Download depolarization waves.

CARDIAC MONITORING

Electrocardiography (EKG) and
phonocardiography are the two most
important techniques for observing the
condition of the heart and its associated
arteries.
• Since the electrical and the acoustic signals
are generated by the same events, it makes
sense to discuss both of them at the same
time.

Plots of the action of
the heart, the
associated heart
sounds, and the
accompanying ECG
signals.




The P wave is associated with atrial
contraction.
The QRS complex signals the atrial
repolarization and ventricular contraction
sequence.
The T wave is generated by the repolarization
of the ventricles.
The U wave is not completely understood, but
it may be associated with some after-potential
phenomenon.

The R-wave frequency is normally taken
as a measure of the heart rate.
• The P-wave frequency should equal that of
the R waves, and the P-to-R interval, which is
normally about 0.12 to 0.22 second should
not vary from beat to beat.

The appearance of ectopic R waves or
missing R waves is usually taken as a
serious sign.

The measurement and recording of
acoustic signals associated with the
action of the heart is known as
phonocardiography.
• Listening to the sounds of various
phenomena inside the body is one of the
oldest medical arts.
• Today, the hearing of the nurse or physician
may be aided by various electrical and
electronic devices, and a graphic record can
be obtained of the sounds heard on monitoring
the heart.
PHONOCARDIOGRAPHIC
TECHNIQUES

The acoustic signals that accompany the
action of the heart can be detected with
either a stethoscope or a microphone.
• The use of a microphone provides some
significant advantages in that the signals can
be

An electrocardiogram is a recording of
the rhythmic electrical activity of the
heart.
• The abbreviation for electrocardiogram (EKG)
is derived historically from the German
spelling "electrokardiogram".

The electrical activity of the heart is
based on the ability of excitable tissue,
such as heart muscle (myocardium), to
change its membrane permeability to
sodium (Na+) and potassium ions (K+).
• When these ions move across the cell
membrane, a changing electric field (dipole)
results which is recorded as electrical activity
of the heart.

Metal electrodes in contact with the skin
surface are used to pick up weak EKG
signals that are amplified and displayed
by an oscilloscope and/or Strip Chart
Recorder.
• Cardiac muscle goes from relaxed (diastole)
to the contractile (systole) state after the
onset of "electrical depolarization".
• The return from contraction to relaxation occurs
after "electrical repolarization".

The normal heart rhythm is established
by a specialized bundle of cells called
the "pacemaker" or the sinoatrial (SA)
node of the heart.
• Electrical impulses are generated
spontaneously by the pacemaker, initiating
the heart cycle.

At the onset of the heart cycle, impulses
from the SA node induce the right atrium
to depolarize.
• This depolarization spreads across the atrial
muscle causing atrial contraction, increasing
atrial pressure and forcing blood into the
ventricles.

The ventricular contraction phase of the
heart cycle is brought about by
depolarization of the ventricles via the
atrioventricular (AV) node, Bundle of His
and the Purkinje fibers.
• The AV node provides a delay time allowing
the atria to pump blood into the ventricles
before ventricular contraction.

This Bundle of His and Purkinje fibers
permit the ventricles to be depolarized in
a relatively short time.
• If the heart relied on impulse conduction
through ventricular musculature, total
contraction would not occur in a short interval
due to long conduction delay of muscle.
• Impulse velocity through the conduction system is
much faster than through the cardiac muscle itself.

The heart goes through a periodic
sequence of electrical depolarizations
and repolarizations that initiate the
mechanical events of pumping blood.
• Mechanically the heart cycle can be divided
into two phases, diastole and systole.
• During diastole the atria contract, emptying blood
into the ventricles.
• During systole the ventricles contract.
THE STANDARD
ELECTROCARDIOGRAPHIC
LEADS

A complete EKG consists of 12 tracings,
• The standard three leads (I, II, and III),
• The precordial or chest lead (V1 - V6) and
• The augmented unipolar limb leads (aVR,
aVL and aVF).

To record the electrical
activity of the heart, the
use of three standard
leads has long been
routine.
•
Electrodes are placed on
the right arm, left arm and
left leg, and voltage
differences constitute the
three standard leads as
shown.
Einthoven Triangle

In summary, the three voltages I, II, and
III are measured on the body as:
• I—The voltage drop from left arm to right arm.
• Il—The voltage drop from left leg to right arm.
• Ill—The drop from left leg to left arm.
• Anatomically, these points form a triangle on the
body, known as the Einthoven triangle.

