Download Mean 24 hour heart rate, minimal heart rate and pauses

European Heart Journal (1983)4,44-51
Mean 24 hour heart rate, minimal heart rate and
pauses in healthy subjects 40-79 years of age
P. BJERREGAARD
University Department of Cardiology, Aarhus Kommunehospital, Aarhus, Denmark
KEY WORDS: Ambulatory electrocardiography, heart rate, sinus bradycardia, A-V block, healthy
subjects.
Normal sinus rhythm has been denned as having
atrial rate of 60 to 100 beats/min with antegrade P
wave contour and adjacent cycle lengths varying by
less than 10%!').
Several factors may, however, raise the heart rate
above 100 beats/min, and sinus tachycardia is
rarely a manifestation of sinus node dysfunction!2].
It has been stated, that females usually have higher
heart rates than males, and smokers a higher heart
rate than non-smokers!3!.
A heart rate below 60 beats/min is common in
healthy adult subjects. Great variability in heart
rate and methodological differences between vanous studies of heart rates in healthy subjects has
made a clear definition of the normal limit for the
heart rate in adult subjects during a 24 h period difficult. Sinus bradycardia is one of the features of
Received for publication 20 August 1981; and in revised form 25
November 1981.
Request* for repnnts to: Dr Preben Bjerregaard, Cardiologist
Afdeling, Aarhus Kommunehospital, 8000 Aarhus C, Denmark.
0195-668X/83/010044+08J02.00/0
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In order to establish normal limits for mean 24 h heart rate (HR24h), minimal heart rate trends (HRJ
and pauses, 24 h ambulatory ECG recordings from 260 healthy subjects 40-79 years of age were analysed.
The HR24h varied from 53 to 95beats/min (mean±2s.d.: 74±18beats/min). The minimal HR,
varied from 36 to 78 beats/min (mean± 2 s.d.: 56 ±16 beats/min). Analysis of variance showedan additive
effect of smoking, sex, leisure-time physical activity and age on both HR24h ond minimal HR,, and the
effect of the three first factors was statistical significant at the 1% level for_ both heart rate variables. The
males, the non-smokers and the physically active subjects had a lower HR24f, ond a lower minimal HR,
than females, smokers and passive subjects. Older subjects had a lower HR24h than younger subjects, but
the effect of age on minimal HR, was non-significant.
A total of 77 subjects (30%) had a pause (R-R intervals. 1500 ms), but in only 12 (5%) did the pause
exceed 1750 ms with the longest pause measuring 2040 ms. Further analysis of the longest pause in each
of the 77 subjects with pauses showed that 46 of the longest pauses occurred at night following a gradual
decrease in the R-R intervals for a few beats ('post-acceleration pauses'). In 12 subjects the longest pause
was caused by sinus arrest, and in nine cases a blocked atrial premature beat was thought to be present.
Wenckebach A- V block was seen in only two subjects.
It is concluded that sex, age, smoking and leisure-time physical activity are all factors that have to be
considered for a thorough evaluation of heart rate variables in the 24 h ambulatory ECG.
sick sinus syndrome!4], but it is still unknown
whether the minimal heart rate or longest pause
observed in a 24 h ambulatory ECG can be used to
differentiate between normal and abnormal sinus
node function.
The purpose of the present study was
(1) to determine the mean 24 h heart rate, minimal
heart rate and duration of the longest pauses in^.
healthy subjects 40-79 years of age,
(2) to elucidate the effect of sex, age, smoking and
leisure-time physical activity on these variables,
(3) to determine the aetiology of pauses in these
subjects, and
(4) to compare different methods of analysing the
minimal heart rate,
_ „ ..
Definitions
HEART RATE VARIABLES (BEATS/MIN)
M e a n h e a r t rate
(SRmin) = the number of heart
beats in one minute period. Mean heart rate-trend
C 1983 The European Society of Cardiology
Twenty-four hour ambulatory ECG recordings
QUANT1TATION OF SINUS BRADYCARDIA
Sinus bradycardia (SB) = any of the heart rate
variables above indicating a value less than
60beats/min. Mild SB = SB > 50 beats/min.
Moderate SB = SB <50, but > 40 beats/min.
Marked SB = SB < 40 beats/min. (SB may be
further specified as for flfR, e.g. SB| is SB based on
HR,.)
