Heart rate monitor, method and computer software product

The invention provides a heart rate monitor, a method and a computer software product. The method determines from the user's electrocardiogram a reference value of a heart rate variable characterizing the heart rate; determines at least one environmental parameter value obtainable from air pressure using air pressure measurement; associates the reference value of the heart rate variable with at least one environmental parameter value; and records in a register at least one environmental parameter value and the reference value of the heart rate variable associated with the at least one environmental parameter value.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims priority based on Finnish Patent Application No. 20045392, filed on Oct. 15, 2004, which is incorporated herein by reference.

FIELD OF THE INVENTION

The invention relates to a method for providing heart rate information, to a heart rate monitor and to a computer software product.

BRIEF DESCRIPTION OF THE RELATED ART

As the geographical altitude increases, the atmospheric pressure decreases, which results in reduced air density. As the air density reduces, the oxygen, content per volumetric unit decreases, which produces reactions in human physiological mechanisms, such as blood circulation, respiration and metabolism. At the same time the human aerobic performance declines. Moreover, pressure reduction has particular physiological effects on the human fluid balance, for instance.

Clearly detectable geographical-altitude-related effects on human physiology typically start above 1500 meters, which can be detected as changes in heart rate variables characterizing the heart rate, such as resting heart rate, heart rate during exercise and variations of heart rate. As the altitude increases the resting heart rate and the heart rate during exercise increase as the human body compensates for the oxygen deficit in muscles, whereas the variation of the heart rate and the maximum heart rate generally decrease instead.

In general, the reference values of the heart rate variables are determined at normal pressure. As altitude increases or air density otherwise decreases, the reference value determined at normal pressure does not correspond to pressure conditions, whereby the heart rate monitoring becomes more difficult and the user is not able to monitor his physiological condition on the basis of a variable value characterizing an instantaneous heart rate.

Thus it is useful to examine various manners to provide heart rate information.

SUMMARY OF THE INVENTION

The object of the invention is to provide a heart rate monitor, a method and a computer software product so as to enable the user to take the prevailing pressure conditions into account while he or she monitors his/her physiological condition.

A first aspect of the invention is to provide a user-specific heart rate monitor comprising: characterizing means for determining from the user's electrocardiogram a reference value of a heart rate variable characterizing the heart rate, the reference value of the heart rate variable comprising the user's resting heart rate or a reference value of the user's heart rate variation; determining means for determining at least one environmental parameter value obtainable from the air pressure using air pressure measurement; associating means, connected to the characterizing means and to the determining means, for associating the reference value of the heart rate variable with at least one environmental parameter value; and a register, connected to the associating means, for recording in the register the at least one environmental parameter value and the reference value of the heart rate variable associated with at least one environmental parameter value.

A second aspect of the invention is to provide a user-specific heart rate monitor comprising: characterizing means for determining from the user's electrocardiogram a reference value of a heart rate variable characterizing the heart rate, the reference value of the heart rate variable comprising the user's resting heart rate or a reference value of the user's heart rate variation; determining means for determining one environmental parameter value obtainable from the altitudinal location of the heart rate monitor using altitudinal location measurement; associating means, connected to the characterizing means and to the determining means, for associating the reference value with at least one environmental parameter value; and a register, connected to the associating means, for recording in the register at least one environmental parameter value and a reference value of the heart rate variable associated with the at least one environmental parameter value.

A third aspect of the invention is to provide a user-specific heart rate monitor comprising: characterizing means for determining from the user's electrocardiogram a reference value of a heart rate variable characterizing the heart rate, the reference value of the heart rate variable comprising the user's resting heart rate or a reference value of the user's heart rate variation; determining means for determining an environmental parameter value proportional to air density using measurement of a physical variable proportional to air density; associating means, connected to the characterizing means and to the determining means, for associating the reference value of the heart rate variable with at least one environmental parameter value; and a register, connected the associating means, for recording in the register at least one environmental parameter value and a reference value of the heart rate variable associated with the at least one environmental parameter value.

