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Original Article
Polar S810 as an Alternative Resource to the Use of the
Electrocardiogram in the 4-Second Exercise Test
Alan Santos Pimentel1, Eduardo da Silva Alves1, Rafael de Oliveira Alvim1, Rogério Tasca Nunes1, Carlos Magno
Amaral Costa1, Júlio Cesar Moraes Lovisi2, Jorge Roberto Perrout de Lima1
Universidade Federal de Juiz de Fora - Laboratório de Avaliação Motora1; Clínica de Medicina do Exercício e Reabilitação Cardiovascular CUORE2, Juiz de Fora, MG - Brazil
Abstract
Background: The 4-second exercise test (T4s) evaluates the cardiac vagal tone during the initial heart rate (HR)
transient at sudden dynamic exercise, through the identification of the cardiac vagal index (CVI) obtained from the
electrocardiogram (ECG).
Objective: To evaluate the use of the Polar S810 heart rate monitor (HRM) as an alternative resource to the use of the
electrocardiogram in the 4-second exercise test.
Methods: In this study, 49 male individuals (25 ± 20 years, 176 ±12 cm, 74 ± 6 kg) underwent the 4-second exercise
test. The RR intervals were recorded simultaneously by ECG and HRM. We calculated the mean and the standard
deviation of the last RR interval of the pre-exercise period, or of the first RR interval of the exercise period, whichever
was longer (RRB), of the shortest RR interval of the exercise period (RRC), and of the CVI obtained by ECG and HRM.
We used the Student t-test for dependent samples (p ≤ 0.05) to test the significance of the differences between means.
To identify the correlation between the ECG and the HRM, we used the linear regression to calculate the Pearson’s
correlation coefficient and the strategy proposed by Bland and Altman.
Results: Linear regression showed r2 of 0.9999 for RRB, 0.9997 for RRC, and 0.9996 for CVI. Bland e Altman strategy
presented standard deviation of 0.92 ms for RRB, 0.86 ms for RRC, and 0.002 for CVI.
Conclusion: Polar S810 HRM was more efficient in the application of T4s compared to the ECG. (Arq Bras Cardiol
2010;94(5):545-549)
Key words: Muscle tonus; heart rate; exercise; measures; heart.
Introduction
A reduced vagal tone is strongly associated with risk of
death by cardiovascular events1-5. Because of its relationship
with cardiac vagal integrity, the behavior of heart rate (HR)
has been widely studied in the initial transient of different
modalities and conditions associated with exercise. The
4-seconds exercise test (T4s) designed to assess cardiac vagal
tone calculates the cardiac vagal index (CVI) from the analysis
of the electrocardiographic recording made during the initial
transient of HR in sudden dynamic exercise performed under
controlled respiratory conditions1.
In the 1980s, the first heart rate monitors (HRM) to be used
during exercise were created. Later, they were validated to
monitor the intensity of aerobic activity. In the following years,
many studies have confirmed the correlation between HR time
Mailing address: Rafael de Oliveira Alvim •
Av. Engenheiro Heitor Antônio Eiras Garcia, 79/21B - Jardim Esmeralda 05588-000 - São Paulo, SP - Brazil
E-mail: [email protected], [email protected]
Manuscript received on March 25th, 2009; revised manuscript received on
July 29th, 2009, accepted on September 1st, 2009.
545
series obtained by HRM and the electrocardiogram (ECG)
and Holter monitoring during various intensities and forms of
exercise6-15. With the development of electronic possibilities,
some models of HMR as the Polar S810 incorporated, in
addition to HR, the ability to record RR intervals. Therefore,
Nunan et al16 found a strong correlation when comparing the
recording and issue of RR intervals between the conventional
ECG and the Polar S810 HRM at rest, thus proving the
capability and applicability of this instrument in the acquisition
of data. Finally, relating the validity of those records at various
times and maneuvers, Kingsley et al17 found no significant
differences between the measures of Polar S810 HMR and
the ECG for all exercise intensities tested.
The T4s protocol includes sudden exercise in cycle
ergometer or upper-limb ergometer18 for 4 s, after initial
immobility of 4 s. This maneuver causes rapid and large
variations in RR intervals. The use of Polar S810 HMR has
not yet been studied in a similar situation. Moreover, there is
the need to observe whether the amplitude of the agreement
interval, ocasionally found between the HMR and the ECG,
can interfere with the identification of the CVI. Therefore, the
objective of this study was to test the operational viability and
Pimentel et al
Use of Polar S810 heart rate monitor in 4-second exercise test
Original Article
use of the Polar S810 heart rate monitor as an alternative to
the ECG in the application of the T4s.
Methods
Calculation of cardiac vagal index (CVI)
Volunteers
The subjects signed an informed consent about the
procedures used in the study. The project was approved by
the ethics committee of the institution under No. 171/2008.
Subjects with different clinical conditions participated in the
study: athletes, beta-blocker users and post-MI patients.
4-second test
Each individual, after a 5-minute rest, underwent 3
consecutive T4s trials, as described in the original protocol19.