PAUSES
A pause = an R-R interval ^ 1500 ms excluding
compensatory pauses following premature QRS
complexes and A-V block. Post-acceleration pause
(PAP) = a pause preceded by an increase in HR|
for a few beats and followed by a gradual return in
HR| to pre-pause level. Sinus arrest (SA) = a pause
preceded by a regular sinus rhythm and followed by
a gradual increase in HR] to pre-pause level.
.Blocked atrial premature beats (APB) = a pause
with or without a visible premature P wave, preceded and followed by a regular sinus rhythm
excluding SB and sinoatrial block.
Some of the definitions are arbitrarily chosen in
order to describe specific type of events for this
paper. Due to the limitations inherent in a single
' bipolar precordial lead and the often less than optimal quality of ambulatory ECGs (e.g. lack of well
denned P waves), commonplace definitions for
arrhythmias in the ordinary 12-lead ECG may not
always be applicable, and universally accepted
definitions are in several instances lacking.
Materials and methods
A total of 310 volunteers between 40 and 79
years of age had a 24 h ambulatory ECG recorded.
Forty-one were excluded for reasons previously
detailed!5], and nine ECG recordings failed leaving
260 ambulatory ECG recordings for detailed analysis. There were 170 males (mean age 53 years) and
90 females (mean age 56 years) with a sex and age
distribution as previously documented!5!. The
subjects were divided into three groups according
to their participation in leisure-time physical
activityt5h
(1) highly active participants in competitive sport,
(2) participants in leisure-time physical exercise
(jogging, tennis or badminton) on a twice weekly
basis, and
(3) subjects who did not participate in such activities.
They were also divided according to smoking habit:
(1) non-smokers,
(2) 1-10 cigarettes per day, and
(3) more than 10 cigarettes per day.
Ambulatory ECG recordings were obtained by
2-channel 24 h portable ECG tape recorders
(Oxford Medilog), while the subjects followed their
normal daily routines. The tapes were analysed
using a semi-automatic arrhythmia analyser
(Reynolds Pathfinder). The overall accuracy for
QRS complex detection by this analyser has been
reported as 98-9%!6!. At a paper speed equivalent to
2-5 mm/min real time a heart rate trend curve was
recorded on a Mingograph 33 ink-jet recorder.
With a time constant of 1 min real time it will by
this method take 3 min (three time constants) to
bring the heart rate value to within 5% of a change
in heart rate. Periods when analysis had been inhibited showed up in the curve as characteristic
jags. The summed duration of these subtracted
from the total recording time provided the length
of the ECG recording analysed. The heart rate trend
curve_was finally divided into hourly sections and
the HRh determined by free-hand averaging.
Recordings with pauses were subjected to a more
detailed analysis. During additional replay the
total number of heart beats/minute were counted
(counter designed at our medicotechnical department), and the HR min recorded at a paper speed
equivalent to 30 mm/h real time. Hereby the minimal HR min was determined. The lowest HR| was
calculated from the longest pause, and the lowest
HRj-2 was calculated from two adjacent R-R intervals including the longest pause. In an attempt to
improve the accuracy of diagnosing the longest
pauses observed in our subjects, an ECG diagnosis
was obtained by two different methods. On an
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(HR,) = the mean number of heart beats/minute
indicated by a heart rate trend curve (time constant
of 1 min). Minimal HRt = the lowest HR, during a
24 h period. Mean hourly heart rate (HRh) = the
mean number of heart beats/minute estimated from
either hourly sections of a heart rate trend curve or
the total_number of beats per hour. Mean 24 h heart
rate (HR24h) = the mean number of heart beats/
minute estimated from the total number of beats in
a 24 h period. Instantaneous heart rate (HR]) = the
duration of a single R-R interval expressed as beats/
minute. Mean instantaneous heart rate (HR]) = the
mean R-R interval duration calculated from two or
more consecutive R-R intervals and expressed as
beats/minute. (The number of R-R intervals used
may be stated by a suffix, e.g. as HR[-2 meaning
HR| based upon two consecutive R-R intervals.)
45
46
P. Bjerregaard
ink-jet recorder the ECG, computer indications of
pauses and R-R intervals were recorded simultaneously at a paper speed equivalent to 8-3 mm/s
real timePl. R-R interval variations in close proximity to the pauses were hereby easily evaluated
and made it possible to group the pauses as shown
in Table 1. The diagnosis based on R-R interval
variations were, however, in all cases confirmed by
an ECG rhythm strip recorded at a paper speed of
25 mm/s real time.