A fourth aspect of the invention is to provide a method for generating heart rate information, the method comprising: determining from the user's electrocardiogram a reference value of a heart rate variable characterizing the heart rate, the reference value of the heart rate variable comprising the user's resting heart rate or a reference value of the user's heart rate variation; determining at least one environmental parameter value obtainable from air pressure using air pressure measurement; associating the reference value of the heart rate variable with at least one environmental parameter value; and recording in the register at least one environmental parameter value and the reference value of the heart rate variable associated with the at least one environmental parameter value.

A fifth aspect of the invention is to provide a computer software product that includes coded instructions for executing a computer process in a computer of a heart rate monitor, the computer process comprising: determining from the user's electrocardiogram a reference value of a heart rate variable characterizing the heart rate, the reference value of the heart rate variable comprising the user's resting heart rate or a reference value of the user's heart rate variation; determining at least one environmental parameter value obtainable from air pressure using air pressure measurement; associating the reference value with at least one environmental parameter value; and recording in the register at least one environmental parameter value and the reference value of the heart rate variable associated with the at least one environmental parameter value.

The invention is based on the idea that by associating a reference value of a heart rate variable, such as resting heart rate, with an environmental parameter value and by recording said values in a register a stored logical data structure will be provided. The logical data structure can be used subsequently to generate heart rate reference information corresponding to prevailing environmental conditions while the heart rate is monitored.

Several advantages are achieved with the heart rate monitor, the method and the computer software product of the invention. One advantage of the invention is to enable calibration of the resting heart rate in the heart rate monitor as a function of pressure, the calibration being available to the user when the determination of the resting heart rate is not possible.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1shows an example of a structure of a user-specific heart rate monitor100based on the use of telemetry. The user-specific heart rate monitor100comprises electrodes106A,106B, an ECG (ElectroCardio-Gram) preamplifier108provided with differential feed poles, a transmitter amplifier (TX AMP)110, a transmitter coil (TX CL)112, a receiver coil (RX CL)116, a receiver amplifier (RX AMP)118, a processing unit (PU)120, a memory unit (MEM)122and a user interface (UI)124.

Electrodes106A,106B sense the electric potential generated by the electric activity of the heart muscle and produce an ECG signal characterizing the electric activity of the heart muscle. The ECG signal is applied from the electrodes106A,106B to the ECG preamplifier108.

The ECG preamplifier108pre-amplifies the ECG signal and feeds the pre-amplified ECG signal to the transmitter amplifier110. The transmitter amplifier110may include a plurality of successive amplification stages, such as an AGC (Automatic Gain Control) amplifier and a power amplifier.

The amplified ECG signal is applied to the transmitter coil112that generates an electromagnetic field114transferring ECG data. The ECG data may include, for instance, the ECG as such, part of the ECG and/or heart rate timing information. The timing information may include a timing impulse that represents the timing of a predetermined ECG part.

In the given example a magnetic component of the electro-magnetic field114serves as a transfer mechanism for a wireless connection. The solution presented is not restricted, however, to the use of the magnetic component of the electromagnetic field114, but the ECG data transfer may use any form of telemetry.

The receiver coil116detects the electromagnetic field114generated by the transmitter coil112and produces an induced electric signal that is applied to the receiver amplifier118.

The receiver amplifier118performs electric signal processing such as filtering and amplifying. In addition, the receiver amplifier may comprise a plurality of successive regulation stages.

The receiver amplifier118feeds the electric signal to the processing unit120that may perform analog signal modification of the electric signal, such as filtration and analog-to-digital conversions. In addition the processing unit120may perform digital processing such as digital filtering, signal shaping, ECG signal detecting, and ECG signal analysing.

In the processing unit120it is possible to determine a value for a heart rate variable characterizing the heart rate and/or a reference value for a heart rate variable. The heart rate variable may be a heart rate beat interval, a heart rate frequency, a variation of the heart rate interval and/or a variation of the heart rate frequency. The reference value for the heart rate variable may be a heart rate interval at rest, a heart rate frequency at rest, a reference value of the variation of the heart rate interval and/or a reference value of the variation of the heart rate frequency.

The processing unit120can be implemented using analog circuits, ASIC circuits (Application Specific Integrated Circuit), a digital processor, a memory and computer software. The processing unit120may be part of the computer in the user-specific heart rate monitor100.

Part of the data produced by the processing unit120can be stored in a memory unit122connected to the processing unit120. In addition the memory unit122may include coded instructions for executing a computer process in the processing unit120.

