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Chapter 4: Rate
Before you begin, look at this chapter's summary on pages 334 and 335.
When reading an EKG, you should first consider the rate.
Note: The sign in this picture is not informing the driver* about the rate of his race car. The man holding the sign is a physician who has been monitoring the driver's transmitted EKG. The sign is telling the driver about his current heart rate (he's a little excited).
When examining an EKG, you should determine the first.
The rate is read as cycles per
Now, let's examine where and how the normal heart rate originates...
- With a sincere dedication to Billy Occam, deceased long ago, who made simplicity a virtue of science.
rate
minute
65
Sinus Rhythm
SA Node "Sinus Node"
i i
Normal
60 -1 00/min. ^
Vftr
The SA Node (Sinus Node), the heart's pacemaker and the dominant center of automaticity, generates a Sinus Rhythm. The SA Node paces the heart in the normal rate range of 60 to 100 per minute.
The heart's normal pacemaker, the , generates a continuous series of regular, pacemaking stimuli (this is its "automaticity").
The SA Node is located within the upper-posterior wall of the right. The SA Node emits a regular series of pacemaking (depolarization) stimuli.
Note: The Sinus Node (SA Node) is the heart's dominant center of automaticity, and the normal, regular rhythm that it generates is called the Sinus Rhythm.
At rest, the Sinus Rhythm maintains a rate of 60 to beats per minute, which is the normal range of the pacing rate.
SA Node
atrium
100
Sinus Tachycardia
** (^) j^ .^ ^
•"'' \:
UM
^iY:
m&
•n\V^ Ux*
Sinus Rhythm
rate more than 100/min
If the Sinus Node (SA Node) paces the heart at a rate greater than 100 per minute, this is Sinus Tachycardia.
Note: "Tachy" = fast; "cardia" = heart.
A rhythm originating in the SA Node (Sinus Node)
is called Sinus Tachycardia if the rate is greater
than per minute. (^100)
Exercise produces sympathetic stimulation of the SA Node; this is the most common cause of Sinus (^) Tachycardia
Note: There are focal areas of automaticity in the heart known as
automaticity foci. * They are potential pacemakers that are capable
of pacing in emergency situations. Under normal circumstances, these foci are electrically silent (that's why they are referred to as "potential" pacemakers).
"Automaticity foci" refers to more than one *"automaticity focus" * in fact, when the word "foci" is used alone, "automaticityfoci" is understood. Foci is pronounced "FOE-sigh".
Automaticity Foci (Potential Pacemakers)
sometimes called "ectopic" foci
If normal SA Node pacemaking fails, other potential pacemakers known as automaticity foci (also called "ectopic" foci) have the ability to pace (at their inherent rate). They are in the atria, the ventricles, and the AV Junction.
If the SA Node ceases to function, one of the potential pacemakers,
known as an automaticity focus, will assume pacemaking activity
at its inherent (only one focus assumes pacing responsibility).
The atria have automaticity of potential pacemakers that are within the atrial conduction system (see page 101), and they are called atrial automaticityfoci.
Note: The proximal end of the AV Node has no automaticity foci,
however the middle and distal regions of the AV Node, an area known as the AV Junction does have automaticity foci that are called Junctional automaticity foci.
rate
foci
Purkinje fibers have automaticity foci, so there are foci of
these potential in the His Bundle and in the Bundle Branches and their subdivisions; these foci are called ventricular automaticity foci.
pacemakers
69
SA Node overdrive-suppresses all foci
(since all foci have a slower inherent pacing rate)
CD > o
f Atrial fOCi (inherent rate 60 - 80 per mm.
C Junctional fOCi (inherent rate 40-60 permin.)
( Ventricular fOCi (inherent rate 20 - 40 per mm.
Rapid automaticity (pacemaking activity) suppresses slower automaticity (pacemaking activity) - this is overdrive suppression, a very important fundamental characteristic of all automaticity centers.