Table 1 The ECG diagnosis of the longest pauses in 77
subjects with pauses in the ambulatory ECG
46 (60)
12(16)
9(12)
3 (4)
7 (9)
Electrocardiographic diagnosis
Post-acceleration pause
Sinus-arrest
Blocked itrial premature beat
Marked sinus bradycardia (SB,)
Miscellaneous
2h
The HR24 h varied between 53 and 95 beats/min
(mean±2s.d.: 74± 18 beats/min). There was no
statistically significant association between sex,
smoking and physical activity, and the influence of
age was independent of the levels of these factors.
Analysis of variance showed an additive effect of
a g e ^ x , smoking and leisure-time physical activity
on HR24 h with a statistically significant effect of all
four factors at the 1% level. Males, non-smokers,
physically active and older subjects had a lower
HR24h than females, smokers, physically passive
and younger subjects. The heart rate was consistently higher in females than in males over a 24 h
period, as shown in Fig. 1. Similar sex related
differences were found for other age groups.
100
90
Statistics
For both HR24h and minimal HRt, a 4-sided
analysis of variance was performed. In the analysis
there were 3 factors:
(1) sex,
(2) leisure-time physical activity, and
(3) smoking and a regression variable: age.
It was tested, whether there was interaction between
the factors, and whether the influence of age was
dependent upon the levels of the factors. It was also
tested, whether any of the factors and age had a
significant influence on both types of heart rate
measurements studied. The Chi square test was
used to compare the proportions of males and
females, smokers (smoking group 3) and nonsmokers (smoking group 1), active (activity group
2) and passive (activity group 3) and subjects below
the age of 60 years and subjects above that age
having pauses. The computing was done with the
statistical package GLIM (Generalised Linear
Interactive Modelling).
Results
A HR24 h was estimated in 227 cases with more
than 22 h of the 24 h recording suitable for computer analysis, and a minimal HR, was available in
|
80
70
60
I I I I I I I I
10 12 14 16 18 20 22 24
Hour of doy
I
I
I
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Pauses
No. of
subjects
253 cases. In seven cases a high noise level prevented a reliable definition of the minimal HRt.
The presence of pauses and second degree A-V
block was evaluated in all 260 subjects.
2 4 6 8 10
Figure_l The circadian variation in mean hourly heart
rate (HRh) for 29 females (O) and 69 males (•) 40-49
years of age.
The normal limits (mean±2s.d.) for HR24h in
various subjects 40-79 years of age can be estimated
using the method described in Appendix 1.
MINIMAL HRt
The minimal HR, varied between 36 and
78 beats/min (mean ± 2 s.d.: 56 ± 16 beats/min).
There was no statistically significant association
between sex, smoking and physical activity, and the <
influence of age was independent of the levels of
these factors. Analysis of variance showed an additive effect of age, sex, smoking and leisure-time
Twenty-four hour ambulatory ECG recordings
physical activity on minimal HR( with a statistically significant effect of the three latter factors at
the 1% level, whereas the effect of age was nonsignificant. Males, non-smokers and physically
active subjects had a lower minimal HRj than
females, smokers and physically passive subjects.
_ J h e normal limits (mean±2s.d.) for minimal
HR, in various subjects 40-79 years of age can be
• estimated by the method described in Appendix 2.
PAUSES
of 1747 ms (range: 1600-1880 ms) were diagnosed
in nine subjects. A premature P wave was, however,
visible in only two of these cases, but in four cases
the diagnosis was supported by the presence of
frequently conducted APBs. In three cases the diagnosis was based entirely on the R-R interval
pattern. An exact doubling of the R-R interval
suggesting sino-atrial block was never encountered.
In the miscellaneous group, three subjects had their
longest pause during the daytime in relation to a
variation in HRj suggesting a Valsalva-like
manoeuvre. One pause followed a gradual deceleration in HR[ leading to two nodal escape beats,
and another subject revealed a pause during transition from an accelerated idionodal rhythm to sinus
rhythm. Finally, two pauses occurred following
short bursts of supraventricular tachycardia.
SECOND DEGREE A-V BLOCK
Second degree A-V block (type I) was only
encounted in two subjects, who both had a PR
interval of 0-20 s in the standard ECG, and all
episodes of A-V block occurred at night. A 52-yearold female had two episodes at 2.43 and 3.23 a.m.,
respectively, and a 46-year-old male had five
episodes between 0.22 and 2.25 a.m. All episodes
occurred in connection with transitory slowing of
the sinus rate with the longest interval between
conducted beats 2-24 s.
