# ecg-interpretation by lanyuehua

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```									                              ECG INTERPRETATION
When using ECGs to determine what arrhythmia (if any) a patient has, it would be great
if we could simply look at it and immediately recognize the rhythm. Perhaps you can do
so successfully, but this is not to be encouraged. Instead, you should use a system so
that your interpretations of ECG after ECG will be consistent.

The Paper

Figure 4-1 : The classic ECG rhythm

An ECG is printed on paper covered with a grid of
squares. Notice that five small squares on the paper form
a larger square. The width of a single small square on
ECG paper represents 0.04 seconds. To successfully
interpret ECGs, you must have this value committed to
memory. Do this now. If each small square represents
0.04 seconds, then a second will be 25 small squares
across. If you print out a minute's worth of your heart's
electrical activity, the paper would be 1500 small squares
wide. If something on an ECG is, let's say, 12 small
squares in width, that means that it lasted 12 x 0.04, or
almost half a second. A common length of an ECG
printout is 6 seconds; this is known as a "six second
strip."
Figure 4-2 : A small square
is 0.04 seconds

The width of one small square is 0.04 seconds (which is equal to 40 milliseconds).
In figure 4-1, look for the baseline. It is the line that would be perfectly straight and
horizontal if not for those vertical deflections. This is called the isoelectric line. On a
single ECG strip, the isoelectric line is usually about mid-height. The exact vertical
position on the paper can vary from ECG to ECG. When looking at a rhythm, imagine
the isoelectric line superimposed on top of the rhythm.

The "height" of an ECG wave is called its amplitude. The isoelectric line is considered
to have an amplitude of zero. Anything above the isoelectric line is positive; below the
line is negative.

While the horizontal parts (i.e. the X-axis) of an ECG measure time, the vertical parts
(a.k.a. the amplitude) represent the "strength" of the electricity at a given time. I put
strength in quotations because what it actually measures is "voltage" along a certain
path. I won't go into why I put voltage in quotations. Going back to the previous
analogy, note that the heart doesn't act as one big firecracker. Instead, it's like a series
of little firecrackers that go off at different times. At any given time, the electricity
around the beginning of the "fuse" could be going a different direction from the
electricity at the end of the fuse. (For example, when the pacemaker first fires, it is
unlikely that there is any major electrical activity around the end of the path. When
electricity finally reaches and depolarizes the areas near the end, the areas that were
depolarized before it could already be in the process of repolarizing). The amplitude
(i.e. how high or low the ECG line is) at any given time represents the sum of all of the
little regions of electrical activity.

When the sum of all voltages is equal to zero, it remains on this line. Hence the
dramatic "flat-lining" that is so common in television shows. Just because there are no
deflections (and thus the ECG is a flat line) does not necessarily mean the patient has
no electrical activity in his heart. Know that voltage can be negative. Think of negative
voltage as pointing the opposite direction as its positive equivalent. If all of the voltages
in the heart were cancelled out by an equal but opposite voltage, you might see a net
voltage of zero. Also, if all of the voltage is pointed perpendicularly to the line of view, it
will register as zero. Perhaps a more likely reason for the flat line is that the treatment
provider forgot to hook the patient up to the machine. (Remember to always treat the
patient and not the machine.) When the net voltage is positive, the ECG deflects above
this line. When the net voltage is negative, it deflects below the line.

For a demonstration of how waves add together, see my wave summation
demonstration.
The Waves

Look at the diagram below. (Note that I have superimposed a green line over the
isoelectric line.) What you see is the textbook, ideal Lead II representation of the
electrical activity in one heart beat. That means that this person should see about
seventy five of these patterns in one minute. In the pattern, there are five letters :
P,Q,R,S,T. They are in alphabetical order so they are not that hard to remember. Each
one of these letters represents what is known as an ECG wave. Notice how each one
of them starts and stops on the isoelectric line. The term complex is usually used to
mean a group of adjacent waves.

The first little hump is known as the P wave. It
occurs when the atria depolarize (i.e. trigger).
The P wave (or its absence) will give you clues
to where this electrical activity started (ask
yourself : did it start in the sinus node, the atria,
the ventricles, or somewhere in between?)

The next three waves constitute the QRS complex. They represent the ventricles
depolarizing. These three are lumped together because a normal rhythm may not
have all three. Many times, you'll only see a R and an S. This is not abnormal. If
there are less than three, how do we know which one is which? Well, the R wave
is the first wave ABOVE the isoelectric line. You then name the waves in relation
to the R wave. If it falls before the R wave, it is called the Q wave; after the R
wave is the S wave.
Figure 4-3 :
The waves of
an ECG (Note
: green line is
superimposed
over the
isoelectric
line.)

If there are no upright waves in the QRS complex (and you are sure you aren't
mistaking the T wave for part of the QRS), you call the second upright wave R'
(pronounced "R prime").

Everything that depolarizes must repolarize if it intends to be triggered again. The
repolarization of the atria usually occurs while the QRS is occurring and is not really
noticeable. The repolarization of the ventricles, however, is represented by that large
hump after the QRS complex. This is known as the T wave.

Sometimes a U wave appears after the T wave. This is uncommon.

One of the first things you want to measure is the electrical rate of the heart. I have
used the term "electrical" because you CANNOT measure the true heart rate with an
ECG machine. The true heart rate is the rate at which the heart pumps blood. This is
detected by taking a pulse. In the normal, healthy heart, the true rate will coincide with
the electrical rate. However, there are many reasons why there might not be a
corresponding pulse to go with an electrical beat.

The Caliper

An ECG caliper (sometimes plural like
scissors) is a tool that helps measure
certain values. It usually has no measuring
ability of its own, but allows you to set its
width and then measure it against a more
convenient portion of the ECG paper.
Although the tool is often marketed as an
"ECG caliper", it is really a general tool that
can be used for many different things. In
the non-ECG world, I've heard them called
"dividers."
Figure 4-4 : A typical ECG caliper
Step 1. Position the two arms of the
caliper on the each end of the width you
intend to measure. Make sure that the
arms are perfectly horizontal (i.e. on the
same vertical line).