The first trial served as a means of familiarization with the
procedure, and the best result of the two subsequent trials
was chosen as representative of the CVI. The interval between
repetitions of the protocol was 1 to 2 minutes, and the
subject waited for the HR to return to its pre-maneuver level
before repeating the protocol. T4s consisted of a dynamic
sudden exercise on a cycle ergometer (Funbec, Brazil) with
no resistance (zero charge), from the 4th to the 8th second
of maximal inspiratory apnea of 12 s. The individual, after
adjusting the saddle and resting for 5 minutes, followed the
four consecutive commands, separated by an interval of 4 s,
as shown in the diagram in Table 1.
Acquisition of ECG and HMR signals
The RR intervals were recorded simultaneously throughout
the test, by the ECG and the HR monitor. We used a digital
ECG (Micromed, Wincardio, Brazil) whose signals, recorded
at a speed of 25 mm/s, were analyzed by a specific software
(PC Ergo Elite version 3.2.1.5). We also used a HMR (Polar
S810, Finland) which had an interface (IR interface, Polar
Electro OY, Finland) for data transfer to a computer, in which
they were analyzed by a specific software (Polar Precision
Performance version 3.01). We used verbal commands as
event markers so as to begin and end the recordings of both
devices simultaneously. We connected the adhesive electrodes
Table 1 - Schedule of application of the 4-second exercise test (T4s)
Command
to the chest of the subjects (Blue Sensor, Brazil) of the ECG, in
CM5 lead, and the HMR standard transmitter (TM 31, Polar
Electro OY, Finland).
Time (s)
Action
1
0
Pre-exercise
After a quick inhalation through the mouth
with maximum inspiration, maintain apnea
(until the end of the test), without pedaling,
sitting on the bike for 4s.
2
4
Exercise
Maintaining apnea, pedal as fast as possible
for 4s (at least 5 rotations of the pedals).
3
8
Postexercise
Stop pedaling abruptly and remain seated on
the bike for 4s, maintaining apnea.
4
12
End of the test
Resume normal breathing and finish the test.
The first step in the calculation of the CVI was the
identification of the last RR interval of the pre-exercise period
or the first period of exercise (whichever was longer) called
RRB, and the shortest RR interval of the exercise period, called
RRC . The CVI is obtained by the RRB/RRC ratio. Initially, so
as to identify the RRB and RRC the time series of RR intervals
obtained by both devices were subjected to visual inspection
and removal of artifacts. Then the records underwent an HMR
filtering performed automatically by a specific software (Polar
Precision Performance version 3.01). Of the time series with
valid RR intervals, the RRB and RRC intervals, both of the ECG
and the HR monitor, were selected to calculate the CVI1 using
as reference the time of the commands.
Statistical treatment
The mean and the standard deviation of the sample
characteristics were calculated, and the values of RRB, RRC,
and CVI were obtained by ECG and HMR. We used the
Student t-test for paired data (p ≤ 0.05) to test the significance
of the differences between means. To identify the correlation
between the measurements made by ECG and HMR, we used
linear regression to calculate the coefficient of correlation, and
also applied the strategy proposed by Bland and Altman20,
which consists of averaging the individual measurements
of ECG (ECG) and HMR (HMR) ([ECG + HMR] / 2) and the
plotting of the differences between measurements made by
the HMR and these means.
Results
The T4s was applied to 49 males (25 ± 20 years, 176
± 12 cm, 74 ± 6 kg). The ECG and the HMR recordings
were synchronized by time markers (verbal commands).
After visual inspection of the records, when the artifacts and
the characteristic intervals of ventricular premature beats
were removed, each observer independently identified
the RRB and RRC of each evaluated interval among the
valid RR intervals. There was a coincidence of 100% on
the location of the identified intervals, and there were no
significant differences (p ≤ 0.05) between the means of the
measurements made by the ECG and the HMR for RRB,
RRC, and CVI, as shown in Table 2.
The linear regression of the values obtained by the ECG
in function of those obtained by the HR monitor had r2 of
0.9999 for the RRB, r2 of 0.9997 for the RRC, and r2 of 0.9996
for CVI. The plotting of the differences between the values
measured by the HMR and the mean measurements had a
standard deviation of 0.92 ms for the RRB, of 0.86 ms for the
RRC, and of 0.002 for the CVI (Fig. 1, 2, and 3).
Discussion
Heart monitors, such as the Polar S810, have been used
for measuring HR. In this study, when comparing the results
Arq Bras Cardiol 2010;94(5):545-549
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Pimentel et al
Use of Polar S810 heart rate monitor in 4-second exercise test
Original Article
Table 2 - Values of RRB, RRC and CVI obtained by ECG and HMR
RRB (ms)
RRC (ms)
CVI
HMR
ECG
HMR
ECG
HMR
ECG
Mean
869.5
869.3
684.3
684.5
1.280
1.279
SD
154.5
154.2
107.2
107.3
0.191
0.190
RRB - last RR interval of the pre-exercise period or the first exercise period,
whichever was longer; RRC - shortest RR interval of the exercise period; CVI cardiac vagal index; ECG - electrocardiogram; HMR - heart rate monitor.