Table 2 Pauses in the 24 h ambulatory electrocardiogram in healthy adult subjects
COMPARISON OF DIFFERENT METHODS FOR ANALYSIS
Age groups
Pauses
None
2:1500 ms
S 1750 ms
£2000ms
OF THE MINIMAL HEART RATE IN A 24 HOUR
4(M9
50-59
60-79
All
years
years
years
No.
No. of No. of No. of
of
subjects subjects subjects subjects
62(63)
36(37)
3 (3)
1 (1)
59(69)
27(31)
4 (5)
1 (1)
62(82)
14(18)
5 (7)
0
183(70)
77(30)
12 (5)
2 (1)
Detailed analysis of the longest pause in each of
the 77 subjects with pauses showed (Table 1) that
the vast majority of pauses (60%) were PAPs (Fig.
•t 2) with a mean duration of 1607 ms (range:
1500-2040 ms). SA was seen in 12 subjects, and
had a mean duration of 1633 ms (range: 15001920 ms), and blocked APBs with a mean duration
AMBULATORY ECG
In contrast to the frequent occurrence of pauses
(HRi^40beats/min), a minimal HR,^ 40 beats/
min was seen in only eight subjects. This illustrates
the influence of different definitions of heart rate in
the estimation of the incidence of SB. The results
of a comparison between figures obtained by four
different methods for minimal heart rate and incidence of various degrees of SB in 77 subjects with
pauses are presented in Table 3. There were only
minimal differences between^ values obtained from
the heart rate trend curve (HRJ and an_actual count
of the number of heart beats per min (HRmj,,). Were
one (HR]) or two R-R intervals (HR r 2) were used
for estimation of the heart rate, a decrease in minimal heart rate and consequently a higher proportion of subjects with various degrees of SB were
found.
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A pause was present in 77 (30%) subjects (Table
2), but in only 12 (5%) did the longest pause
exceeded 1750 ms. There were two subjects who
had a pause in excess of 2000 ms. One subject had
a pause measuring 2020 ms, and the longest pause
observed in any of these subjects measured
2040 ms. Except for three subjects who had their
longest pause during the daytime, all occurred
between 10 p.m. and 8 a.m. and most presumably
during sleep. The proportions of subjects with
> pauses were significantly higher for males (34%)
than for females (21%; / ) <0-05), for non-smokers
(57%) than for smokers (15%; P<0025) and for
physically active in activity group 2 (48%) than for
physically passive (26%; 7>< 0-001). Subjects under
60 years of age had a significantly higher incidence
of pauses (34%) than subjects above that age (19%;
i><0025).
47
48
P. Bjerregaard
(a)
i
i
ECG - CM,
d E
If 1
i
J, I,*v> ii i i l l
\
*
- ' - •
I I.
i.
L 1 Hi 1.
ri
—1_..
1
n
1, 1. Ii i
i
i
L
-7-4-
-
j-—
(c)
ECG-CM
Pause
R - R interval
Figure 2 Three examples of simultaneous recording at a paper speed equivalent to
8-3 mm/s real time of ECG lead CM5, computer indication for pauses and horizontal levels
for R-R interval duration, (a) Post-acceleration pause, (b) sinus-arrest and (c) an example
of a blocked atrial premature beat in a subject also with several conducted atrial premature
beats (shown by *).
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I
Twenty-four hour ambulatory ECG recordings
49
Table 3 Comparison between figures obtained by four different methods for minimal heart rate and frequency of various degrees of sinus bradycardia in 77 healthy subjects with pauses in the ambulatory electrocardiogram
Heart rate variable
HR,
HR min
HRi-2
HR[
Minimal
heart rate
Beats/min
Mean ± s.d.
50
49
41
37
±8
±7
±3
±3
Mild sinus
bradycardia
No. of subjects
Moderate sinus
bradycardia
No. of subjects
Marked sinus
bradycardia
No. of subjects
66 (86)
71(92)
77(100)
77(100)
35(45)
39(51)
77(100)
77(100)
3 (4)
5 (6)
24(31)
61 (79)*
•There were 16 subjects with HR| = 40 beats/min.