Figure 4-5 : Positioning the arms of a caliper on

Step 2. After you have positioned the
caliper arms, do not let them move
(relative to each other). This would
defeat the purpose of using the
calipers. Find a place on the paper that
it will be easy to see the lines. Keeping
the two caliper arms on the same
vertical level, place the left arm of the
caliper on the left side of a big square.
Count the number of small squares
between the two arms. (Remember that
each big box is five small squares.)
Multiply this by 0.04 to convert the
number of small squares into seconds.

Figure 4-6 : Moving the caliper to a more
convenient place to count squares.

Of course, calipers are not absolutely necessary but they do make the job of ECG
interpretation much less tedious.

What is there to measure on an ECG? Well, for basic Lead II rhythm interpretation,
there are three things you want to be sure to measure every ECG. They are : R-R
interval, the PR interval, and the QRS width. We will learn later what these values
mean. For now, concentrate simply on being able to measure them correctly.

The R-R interval

This is a measurement of the distance between two consecutive beats. The R wave is
usually chosen to do this because it is the tallest and most conspicuous. In most
rhythms (including normal rhythms), the R-to-R interval will be the same as the P-to-P
distance, or the distance between any two analogous points on consecutive beats.

If the R-R interval is constant, no matter what two consecutive beats we chose, we say
the rhythm is regular. Do not confuse the terms regular and normal. The term regular
does not indicate whether the rate is normal, fast, or slow, just that the beats are evenly
spaced. Irregular, therefore, means that waves are not evenly spaced. Think of
someone you know who has absolutely no rhythm. Imagine this person trying to clap
his or her hands to the beat of a song but failing miserably. Some claps are too close
together, some are too far apart. This is what should come to mind when you hear the
word irregular in the context of ECG.

Rate is expressed in beats/min. It probably isn't accurate to say "beats/min" in this case
because you cannot tell if the heart is actually pumping blood simply from an ECG.
When using only the ECG, it is best to refer to what you measure as the electrical rate
and leave out the word beats, saying simply "per minute" as in "The electrical rate is 75
per minute."

How do you determine rate of the rhythm?

1. The more accurate method : 1500 / (# of small squares in R-R interval)

This should only be used if the rhythm is regular. If the R-R interval fluctuates
from beat to beat, then it should be obvious why you cannot use this formula.
(We use the number 1500 because there are 1500 small squares in a minute.)

2. The quick estimation : count the number of electrical beats in a six-second strip
and multiply that number by 10.

This should be used if the rhythm is not regular. It is not as accurate as method
#1. (If you use this method, every rate you get will be evenly divisible by 10. The
true heart rates tend not to be so "computationally friendly.") Never assume
that a strip is six seconds; check for tick marks or count the large squares.
The PR interval

Often abbreviated PRI, the PR interval is the distance
(time) between the beginning of the P wave and the
beginning of the QRS complex. The R in PR interval refers
to the R wave, but it is not always measured to that wave.
It actually ends at the beginning of whatever the first wave
in the QRS complex is. It might have been more accurately
called "P-to-QRS interval," but that would have been a
mouthful.

A normal PRI should be in the range of 0.12 - 0.20
seconds. Memorize this. Since we already know that
each square represents 0.04 sec, we are able to calculate
that 0.12 seconds is three squares and that 0.20 seconds
is five squares.

Figure 4-7 : The PRI

In a normal rhythm, every P wave will be followed by a QRS complex. This is the same
thing as saying that the ratio of P waves : QRS complexes is 1 : 1. In some rhythms,
however, you don't see this 1:1 ratio. The absence of this 1:1 ratio provides a major
clue as to what rhythm may be present. In some cases, it will be impossible to measure
the PRI.

The QRS width

Another important measurement is QRS width (also called QRS duration). This is the
measurement from the beginning of the first wave in the QRS to the end of the last
wave in the QRS. A normal QRS width should be less than 0.12 s. Some people say it
should be less than 0.10 s, but both groups agree that it should be less than 0.12 s. (In
order to confuse the reader, I have decided to use both values and frequently switch
back and forth.) Below this value (and thus of normal duration) it is called a narrow
QRS. When a QRS is longer than 0.10 - 0.12 seconds, it is called a wide QRS.

The QRS begins at the point where the PRI
stops. This is either the beginning of the Q wave;
if there is no Q wave, it starts at the beginning of
the R wave.

The end of the QRS can be tricky to find. The S
wave (or whatever the last wave in the QRS is)
ends at the beginning of what is called the ST
segment. This point is sometimes called the J
point. In the usual "textbook" ECG, the S wave
ends on the isoelectric line. (On figure 4-7, the
QRS is easy to spot.) However, in many cases,
the ST segment is not so flat. In Figure 4-8,
notice that the ST segment starts below the
isoelectric line. The J point (i.e., the end of the
QRS complex) appears like a bend in the road.
Figure 4-8 : The width of the QRS complex (Note
the non-horizontal ST segment)

Sinus rhythms are those that arise from the pacemaker in the sinus node (also called
the SA node). The sinus pacemaker is our heart's normal pacemaker. We have three
basic rhythms that originate in the sinus node. If the heart's rate is below 60 beats/min,
we call it sinus bradycardia. If the rate is above 100 beats/min, the rhythm is called
sinus tachycardia. If everything is just right, the rhythm is called normal sinus
rhythm (NSR). (These rates apply to human adults. Pediatrics is a whole 'nother
world.)
Of course, if all rhythms started in the sinus node, we probably wouldn't have a section
devoted specifically to sinus rhythms. While the normal heart rhythm is of sinus origin,
there are many arrhythmias that do not start in the sinus. Why is the sinus the best
place to start?

One reason is that an impulse that originates in the sinus node follows a certain path
that allows the atria to contract before the ventricles. If the two sets of chambers
contracted at the same time, the atria would push against closed valves. Because there
are no valves that separate the atria from their veins (vena cava and pulmonary vein),
blood would flow backwards. This often causes the jugular veins in the neck to pulse.

Between the atria and the ventricles is the A.V. node. On the diagram of the
firecracker (figure x-x), this node is represented by the yellow tunnel. Conduction of the
electrical impulse slows down in the A.V. node, allowing the atria to completely
depolarize before the ventricles so that atria may contract first. On the ECG, the
isoelectric part between the P wave and the QRS complex demonstrates this pause. If
the conduction through the A.V. node were slowed too much, the ECG would show this
as a PR interval that is longer than 0.20 seconds.

