United States Patent [19]
`Bernard et al.
`
`[11] Patent Number:
`[45] Date of Patent:
`
`4,883,063
`Nov. 28, 1989
`
`[54] jº, wº PROCESS FOR
`
`[75] Inventors: Thomas E. Bernard, Pittsburgh; Gary
`W. Sherwin, Yukon; William L.
`Kenney, Jr., Boasburg; Debra A.
`Lewis, State College, all of Pa.
`-
`-
`*
`[73] Assignee: Electric Power Research Institute,
`Inc., Palo Alto, Calif.
`-
`[21] Appl. No.: 338,374
`[22] Filed:
`Apr. 13, 1989
`
`FOREIGN PATENT DOCUMENTS
`117330 9/1984 European Pat. Off. .
`OTHER PUBLICATIONS
`Journal A, vol. 24, No. 4, Oct. 1983, (Antwerpen, BE),
`M. Woerlee: “Measurements of Human Work and
`Work Environment”, pp. 195–204, see pp. 196–198: 2.
`Measurements in the Blood Circulation During Work;
`FIGS. 2, 4; pp. 200–203; 5. Measuring the Thermal
`Loading During Work and 6. Measurement in the Work
`Environment.
`Primary Examiner—Angela D. Sykes
`Attorney, Agent, or Firm—Flehr, Hohbach, Test,
`Albritton & Herbert
`Related U.S. Application Data
`[57]
`ABSTRACT
`Continuation of Ser. No. 55,654, May 29, 1987, aban-
`A personal monitor (10, 100, 200) for work and heat
`doned.
`stress has a heart beat sensor (12) producing output
`§ §§ºiãº.; electrical signals indicating a user's heart beats. A mem
`**-** *** * * * * * --------------- - - - - as a sa e = * * * * *
`3. 128/73 6
`ory (127) stores heartbeat information corresponding to
`º
`-
`the heart beat signals produced over predetermined
`[58] Field of Search "1237&#390, 736 §§ time intervals. A microprocess (126) is connected to
`3 * ~ *, *.x vy
`receive the heart beat information from the memory
`[56]
`References Cited
`(127). The microprocessor (126) analyzes the heart beat
`information incrementally over the predetermined time
`U.S. PATENT DOCUMENTS
`intervals under program control to obtain a physiolog
`3,675,640 7/1972 Gatts ................................... 128/671
`ical demand during the predetermined time intervals
`3,978,849 9/1976 Geneen .
`and compares the obtained physiological demand
`4,090,504 5/1978 Nathan .
`against a stored physiological demand limit. Light emit
`4,117,834 10/1978 McPartland et al. .
`ting diodes (208) and a sound producing diaphragm
`4,129,125 12/1978 Lester et al. .
`(186) provide indications to the user when the stored
`; #: §:
`- - - - - - - º: physiological demand limit has been exceeded. The
`# /; ...". ºx microprocessor (126) controls operation of the LEDs
`4,463,764 3/1984 Anderson et al. 123/671 x (208) and the diaphragm (186).
`4,595,020 6/1986 Palti ..................................... 128/736
`4,679,566 7/1987 Tamm ................................. 128/671
`19 Claims, 12 Drawing Sheets
`
`[63)
`
`
`
`FITBIT, INC. v. LOGANTREE LP
`Ex. 1010 / Page 1 of 35
`
`

`
`U.S. Patent Nov. 28, 1989
`U.S. Patent
`Nov. 28, 1989
`
`Sheet 1 of 12
`Sheet 1 of 12
`
`4,883,063
`4,883,063
`
`
`
`Ex. 1010 / Page 2 of 35
`
`

`
`U.S. Patent
`U.S. Patent
`
`Nov. 28,1989
`Nov. 28, 1989
`
`Sheet 2 of 12
`Sheet 2 of 12
`
`4,883,063
`4,883,063
`
`
`
`“FIG.-2
`
`Ex. 1010/ Page 3 of 35
`
`Ex. 1010 / Page 3 of 35
`
`

`
`U.S. Patent
`U.S. Patent
`
`Nov. 28, 1989
`Nov. 28, 1989
`
`Sheet 3 of 12
`Sheet 3 of 12
`
`4.883,063
`‘ 4,883,063
`
`XOONBx
`
`v:wmm
`
`N
`/
`
`ç-‘914
`mid:
`
`
`
`
`
`
`
`Ex. 1010/ Page 4 of 35
`
`Ex. 1010 / Page 4 of 35
`
`
`