The user-specific heart rate monitor100further comprises a measurement module (MM)130, which typically measures a value of a physical variable proportional to air density.

In one embodiment the measurement module130includes a pressure sensor that measures the ambient air pressure. The pressure sensor may generate a voltage level that is proportional to the pressure and that is converted into numeric format in the processing unit120, for instance. Small pressure sensors employed in the user-specific heart rate monitor100can be implemented, for instance, by piezo resistive silicon components and they represent commercially available technology known per se. Miniature pressure sensors of MS54 series manufactured by Intersema can be given as an example.

In another embodiment the measurement module130includes a satellite navigator that determines the location coordinates of the heart rate monitor100including altitude data and optionally the time. For instance, the satellite navigator may record the altitude coordinate of the heart rate monitor100and optionally other determined coordinates into the processing unit120. The satellite navigator may operate, for instance, in the GPS system (Global Positioning System), in the GLONASS (Global Navigation Satellite System) system or in another commonly used satellite positioning system. The implementation of the satellite navigator in the user-specific heart rate monitor system applications is technology known per se.

The user interface124typically includes a display unit126and a display controller. The display unit126may include, for instance, LCD components (Liquid Crystal Display). The display unit126may display graphically and/or numerically an instantaneous heart rate variable value, a reference value of the heart rate variable and/or measurement information produced by the measurement module130, such as instantaneous air pressure and/or instantaneous location altitude.

The user interface124further includes a keypad128by means of which the user may record commands into the user-specific heart rate monitor100.

The user-specific heart rate monitor100is characterized in that the user of the heart rate monitor monitors his or her own condition by means of the heart rate monitor.

The user-specific heart rate monitor100shown inFIG. 1can be divided into a transmitter part102and a receiver part104. The transmitter part102typically includes device parts106A to112and it performs the ECG measurement and transmission of the ECG information to the receiver part104. In some embodiments the transmitter part102may include a heart rate detector that detects a predetermined part of the ECG, generates a transmitter burst and/or bit stream representing the timing of the predetermined ECG part and transmits the transmitter burst to the receiver part104.

The receiver part104typically includes device parts116to128that process the electric signal used in telemetry and the ECG information and provide a user interface. In addition the receiver part104typically includes a measurement module130, but in some embodiments the measurement module130may also be located in the transmitter part102. In that case the information generated by the measurement module130can be transmitted telemetrically to the receiver part104.

With reference to the embodiment ofFIG. 2the transmitter part102is placed in a transmitter belt202that is worn around the user's200chest. The ECG information is delivered telemetrically from the transmitter belt202to the receiver unit204that is typically a wrist-worn device. In some embodiments the receiver unit204is attachable to bicycle structures such as a handle bar. The placement of the receiver unit204is not restricted, however, to the wrist or the handle bar, but it can be placed anywhere with the proviso that a telemetric connection between the transmitter unit202and the receiver unit204is maintained and that the user is able to use the receiver unit204.

In one embodiment the transmitter part102and the receiver part104are integrated in the same heart rate monitor, whereby a user-specific heart rate monitor worn on the wrist or held on the handle bar, for instance, is obtained. In that case some device parts ofFIG. 1, such as the coils112,116and the amplifiers110,118, are not necessarily needed. In one embodiment the transmitter part102and part of the receiver part104are integrated in the transmitter belt202, whereby the transmitter belt202may collect ECG data, process the ECG data and determine values of variables characterizing the heart rate. In that case the telemetric data transmission conveys processed data, such as variable values characterizing the heart rate and commands given by the user, from the transmitter belt202to the receiver unit204. In that case the receiver unit204may also be telemetrically, optically or galvanically connected to the transmitter belt202.

Each pulse304A,304B corresponds to one heartbeat with timing316A,316B. The interval between the pulses304A,304B is called a heart rate interval318.

The pulse304A,304B has pre-detectable parts, such as P wave306A,306B, Q wave308A,308B, R wave310A,310B, S wave312A,312B and/or T wave314A,314B, which represent various phases of a heartbeat.

The generation mechanisms of P, Q, R, S and T waves are known per se. The R wave310A,310bproduces a strong and thus easily detectable structure in the pulse304A,304B, so the R wave310A,310B is generally used for detecting a QRS complex and for determining the pulse timing316A,316B.