Figure 5-1 :The match represents the sinus pacemaker. The yellow tunnel represents the AV node.
How can you tell if an ECG rhythm originates in the sinus node? One thing to do is to
look at the P wave. A rounded, upright P wave is often indicative of the sinus
pacemaker. (Occasionally, the amplitude of an ECG is so great that the P wave will end
up looking pointed even though it is sinus. Be very careful with this.) In non-sinus (i.e.
ectopic) pacemakers, the P waves are often either notched, very pointed, inverted, or
absent. To know what a normal rhythm looks like, it is best to be familiar with abnormal
rhythms.

Don't be caught off guard by a weird looking QRS complex. Just because it doesn't
look like the "textbook example" of the QRS does not mean that it is abnormal. A
normal QRS is less than 0.12 seconds, but it is not limited to a single shape. Although
the QRS complexes can differ from ECG to ECG, it should not be considered normal if
they were to differ in the same ECG.

Normal sinus rhythm

A normal sinus rhythm (NSR) is the common, everyday rhythm. It must be, of course,
sinus in origin. It must be regular and have a rate between 60 - 100 per minute. It must
have a normal PRI and QRS duration.

Figure 5-2 : A normal sinus rhythm

Often times, an arrhythmia is described by saying the "underlying rhythm" and adding to
that anything abnormal (e.g. sinus rhythm with first degree heart block). If any
abnormalities exist, do not include the word normal when designating the underlying
rhythm.

This is just like a normal sinus rhythm except that the rate is slower than 60 per minute.

Remember that the limits of 60 and 100 are arbitrary. A person who has a rate of 59
beats/min would not feel much different than he would at a rate of 60 beats/min. Some
people (especially athletes) have a normal resting heart rate below 60. When President
(George W.) Bush passed out after choking on a pretzel, it was revealed that his normal
heart rate was around 45 beats/min. Many pundits became alarmed and criticized the
president for not revealing his "disease" prior to the incident. The talking cardiologist
heads were quick to point out that "disease IS as disease DOES"- that a disease is
based on the patient and not always on standard one-size-fits-all guidelines. Some
people may become symptomatic when there heart rate falls even though it is still
above 60. We would call this relative bradycardia.

This would be a good time to reiterate : treat the patient, not the machine.

Sinus tachycardia

This is just like a normal sinus rhythm except that the rate is faster than 100 per minute.
Figure 5-4 : Sinus tachycardia

Sinus tachycardia is common in everyone. If you are a paramedic or EMT, you will
probably find plenty of patients in sinus tachycardia simply because they are nervous or
excited. In more serious cases, a person in the early stages of shock may have a fast
heart rate to compensate for the would-be-fall in blood pressure. Sinus tachycardia in
itself is not always a bad thing; treatment should be aimed at the underlying cause.

Sinus arrhythmia

Sinus arrhythmia is similar to normal sinus rhythm except that it the rate is irregular. It
often matches the patient's breathing pattern, speeding up when the patient inhales and
slowing down when the patient exhales.

Sinus arrhythmia can be relatively common in young and is often asymptomatic.

How irregular is irregular? The criterion used by many is that the longest R-R interval
should differ from the shortest by at least 0.16 seconds.

Figure 5-5 : Sinus arrhythmia
Atrial rhythms are rhythms that originate in the atria. The atrial rhythms that fall under
the category of PSVT are mentioned elsewhere.

What is the difference between an electrical beat originating in the sinus and one
originating in the atria? For one, the P wave tends to be a different shape. Sinus P
waves tend to be rounded, upright, etc. while atria P waves tend to be weird shaped.
Sometimes they are pointy, sometime flat. They can have a notch running down the
middle of them. Some are diphasic, which means that one part is above the isoelectric
line while one part is below it.

Premature atrial complex

When you feel your heart has "skipped a beat," it very well may have been due to a
PAC. A premature atrial complex (PAC) describes a wave or set of waves caused by
an atrial pacemaker that interrupts the underlying rhythm. Figure 6-1 shows a sinus
rhythm that is twice interrupted by PACs. The PACs shown consist of an atrial P wave
along with a normal QRS complex and a normal T wave. Compare the P waves on
both the sinus complexes and the PACs.

Premature describes the fact that the beat occurred before the regular one would have.
In figure 6-1, you can imagine that the sinus pacemaker was firing along at a regular
rhythm until it was unexpectedly (and rudely) interrupted by the atrial pacemaker. The
atrial pacemaker causes the wave of depolarization that resets the sinus node. Look at
the R-R intervals. The ones from the sinus to the following atrial complexes are shorter
than the others.

Figure 6-1 : A sinus rhythm with two premature atrial complexes
When an atrial pacemaker fire prematurely, it is entirely possible that the AV node is still
refractory. In these situations, you might find a single atrial P wave without the QRS
complex and T waves. We describe this as non-conducted.

When these impulses conduct, you may see either a normal QRS or a wide QRS. The
wide QRS in this case is due to what is called aberrancy. This is often due to one of the
bundle branches still being refractory.

PACs are also covered in Section 10 : Premature Complexes.

Atrial fibrillation

Atrial fibrillation (often called "a-fib") is relatively common among the elderly. It occurs
when the electricity in the atria follows a seemingly random and repetitive path, causing
the atria to quiver. (The word fibrillation means quivering.) Because the atria are
quivering, they are unable to pump blood. The ventricles are still functional, but are
depolarized at irregular intervals. This rhythm can last for years and is not always
symptomatic.

There are a few reasons why this rhythm is bad. Some of them are :

The ventricles may be depolarized at too high a rate.
The atria are unable to perform their normal function.
Clots may form in the left atrium, predisposing the patient to a stroke. The old saying
"a rolling stone gathers no moss" might be applied to blood. Instead of gathering
moss, stagnant blood tends to form thrombi (clots).
Figure 6-2 : Atrial fibrillation

One of the major questions that doctors are asking themselves is : is it better to control
or convert?