`
`U.S. Patent
`U.S. Patent
`
`Nov. 28, 1989
`Nov. 28, 1989
`
`Sheet 4 of 12
`Sheet 4 of 12
`
`4,883,063
`4,883,063
`
`
`
`Ex. 1010/ Page 5 of 35
`
`Ex. 1010 / Page 5 of 35
`
`

`
`U.S. Patent Nov. 28, 1989
`U.S. Patent
`Nov. 28, 1989
`
`Sheet 5 of 12
`Sheet 5 of 12
`
`4,883,063
`4,883,063
`
`82
`82
`
`84
`84
`
`‘
`-
`
`as
`86
`
`88
`
`+5V
`H -
`*—————‘ 0V
`
`+ 5V
`
`OV
`
`(A)
`(A)
`
`(B)
`(B)
`
`(C)
`(C)
`
`(D)
`
`(D)
`
`FIG.-5
`FIG.-5
`
`Ex. 1010/ Page 6 of 35
`
`Ex. 1010 / Page 6 of 35
`
`

`
`U.S. Patent
`U.S. Patent
`
`Nov. 28, 1989
`Nov. 28, 1989
`
`Sheet 6 of 12
`Sheet 6 of 12
`
`4,883,063
`4,883,063
`
`
`
`whom
`
`NO.
`
`Ex. 1010/ Page 7 of 35
`
`Ex. 1010 / Page 7 of 35
`
`

`
`U.S. Patent
`U.S. Patent
`
`Nov. 23, 1989
`Nov. 28, 1989
`
`Sheet 7 of 12
`Sheet 7 of 12
`
`4,883,063
`4,883,063
`
`
`
`AL
`hl.0_....
`
`CS:N2
`
`29]
`
`Ex. 1010/ Page 8 of 35
`
`Ex. 1010 / Page 8 of 35
`
`

`
`U.S. Patent
`U.S. Patent
`
`Nov. 28, 1989
`Nov. 28, 1989
`
`Sheet 8 of 12
`Sheet 8 of 12
`
`4,883,063
`4,883,063
`
`cm.
`
`5pa0%Inn_<_U
`
`2.0
`NU
`
`mvov
`
`mm.mm:
`
`Nm_
`
`<m:
`
`m+
`
`mvov
`
`mm:
`
`mvov
`
`
`
`5%ammvov
`
`on:
`
`mvoc
`
`mm:
`
`I!
`
`mwov
`
`m....o_n_
`
`Ex. 1010/ Page 9 of 35
`
`Ex. 1010 / Page 9 of 35
`
`

`
`U.S. Patent Nov. 28, 1989
`U.S. Patent
`Nov. 23, 1989
`
`Sheet 9 of 12
`Sheet 9 of 12
`
`4,883,063
`4,883,063
`
`
`
`FIG.-9
`FIG.-9
`
`Ex.1010/Page10of35
`
`Ex. 1010 / Page 10 of 35
`
`

`
`U.S. Patent Nov. 28, 1989
`
`Sheet 10 of 12
`
`4,883,063
`
`START COMPUTER
`
`23O
`
`GREEN ARED LEDs
`WAIT FOR COMMAND
`FUNCTION SWITCH
`
`236
`
`RUN
`
`GREEN LED
`READ AGE/
`CLOTHING SWITCH
`|NITIALIZE SYSTEM
`
`
`
`
`
`SOUND ALARM
`YELLOW #RED LED
`
`SOUND ALARM
`27O SEQUENCE LEDs
`
`2 48
`
`
`
`
`
`
`
`
`
`
`
`
`
`25O
`READ & RESET HR
`
`256
`
`58
`2
`
`52
`2
`
`READ TEMPERATURE
`6O
`2
`
`262
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`
`SEND DATA TO:
`-PRINTER
`•COMPUTER
`
`CHECK BATTERY VOLTAGE
`
`|F LOW (FIRST TIME}
`-SOUND ALARM
`TURN LEDs OFF
`-PRINT MESSAGE
`
`
`
`
`
`WAIT #5 SEC
`
`|F
`COMMAND
`
`
`
`PRINT HEART RATE,
`DISK TEMPERATURE,
`
`
`
`264
`|F COMMAND DURING RUN
`FUNCTION SWITCH A RUN
`FUNCTION SWITCH = RUN
`254.2/||NITIATE RECOVERY ROUTINE
`FIG.-IO
`
`Ex. 1010 / Page 11 of 35
`
`