The QRS complex can be detected with a pulse detector, for instance. The transmitter part102may generate, for instance, a burst corresponding to the timing of each pulse304A,304B, the burst being transmitted to the receiver part104. The receiver part104receives the bursts and may determine, for instance, the heart rate interval318between the successive bursts. On the basis of the heart rate interval318it is possible to generate heart rate variables characterizing the heart rate, such as heart rate frequency, resting heart rate, variation in the heart rate interval and/or a reference value of the variation in the heart rate interval. Determination of the heart rate variables is known per se, and therefore it is not described in greater detail herein. The values of the heart rate variables can be recorded in the memory unit122for processing or for subsequent use.

The environmental parameter is typically a parameter characterizing air density in the ambient air of the heart rate monitor, so at the same time the environmental parameter characterizes the oxygen content in a volumetric unit. The environmental parameter may be the pressure prevailing in the vicinity of the heart rate monitor. In one embodiment the environmental parameter is the altitudinal location or the effective altitudinal location of the heart rate monitor. The effective altitudinal location corresponds to a free atmospheric altitude. The effective altitudinal location can be attained under controlled conditions such as in a pressure chamber or in an artificial, low-pressure “alpine hut” (??). The effective altitudinal location can also be linked to the oxygen content of breathing air in a volumetric unit.

As an example of an environmental parameter curve is given an altitude curve406that represents the altitudinal location of the heart rate monitor, for instance, in kilometers. Thus the length variable unit such as kilometer appears on the vertical axis404. The starting point410of the altitude curve406corresponds, for instance, to the altitude to which the user is acclimatized. The starting point410of the altitude curve is the sea level or the altitude of the user's domicile, for instance.

As another example of an environmental parameter is given a pressure curve408that is typically a function of the altitudinal location or the effective altitudinal location. Thus the pressure variable unit such as millibar appears on the vertical axis404. The starting point412of the pressure curve408is the normal atmospheric pressure (1013 mbar), for instance.

The heart rate variable curve414presents the reference value of the heart rate variable as the altitude/pressure conditions change. Thus the heart rate variable unit such as pulse per minute appears on the vertical axis404. The starting point416of the heart rate variable reference value is the resting heart rate at normal atmospheric pressure, for instance.

At the altitude of 2000 meters the heart rate frequency typically increases 10% as compared with the heart rate frequency at the sea level. When reaching the altitude of 4500 meters the heart rate frequency increases about 50% as compared with that at sea level.

The situation inFIG. 4Amay represent, for instance, a take-off of an aircraft, whereby the time scale on the horizontal axis is a few minutes. The pressure curve408thus corresponds to the pressure inside the aircraft cabin, which does not correspond to the actual altitude of the plane due to the pressurization. Thus, the altitude curve406represents the effective altitudinal location. In this case there is no time for acclimatization, and the obtained reference values of the heart rate variable characterize the human's fast physiological response to changes in pressure.

The situation ofFIG. 4Amay also represent a car drive or a cable car ride in the mountains.

When the environmental parameter varies on a relatively fast time scale, that is, in the order of less than 24 hours, the user's body has not time enough to adapt to the prevailing environmental conditions. Then, the reference value of the heart rate variable does not include the effect of acclimatization on the heart rate variable.

FIG. 4Aalso shows a sampling point418,420of the environmental parameter curve406,408and a sampling point422of the heart rate variable curve414. The sampling point418,420of the environmental parameter curve406,408is obtained from the determination of a physical variable proportional to air density, such as pressure and/or altitude, at the moment of determination tm. The sampling point422of the heart rate variable414is obtained from the determination of the reference value of the heart rate variable at the moment of determination tm.

The sampling point418,420of the environmental parameter curve406,408can be determined by measuring a plurality of physical variable values proportional to air density and by calculating the value of the sampling point418,420of the environmental parameter curve406,408corresponding to the moment of determination tmfrom the average of the physical variable values proportional to air density. In that case the air pressure is measured, for instance, within a time interval t1to t2, during which the pressure varies within the pressure range p1to p2. The pressure value pmcorresponding to the moment of determination tmwill be pm=(p1+p2)/2. The pressure value pmmay be as such the environmental parameter value or the altitudinal location hmcan be generated therefrom. In the corresponding manner it is possible to determine the altitudinal location within the range of h1to h2by direct measurement, for instance, by means of a satellite navigation system, and to calculate the altitudinal location hmcorresponding to the moment of determination tmfrom the expression hm=(h1+h2)/2.