Note: Overdrive suppression is characteristic of all centers of automaticity (including the SA Node and all automaticity foci). Simply stated: any automaticity center will overdrive-suppress* all others that have a slower inherent pacemaking rate.
The SA Node overdrive-suppresses the (slower) inherent pacemaking activity of all the automaticity below it; this provides the SA Node with the luxury of not having to compete with slower pacemaking activity of lower automaticity foci.
In fact, once an automaticity focus actively begins pacing, it will overdrive- all lower (slower) foci, including slower foci at the same level...
...eliminating any competition. Well Designed!
- When used as a verb, "overdrive-suppress" is hyphenated... so says the publisher.
foci
suppress
71
Overdrive Suppression
provides emergency backup pacing
at 3 separate levels
Emergency Failsafe Pacing Mechanism
sAkNode
If Failure... -> Atrial focus assumes pacing responsibility.
If Failure...
c •> Junctional focus assumes pacing responsibility.
If Failure...
c > Ventricular focus assumes pacing responsibility.
Overdrive suppression is the heart's failsafe pacing mechanism, providing three separate levels of backup pacing, by utilizing automaticity foci in the atria, the ventricles, and the AV Junction.
Note: An automaticity focus actively pacing at its inherent rate, overdrive-suppresses all slower foci including slower foci at its own level.
Should normal SA Node pacing fail (pacemaker failure), a backup pacemaker (i.e., an automaticity focus from a lower level) - no longer overdrive-suppressed - will emerge to pace at its inherent rate; and it conveniently overdrive-suppresses potential pacemaking activity at all levels that are it.
Therefore, an automaticity focus only emerges to function as a pacemaker when it is no longer -suppressed. For instance, in SA Node failure...
.. .a focus from a lower level - no longer overdrive-suppressed by regular pacing stimuli from above - can emerge to pace. Very well Designed!
Let's do that once again, slowly.
below
overdrive
(AV) Junctional Automaticity Focus (potential pacemaker)
proximal end of AV Node (no foci)
AV Node
inherent rate
40 - 60/n rnrnn.
The AV Junction has automaticity foci (potential pacemakers), one of which will emerge to actively pace in its inherent rate range of 40 to 60 per minute if there is an absence of regular pacing stimuli progressing down from the atria.
Note: The AV Junction is that portion of the AV Node that has foci of automaticity. The proximal end of the AV node has no foci. The AV
Junction has foci of automaticity called "Junctionalfoci."
An automaticity focus in the AV Junction begins active backup pacing
only in the absence of pacing stimuli coming down from the atria. Then,
no longer overdrive-suppressed, it emerges to actively pace in its inherent
range of 40 to 60 per minute, and it overdrive-suppresses all
lower (slower) automaticity foci, becoming the dominant pacemaker.
An Junctional focus actively pacing at its inherent rate
( to 60 per minute range), produces an idio-junctional* rhythm.
Note: A Junctional focus (that is, an automaticity focus in the
AV Junction) emerges as the active pacemaker if it is no longer
overdrive-suppressed by regular pacing stimuli from above. This can occur if the SA Node and all atrial foci fail. But wait —
something else can prevent a Junctional focus from being
depolarized by regular pacing stimuli from above. Next page!
- The prefix "idio" is of Greek origin, and it means "one's own." Idiojunctional is usually not hyphenated.
rate
40
(AV) Junctional Automaticity Focus (potential pacemaker)
Complete conduction block
AV Node
inherent rate
40 - 60/min.
If there is a complete conduction block in the AV Node above the AV Junction, then no regular paced depolarization stimuli from above reach the automaticity foci in the AV Junction.
Note: You will recall that the AV Node is the only conduction link between the atria and the ventricular conduction system below.
With a complete conduction block in the AV Node above the AV Junction, an automaticity in the AV Junction just below, receives no pacing stimuli from above...
... so, no longer overdrive-suppressed, the Junctional focus escapes to become the active pacemaker for the ventricles. And that Junctional focus paces the ventricles at its inherent rate of - per minute while overdrive-suppressing lower, (slower) ventricular foci.