The voltages may be represented by
vectors, having the tail on the negative
pole of the potential and the arrowhead
on the positive pole of the potential.

Thus, in clinical practice,
the three voltages are
represented as vectors,
which are called frontalplane vectors and
illustrated.

The algebraic relationship between
these voltages comes from circuit theory
applied to the human thorax.
• It is the fundamental law of voltage drops that
the voltage drop between two points is the
same regardless of the path traveled between
those two points.
• This is known as Kirchhoff’s law of voltages.
LEAD I

In recording limb lead I,
• The negative (-) terminal of the
•
•
electrocardiograph is connected to the right
arm (RA),
The positive terminal (+) to the left arm (LA),
and
The ground (G) to the left leg (LL).

Therefore, when the point on the chest
where the right arm connects to the
chest is electronegative with respect to
the point where the left arm connects,
the electrocardiograph records positively
(i.e. above the zero voltage line in the
electrocardiogram).
• When the opposite is true, the
electrocardiograph records below the line.
LEAD II

In recording limb lead II,
• The negative terminal of the electro•
•
cardiograph is connected to the right arm
(RA),
The positive terminal to the left leg (LL), and
Ground to the right leg (RL).
• Thus, when the right arm is negative with respect
to the left leg, the electrocardiograph records
positively.
LEAD III

In recording limb lead III,
• The negative terminal of the electro-
•
•
cardiograph is connected to the left arm
The positive terminal to the left leg and
Ground to the right leg.
• This means that the electrocardiograph records
positively when the left arm is negative with respect
to the left leg.

These three leads give electrocardiograms that show the same
component waves, but the amplitude
(height) and direction of the waves are
different

Kirchhoff’s law applied to the
figure implies that the
voltage drop as one travels
from the left arm to the right
arm equals the drops
measured as one travels
from the left arm to the left
leg and then to the right arm.

In equation form, this implies that
I  II  III
• The minus sign appears because III is a
negative drop, in accordance with the
polarities assigned in the figure.
• The three voltages as arranged on the figure have
traditionally been called Einthoven’s tnangle, in
honor of Willem Einthoven, the physiologist and
inventor, who studied ECG voltages in 1903.

The equation means that only two leads
are needed to gather all of the
information available to the three leads.
• This follows from the fact that, the voltage on
any one of the leads can be calculated from
the other two.
• In other words, one of the leads is redundant.

This is not wasteful, though, because if
one of the leads is poorly connected, the
information will still be available for
diagnosis.
• This is especially important in ECG units that
do diagnosis automatically.
CHEST LEADS
(PRECORDIAL LEADS)

Often electrocardiograms are recorded
with one electrode placed on the anterior
aspect (front) of the chest over the heart.

This electrode (exploring) is
connected to the positive
terminal and the negative
electrode (i.e. indifferent
electrode) is normally
connected simultaneously
through electrical resistances
to the right arm, left arm, and
left leg.

Usually six different
standard chest leads are
recorded from the
anterior chest wall, the
chest electrode being
placed respectively at
the six points.

The sites of the six possible precordial leads,
are as follows:
•
•
•
•
•
•
V1—Fourth intercostal space, on the right sternal
margin.
V2—Fourth intercostal space, on the left sternal
margin.
V3—Midway between V2 and V4.
V4—Fifth intercostal space on the midclavicular line
(MCL).
V5—Fifth intercostal space on the anterior axillary line.
V6—Fifth intercostal space on the midaxillary line.

The different leads recorded by the
method are known as leads V1, V2, V3,
V4, V5, and V6.
• Because the heart surfaces are close to the
chest wall, each chest lead records mainly the
electrical potential of the cardiac musculature
immediately beneath the electrode.
• Therefore, relatively minute abnormalities in the
ventricles, particularly in the anterior ventricular
wall, frequently cause marked changes in the
electrocardiograms recorded from chest leads.

In leads V1 and V2, the QRS recordings of
the normal heart are mainly negative
because the chest electrode in these leads is
nearer the base of the heart than the apex,
which is the direction of electronegativity
during most of the ventricular depolarization
process.
•
On the other hand, the QRS complexes in leads V4,
V5, and V6 are mainly positive because the chest
electrode in these leads is near the apex, which is the
direction of electropositivity during depolarization.
AUGMENTED UNIPOLAR
LIMB LEADS

Another system of leads in wide use is
the "augmented unipolar limb lead".
• In this type of recording, two of the limbs are
connected through electrical resistance to the
negative terminal of the electrocardiograph
while the third limb is connected to the
positive terminal.