`
`U.S. Patent
`
`Nov. 23, 1939‘
`
`Sheet 11 of 12
`
`4,883,063
`
`Minute of Delay
`
`Most Recent HR
`
`‘axooccnboobooo'ucco_A00'(,D'°'U‘|&Q)l’\)-SQoto<9.
`
`9
`
`9
`
`4
`
`MTA-5 End Point
`
`MTA-10 End Point
`
`MTA-20 End Point
`
`MTA-30 End Point
`
`MVTA-45 End Point
`
`MTA-60 End Point
`
`MTA-90 End Point
`One Minute Past Values for MTA-90
`
`FIG.--II
`
`Ex. 1010/Page 12 Of35
`
`Ex. 1010 / Page 12 of 35
`
`

`
`U.S. Patent
`U.S. Patent
`
`Nov. 28, 1989
`Nov. 28, 1989
`
`Sheet 12 of 12
`Sheet 12 of 12
`
`4,883,063
`4,883,063
`
`
`
`Emmamc_Em>>Q_dflwmm_u_H._.mm_m
`
`In:5
`
`ooooNEN>>
`
`em53
`
`oooo
`
`asma
`
`Iu|.||l..|m..2d
`
`rm.D...
`
`Nom-<:,_
`
`no2-<:2
`
`mo8-35.58.52
`
`onmv<E
`
`on8&558.5.2
`
`N.Id_u_
`
`Ex. 1010/Page 13 Of35
`
`Ex. 1010 / Page 13 of 35
`
`

`
`PERSONAL MONITOR AND PROCESS FOR HEAT
`AND WORK STRESS
`
`This is a continuation of application Ser. No. 055,654,
`filed May 29,1987, now abandoned.
`BACKGROUND OF THE INVENTION
`
`5
`
`1. Field of the Invention
`This invention relates to a device and process which 10
`helps a user avoid excessive heart rates and increases in
`body temperature during heat stress and heavy work
`exposures. More particularly, it relates to a device and
`process that measures physiological responses to work
`and heat stress and provides recommendations to the 15
`user in real time. Most especially, it relates to such a
`device and process in which the user is provided the
`recommendations to make an intelligent decision to
`control his or her exposure to the stress. It further re-
`. lates to such an apparatus and process which is useful in
`an industrial environment.
`2. Description of the Prior Art
`Two approaches are conventional to avoid excessive
`heart rates and increases in body temperature during
`heat stress exposures. One is to limit
`the exposure
`through administrative controls, such as stay times. The
`other is to use self-determination. Stay times are usually
`very conservative to protect most people. Self-deterrni-
`nation often leads an individual to overextend himself
`or herself.
`A variety of measuring devices are known in the art
`for measuring exertion and related parameters. Body
`core temperature is a basic physiological measure of the
`body’s response to heat stress. Rectal temperature is
`commonly accepted as a primary measure. Because 35
`rectal temperature is measured by an inserted probe, it is
`not practical or acceptable for routine evaluation of
`core temperature. A substitute measure is required.
`Commonly used substitutes for rectal temperature are
`ear, esophageal, and oral temperatures. While the tem-
`perature measuring technology for these is
`readily
`available, they pose many practical problems that make
`them unsuitable for a personal monitor. Swallowable or
`ingested therrnotransmitters are possible, but unsuitable
`because of cost, potential liability and questionable ac-
`ceptability.
`A commerciallyavailable device can predict body
`core temperature by equalizing the heat flux from the
`core to the surface by placing a heater on the outside.
`However, this device requires too much power to be
`fully practical in this application, and it would give
`erroneous results in high ambient temperatures.
`Heart rate is another physiological measure that is
`related to work and heat stress. There are several com-
`mercially available devices to measure heart rate in
`work environments. The primary markets for these
`devices are sport, exercise and rehabilitation. While the
`methods and embodiments vary somewhat, the primary
`method of monitoring is to provide a low and high
`threshold for heart rate, so that an alarm can be given if 60
`heart rate goes below or above the specified window.
`For industrial environments, the low threshold does
`not have any real purpose. The high threshold is more
