... drop from left leg to right arm. Ill The drop from left presentation

About This Presentation

Transcript and Presenter's Notes

Title: ... drop from left leg to right arm. Ill The drop from left

1

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.

8
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.

20
Einthoven Triangle

In summary, the three voltages I, II, and III are
measured on the body as
IThe voltage drop from left arm to right arm.
IlThe voltage drop from left leg to right arm.
IllThe 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
frontal-plane 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 Kirchhoffs law of voltages.

24
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.

26
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.

27
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 electro-cardiograms that
show the same component waves, but the amplitude
(height) and direction of the waves are different

Kirchhoffs 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
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 Einthovens 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
V1Fourth intercostal space, on the right sternal
margin.
V2Fourth intercostal space, on the left sternal
margin.
V3Midway between V2 and V4.
V4Fifth intercostal space on the midclavicular
line (MCL).
V5Fifth intercostal space on the anterior
axillary line.
V6Fifth 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.

Write a Comment

User Comments (0)

About PowerShow.com

... drop from left leg to right arm. Ill The drop from left PowerPoint PPT Presentation