitSense Technologies (Southborough, MA), but it does not support the recording and transmission of electrocardiograms. However, FitSense does provide a good solution for low-bandwidth vital signs transmission with their proprietary BodyLAN system and an evaluation of their system during a Mount Everest expedition has been reported in [7] which also provides an excellent description of the evolution of wearable health data monitoring.

1089-7771/$20.00 2005 IEEE

MUNDT et al.: MULTIPARAMETER, WEARABLE PHYSIOLOGIC MONITORING SYSTEM

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A number of efforts to develop wearable monitors have been reported [8][21]. These include transtelephonic ECG monitors [8], personal computer memory card industry association (PCMCIA), and Compact Flashcard-based Holters [9], [10], Holters transmitting ECG wirelessly through a global system for mobile communications GSM() modem [11], [12], wearable ECG monitors with real-time user feedback [13], and monitors that transmit real-time ECG data over the Internet [10]. Rhee et al. developed a wearable photoplethymographic ring-sensor and integrated it with an ad-hoc self-organizing wireless network [14], and Paradiso is developing wireless, instrumented, smart-ber garments [15]. Most of these systems demonstrate new technologies and methods that advance the state of the art, but are generally prototypes that lack the integration and robustness to meet space ight requirements. Despite recent advances in medical technology, none of the currently available devices provides a combination of wearability, size, and functionality that satises demanding NASA requirements or allows the devices to be used in clinical studies where body motion and physiologic parameters are to be investigated simultaneously. The goal of this effort was to design a small, lightweight, wearable, ergonomic device that not only records and streams a comprehensive set of diagnostic-quality physiologic parameters, but can also record body position and orientation, acceleration in three axes, and can be used to mark events. This feature set, combined with wearability, alarm indicators, fault detections, and the ability to stream data to handheld Bluetooth-enabled devices, forms a compact and reliable system that not only improves medical care in space ight, but also enables a new range of physiologic studies to be performed in terrestrial applications, including medical care. II. METHODS AND MATERIALS The LifeGuard system consists of the CPOD device and a portable base station computer. The CPOD device, the core component of the system, is a small, lightweight, easy-to-use device that is worn on the body along with the physiologic sensors described below. It is capable of logging physiologic data as well as wirelessly transmitting data to a portable base station computer for display purposes and further processing [Fig. 1(a)]. A. Physiologic Sensors A matrix listing typical physiologic parameters for a number of application scenarios, including commercial and military scenarios, was generated since it was anticipated that such a system might prove very useful in terrestrial medical applications as well. This matrix was then used to choose a nal set of physiologic parameters representing a common denominator for all investigated applications. Most physiologic parameters supported by LifeGuard are measured with sensors that are external to the CPOD wearable device and can be congured as needed. The only sensors that are integrated into the CPOD are the accelerometers. Temperature can be measured in one of two ways: either with an ambient temperature plug, or with a probe (cable) for skin