`valuable. If a person’s heart rate exceeds the threshold,
`an alarm can warn him or her of possible overexertion.
`The problem with these high threshold warnings is how
`to set the threshold. Workers can and do have momen-
`tary peaks of high heart rates due to sudden bursts of
`
`20
`
`25
`
`30
`
`45
`
`50
`
`55
`
`65'
`
`1 .
`
`4,883,063‘
`
`2
`activity or isometric work. These peaks do not repre-
`sent sustained levels of work, but they result in an alarm
`that would be reasonable for sustained levels of high
`demand. However, with a simple high threshold, there
`would be many alarms that do not represent significant
`physiological strain. The situation is further compli-
`cated by prolonged work at an intermediate level. The
`high threshold would miss prolonged heart rates just
`below the threshold that are very significant physiolog-
`ical strains.
`
`Examples of prior art devices for measuring heart
`rates, body temperature and related parameters are
`disclosed in the following issued U.S. Pat. Nos.:
`4,513,753,
`issued Apr. 30, 1985 to Tabata et
`a1.;
`4,450,843,
`issued May 29, 1984 to Barney et al.;
`4,425,921,
`issued Jan.
`17, 1984 to Fujisaki et al.;
`4,409,985,
`issued Oct. 18, 1983 to Sidorenko et al.;
`4,378,111,
`issued Mar. 29, 1983 to Tsuchida et al.;
`4,367,752,
`issued Jan.
`11, 1983 to Jimenez et al.;
`4,343,315,
`issued Aug.
`10,
`1982 to O’Leary and
`4,312,358, issued Jan. 26, 1982 to Barney. The state of
`the art
`is
`further indicated by Humen, D.P. and
`Boughner, D.R., “Evaluation of Commercially Avail-
`able Heart Rate Monitors,”The Canadian Medical Asso-
`ciation Journal, Vol. 131, Sept. 15, 1984, pp. 585-589
`and by commercially available heart rate monitors from
`Computer Instruments Corporation, Hempstead, L.I.,
`N.Y. 11550; Biosig Instruments, Inc., Champlain, N.Y.
`12919 and Dak Industries,
`Inc., Canoga Park, CA
`91304.
`
`While the art relating to such devices is therefore a
`well-developed one, a need still remains for a personal
`monitor and process capable of meeting the demands of
`an industrial environment.
`
`SUMMARY OF THE INVENTION
`
`Accordingly, it is an object of this invention to pro-
`vide a personal monitor and process for work and heat
`stress which will measure physiological responses to the
`stresses and provide recommendations to the user in an
`industrial environment.
`
`It is another object of the invention to provide such a
`personal monitor and process which will neither pro-
`vide the user with a multitude of inappropriate warn-
`ings, nor allow the user to exceed safe limits without
`providing a warning to the user.
`It is a further object of the invention to provide a
`novel sensor for measuring body core temperature.
`It is still another object of the invention to provide
`such a personal monitor and process which combines
`measurements of different body parameters to provide
`reliable indication of work and heat stress to the user.
`The attainment of these and related objects may be
`achieved through use of the novel personal monitor,
`temperature sensor and process of this invention. A
`personal monitor for work and heat stress in accordance
`with this invention has a heart beat sensor producing
`output electrical signals indicating a user’s heart beats.
`Means in the personal monitor stores heart beat infor-
`mation corresponding to the heart beat signals pro-
`duced over a predetermined time interval and is con-
`nected to receive the heart beat information. An infor-
`mation processing means is connected to receive the
`heart beat information from the storing means. The
`information processing means is configured to analyze
`the heart beat information incrementally over predeter-
`mined time intervals to obtain a physiological demand
`during the predetermined time intervals and to compare
`
`Ex. 1010/Page 14 Of35
`
`Ex. 1010 / Page 14 of 35
`
`