Note: It is possible for the AV Junction (together with all its automaticity foci) to suffer a complete block. In that instance, only an automaticity focus in the Purkinje fibers of the ventricles can come to the rescue to pace the ventricles. Let's see how...
focus
40-
75
OVERDRIVE SUPPRESSION
provides emergency backup pacing at 3 separate levels
SftNode
If Failure...
Emergency Failsafe Pacing Mechanism
Atrial focus assumes pacing responsibility.
If Failure... Junctional focus assumes pacing responsibility. If Failure... ^—• Ventricular focus assumes pacing responsibility.
Range of Inherent Pacing Rates of Automaticity Foci t£ (^) * Atrial | Junctional Ventricular
If normal SA Node pacing fails, an automaticity focus in the atria, or the AV Junction, or even the ventricles (m that order) is available to assume the pacemaking responsibility at its own inherent rate. This provides three levels of backup pacing.
If the SA Node should cease pacing, an atrial automaticity focus
can pace in its inherent rate range of 60 to 80 per minute; failing
that, backup pacing by a Junctional focus will assume the active
responsibility in its (slightly slower) inherent rate range pacing
of 40 - 60 per minute.
The ventricles can be paced by a ventricular automaticity focus
in its rate range of 20 to 40 per minute, if the focus
is not regularly depolarized by paced stimuli. The lack of properly
paced stimuli to the ventricular focus can be due to failure of all
automaticity centers above, or due to an intervening complete conduction block that prevents pacing stimuli (from above) from conducting to the ventricles. What a miracle of Nature!
Note: In a physiological or pathological emergency, an irritable automaticity focus may suddenly discharge at a rapid rate. This emergency rate (150 to 250 per minute) is approximately the same for foci of all levels.
Now let's try something real easy...
inherent
Rate is?
r-*
Our main objective is to rapidly determine the heart rate.
After finishing this chapter you will be able to determine the rapidly.
No special devices, calculators, rulers, or awkward mathematical computations are needed in order to the rate.
V-V^
Note: In emergency situations, you probably will not be able
to find, much less use a calculator; and you may not have the
presence of mind (or the time) to do mathematical calculations.
Observation alone can give us the
r~<
rate
determine
rate
80
Chapter
4: Rate
<#
4?
^
yo
A
„wN
Next:
Count
off
"300,
150, 100" for the three thick lines that follows the start line,
naming
each
line
as
shown.
Memorize
these
numbers.
An R wave peaks on a heavy black start line...^
.the
next
heavy
black
line is
named
_"...
followed
by
"^
and
"
_" for the next two heavy black lines.
Note:
The
line that the R wave peaks upon is the start line;
we
only
name
the
heavy
lines that
follow
the
start line.
The
three lines following the start line (where the R
wave
falls)
are
named
"
,^
.^
" in
succession.
(Say them out loud!)Again!
300
150,
100
300,
150,
100
Chapter 4: Rate
Then: Count off the next three lines after "300, 150, 100" as "75, 60. 50."
The next three lines after "300, 150, 100" are named " , 60, 50."
Remember the next three lines together as:
Once more out loud, please.
Very good!
SI
75
75, 60, 50
Where the next R wave falls, determines the rate. It's that simple.
Find an R wave peaked upon a heavy black (start) line, then look for the R wave.
Where the next R wave falls gives the. There is no need for mathematical computations.
If the next R wave falls on "75"... the rate is 75 per.
Note: You may have noticed that the illustration shows the normal rate range of 75 to 100.
83
next
rate
minute
84
Start
J^
Start
ju^
300 150 100
J^
Rate: 100/min.
300 150 100 75 60 Rate: 60/min.
By knowing the triplets "300, 150, 100" then "75, 60, 50" you can merely look at an EKG and tell the approximate rate immediately.
The triplets are: first "_
then "
By simply naming the lines following the start line,
you can identify the rate immediately using the
300, 150, 100
75, 60, 50
triplets