The sampling point422of the heart rate variable curve414can be determined by measuring the heart rate variable within the time interval t1to t2. The time interval t1to t2is typically the time interval used in the determination of the sampling point418,420of the environmental parameter curve406,408. From the heart rate variable values determined within the time interval t1to t2it is possible to determine a reference value of the heart rate variable, for instance, as an average value. In one embodiment within the time interval t1to t2there is measured a heart rate average that corresponds to the resting heart rate.

The environmental parameter value and the reference value of the heart rate variable corresponding to the same moment of determination tmconstitute an associated pair, in which the reference value of the heart rate variable is associated with the environmental parameter value.

When a plurality of sampling points418,420of the environmental parameter curve406,408are measured as the value of the environmental parameter changes the presentation ofFIG. 4Bis obtained. InFIG. 4Bthe horizontal axis424represents pressure p and the vertical axis426represents the reference value HRrefof the heart rate variable. Sampling points430and432, which correspond to coordinates (pm1, HRref,m1) and (pm1, HRref,m1) in said order, are given as examples. Through the coordinates it is possible to form a heart rate variable curve428.

FIG. 5is a block diagram of a heart rate monitor500. The heart rate monitor500comprises a characterization unit (CU)502that determines a reference value514of a heart rate variable from the user's electrocardiogram. The characterization unit502may include, for instance, device parts116to122of a transmitter part102and a receiver part104shown in the figure.

The heart rate monitor500also comprises a determination unit (DU)504that determines an environmental parameter value516.

The determination unit504comprises a measurement module130shown inFIG. 1and optionally parts of a processing unit120.

In one aspect of the invention the measurement module130includes a pressure sensor. In that case the processing unit120receives pressure information generated by the pressure sensor and generates from the pressure information an air pressure and an altitudinal location.

In a second aspect of the invention the measurement module130includes means, such as a satellite navigator, for measuring the altitudinal location. In that case the processing unit120receives from the measurement module130location information and generates from the location information an altitudinal location or an air pressure.

It is possible to use the air pressure and/or the altitudinal location as the environmental parameter value.

Conversions between the air pressure and the altitudinal location can be carried out in the processing unit120using the digital processor and the computer program of the processing unit120.

The characterization unit502feeds the reference value514of the heart rate variable to an association unit (AU)506. The determination unit504feeds the environmental parameter value516to the association unit506.

The association unit506provides a logical link between the environmental parameter value516and the reference value514of the heart rate variable associated with the environmental parameter value516. The logical link may be based, for instance, on indexing, by which the reference value514of the heart rate variable associated with the environmental parameter value516is indexed.

The association unit506can be implemented, for instance, in the processing unit120by means of a digital processor and a computer program.

The association unit506records a data element518comprised by the environmental parameter value516and the reference value514of the heart rate variable associated with the environmental parameter value516into a register (REG)508. Thus the register508includes the environmental parameter516value and the reference value514of the heart rate variable associated with the environmental parameter value516. The register508can be implemented, for instance, in the memory unit122ofFIG. 1. In addition the processing unit120may perform some register508functions such as indications to the memory of the register and restoration of data stored in the indicated memory locations.

In one embodiment the determination unit504determines a plurality of environmental parameter values516of different magnitudes and the characterization unit502determines a plurality of reference values514of the heart rate variable, for instance, by performing determinations of the sampling points418,420,422ofFIG. 4Afor a plurality of successive determination moments tm. Thus is obtained, for instance, the sampling points430,432of the heart rate variable curve428shown inFIG. 4B, which sampling points characterize the reference point of the heart rate variable as a function of air pressure.

The association unit506associates the reference values of the heart rate variable with the values of the environmental parameter such that each reference value514of the heart rate variable is associated with the environmental parameter value516determined at the moment of determination tmof the heart rate variable reference value514.