Control refers to controlling the rate. Keeping the rate of ventricular contraction under
100/min. often minimizes the symptoms of this rhythm. This is usually done with
medication. When the rate of QRS complexes (and thus ventricular depolarizations)
exceeds 100, we call this rhythm uncontrolled atrial fibrillation.

Convert refers to converting the rhythm into a sinus rhythm. You might think this would
be the obvious choice. The problem is, however, that someone who has been in atrial
fibrillation for a while has a high chance of "throwing clots" in the left atrium. These
thrombi (clots) are thought to form in the atria when they are fibrillating. In the process
of conversion to a sinus rhythm, the thrombi may become dislodged from the atrium.
They are then likely to be sent through the left ventricle, the aorta, and into the arteries
that supply the brain. This could lead to a stroke. Patients who undergo this
"cardioversion" are often put on anticoagulants for weeks before the procedure.

There are usually two readily apparent things in an ECG of atrial fibrillation.

1. There is an extremely irregular QRS rate. The R-to-R values often differ with
each beat with no visible pattern to their timing.
2. There are no easily discernible P waves. Instead, the baseline usually appears
chaotic. It may have an appearance ranging from coarse (large and jagged) to
fine (relatively smooth). This is the atrial contribution to the ECG.

The PRI cannot be measured.

Atrial flutter

The buzzword that everyone loves to use when describing atrial flutter is "saw tooth."
The term is used for good reason : the P waves often looks like the teeth of a saw when
viewed in Lead II. These P waves are often called "flutter waves."

In atrial flutter, the atria are depolarizing at an extremely rapid rate. The AV node will
normally only conduct impulses up to a rate around 220 per minute. In atrial flutter, the
atria are depolarizing about 250 - 350 times per minute. This means that not all of the
impulses will be conducted. The conduction ratio (P:QRS ratio) is often relatively
constant at 2:1 or 4:1, although it can also vary. With 2:1 conduction, the ventricular
rate is usually about 150. Unlike atrial fibrillation, atrial flutter QRS complexes tend to
appear at regular intervals.

Figure 6-3 : Atrial flutter (2:1 conduction)
Junction here refers to the AV junction, the area around the AV node and the bundle of
His. The exact definition of the AV junction often varies, depending on who you ask. As
I've mentioned before, the AV node is an electrically conducting path that connects the
atria to the ventricles. Emerging from the AV node is the bundle of His (also called the
AV bundle). Because the "anatomy people" seem to disagree on where the AV node
starts and stops, the "physiology people" call the general area the AV junction. This
would include the AV node and at least part of the bundle of His.

Figure 7-1 : A firecracker showing the firing of a junctional pacemaker

What is a junctional rhythm? A junctional rhythm is one that starts in the AV junction.
Figure 7-1 illustrates what happens when a pacemaker in the junction fires. The match
lights the fuse, causing the fire to travel both ways. In the atrial part of the fuse, the
activation travels in the reverse direction (from right to left on the diagram). We know
that when it travels "forward" it produces an upright P-wave. When it travels retrograde
(which is a snooty, polysyllabic way of saying "backward"), the P wave is inverted (i.e.
upside down, below the isoelectric line). Thus, an inverted P wave strongly indicates
that the electrical impulse originated in the AV node or beyond.

In addition to being upside-down, the junctional P wave may not be before the QRS.
When the sinus fires, the atria are depolarized before the ventricles, and thus the P
wave is first. In figure 7-1, the atrial explosives are lit before those of the ventricles.
While simply looking at the figure, however, it is hard to predict which set of explosives
will be ignited first. Perhaps they will be ignited at the same time. The ignition of the
explosives in the atria (i.e. atrial depolarization) is the P wave while the ignition of the
ventricular explosives (i.e. ventricular depolarization) is the QRS complex. In a
junctional rhythm, the P wave may occur before, during, or after the QRS complex. This
depends on the exact location of the pacemaker, which may vary. When two waves
occur at the same time, they add together. Anything above the isoelectric line counts as
positive, below negative. Because the P wave in this case is negative, it will subtract
from whatever the QRS is.

Premature Junctional Complex

A premature junctional complex (PJC) is not a rhythm but rather denotes a complex
caused by a junctional pacemaker that interrupts the underlying rhythm.

Figure 7-2 : Sinus rhythm with a PJC
Junctional escape

We have talked about the "backup" pacemakers in the heart. There happens to be one
of these pacemakers in the AV junction. This junctional pacemaker's intrinsic rate is
between 40 and 60 times/min. Remember the rule, "the fastest pacemaker calls the
shots"? That means that the sinus pacemaker (usually around 75/min) will normally
prevent the junctional pacemaker from firing. What might happen if the sinus rate were
to fall to, let's say, 30 times/minute? Well, the junctional pacemaker may start calling
the shots. We call this type of "backup rhythm" an escape rhythm. Junctional escape
will often have a rate between 40 and 60 beats/min.

Figure 7-3 : Junctional escape

Junctional tachycardia

(I am using the term junctional tachycardia to refer specifically to the type caused by a
junctional pacemaker. The type of junctional tachycardias caused by reentry are dealt
with in the PSVT section.)
Let's suppose an ectopic pacemaker in the AV junction decides to overtake the sinus
node. This is equivalent of a coup, led by the a trouble maker in the AV junction. If the
rate is faster than 100, we call this junctional tachycardia.

Figure 7-4 : Junctional tachycardia

Accelerated junctional rhythm

Occasionally, an ectopic pacemaker in the AV junction will have a rate that is too fast to
be considered junctional escape but too slow to be considered junctional tachycardia.
We call this an accelerated junctional rhythm.

Figure 7-5 : Accelerated junctional rhythm
The term supraventricular tachycardia (SVT) has at least two different meanings that
are commonly in use.

1. MORE GENERAL : Any tachycardia that originates in or depends on parts above
the ventricles. In other words, any tachycardia that is not from the ventricles.
This is mostly used in the context of describing an unknown tachycardia. All
tachycardia will fall under the categories of ventricular or supraventricular. Under
this definition, sinus tachycardia is a type of supraventricular tachycardia.
2. MORE SPECIFIC : This group of rhythms is often called "paroxysmal SVT", or
PSVT. A group containing certain tachycardia rhythms (of supraventricular
origin) which all have a similar appearance. Some in the rhythms in this group
are AV nodal reentrant tachycardia, unifocal atrial tachycardia, and sinus reentry
tachycardia. The different rhythms of this group are usually classified under the
term SVT because they cannot be easily distinguished from one another.

