today I'll be talking about the issues
and misconceptions regarding the most
basic of physical findings what is a
normal heart rate that is the number of
times per minute that your heart
contracts pushes blood through the body
and generates a pulse now if you're
thinking everyone knows it's 60 to 100
what's the big deal well you may be
right that everyone knows it but the
problem is that everyone is also wrong
when talking about what constitutes a
normal heart rate there are a couple
things I need to clarify first I'm
referring to a normal resting heart rate
the resting heart rate is the rate you
experience when you've been sitting or
lying still for several minutes with no
significant physical activity beyond
breathing at the other end of the heart
rate spectrum does exist something
called the maximum predicted heart rate
which as name implies is the fastest
rate your heart can achieve during the
most strenuous activity your body can
perform the commonly cited equation for
predicting the maximum heart rate of a
person is 220 minus their age and years
unfortunately while it's simple to
remember and even more simple to apply
most exercise physiologists feel it's
too inaccurate and have been many
attempts to better refine it but for
today we'll be focusing on the heart
rate at rest the next thing to clarify
is the age of the person we're talking
about just as age impacts the maximum
predictive eight it also impacts the
resting rate specifically children have
faster rates than adults and infants
have faster rates than children in fact
the age at which the resting heart rate
is the fastest nine to ten weeks after
conception when the average rate tops
out at around 170 to 180 beats per
minute over the next several weeks the
rate slows down just a little
until settling out somewhere between 120
and 160 but unless you're an
obstetrician or a neonatologist you
won't need to worry about fetal heart
rates so for the rest of the video I'm
going to focus just on the normal
resting rates in adults since there are
so many adults in the world to examine
and the study and since the heart rate
or the pulse is arguably the most
fundamental and basic of all physical
exam findings you might assume that
there's already a consensus about it in
fact there's actually a very strong
consensus among major physical exam
textbooks among guides to ECG
interpretation and among instructors in
medical and nursing schools
that consensus is that the normal
resting heart rate in adults is between
60 and 100 beats per minute any rate
below 60 is considered to be a
bradycardia which is literally Greek for
slow heart any rate above 100 is
considered to be a tachycardia which of
course means fast heart this 60 to 100
consensus is so strong that have
honestly never heard a member of a
medical school faculty excluding
cardiologists teach to the contrary so
what's the problem
well there's three of them the first
problem is the evidence the largest
relevant study was performed in 2007
when investigators sought to remeasure
and potentially redefine the normal
values for a number of aspects of
resting electrocardiograms
or ECG s this included the duration of
certain waveforms the average
directionality of the electrical
potential vector which is called the QRS
axis and yes the heart rate their study
subjects included 44,000 adults who had
been identified as having very low
probability of cardiovascular disease so
what did they find the median adult
heart rate which isn't exactly the same
as average heart rate was 67 with 96% of
the subjects falling between 48 and 96
there was an extremely slight gender
difference and that woman's rates were
one to two points higher than men's
these findings have a slower and
slightly broader range of normal as well
as a gender difference were consistent
with previous smaller studies the second
problem is expert opinion in my own
personal experience every cardiologist
I've ever talked to about this issue has
voiced the opinion that 60 to 100 is too
high particularly the lower bound of 60
in 1992 a survey was sent to 150
prominent cardiologists asking a simple
question should the limits of a
conventionally defined normal heart
rates be changed from 60 to 102 instead
50 to 90
should it remain the same or should a
different range be used 136
cardiologists responded of those 123 or
90% agreed with a proposal of 50 to 96
voiced preference of 50 to 100 and only
to prefer the traditional range and the
remaining 5 had a variety of responses
all of which were slower additional
evidence of expert opinion on this issue
comes from critical care medicine in
1992 a consensus committee from the
American College of Chest Physicians and
the Society of critical care medicine
sought to establish uniform definitions
of sepsis and the systemic inflammatory
response syndrome or sirs for
consistency this committee described the
sirs and sepsis as states in which at
least two of the following physiologic
arrangements for present fever or
hypothermia
heart rates above 90 respiratory rates
above 20 and either leukocytosis or
leukopenia although it's not explicitly
described in the consensus statement why
would they have set the heart rate
cutoff at 90 if they believed the normal
range to extend above this it's logical
to conclude that the committee also felt
that 90 was the upper limit of normal