`
`3
`
`4,883,063
`
`4
`
`the obtained physiological demand against a stored
`physiological demand limit. A means provides an indi-
`cation to the user when the stored physiological de-
`mand limit has been exceeded. The information process-
`ing means is configured and connected to control opera-
`tion of the indication providing means.
`A temperature sensor in accordance with the inven-
`tion has a thermally conducting member with a surface
`configured to make direct contact with an object the
`temperature of which is to be measured. A device is
`thermally connected to the thermally conducting mem-
`ber for producing an output ‘electrical signal which is a
`function of temperature. A body of thermally insulating
`material surrounds a remainder of the thermally con-
`ducting member.
`A process for monitoring heat and work stress of an
`individual in accordance with the invention includes
`measuring the individual’s heart beats. The heart beat
`information is analyzed incrementally over predeter-
`mined time intervals to obtain a physiological demand
`. during the predetermined time intervals. The obtained
`physiological demand is compared against a predeter-
`mined physiological demand limit. An indication to the
`individual is provided when the predetermined physio-
`logical demand limit has been exceeded.
`The attainment of the foregoing and related objects,
`advantages and features of the invention should be more
`readily apparent to those skilled in the art, after review
`of the following more detailed description of the inven-
`tion, taken together with the drawings, in which:
`
`BRIEF DESCRIPTION OF THE DRAWINGS
`
`FIG. 1 is a perspective view of a first portion of a
`personal monitor in accordance with the invention.
`FIG. 2 is a perspective view of a second portion of
`the personal monitor, with a partial cutaway to show
`interior detail.
`FIG. 3 is a schematic diagram of a first electrical
`circuit used in the portion of the personal monitor
`shown in FIG. 1.
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`40
`
`FIG. 4 is a schematic diagram of a second electrical
`circuit used in the portion of the personal monitor
`shown in FIG. 1.
`
`45
`
`FIG. 5 are waveform diagrams useful for understand-
`ing operation of the electrical circuit of FIG. 4.
`FIG. 6 is a perspective view of a second portion of 50
`the personal monitor of FIGS. 1-4.
`FIG. 7 is a schematic diagram of a third electrical
`circuit used in the second portion of the personal moni-
`tor shown in FIG. 6.
`
`FIG. 8 is a schematic diagram of a fourth electrical
`circuit used in the second portion of the personal moni-
`tor shown in FIG. 6.
`
`FIG. 9 is a perspective view of a third portion of the
`personal monitor of FIGS. 1-4 and 6-8.
`
`FIG. 10 is a flow chart useful for understanding oper-
`ation of software forming a part of the invention.
`FIG. 11 is a diagrammatic representation of certain
`information stored and used in operation of the inven-
`tion.
`
`FIG. 12 is a diagrammatic representation of certain
`other information stored and used in operation of the
`invention.
`
`55
`
`60
`
`65
`
`DETAILED DESCRIPTION OF THE
`INVENTION
`
`Physiological Basis
`
`This section describes the physiological basis on
`which the PM algorithm was based. Physiological
`strain is the body’s response to a stress. The stress in
`heat stress is the combination of work demands, cloth-
`ing requirements and the environment. In addition to
`sweating,
`the most apparent physiological strains in
`response to heat stress are body temperature and heart
`rate. Basically, the accumulation of heat is reflected in
`body temperature while the ability to perform work and
`dissipate heat is reflected in heart rate. While body core
`temperature and heart rate correlate with each other,
`they measure different physiological strains, and there-
`fore add information about physiological state when
`both are considered. The Personal Monitor (PM) at-
`tempts to account for both body temperature and heart
`rate in order to present an alert for excessive physiolog-
`-ical strain. Based on laboratory evaluation there is also
`evidence the PM is sensitive to the onset of symptoms
`which is also reason to terminate a heat stress exposure.
`The algorithm uses insulated skin temperature and
`heart rate history to determine the alert state. These two
`physiological measures are treated independently and
`the alert state is determined by the measure that reaches
`the alert threshold first.
`
`The insulated skin temperature (called disk tempera-
`ture) is a physiological variable developed to predict
`overexposure to heat stress as a surrogate measure for
`body core temperature. There is a very high correlation
`between core temperature under two different condi-
`tions of heat stress.
`The next step is to establish a safe limit for core tem-
`perature. The National
`Institute for Occupational
`Safety and Health (NIOSH) and the World Health
`Organization have recommended limiting heat expo-
`sure so that core temperature does not exceed 38.0“ C.
`for daily, prolonged periods,
`i.e., for chronic work
`under heat stress conditions. WHO suggested that “in
`closely controlled conditions the deep body tempera-
`ture may be allowed to rise to 39° C. (l02° F.)”. There
`is an increased risk for heat illness at the higher core
`temperature as well as increasing incidence of unsafe
`behavior linked to heat stress. Heat stroke is associated
`with core temperatures above 40° C.
`The action alert threshold for core temperature was _
`selected to be 38.5“ C., because the PM represents a
`means to closely control the heat stress exposure and
`the alert threshold allows time to terminate the expo-
`sure. First, this limit allows a greater exposure to heat
`stress above acceptable levels for chronic exposures.
`Second, it allows five to ten minutes to leave an area of
`heat stress.
`
`With a core temperature limit of 38.5“ C. specified, a
`disk temperature threshold for the alert could be deter-
`mined. To set an action alert threshold for disk tempera-
`ture that protects most workers from exceeding a core
`temperature of 38.5‘ C., two decisions were made. First,
`analysis indicated that the relationship of core tempera-
`ture to disk temperature is different for different cloth-
`ing. For this reason, two classes of clothing were de-
`fined: (1) work clothes or single cotton coveralls, and
`(2) double cotton coveralls or impermeable clothing
`(plastics) over coveralls. Second, disk temperature is
`not an exact measure of core temperature, but a surro-
`
`Ex. 1010/Page 15 Of35
`
`Ex. 1010 / Page 15 of 35
`
`