The normal variability of the ECG has to be
taken into account for the appropriate differentiation between normal and abnormal in clinical
electrocardiography. New methods for ECG
recording over an extended period of time have
called for new limits for normality, since it has become more and more apparent that data based
upon a conventional ECG of 1 or 2 min duration
cannot be applied. Within the last six years studies
concerning ambulatory ECG recording in healthy
subjects have emerged!3-3'8"14]. Small population
samples (usually less than 100 subjects), a wide
range in age (16-80 years) and methodological
differences have, however, prevented a precise definition of what is normal in a 24 h ambulatory
.electrocardiogram. The great variability in heart
rate during a 24 h period is well known from earlier
reports!3-"-13]. The highest heart rate is usually
recorded early in the morning, but is to a great
extent dependent upon physical activity, whereas
the lowest heart rate is consistently recorded at
night usually around 5 o'clock in the morning (after
'approximately 6 h of sleep). The present study has
confirmed the statistically significant effect of age,
sex, smoking and leisure-time physical activity for
the heart rate level over a 24 h period. Figure 1
shows that the lowest HR24jj_in males compared to
females is due to a lower HRh in males compared
,to females during all 24 h. The reduction in HR24 h
with age may be explained by a diminished physical
activity level for older subjects than for the younger
during the_recording period, since the reduction in
minimal HR, with age is less pronounced and the
proportion of subjects below 60 years of age with
pauses is higher than for subjects above that age.
Since the heart usually beats more or less irregularly, a value for heart rate (conventionally
expressed as beats/min) has little meaning without
information about the method used for heart rate
estimation. Various methods based upon either a
certain number of R-R intervals (HR|) or the
number_of heart beats within a certain period of
time (HR) may provide different results. These
methodological problems are illustrated by the
results in Table 3, where the actual number of heart
beats/minute (HRmjn) is compared with figures for
mean heart rates obtained by three other methods.
It can be seen that the lowest value on a heart rate
trend curve with a time constant of 1 min real time
gives a fairly good estimate of the minimal number
of heart beats/minute. It is also seen, that an estimation of the minimal Heart rate based upon only
one or two R-R intervals will provide much lower
values and hence a higher number of subjects with
various degrees of SB. The values for minimal heart
rates (43 ± 5 beats/min) obtained by Brodsky et a/.Pl
in 50 healthy medical students 23-27 years of age
are much lower than in our subjects (56 ±8 beats/
min) and may be due not only to an actual lower
number of heart beats/minute in these younger subjects, but may also be due, to a certain extent, to
the method Brodsky et al. used to estimate the heart
rates, from only five consecutive R-R intervals
(HR]-5). Djiane el a/.M in 50 healthy subjects 32 ±9
years of age and Leitnere/a/.['2lin 100 healthy subjects 40-65 years of age using the same method as
in the present paper found values for minimal heart
rate almosHdentical to the values in our study.
Marked SB, was seen_in only three subjects in our
study with a lowest HR, of 36 beats/min. Brodsky
et a/.Pl in their group of medical students found
marked SBr5_in 24% compared to a figure of 9%
with marked SB|-2 in the present study. Despite the
difference in the applied heart rate variable these
figures suggest a decrease in the proportion of sub-
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Discussion
50
P. Bjerregaard
mainly been observed during sleep!3'8'10] and in
accordance with my results have been a rare event
occurring in only a few per cent of healthy adult'
subjects.
Differentiation between 'normal' and 'abnormal'
in the 24 h ambulatory ECG_ of subjects 40-79
years of age with regard to HR24 h> minimal H"R,
and pauses may be performed from the results in
the present study. Similar studies in patients with
well-defined pathological conditions (e.g. sick sinus
syndrome) are, however, necessary before a precise
evaluation of the diagnostic accuracy of the
presented results can be made.
Appendix 1
METHOD
FOR
ESTIMATION
( M E A N ± 2 S.D.) FOR H R 2 4 h
IN
OF
NORMAL
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jects with marked SB with age within the age range
from 23 to 79 years of age.