the last problem with the traditional
range of 60 to 100 is its history where
exactly did this range come from
originally well it goes back to the
earliest days of electrocardiograph II
in 1928 the New York Heart Association
published their first edition of
nomenclature and criteria for diagnosis
of diseases of the heart in this book
they define something called quote
regular sinus rhythm as a heart rhythm
originating from a structure in the
right atrium called the sinus node from
where it normally originates and is
having a rate between 60 and 100 beats
per minute but 1928 was decades before
any formal study was conducted into the
normal distribution of heart rates in
the population
so how and why could the New York Heart
Association come up with such a range
well as put by charles kaufmann an early
member of that group it was for quote
the purposes of standardizing
electrocardiographic nomenclature these
limits and terminology are used
principally for convenience and
uniformity of designation they do not
define normality except in a gross way
however in 1943 that same New York Heart
Association renamed regular sinus rhythm
as normal sinus rhythm a decision which
may have been responsible for creating
this whole misconception to begin with
and if that didn't feel arbitrary enough
consider what a specific range of 60 to
100 was the one chosen for
Standardization standard
electrocardiograms are printed on graph
paper which is divided into large boxes
five millimeters on a side which are
then further subdivided into twenty five
smaller boxes one millimeter on a side
it's the large boxes that were
interested in the ECGs are printed out
at a rate of 25 millimeters per second
therefore along the x-axis each large
box of five millimeters represents 1/5
of a second or 200 milliseconds this has
been standard convention since before
electrocardiography was really
introduced commonly into clinical
practice as a consequence of this
convention an interesting trick for
quickly estimating heart rate becomes
apparent the heart rate is equal to the
number of large boxes between successive
spikes known as QRS complexes divide it
into 300 so if there are too large boxes
between each QRS complex the rate is 300
divided by 2 or 150 beats per minute if
there are 4 large boxes between each QRS
complex the rate is 300 divided by 4 or
75 beats per minute how this becomes
relevant to our 60 to 100 range is that
when one is trying to determine if
something is normal sinus rhythm or a
tachycardia that is a rate that's too
fast if we use 100 as our cutoff one
only needs
see if the number of large boxes
separating complexes is more or less
than three more than three the patient
has normal sinus rhythm less than three
the patient has a tachycardia that can
be determined by inspection
instantaneously however if we use 90 as
our cutoff we would need to see if
number of large boxes separating
complexes was more or less than three
point three three a little more awkward
and not quite as automatic so in essence
the decision to make the upper bound of
a normal sinus rhythm at 100 was because
it allowed for a trivially easier time
interpreting ECGs and even this
ridiculous reason has been made obsolete
by the use of ECG machines which now
automatically calculate the rates with
extremely high reliability so the
traditionally taught range for normal
heart rates is not supported by evidence
it's not supported by expert opinion and
originates from a completely arbitrary
work and how ECGs are recorded there's
just one more issue to consider that is
what do we even mean by the word normal
when we discuss the normal heart rate
are we referring to the statistical
average in the general population or are
we referring to the statistical average
only in those patients who are known or
suspected to have no cardiovascular
disease or are we referring to a heart
rate that will minimize chance of future
cardiovascular disease put more simply
does normal mean typical or does normal
mean healthy a 1990 study looked at the
association between heart rates and
mortality in 1,800 patients who were
admitted to the hospital with a heart
attack this graph here shows the
patient's mortality in the first year
after the heart attack as a function of
their final recorded heart rates
immediately prior to hospital discharge
we see that the mortality for heart
rates of 50 through 89 are uniform
but once the rate at discharge creeps
above 90
where talent he starts to climb pretty
rapidly but the danger of faster heart
rates isn't limited to just patient
who've had heart attacks in a study of
56,000 patients without known heart
disease
the faster the resting heart rates the
increased risk of a major adverse
cardiovascular event such as a heart
attack or cardiac arrest and this trend
persisted even after statistical
adjustments were made to account for the
presence of high blood pressure high
cholesterol diabetes smoking obesity and
lung disease so even if one believes
that resting heart rates of 90 to 100
are statistically within two standard
deviations of average which they
probably aren't they don't seem to be
markers of good health so going back to
our original question what is a normal
heart rate well it depends on who your
patient is and what you mean by normal
but it's probably not 60 to 100 thanks
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