When the positive
terminal is on the right
arm,
•
The lead is known as the
aVR lead;

When on the left arm, as
the aVL lead;

When on the left leg, as
the aVF lead.

Normal recordings of the augmented
unipolar limb leads are similar to the
standard limb lead recordings except
that the aVR lead is inverted.
• The reason for this inversion is that the
polarity of the electrocardiograph in this
instance is connected backward to the major
direction of current flow in the heart during the
cardiac cycle.

Each augmented unipolar limb lead
records the voltage of the heart on the
side nearest to the respective limb.
• Thus, when the recording in the aVR lead is
negative, the side of the heart nearest to the
right arm is negative in relation to the
remainder of the heart.

Generally speaking, the P
wave results from electrical
currents generated as the
atria depolarize prior to
contraction, and the QRS
complex is caused by
currents generated when
the ventricles depolarize
prior to contraction.

Therefore, both P wave
and the components of
the QRS complex are
DEPOLARIZATION
WAVES.

The T wave is caused by currents
generated as the ventricles recover from
the state of depolarization.
• This process occurs in the ventricular muscle
about 0.25 sec after depolarization, and this
wave is known as a REPOLARIZATION
WAVE.
• Thus, the electrocardiogram consists of
depolarization and repolarization waves.

The P Wave. --The P wave represents
the spread of excitation and contraction
of atrial tissue of both atria.
• It is normally upright, of amplitude about 0.2
mV (between 0.1 - 0.3 mV) and lasts about
0.1 sec.

Atrial repolarization takes place in the
period when ventricular depolarization is
occurring;
• That is, the "Ta" would fall within the QRS
complex of the ventricles and so is obscured
in the record.

The QRS Complex. --This complex
signals the depolarization of conduction
system (Q) and of the ventricular
muscle.
• "Q" is an initial downward deflection;
• "R", a large upward deflection;
• "S", a downward deflection that follows,
sometimes, to below the base line, when the
small upward deflection that follows is called
R1.
• The duration of the QRS complex is approximately
0.08 sec.

The S-T Segment.--The S-T segment
represents the depolarized state, when
all of the ventricular muscle is
depolarized.
• Its level is normally very close to the baseline.
The duration of the S-T segment is
approximately 0.24 sec.

The T Wave.--The T wave represents the
final difference in rate of repolarization of
the different parts of the ventricular
muscle.
• Its amplitude and form is the most variable of
all the waves in the electrocardiogram, and it
is the most sensitive index of disturbances in
normal conduction.

This is illustrated by the range given for
normal amplitude in lead I: from 0.05 to
+0.55mV.
• A minus sign would mean that the T wave in
lead I was inverted, that is, downward.
• The duration of the T wave is approximately 0.12
sec.

The time intervals between the different
waves give valuable physiological
information.
• The two important intervals that are routinely
used are the P-R and Q-T intervals or
segments.

The P-Q Interval (sometimes called P-R
interval because the Q wave is
frequently absent) is measured from the
beginning of the P wave to the beginning
of the R wave (or QRS complex).

It represents the time taken from the
start of the excitation at the pacemaker
(sinoatrial node) to the beginning of
ventricular depolarization (i.e.
depolarization of the atrium, conduction
through the atrioventricular node and
through the conduction system to reach
the ventricular muscle).

This is normally 0.16 - 0.20 sec.
• An increase in the P-R interval indicates a
slowing of the conduction system, usually in
the atrioventricular node.

The Q-T Interval represents the total time for
the ventricular muscle to depolarize and
repolarize, from the beginning of the Q wave to
the end of the T wave.
•
This interval is longer for men and children than for
women, and it is usually reduced as the heart rate
increases (but not proportionally to the decrease in
total period of the heartbeat).

Speeding (Tachycardia) of the heart is
thus accomplished more by shortening
the electrical rest period of the heart
muscle than by shortening the period of
electrical activity.
• The normal duration of the Q-T interval is 0.30
sec.
WHAT THE
ELECTROCARDIOGRAM
CANNOT TELL US

The electrical activity of the heart is due
to the depolarization and repolarization
of the physiological membranes of the
neuromuscular and muscular tissues of
the heart.
• Depolarization normally is accompanied by
contraction of the muscle beneath these
membranes.

The magnitude of the voltages recorded by
local or distant electrodes depends on the
amount of the resting and action potentials.
•
In contrast, the strength of the contraction depends on
the amount and state of the muscle contractile
substance.
• Thus, it is a mistake to expect that the amplitude of the
electrocardiogram can tell us, except in extreme cases,
anything about the strength of contraction or the force of
the heartbeat (e.g. level of arterial pressure pulse
produced).