`
`5
`gate. This means that there is a certain amount of error
`associated with limiting core temperature by disk tem-
`perature. For each clothing class or ensemble, a disk
`temperature was selected so that for 95% of the cases
`core temperature was less than 38.5‘ C. The selected
`disk temperature was the action alert threshold. The
`action threshold for work clothes or cotton coveralls is
`38.2° C., and for double cotton coveralls or imperme-
`able coveralls over cotton coveralls, it is 38.5° C. These
`values are the action alert thresholds in the PM algo-
`rithm, which indicate a time to terminate the exposure.
`The design objectives for the PM called for a warn-
`ing alert to give preliminary notice that there has been
`an increase in physiological strain, and reset criteria that
`would allow the alert status to change from warning to
`acceptable. The warning alert should allow about 10
`minutes warning prior to an action alert under typical
`conditions of heat gain. The warning thresholds were
`38.0° and 38.l° C. for work clothes/cotton coveralls
`and double cottons/impermeable clothing, respectively.
`If the strain is reduced after a warning alert, the PM will
`reset to the acceptable indicator. The reset thresholds
`were set to be 0.2‘’ C. less that the warning level. The
`0.2“ C. drop indicates a significant drop in temperature
`and avoids problems with temperature sensor fluctua-
`tion or toggling around the threshold (e.g., changing
`between 37.9° and 38.0“ C.).
`The purpose of the heat rate criteria is to relate the
`information gained from monitoring heart rate to the
`cardiovascular strain of heat stress exposure. This is
`accomplished by examining the relationship between
`work demand and endurance time, then relating endur-
`ance time and heart rate through work demand, and
`finally proposing heart rate criteria to mark extensive
`strain. In addition, recovery heart patterns can be used
`to judge the level of cardiovascular adjustment to heat
`and work stress.
`
`As a person performs physically demanding work,
`the muscles demand oxygen, which is delivered by
`circulating blood. The increased demand for oxygen
`and, therefore blood flow, is achieved by increasing the
`heart rate. As work demands increase, oxygen demand
`increases to support muscle metabolism and heart rate
`rises to increase blood flow. If a person is subjected to
`increasing levels of work (and oxygen demands), his or
`her heart rate will reach a maximum value, which is also
`marked by a maximum utilization of oxygen (or maxi-
`mum aerobic capacity, MAC).
`A common observation is that the same task can be
`easily performed by one worker while another will
`have difficulty. The relative difficulty of performing a
`work task depends on how the oxygen demand of the
`task compares to the individual’s MAC. For work de-
`mands and commensurate oxygen demands less than the
`MAC, the relative difficulty is related to the oxygen
`demand expressed as the percent of the maximum aero-
`bic capacity (%MAC). A very fit worker (high MAC)
`will have less difficulty performing the same task than a
`less fit one because the former worker will be working
`at a lower fraction of his MAC.
`If the work requires an oxygen demand that is 30% of
`the maximum, the task is considered light and can be
`done for hours, but if it is 75% of the MAC, the task can
`only be performed for about 20 minutes under a great
`deal of physical duress that will lead to exhaustion.
`Many laboratory studies have been performed to exam-
`ine the relationship between time to exhaustion (endur-
`ance time) and % MAC. In the typical study, an indi-
`
`10
`
`15
`
`20
`
`25
`
`30
`
`35
`
`45
`
`50
`
`55
`
`60
`
`65
`
`4,883,063
`
`6
`vidual is given a constant work task requiring a steady
`rate of oxygen consumption (e.g., treadmill walking or
`pedaling a bicycle-ergometer). The work is performed
`until exhaustion, and the time is noted. The endurance
`time plotted against % MAC gives similar curves for all
`individuals. Endurance time decreases exponentially
`with % MAC.
`An average person can work about 20 minutes at
`80% of his/her MAC before exhaustion. This increases
`to 40 minutes at 70% MAC and 80 minutes at 60%
`MAC. From the job design point of view,
`the job
`should not require more time than the endurance time.
`So a work task requiring 70% MAC of a person should
`not exceed 40 minutes. Alternatively,
`if a minimum
`work time is required,
`the actual % MAC must be
`lower than that % MAC associated with the endurance
`time that equals the work time. For instance, if 40 min-
`utes are required to perform a task, the actual % MAC
`should be less than 70% MAC for any worker who is
`expected to complete the task.
`There is a range of endurance times (ET) for any
`particular value of % MAC. In order to be protective of
`most of the population, a conservative line can be
`drawn to include all of the ET data for a given % MAC,
`which follows the equation
`
`Iog1o(ET)=4.0-—4.0(% MAC/l00%)
`
`% MAC: 25%[Iog1o(ET)——4.0]
`
`(la)
`
`(lb)
`
`where ET is endurance time in minutes and the term
`(% MAC/ 100%) is the ratio from O to 1 that the oxygen
`demand is to the MAC. Equations la and lb are the
`same relationship expressed in two ways. The meaning
`of the relationship can be interpreted as follows. For
`any given % MAC, most workers would be able to
`perform at that relative level of work for at least a time
`of ET from Equation la. Or, if a work demand required
`a time of ET, it can be done by anyone who is working
`at or below the respective value of % MAC from Equa-
`tion lb. So a worker can work at least 10 minutes at
`75% MAC; or a job requiring 10 minutes of work can
`be performed by anyone working at or below 75%
`MAC.
`
`As a practical note, a pattern of different work de-
`mands with different oxygen demands can be averaged
`together over a time interval, and the average %MAC
`computed. The average % MAC is used to estimate the
`expected endurance time, and this endurance time is
`compared to the actual time requirement (the time base
`for the average). For instance, 5 minutes at 80% MAC
`can be averaged with 20 minutes at 50% MAC for an
`average demand of 56% MAC over 25 minutes. This
`average rate can be sustained because it is less than the
`endurance time of 100 minutes associated with 56%
`MAC. This averaging technique cannot be used if the
`work demand in a shorter period would mean the en-
`durance time is exceeded. For instance, 30 minutes at 80
`% MAC cannot be averaged with 30 minutes at 40%
`MAC for an average of 60% MAC over 60 minutes.
`The individual would reach exhaustion during the 80%
`MAC segment, because the endurance time for 80%
`MAC is twenty minutes.
`Based on this averaging principle, an incremental
`analysis can be performed to assess whether a job may
`cause exhaustion. In the incremental analysis, the rela-
`tive oxygen demands (% MAC) over small intervals
`(i.e., 5 minutes) are examined to determine if the associ-
`
`Ex. 1010/Page 16 Of35
`
`Ex. 1010 / Page 16 of 35
`
`