Figure 2 - RRC ECG - the shortest RR interval of the exercise period obtained by
ECG; RRC HMR - the shortest RR interval of the exercise period obtained by the
heart rate monitor; mean HMR-RRC - mean value measured by ECG and HMR.
0.92 ms for the RRB, -0.10 ± 0.86 ms for the RRC, and 0.000
± 0.002 for CVI. There was no significant difference between
the measurements (p > 0.05).
Figure 1 - RRB ECG - last RR interval of the pre-exercise period or the first
exercise period (whichever is longer) obtained by ECG; RRB HMR - last RR
interval of the pre-exercise period or the first exercise period (whichever was
longer) obtained by the heart rate monitor; mean HMR-RRB - mean value
measured by ECG and HMR.
of T4s obtained by both devices (ECG and HMR), we found a
strong correlation, r2 of 0.9999 for the RRB (last RR interval of
the pre-exercise period or the first exercise period, whichever
was longer), and r2 of 0.9997 for the RRC (shortest RR interval
of the exercise period). The high correlation observed in
the RRB and RRC resulted in a correlation of 0.9996 in the
identification of the CVI, which was obtained by the RRB/RRC
ratio. As for the limits of agreement determined by Bland and
Altman20 strategy, we obtained the following results: 0.08 ±
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Arq Bras Cardiol 2010;94(5):545-549
In this study, the statistical treatment used was similar to
others reported here, in order to facilitate the comparison
of results. It should be noted that in the present study, we
used the RR interval in the initial transient of exercise with
sudden onset. Nevertheless, we found values similar to those
reported by Kingsley et al17 and Gamelin et al21 who studied
the validity of HMR in progressive exercise and at rest. The
limits of agreement found here were lower than those reported
in the studies cited above, perhaps because, in this study
we used, in the comparison, only RR intervals used in the
identification of the CVI, rather than the complete series (>
5 min), usually used.
There are few data in the literature referring to the
efficiency of the Polar S810 HMR to capture the RR intervals
of HR for determining the values of the CVI. However,
recent studies22-24 have demonstrated the effectiveness and
applicability of this device in the analysis of the HR variability
indexes, in both the time domain and in the frequency
domain, thus allowing a reliable, noninvasive, and low cost
Pimentel et al
Use of Polar S810 heart rate monitor in 4-second exercise test
Original Article
complex (ventricular depolarization), regardless of whether this
was determined by a normal beat originated in the sinus node
or by a ventricular premature beat. Therefore, the occurrence
of ventricular premature beats during the T4s may limit the
interpretation of the RR intervals by the HMR. In our study,
this was not a limiting factor, because, upon the occurrence of
ventricular premature beats with a too short coupling interval
(very early ES), the observer responsible for recording the HMR
was able to clearly identify the artifact generated, excluding it
from the measurements, which, ultimately, can be confirmed
by the high correlation coefficients observed here. However,
in ventricular premature beats with a slightly longer coupling
interval (close to the baseline measurements of RR), the
identification of artifacts by the Polar Precision Performance
software becomes significantly more difficult, which may
eventually limit the analysis of the measurements used in
the method. In this sample of heterogeneous characteristics,
with great variability of the CVI, the occurrence of ventricular
premature beats became undiscriminated in healthy subjects
and in patients with heart disease, and although they were
more frequent in the latter, there was no interference in the
correct interpretation of the indexes by the HMR observer.
Conclusion
Figure 3 - CVI ECG - cardiac vagal index obtained by ECG; CVI HMR - cardiac
vagal index obtained by the heart rate monitor; mean HMR-CVI - mean value
measured by ECG and HMR.
assessment of the autonomic balance. Although these are
different evaluation methods, both the CVI obtained by T4s
and the indexes adopted for HRV analysis arise from the
measurement of RR interval, which allows us to associate the
results found in previous studies.
According to the authors’ clinical experience, the
application of T4s is a very safe procedure. Although the test
has a high arrhythmogenic potential due to rapid changes
observed in autonomic activity, no cardiovascular events were
observed. Moreover, even if arrhythmias occur, in most cases
they are self-limited and of low complexity.
It should be noted that the measurement of the RR intervals
monitored by the Polar S810 HMR and transferred to the Polar
Precision Performance software is derived from the capture of
the electrical pulses generated by the R wave peak of the QRS
We concluded that the Polar S810 HMR can be used as an
alternative to the ECG in the application of the T4s. The use
of HR monitors increases the possibilities of the application of
the T4s. However, it should be emphasized that their use does
not replace the ECG in cardiac diagnosis. As the reduction
of cardiac vagal tone is associated with an increased risk of
death from cardiovascular events, the application of a simple
and safe method, such as the T4s, through a more accessible
methodology, using the HMR, could permit the identification
of a greater number of patients with these characteristics,
increasing the benefits derived from this observation. In
healthy subjects, it can be applied by health professionals as
a routine procedure in the assessment and monitoring of the
autonomic changes caused by aerobic training.
Potential Conflict of Interest
No potential conflict of interest relevant to this article was
reported.
Sources of Funding
There were no external funding sources for this study.
Study Association
This study is not associated with any post-graduation
program.
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