Where changes in the ionic environment
(e.g. abnormal K+) or in the metabolic
state of the tissue have altered the
resting and action potentials of the
muscle, there will, of course, be some
correlation of the amplitude of the
electrocardiogram with the strength of
the beat.

The amplitude of the waves is also affected by
the electrical resistance of the pathways to the
distant electrodes; in addition, as has been
already pointed out, the recorded voltages
represent only the difference between the
influences of simultaneous and opposite
electrical dipoles in a complicated pattern.
•
The resultant depends as much on the synchrony or
asynchrony of the component dipoles as on the
magnitude of the original potentials.
VARIETIES OF HEART
BLOCK

Since the primary information from the
electrocardiogram concerns the conduction
pathways, it is most useful in the diagnosis of
cases of interruption of normal pathways.
•
Bundle-branch block means that the impulses have
come from the atrial pacemaker (sinoatrial node),
reached and passed the atrioventricular node, but
travel down only one of the two main branches of the
conduction system (left or right bundle-branch block).

Excitation of the "blocked" ventricle still
occurs, but by spread from the normal
ventricle.
• The QRS complex of the electrocardiogram is
greatly prolonged from the normal 0.06 - 0.10
sec to more than 0.12 sec (First Degree).
• This is because conduction takes longer route to
the block ventricle and because the velocity is less
in the muscle than in the conduction system.

In total sinoatrial block (Third Degree), the
ventricles may continue to beat in a new
rhythm, called a "nodal, or ideoventricular,
rhythm," according to whether the initiation of
the impulse is taken up by the sinoatrial node
itself or an ectopic focus in the ventricular
muscle becomes the pacemaker.
•
Such varieties of block are diagnosed from the
electrocardiogram by noting a dissociation of the P
waves (atrial activity) from the QRST waves
(ventricular activity).
• The P wave may be absent (atrial standstill) or present
from an abnormal origin in the atrium.

In cases where the atrioventricular node
is partly blocked, the atrial rhythm may
be conducted to the ventricles only every
second beat, or in-groups of two or three
beats (bigeminal and trigeminal rhythms,
meaning "two twins" and "three twins",
respectively – Second Degree).
FLUTTER AND
FIBRILLATION ARRHYTHMIAS

The normal rhythms dominated by the
atrial pacemaker are known as "sinus
rhythms".
• Nodal and ventricular rhythms are examples
of escape from the dominance of the normal
pacemaker.
• In a different category of arrhythmias are atrial
flutter and fibrillation and ventricular fibrillation.

In atrial flutter, a regular succession of P
waves is seen in the electrocardiogram
at a rate many times the normal sinus
rhythm.
• Only every second, third, fourth, or fifth of
these waves may be followed by the
ventricular complex (QRST complex).

One explanation for flutter is that the
wave of depolarization is following some
unusual path in the atrial tissue and is
returning to re-excite repetitively the
pacemaker tissue, which is capable of
responding to stimuli at intervals much
shorter than those of its own normal
spontaneous rhythm.

If the wave of depolarization spreads
normally from the node in all directions, it
could obviously not so return (because
of the refractory period); but if
conduction were blocked in certain
areas, one could conceive of such a
"circus" route.

In fibrillation, which often develops from
flutter, the whole atrium beats in an
uncoordinated manner with multiple
apparent foci of the impulses (it quivers
like jelly).
• The atrioventricular node is thus bombarded
with hundreds of impulses every minute, and
only now and then does an impulse find the
atrioventricular node out of its refractory state
(i.e. susceptible to stimulation).

The ventricular rhythm that results is
very irregular.
• The whole base line of the electrocardiogram
shows tiny irregular waves, less regular and
more frequent than those of flutter.
• The atria no longer achieve a significant pumping
of blood; but it turns out that this does not, per se,
reduce the output of the heart greatly, although the
accompanying irregularity of the ventricular
contractions does significantly reduce the output.

Ventricular fibrillation is a similar
quivering of the ventricles (the ventricle
feels to hand like a sack of worms).
• In this case the cardiac output ceases, and
this is an emergency with fatal outcome
unless cardiac massage and a means of
defibrillation is at once employed.

Defibrillation is accomplished by
administering a severe electrical shock
to the heart, which arrests all excitation
and conduction.
• On recovery from the refractoriness produced
by the shock, a normal rhythm may be taken
up.
• It is not surprising that electrocardiographic records
of human ventricular fibrillation are not readily
available for illustration.