`
`7
`ated endurance time is less than the time interval. That
`is, for any 5 minute interval,
`the average % MAC
`should not exceed 83 for the individual as calculated
`from Equation lb, using 5 minutes for ET. The time
`step is then increased (e.g., 10 minutes) and the average
`% MAC for that interval is calculated. Once again, if
`the average % MAC is less than 75% MAC (value from
`Equation lb for ET= 10 min), there should not be any
`excessive physiological demand (exhaustion). By incre-
`menting the time base of the average out to the working
`time itself, all possible combinations of work can be
`examined in terms of their effect on exhaustion. This
`means that short periods of high demand are considered
`as well as long periods of lower average demands.
`Because of the exponential nature of the relationship,
`the time intervals can be increased geometrically to
`reduce the number of averaging intervals without los-
`ing protective ability. So starting at 5 minutes, which is
`the smallest practical interval, the intervals can follow
`10, 20, 40, 80, .
`.
`. - minutes. As an approximation of the
`geometric progression, 7 averaging intervals were se-
`lected for the PM algorithm: 5, 10, 20, 30, 45, 60, and 90
`minutes. From Equation lb, the highest % MAC that
`can be sustained for these intervals is 83, 75, 67, 63, 59,
`56, and 51 % MAC, respectively. For work scenarios
`where the PM would be used, there is no need to extend
`the averaging intervals beyond 90 minutes because few
`exposures will go to 3 hours (180 minutes) of steady
`work.
`
`The next step to developing an algorithm for heart
`rate is to relate heart rate to % MAC. This step is re-
`quired because it is not feasible to know the MAC of
`each potential user of the personal monitor as well as
`the oxygen demands for all possible tasks to be per-
`formed so that % MAC can be known.
`_
`As a person moves from a resting oxygen demand to
`MAC, heart rate increases from the resting rate (HR,,,,,)
`to the maximum heart rate (HR,,,,,,,) The average resting
`heart rate is 75 beats per minute, and does not change
`much for age, race or sex. For the general population,
`HR,,,ax decreases with age, and is reasonably well pre-
`dicted by:
`
`(2)
`HRmgx= 220-age(year:)
`An implication of equation (2) for the population at
`large is that the average HR,,,,,x for a given age is 1 beat
`per minute (bpm) less than those a year younger. This
`also means average MAC decreases because of a lower
`ability to deliver blood and oxygen to the muscles, as is
`the case.
`
`Heart rate reserve (HRR) is the difference between
`HRM, and HR,,,,,x. Any heart rate (HR) can then be
`expressed as percent heart rate reserve % HRR by:
`%
`
`HRR=[(HR,,,,,,;—HR)/(HR,,,,,,,—-H-
`Rre.rr)] X 100%
`
`O1‘
`
`(30)
`
`(35)
`
`%HRR = [(HR,,.,,,,—HR)/HRR] x 100%
`% HRR is zero at rest and 100 at HR,,,,,,,.
`There are three factors to consider: (1) MAC(lOO%
`MAC) occurs at HR,,,,,x, (2) the oxygen demand is about
`10% MAC, and (3) heart rate increases linearly with %
`MAC. These three facts suggest a basic approximation
`used in the PM: % HRR is equal to % MAC. If some-
`one is using 50% of his MAC to do work, he will also
`use 50% of his HRR. To illustrate these relationships,
`suppose a 45 year old worker performs a task with a
`
`5
`
`l0
`
`l5
`
`20
`
`25
`
`30
`
`35
`
`40
`
`45
`
`50
`
`55
`
`60
`
`65
`
`4,883,063
`
`8