The term American is similar in that it has two meanings : in its most general sense, it
refers to people and things of North America and South America. In the more specific
sense of the word, it is used to refer to people and things of the U.S.

Paroxysmal supraventricular tachycardia (PSVT)

We will focus on the second definition of SVT (top of page) right now. This definition
includes a number of rhythms- knowledge of each of the individual rhythms is not
usually required for someone learning basic ECG interpretation. Instead, be familiar
with this group. These rhythms tend to be between the rates of 150-250 and are
paroxysmal.

The word paroxysmal means sudden; when used with arrythmias, it denotes one
that begins and ends suddenly. This means that someone can go from a normal sinus
rhythm to a PSVT with a rate of 180 in only second.
Figure 8-1 : A supraventricular tachycardia (SVT)

AV nodal reentrant tachycardia (AVNRT)

This is the most common type of PSVT. As the name suggests, this rhythm is due to a
reentrant impulse at the site of the AV node.

Because the mechanism depends on the AV node, we can reason that if we were to
temporarily disable the AV node, this rhythm might "break," that is, convert to a sinus
rhythm. Vagal maneuvers (e.g. carotid sinus massage, breathing against a closed
glottis) will often slow conduction down at the AV node to the point that this rhythm
breaks. Adenosine is a drug that temporarily blocks conduction in the AV node, also
causing this rhythm to break. If it is your job to offer treatment, then check with your
local protocols on how to proceed.

AV reentrant tachycardia (with accessory pathway)

Like AVNRT, this type also depends on reentry. However, it involves an accessory
pathway. An accessory pathway (in this case) is an abnormal connection between the
atria and ventricles. Patients who are prone to this type of PSVT tend to have two
pathways : the AV node and this accessory pathway.

Circus movement tachycardia, while sometimes applied to any tachycardia involving a
loop, is often used to specifically refer to this rhythm.
Atrial tachycardia

Atrial tachycardia is usually considered a type of PSVT; it can be subdivided even
further based on its mechanism. It can be caused by a intra-atrial reentrant circuit or by
automaticity (i.e. ectopic pacemaker).

The important thing to note is that these tachycardias are not dependent on the AV
node for their survival.

Warning : Use the following information at your own risk. While accuracy is one my goals, there is
always the possibility that some of the information could be wrong. There could be typos. I could also be
severely mistaken in some of my knowledge. This site is meant to help clarify certain concepts of ECG
and at no point should any life-or-death decision be made based upon the information contained within.
Remember, this is just some page on the internet. (If you do find errors, please notify me by feedback.)

The term supraventricular tachycardia (SVT) has at least two different meanings that
are commonly in use.

1. MORE GENERAL : Any tachycardia that originates in or depends on parts above
the ventricles. In other words, any tachycardia that is not from the ventricles.
This is mostly used in the context of describing an unknown tachycardia. All
tachycardia will fall under the categories of ventricular or supraventricular. Under
this definition, sinus tachycardia is a type of supraventricular tachycardia.
2. MORE SPECIFIC : This group of rhythms is often called "paroxysmal SVT", or
PSVT. A group containing certain tachycardia rhythms (of supraventricular
origin) which all have a similar appearance. Some in the rhythms in this group
are AV nodal reentrant tachycardia, unifocal atrial tachycardia, and sinus reentry
tachycardia. The different rhythms of this group are usually classified under the
term SVT because they cannot be easily distinguished from one another.

The term American is similar in that it has two meanings : in its most general sense, it
refers to people and things of North America and South America. In the more specific
sense of the word, it is used to refer to people and things of the U.S.

Paroxysmal supraventricular tachycardia (PSVT)
We will focus on the second definition of SVT (top of page) right now. This definition
includes a number of rhythms- knowledge of each of the individual rhythms is not
usually required for someone learning basic ECG interpretation. Instead, be familiar
with this group. These rhythms tend to be between the rates of 150-250 and are
paroxysmal.

The word paroxysmal means sudden; when used with arrythmias, it denotes one
that begins and ends suddenly. This means that someone can go from a normal sinus
rhythm to a PSVT with a rate of 180 in only second.

Figure 8-1 : A supraventricular tachycardia (SVT)

AV nodal reentrant tachycardia (AVNRT)

This is the most common type of PSVT. As the name suggests, this rhythm is due to a
reentrant impulse at the site of the AV node.

Because the mechanism depends on the AV node, we can reason that if we were to
temporarily disable the AV node, this rhythm might "break," that is, convert to a sinus
rhythm. Vagal maneuvers (e.g. carotid sinus massage, breathing against a closed
glottis) will often slow conduction down at the AV node to the point that this rhythm
breaks. Adenosine is a drug that temporarily blocks conduction in the AV node, also
causing this rhythm to break. If it is your job to offer treatment, then check with your
local protocols on how to proceed.
AV reentrant tachycardia (with accessory pathway)

Like AVNRT, this type also depends on reentry. However, it involves an accessory
pathway. An accessory pathway (in this case) is an abnormal connection between the
atria and ventricles. Patients who are prone to this type of PSVT tend to have two
pathways : the AV node and this accessory pathway.

Circus movement tachycardia, while sometimes applied to any tachycardia involving a
loop, is often used to specifically refer to this rhythm.

Atrial tachycardia

Atrial tachycardia is usually considered a type of PSVT; it can be subdivided even
further based on its mechanism. It can be caused by a intra-atrial reentrant circuit or by
automaticity (i.e. ectopic pacemaker).

The important thing to note is that these tachycardias are not dependent on the AV
node for their survival.
Figure 9-1 : A firecracker showing a number of potential ventricular pacemaker locations. Note that the
fuse representing the ventricular septum is a "slow fuse".