The register508may comprise a logical data structure, such as a table, into which the environmental parameter values516and the reference values514of the heart rate variable associated therewith are recorded.

In one embodiment the heart rate monitor500comprises a controller510connected to the determination unit504and to the characterization unit502. In one embodiment the controller starts the determination of the reference values514of the heart rate variable on the basis of the environmental parameter values516. The controller510may include an algorithm that monitors, for instance, the environmental parameter values516as a function of time. In one embodiment the determination of the reference values514of the heart rate variable is started when a predetermined change rate is detected in the environmental parameter value516. The predetermined change rate is typically defined to be so high that the user will not be able to attain by himself such a big change in pressure and/or effective altitude by walking or cycling, for instance. The selection of the predetermined change rate permits one to detect, for instance, that the user is aboard an aeroplane or in another environment that enables a fast change in pressure and/or effective altitude, whereby the effect of acclimatization on the reference value of the heart rate variable is minimized. The predetermined change rate of the environmental parameter may be in the order of 100 m/min or 10 mbar/min, for instance, but the presented solution is not restricted to those figures, however.

In one embodiment the controller510ends the determination of the reference values of the heart rate variable and the determination of the environmental parameter values516on the basis of the environmental parameter values. The controller may monitor and end the determination of the environmental parameter values and the reference values of the heart rate variable when the environmental parameter value reaches a threshold value. The threshold value may be a predetermined resting heart rate level or it may be a value generated from previous measurements of the environmental parameter values516. Thus, for instance, as the pressure in the aircraft cabin is balanced, the controller detects the balancing of the pressure and ends the determination of the resting heart rate automatically.

The above-described determination, association and recording of the environmental parameter values516and the reference values514of the heart rate variables associated therewith enable calibration of the reference value of the heart rate variable with respect to the environmental parameter value. The logical data structure included in the register508and the environmental parameter values recorded therein and the reference values of the heart rate variable associated therewith can be utilized in a variety of ways.

In one embodiment the determination unit504determines an instantaneous value522of an environmental parameter while the user is experiencing a strain for instance in the mountains. In that case the determination of the actual resting heart rate may be difficult due to a pre-measurement strain, possibly of long duration, and to the acclimatization.

The determination unit504records the instantaneous value522of the environmental parameter into the register508.

The register508restores the reference value520of the heart rate variable to be associated with the instantaneous value522of the environmental parameter by using the environmental parameter values and the reference value of the heart rate variable associated with each environmental parameter value recorded in the register508.

The register508does not necessarily include in advance the exact instantaneous value522of the environmental parameter and the reference value520of the heart rate variable to be associated therewith. So, the register508may determine the reference value520of the heart rate variable to be associated with the instantaneous value522of the environmental parameter, for instance, by using interpolation or optionally extrapolation. This can be performed, for instance, by forming a heart rate variable curve428as shown inFIG. 4Band by searching a point434therein that corresponds to instantaneous pressure piand the reference value HRref,iof the heart rate variable corresponding to the instantaneous pressure.

In one embodiment the instantaneous value522of the environmental parameter is applied to a display unit (DISP)512that is configured to display the instantaneous value522of the environmental parameter and the reference value520of the heart rate variable to be associated with the instantaneous value522of the environmental parameter restored by the register. In that case the user may monitor the prevailing ambient pressure and the value of his resting heart rate corresponding to the pressure.

In one embodiment the characterization unit502determines an instantaneous value524of the heart rate variable characterizing the heart rate, such as the heart rate during exercise. The instantaneous value524of the heart rate variable can be fed into the display unit512and displayed in relation to the reference value520of the heart rate variable associated with the instantaneous value522of the environmental parameter. In that case the user may compare the instantaneous heart rate during exercise to the reference value of the heart rate corresponding to the prevailing pressure and/or altitude and adapt his physical strain in view of the prevailing pressure conditions.

The display unit512may be, for instance, a display unit126as shown inFIG. 1.

With reference toFIG. 6the display unit may comprise a heart rate display segment (HR DISP)602, a time display segment (D/T DISP)604, an environmental parameter display segment (EP DISP)606and a control segment (C DISP)608.

The heart rate display segment602typically shows an instantaneous numerical value of a heart rate variable, such as the instantaneous heart rate frequency or the instantaneous heart rate variation.