or core temperature measurements. ECG and respiration signals are acquired using commercial button electrodes that are connected to the CPOD via snap-leads (such as Red Dot types 2249 and 2237 from 3M, St. Paul, MN). Pulse oximetry (SpO ) is measured with a Nonin pulse oximeter (Nonin Medical Inc., Plymouth, MN), which is typically attached to a nger (nger clip or exible wrap) or ear lobe (ear clip). For applications requiring subject mobility, the ex nger sensor from Nonin (model 8000J) was optimum. Nonin pulse oximeters are used with a mating signal conditioning and digitization unitthe Xpod. The Xpod used for LifeGuard (model 3011) streams serial data to the CPOD at a rate of about one sample per second. A cuff-based device is used to measure systolic and diastolic blood pressures. For applications involving signicant motion or vibration, an auscultatory motion-tolerant device can be used. The LifeGuard system supports the Accutracker II (Suntech Medical, Morrisville, NC), one of the most motion-tolerant devices available. Depending on the application, either all of the external sensors or any desired subset can be used. In many cases, ECG, respiration rate, activity (acceleration), skin temperature, and heart rate (derived from ECG through post-analysis) will sufce. This conguration only requires the ECG/respiration electrode set and provides the greatest degree of mobility. Fig. 1 (b) illustrates how the CPOD device is worn on the body. Amphipod sport packs (Amphipod, Inc., Seattle, WA) are used to secure the device around the waist. B. Prior Research Prototype System Initial development efforts by our group were focused on using commercial portable digital assistant (PDA) technology as the platform for physiologic data acquisition under the Microsoft Windows CE operating system. A prototype system using commercial off-the-shelf components was rst developed to rene requirements and to gain understanding about the real-world use of such a device. This Smart Healthcare Management System (SHMS) consisted of an easy-to-apply physiologic sensor pad (Nexan Ltd., Cambridge, UK), which provided a two-lead ECG and respiration signals. The electronics of the Nexan sensor was modied so that it transmitted its digitized data wirelessly using Bluetooth technology to a personal digital assistant (PDA, a Compaq/HP iPaq Pocket PC) running Windows CE (Microsoft, Redmond, WA). This PDA was worn on the body and could record data locally and/or transmit data in real-time via 802.11b (IEEE wireless local area network standard) over the Internet to a central server (a switchboard) where multiple devices (PCs, PDAs) could view the data live via the Internet. The SHMS system was used for in-lab and eld testing (including a high-altitude research study), and provided valuable real-world experience. A number of shortcomings were identied, including the lack of robustness of connectors, short battery lifetime, and external dependencies on commercial off-theshelf components (iPaqs, WindowsCE) that were not designed to meet the high reliability requirements of medical monitors. As a research prototype system the SHMS provided a mechanism for successfully demonstrating functionality and rening

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IEEE TRANSACTIONS ON INFORMATION TECHNOLOGY IN BIOMEDICINE, VOL. 9, NO. 3, SEPTEMBER 2005

Panel A

Panel B Fig. 1. Panel A: LifeGuard System. 1) (a) CPOD vital signs monitor with physiologic sensors, including (b) electrodes for recording ECG and respiration waveforms, (c) pulse oximeter measuring SpO and heart rate, and (d) blood pressure monitor. 2) Diagram illustrating locations of CPOD monitor and sensors on body. 3) Communication options (RS-232 hardwired or Bluetooth wireless) for data transfer between CPOD and (e) base station computer. Panel B: Photograph of subject with cutaneous disposable electrodes in place on upper chest and left side, as well as a ngertip pulse-oximeter sensor. An Amphipod sport pack (Amphipod, Inc., Seattle, WA) is used to secure the device around the waist.

requirements for an eventual system, but was not capable of meeting the goal of a rugged physiologic monitoring system that was reliable and easy to use. A new, rened and optimized solution was required, and to be derived from careful examination of requirements across a broad range of applications. This effort resulted in the development of the LifeGuard system and the CPOD device. C. Technical Specications of CPOD Device Table I lists the technical specications of the CPOD. Many of these, such as battery lifetime, data storage capacity, and wireless range, are the result of trade-offs between the nal size and form factor of the device, its usability, and its feature set and functionality. For example, AAA batteries represented the best trade-off between battery size and capacity for this particular device. Primary cells were chosen over rechargeable battery packs due to their availability and ease of replacement.

A custom LCD panel displays information on the device status (logging, streaming, connection to base station) as well as cycles through the measured parameters (skin temperature, activity, heart rate, SpO , systolic and diastolic blood pressure, and remaining battery life). A button on the front of the device can be used to create and store event markers (the CPOD also incorporates an internal real time clock), and to enable the Bluetooth module. It can also stop the cycling display to continually show a single parameter. A piezo buzzer is used to alert the user of low battery life, low SpO , or high heart rate values. It also serves to signal an alarm when a sensor is disconnected or not functioning properly. The CPOD acquires and logs physiologic data and can download or st...

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