Several matches have been drawn in figure 9-1 to represent some of the possible
locations of the ventricular pacemaker. These are all portions of the Purkinje system.
Pick one of the matches and predict the path the impulse (fire) would follow. Perhaps
can now imagine why impulses that originate in the ventricles produce wide QRS
complexes. In a normal sinus beat, the impulse forks at the bundle of His. It covers
both ventricles simultaneously. It generally depolarizes the ventricles in less than 0.10
seconds. If you imagine that the impulse starts where the bottom match is lighting the
fuse, you can see it now has to cover both ventricles. The "slow fuse" is where the
impulse can travel through the ventricular septum and into the other ventricle. This
should explain why ventricular pacemakers cause a QRS complex longer than 0.10 s.

What else might cause a QRS to last longer than 0.10 seconds? Imagine that one of
the bundle branches were blocked. (The bundle branches start at the fork, just after the
A.V. node.) An impulse that originates in the sinus node would be normal up until it
reaches the block. If the impulse could only travel down ONE of the branches, it would
have to cut over through the ventricular septum ("slow fuse") to depolarize the other
ventricle. You can see why this would take longer. Not everything that glitters is gold,
and not every QRS that is wide is caused by a ventricular pacemaker.

All QRS complexes of ventricular origin are wide (> 0.12 s). Does it logically follow that
all wide QRS complexes are ventricular? No, it does not. In fact, it is the case that
NOT ALL WIDE QRS COMPLEXES ARE FROM THE VENTRICLES. On the other
hand, virtually all narrow complex QRS complex are supraventricular (i.e. not from the
ventricles). If this didn't "click," you might want to reread this paragraph.
Let's say we can put all rhythms in one of two categories : supraventricular or
ventricular.

Supraventricular QRS complexes : NARROW or WIDE

Ventricular QRS complexes : WIDE only

Premature Ventricle Complex

Premature ventricular complex (PVC) is a term that originally was called premature
ventricular contraction. Because the mechanical contraction of the ventricles cannot be
inferred from the ECG, the word complex has replaced contraction.

These are also referred to as : premature ventricular beats (PVB), VPBs, and VPCs.

PVCs tend to be compensating (i.e. they don't travel back and reset the underlying
pacemaker). In these cases, the distance from the normal P wave before the PVC to
the P wave after the PVC is twice the underlying P-P interval.

Unlike the other three rhythms in this section, PVCs frequently occur in normal healthy
hearts and often go unnoticed. They also may give a person the feeling of having
"skipped a beat". On the other hand, PVCs in an unhealthy heart may be a bad omen.

Figure 9-2 : Sinus rhythm with a unifocal PVC

PVCs are also covered in Section 10 : Premature Complexes.
Ventricular fibrillation

Ventricular fibrillation ("v-fib", often abbreviated VF) describes the electrical activity
associated with the quivering of the ventricles. When Don Corleone is shot, quivering
Fredo is unable to successfully wield a gun. Quivering ventricles are about as effective
as quivering Fredo- they cannot pump blood. I mentioned earlier that, while functional
atria are a prerequisite for playing tennis or jogging, functional ventricles are a
prerequisite for staying alive. Thus, untreated v-fib can progress to death within
minutes. You can understand why v-fib is one of the evil rhythms. When someone
suddenly drops dead from a cardiac arrest, v-fib is likely to blame.

The pattern for ventricular fibrillation is... well, there really is no pattern. It is random
electrical activity. Sometimes the amplitude of the waves is large (coarse VF), while
other times the amplitude is so small (fine VF) that the rhythm is almost asystole. It has
almost a "kindergarten artwork" quality to it.

As the minutes pass, cells in the body become damaged due to the lack of oxygenated
blood. Among these are cells of the heart. As the heart dies, it loses its ability to
conduct electricity. Coarse v-fib will turn into fine v-fib, and fine v-fib will transition into
asystole.

Figure 9-3 : Ventricular fibrillation (coarse)
Figure 9-4 : Ventricular fibrillation (fine)

Ventricular tachycardia

Ventricular tachycardia (V-tach, often abbreviated VT) refers to a rhythm that arises
from the ventricles causing the heart to beat at a rate faster than 100 beats per minute.
The ventricular rate is usually above 120 beats/min and may exceed 250 beats/min. At
some point, the ventricles may beat so frequently that there is not adequate time for the
blood to refill. A patient in ventricular tachycardia MAY or MAY NOT have a pulse.

A heart in VT is vulnerable to going into ventricular fibrillation. In fact, the common
sequence of arrhythmias in patients who die in this rhythm is: V-tach to V-fib to
asystole.

The QRS complex will be wider than 0.12 seconds. Figure 9-5 shows the stereotypical
V-tach, but not all cases look like this.

Unless you have been trained to distinguish a ventricular QRS complex from a wide
QRS of non-ventricular origin, then you should call a tachycardia with wide QRS
complexes a "wide complex tachycardia."
Figure 9-5 : Ventricular tachycardia

Ventricular escape

Ventricular escape, often called idioventricular escape, is when an ectopic "backup"
pacemaker in the ventricles kicks in. A few things can cause this :

1. The sinus and junctional pacemakers have failed
2. There is a block that prevents impulses of the sinus (or junctional) pacemaker
from reaching the ventricles.

Although it is considered a "backup" rhythm, it is only slightly more compatible with life
than asystole. The intrinsic rate of a ventricular pacemaker is 20 - 40 times/minute. In
other words, this is a very bad rhythm.

Figure 9-6 : Idioventricular rhythm
Asystole

Asystole is not a ventricular rhythm. It is the "flat-line"; subsequently, it is the easiest
rhythm to recognize. Because it has no electrical activity, asystole does not readily
belong into any of the other groups of arrhythmias. I have included in this section
because it is often the end result of ventricular fibrillation.

Pronounced uh-SIS-toe-lee.

Always make sure the equipment is connected to the patient. Also, check more than
one lead to confirm that the rhythm is asystole. If an electrically active heart has all of
its activity perpendicular to a given lead, very little will show on that lead's ECG. In
these cases, another lead (pointing a different direction) should pick up on this hidden
activity.

Figure 9-7 : Asystole

I'm frequently asked if there is some way that I can explain the concept of the human
heart's premature ectopic complexes using waterfowl. In response to these requests, I
have put together in this section an extended metaphor that addresses the topics
common to premature atrial, junctional, and ventricular complexes.