The time display segment604displays numerically a time variable such as the date, the time and/or a time variable relating to timing.

The control segment608typically displays menu elements and parameter values representing the operation of the heart rate monitor.

The display unit512may also include a graphic display (GD)610for graphical representation of the heart rate information and the environmental parameter information. The graphic display610may include a horizontal axis618and a scale620A,620B,620C,620D.

In one embodiment the display unit610displays the reference value520of the heart rate variable associated with the instantaneous value of the environmental parameter with a reference value indicator614, the horizontal position of which indicates the resting heart rate corresponding to the prevailing air pressure. In addition the display unit610may display an instantaneous value524of the heart rate variable with a heart rate variable indicator612, the horizontal position of which depends on the instantaneous value524of the heart rate variable. The user may thus monitor the instantaneous value of the heart rate in relation to the resting heart rate corresponding to the prevailing air pressure by comparing the position of the heart rate variable indicator612to that of the reference value indicator614.

The graphic display610may also include a maximum value indicator616that indicates a maximum value of a heart rate variable, such as the maximum heart rate or any other value of a variable characterizing the upper limit of the heart rate variable. The user may compare the position of the heart rate variable indicator612to that of the maximum value indicator616and adapt the strain suitably.

The graphic display610may also include a second reference value indicator622. The second reference value indicator622may indicate the reference value of the heart rate variable stored in the register510in other than prevailing circumstances. In one embodiment the second reference value indicator622indicates the user's acclimatized resting heart rate. The acclimatized resting heart rate is a resting heart rate that is attained when the user has adapted to the ambient conditions. The acclimatized resting heart rate may be, for instance, the resting heart rate in the atmospheric normal pressure.

FIGS. 7 and 8show methods in accordance with the presented solution.

At702a reference value514of a heart rate variable characterizing the heart rate is determined from the user's electrocardiogram.

At704at least one value516of an environmental parameter obtainable from the air pressure is determined using air pressure measurement.

At706the reference value514of the heart rate variable is associated with at least one environmental parameter value516.

At708at least one value516of the environmental parameter and the reference value514of the heart rate variable associated with the at least one environmental parameter value are recorded in the register.

At710an instantaneous value522of the environmental parameter is determined.

At712an instantaneous value522of the environmental parameter is recorded in the register510.

At714an instantaneous value524of a heart rate variable characterizing an instantaneous heart rate is determined from the user's electrocardiogram.

At716the reference value522of the heart rate variable to be associated with the instantaneous value522of the environmental parameter is restored from the register510using the environmental parameter values recorded in the register510and the reference value514of the heart rate variable associated with each environmental parameter value.

At718the instantaneous value522of the environmental parameter and the reference value of the heart rate variable to be associated with the instantaneous environmental parameter value522are displayed.

At720there are displayed the reference value520of the heart rate variable to be associated with the instantaneous environmental parameter value522and the reference value524of the heart rate variable in relation to the reference value520of the heart rate variable to be associated with the instantaneous environmental parameter value522.

The method ends at722.

At802the determination of reference values514of a heart rate variable is started on the basis of environmental parameter values516.

At804a plurality of reference values of the heart rate variable characterizing the heart rate are determined from the user's electrocardiogram.

At806a plurality of environmental parameter values516of different magnitudes are determined.

At808the reference values514of the heart rate variable are associated with the environmental parameter values516such that each reference value514of the heart rate variable is associated with the environmental parameter value516determined at the determination moment of the reference value514of the heart rate variable.

At812the determination of the reference values514of the heart rate variable and the determination of the environmental parameter values516on the basis of the environmental parameter values are finished.

One aspect of the invention is a computer software product that includes coded instructions for executing a computer process in the computer of the heart rate monitor. The embodiments of the computer process appear inFIGS. 7 and 8.

The computer software product can be stored on a distribution medium, such as a magnetic and/or optical storing medium, a hard disk or another means suitable for data storage and/or transfer. In addition the computer software product can be transferred using a computer-readable signal, such as a telecommunications signal.

Even though the invention is described above with reference to the accompanying drawings, it is obvious that the invention is not restricted thereto but it may be modified in a variety of ways within the scope of the attached claims.