There was usually agreement between the two
applied methods for making an ECG diagnosis of
the longest pauses. Only in six cases of PAP was the
diagnosis based entirely on the R-R interval variation, since a premature P wave was not visible in
the ECG. The descriptive term, post-acceleration
pause (PAP), was applicable on the majority of
pauses (Fig. 2). Following a gradual increase in HR|
for a few beats (usually 5-10) the HR| would
decrease for two beats with the second R-R interval
forming a pause prior to a gradual return in HR]
to the pre-pause level. Such pauses were usually
seen during periods at night with enhanced variation in HR] and show great similarity to the heart
rate variability described by others during REM
sleep!15-17]. Baust and Bohnertf'8] in 1969 gave a
detailed description of changes in HR] associated
with bursts of rapid eye movement in experimental
studies on cats. They found that acceleration in
HR] was due mainly to an inhibition of vagal
activity, whereas the following bradycardia was
brought about by inhibition of the sympathetic
output and a simultaneous increase in the vagal discharge. In 1973 Lown et a/.t"l noticed episodes of
nocturnal SB and sinus pauses in man during 24 h
ambulatory ECG monitoring, but they offered no
precise description of these findings. Similar pauses
have been diagnosed as severe sinus arrhythmia by
others!812-16]. The true nature of at least some of
these pauses may in some cases have been missed,
since the acceleration in HR| prior to the pause is
often difficult to detect in an ordinary 10 s ECG
rhythm strip. Pauses not preceded by an acceleration in HR], but followed by a gradual return of
HR] to prepause level was found to be SA (Fig. 2b).
The 'warming-up' of the sinus node following the
pause was felt to be an important clue to depression
of impulse formation!20]. Blocked APBs were evidenced by a non-conducted premature P wave in
only three cases, but were considered to be present
in six additional cases with a similar R-R interval
pattern, but without a visible premature P wave.
This is, however, only a deduction.
Development of second degree A-V block (type
I) and aggravation of first degree A-V block during
phases of parasympathetic overactivity has previously been demonstrated in healthy subjects by
Johnson et a/.P'l, and a Wenckebach A-V block
may develop following heavy physical training!22!.
Pauses in the ambulatory ECG of healthy subjects
due to second degree A-V block have hitherto
LIMITS 1
SUBJECTS 40-79 YEARS
OF AGE
The point of reference is the mean ± 2 s.d. for the
HR24 h in male subjects 40 years of age in smoking group
1 and activity group 1, namely 67-2± 16-6 beats/min.
Due to an additive effect of age^ sex, smoking and
leisure-time physical activity on HR24 h. the mean for
HR24 h m subjects 40-79 years of age is obtained from this
figure by the following calculations:
For Age: subtract 0-1622 beats/min/year in excess of 40.
For Sex: add 3-8 beats/min for female sex.
For Smoking: add 3-1 beats/min for smoking group 2; add
7-4 beats/min for smoking group 3.
For Physical activity, add 1 -4 beats/min for activity group
2; add 6-8 beats/min for activity group 3.
s.d. is the same for all subject groups.
Example
The normal limits for HR24 n in females 50 years of age
in smoking group 2 and activity group 3 is:
67-2 — 1-622 + 3-8 + 3-1+6-8 = 79-3±16-6 beats/min.
Appendix 2
METHOD
FOR
,
ESTIMATION
OF
NORMAL
LIMITS
(MEAN ± 2 S.D.) FOR MINIMAL H R t IN SUBJECTS 40-79
YEARS OF AGE
The point of reference is the mean ±2 s.d. for the mini-1'
mal HRt in male subjects 40 years of age in smoking group
1 and activity group 1, namely 49-3 ± 14-4 beats/min.
Due to an additive effect of age, sex, smoking and
leisure^time physical activity on HRt, the mean for minimal HRt in subjects 40-79 years of age is obtained from
this figure by the following calculations:
For Age. subtract 0-092 beats/min/year in excess of 40*.
•The effea of age is non-significant
Twenty-four hour ambulatory ECG recordings 51
For Sex: add 2-9 beats/min for female sex.
For Smoking: add 1 -4 beats/min for smoking group 2; add
5-7 beats/min for smoking group 3.
For Physical activity, add 2-3 beats/min for activity group
2; add 7-3 beats/min for activity group 3.
s.d. is the same for all subject groups.
Example
The normal limits for minimal HRt in females 55 years
of age in smoking group 3 and activity group 3 is:
49-3-(0-092xl5)+2-9 + 5-7 + 7-3 = 63-8± 14-4 beats/
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Downloaded from http://eurheartj.oxfordjournals.org/ by guest on May 12, 2016
I am grateful to Birgit Dupont for technical assistance
and Anders Hoist Andersen for the statistical analysis.
The study was supported financially by the Danish Heart
Foundation and the Danish Medical Research Council.
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