The normal mallard rhythm
Figure 10-1 shows a number of mallards marching along. Each mallard represents the
ECG complex above it. Let's assume there exists some commander of the ducks (who
is not shown in the diagram). He is very picky when it comes to marching, and he
requires that each duck maintain a specified distance between himself and the duck
before him. This distance is the length of a single ruler (shown below ducks).

Figure 10-1 : Sinus ducks marching. Each duck is ordered to maintain a ruler's length distance between
the duck ahead of it and itself.

Enter the goose

Now let's look at the duck-duck-goose patterns. In figure x-x, you see that a goose
(representing a ventricular complex) has joined the parade. He took ("stole") the place
of one of the "regularly scheduled" ducks. In addition to this, the goose is following a
little too closely (much like bad drivers do). Thus, we call this goose premature.

Remember, each of the ducks was originally given the order to march at a distance of
one ruler's length behind the duck directly in front of him. This puts the duck that follows
the goose in a dilemma. He has two options :

1. The duck can ignore the goose and follow one ruler's length behind where the
missing duck would have been. This is the equivalent of following the length of
two rulers behind the previous mallard (the one before the goose).
Compensating for the too-short distance before the goose, the distance after the
goose would be longer than a ruler. This "distance" is called a compensatory
pause.
2. The duck could march at a distance of one ruler's length behind the goose. This
would mean that the goose has shifted the entire marching formation behind him
a little bit more forward than they would have been. There is no compensatory
pause.

Figure 10-2 : A premature waterfowl (followed by a compensatory pause). Even though the goose takes
the place of one of ducks, it does not alter the overall spacing of the group. This is exhibited by the
compensatory pause following the goose.
Figure 10-3 : Premature waterfowl (no compensatory pause). Notice how the overall pattern is reset by
the blue-headed (premature) ducks. The red rulers indicate a distance that is too short. The mallards
that follow the premature ducks are basing their position on these blue-headed troublemakers.

In the heart, the "option" is usually decided by where the premature complex originates.
Those complexes that cause a compensatory pause are those whose impulse does not
reach the normal (sinus) pacemaker. Lack of the compensatory pause is generally
attributed to the impulse from the premature complex conducting retrograde towards the
sinus node and resetting it. Although this is by no means a fixed rule, PACs (and PJCs)
tend to reset the sinus while PVCs tend not to reset the sinus. Thus, if a premature
complex is followed by a compensatory pause, you should suspect a PVC.

Interpolated complexes

Sometimes, a goose may be able to squeeze between two ducks without messing with
their pattern at all. When this happens with an PVC, we call it an interpolated PVC.
An interpolated complex is a premature complex that is early enough so that no
complex is skipped. Thus, in figure 10-4, there are no missing ducks.
Figure 10-4 : An interpolated goose. This is a premature goose who, rather than take the place of one of
the ducks, has managed to squeeze between two ducks without altering their pattern. No ducks have
gone missing.

Unifocal versus multifocal

Remember the following generalization : different shape means different origin.

The QRS shape often reflects where the impulse entered the ventricular conduction
system. As far as the ventricles are concerned, pacemakers in the sinus node, atria,
and the junction (normally) all share the same path. Thus a sinus complex, a PAC, and
a PJC are all likely to have similarly shaped QRS complexes.

Pacemakers in the ventricular conduction system are not limited to one location; they
can occur in a variety of places. These different places should produce different QRS
complexes. For example, in figure 10-5, we see two different PVCs. We should
assume, because of their dissimilarity, that these PVCs each originated from a different
part of the ventricular conduction system. Thus, we designate them multifocal. If all of
the PVCs had the same general shape, we would refer to them as unifocal.
Figure 10-5 : Mallard rhythm with multifocal geese.

Patterns of premature beats

These terms describe how often the premature complexes appear. If their appearance
seems random, none of the following terms are applicable.

Bigeminy : every
other beat

Trigeminy : every
third beat

every fourth beat
Multiple premature beats

Paired complexes
(also called
couplets)

A salvo of
complexes (also
called a run)

The term salvo (as in barrage) is used to describe the occurrence of multiple premature
complexes in a row. (Many consider "multiple" in this case to mean three or more.)
When many premature complexes occur in a row, you should probably start looking at
them less in terms of premature complexes and more in terms of tachycardia.

Atrioventricular heart blocks (referred to simply as heart blocks from here on) is the
name given to conditions in which electrical conduction at the AV node is somehow
affected. We generally speak of three broad types (or degrees) of heart blocks. In a
nutshell, they are :

1st degree heart block : AV conduction is (excessively) slowed
2nd degree heart block : AV conduction is incompletely (i.e. occasionally) blocked
3rd degree heart block : AV conduction is completely blocked

Before you continue, stop and predict what each type of heart block will look like. Use
what you already know about P waves, QRS complexes, and the space between them.
If AV conduction is slowed, how will this appear on an ECG? If conduction is
completely blocked, will QRS complexes follow P waves?

1st degree heart blocks :

This occurs when conduction at the AV node is slowed beyond the normal amount.
This is manifested as a PRI that is longer than 0.20 seconds. This PRI will generally
remain constant. (If the PRI is changing from beat to beat, you may a second degree
heart block.) When you see a rhythm with a first degree heart block, you generally
name the rhythm according to this pattern : <underlying rhythm> with a first degree
heart block. For example, if you were to see a case of sinus bradycardia but the PRI
consistently measures 0.22, you would call the rhythm : sinus bradycardia with a first
degree heart block.

Figure 11-1 : Sinus rhythm with a first degree heart block

2nd degree heart blocks :

The most confusing thing about second degree heart blocks is that there are two
subtypes and they are called a variety of names. Whenever you see Mobitz, think 2nd
degree heart block. Therefore, Mobitz I is the same thing as 2nd degree heart block
type I.
2nd degree heart block type I : This rhythm is also called Mobitz I. It is also called
Wenckebach. It consists of the PRI getting longer with each electric beat until
eventually a P wave occurs but the QRS never shows (essentially skipping a beat).
The process is then repeated. This is like showing up for work Monday an hour late,
on Tuesday two hours late, and so on until Friday comes around and you don't even
show up at all. The following Monday, you start the same cycle again. You could call
this behavior "pulling a Wenckebach" but nobody would get it and people would just
think you're weird.