`heart rate of 125 beats.per minute (bpm). First, his
`HRm,,x= l75(=220—45, Equation 2) and he has a rest-
`ing rate of 75. He has an HRR of 100 bpm
`(l75—75= 100). If his heart rate is 125 bpm, he is using
`50% of his HRR([(175—125)/lO0]><l00%, Equation
`3b). This means he can work for at least 100 minutes
`(Equation la). Knowing HR then allows the endurance
`time to be estimated through % HRR and % MAC.
`This demonstrates that it is not necessary to know the
`oxygen demand of the task or the individual’s MAC to
`be able to estimate endurance time——only heart rate for
`the task.
`
`The relationships can be used in reverse as well. If a
`minimum work time is known and equated to ET, the
`highest expected work demand expressed as % MAC
`can be estimated from Equation lb, and therefore the %
`HRR. With the % HRR and assumptions about
`HR,es;(=75) and HR,,,,,,, (Equation 2), a limit on heart
`rate can be suggested. As an example, suppose the work
`time is 20 minutes and the worker is 45 years old (as
`above). The highest % MAC is 67 for ET=20 from
`Equation lb. This then means that the average heart
`rate for the 20 minutes should not exceed 67% HRR.
`Equation 4 is a rearranged version of Equation lb.
`
`(4)
`HR =HRre:t+HRR(% HRR/100%)
`Sixty-seven % MAC translates to a heart rate of
`l42(=75 + 100[67/100]), Equation 4.
`The preceding discussion, however, has not ad-
`dressed heat stress. It is known that the added burden of
`heat stress on the cardiovascular system is also reflected
`in heart rate, and that heart rate increases due to heat
`stress can be treated as an increment in % MAC. In
`other words, the physiological strain reflected in heart
`rate is the same whether the heart rate is due to work
`alone or a lesser level of work in combination with heat
`stress. Therefore, the effect of work and heat stress on
`heart rate are additive, and the overall effect on heart
`rate can be used to estimate a % MAC through %
`HRR. The % MAC can then be used to predict a maxi-
`mum work time or endurance time.
`
`Two assumptions come into play, therefore, in using
`heart _rate to monitor physiological strain due to work
`under heat stress. First, the effects of heat stress on heart
`rate are additive to the heart due to work alone. Second,
`heart rate can be used to reflect the equivalent % MAC
`and thus the endurance time is short for high % MAC
`and increases with lower values of % MAC. The result-
`ing principle is that high heart rates (high demands,
`reflected as the combination of metabolism and heat
`stress) can be safely sustained for short periods of time
`and lower heart rates (lower demands) for longer peri-
`ods.
`For the PM, a heart rate threshold is a level above
`which an average heart rate for a specified period of
`time triggers an alert. For a short time, the PM should
`tolerate a high heart rate due to heavy work and/or
`high heat stress. This can be achieved by using a short
`averaging period with a high threshold. In contrast, the
`PM must be sensitive to a moderate demand (lower HR)
`over a longer period. Therefore, a longer averaging
`period can be used with a lower threshold.
`Averaging avoids problems commonly found with
`single, static thresholds used in currently available heart
`rate monitors. If a static criterion is set high enough to
`allow bursts of high demand, then it will miss longer
`terms of moderate strain. If it is set low for moderate
`strain, it will alar