Figure 11-2 : Second degree heart block type I (Wenckebach)

2nd degree heart block type II : This rhythm is also called Mobitz II. This occurs
when a QRS suddenly fails to show up after a P wave. It usually makes an
appearance the next wave. This rhythm lacks the increasing PRI that is seen with the
Wenckebach type. It would be as if you showed up 30 minutes late Monday through
Thursday, but failed to show up Friday. I have always considered this the "duck duck
goose" rhythm because it maintains a relatively constant PRI until it skips.
Figure 11-3 : Second degree heart block type II

It is important to recognize the two subtypes of 2nd degree heartblocks. The second
subtype tends to be much worse than the first subtype (Wenckebach).

Note : For second degree heart blocks, it is common to specify the ratio of P waves to
QRS complexes. This is the conductance ratio. In a second degree heart block with a
2:1 conductance, there will be only one PRI. It will be impossible to distinguish between
the two subtypes of 2nd degree heart blocks using only the ECG.

3rd degree heart blocks :

These are also called complete heart blocks. This is when the atria and the
ventricles are essentially divorced. If no electricity travels through the AV node for a
little while, the ventricle's backup pacemaker starts calling the shots. The atria are
being controlled by one pacemaker, the ventricles by another. This often manifests
itself on an ECG as P waves occuring at regular intervals with QRS complexes occuring
at regular intervals, but no apparent relationship between any P wave or QRS
complex. Sometimes it may look like a P wave follows a QRS, sometimes vice-versa,
but they don't seem to affect each other.
Figure 11-4 : Third degree heart block

In figure x-x, you may have to play "Where's Waldo?" with the P waves. The first two
are clearly visible. The last two are hiding in QRS complexes. Compare the shape of
the QRS complexes. You should notice a slight difference where you expect the P
wave to be. In the middle QRS, the P wave is evident at the very end. In the last QRS
complex, it is at the very beginning.

To be considered a true third degree heart block, the ventricles should be in an escape
rhythm. Why? There are many situations in which the atria and ventricles can be
completely independent when there is no "true block" between the atria and ventricles.
If a ventricular ectopic pacemaker were firing at such a rate that the sinus and ectopic
impulses meet head-on somewhere in the junction, you would see these two impulses
cancel each other out; the atria and ventricles would be doing their own thing despite
no real problem with AV conduction.

The word artifact is similar to artificial in the sense that it is often used to indicate
something that is not natural (i.e. man-made). In electrocardiography, an ECG artifact
is used to indicate something that is not "heart-made." These include (but are not
limited to) electrical interference by outside sources, electrical noise from elsewhere in
the body, poor contact, and machine malfunction. Artifacts are extremely common, and
knowledge of them is necessary to prevent misinterpretation of a heart's rhythm.
Pacing spikes

These are seen in someone whose implanted pacemaker is firing.

The sharp, thin spike seen in figure x-x is an electrical signal produced by an artificial
pacemaker. The wide QRS complex that follows it represents the ventricles
depolarizing. We say that the "(artificial) pacemaker captures" when it is able to
successfully depolarize its intended target. If a pacing spike is not followed by its
intended response, we say that it has failed to capture.

Figure 12-1 : Artificial   pacemaker spikes

The wide QRS suggests that the pacemaker was implanted in the ventricles.

Electrode/lead placement is very important. If one were to accidentally confuse the red
and white lead cables (i.e. place the white one where the red one should go, vice
versa), he might get an ECG that looks like figure 12-2. In this ECG, we can make out a
normal sinus rhythm with all of the waves upside-down. When this happens, you are
essentially viewing the rhythm in a completely different lead.

One must also make sure that the lead wires are actually plugged into the machine. If
your talkative patient shows asystole, you should suspect this. Many machines are
"smart" in that they can sense common errors of this nature, but many such errors aren't

AC interference

Alternating current (AC) describes the type of electricity that we get from the wall. In the
United States, the electricity "changes direction" 60 times per second (i.e. 60 hertz).
(Many places in Europe use 50 Hz AC electricity.) When an ECG machine is poorly
grounded or not equipped to filter out this interference, you can get a thick looking ECG
line (as shown in figure 12-3). If one were to look at this ECG line closely, he would see
60 up-and-down wave pattern in a given second (25 squares).

Figure 12-3 : 60   Hz AC interference

Muscle tremor / noise
The heart is not the only thing in the body that produces measurable electricity. When
your skeletal muscles undergo tremors, the ECG is bombarded with seemingly random
activity. The term noise does not refer to sound but rather to electrical interference.

Low amplitude muscle tremor noise can mimic the baseline seen in atrial fibrillation.
Muscle tremors are often a lot more subtle than that shown in figure 12-4.

Figure 12-4 : Muscle   tremors

Wandering baseline

In wandering baseline, the isoelectric line changes position. One possible cause is the
electrodes, and a variety of other things can cause this as well.

Figure 12-5 : Wandering   baseline artifact
Absolute heart block

Absolute heart block (or 4th degree heart block) results from over-exposure to imported-
show no relationship with the P wave. It occurs very rarely, and even then, only in
fictional settings. This should not be confused with the real arrhythmia complete heart
block.

Figure 12-6 : Absolute   heart block
Acute inferior myocardial infarction
   ST elevation in the inferior leads II, III and aVF
   reciprocal ST depression in the anterior leads

Acute anterior myocardial infarction
   ST elevation in the anterior leads V1 - 6, I and aVL
   reciprocal ST depression in the inferior leads

Acute posterior myocardial infarction
   (hyperacute) the mirror image of acute injury in leads V1 - 3
   (fully evolved) tall R wave, tall upright T wave in leads V1 -3
   usually associated with inferior and/or lateral wall MI

Old inferior myocardial infarction
   a Q wave in lead III wider than 1 mm (1 small square) and
   a Q wave in lead aVF wider than 0.5 mm and
   a Q wave of any size in lead II

Acute myocardial infarction in the presence of left bundle
branch block
Features suggesting acute MI
   ST changes in the same direction as the QRS (as shown here)
   ST elevation more than you'd expect from LBBB alone (e.g. > 5 mm in leads V1 - 3)
   Q waves in two consecutive lateral leads (indicating anteroseptal MI

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