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Examining Cardiac Activity through the ECG (Exercise 6)

Calo, Nino; Cayetano, Jerwin; Lopez, Roxanne; Santiago, James Ian Cornelius; Torres, Katryna Mae Ann

Abstract

The study describes the physiology of cardiac muscle contraction

Keywords:

Introduction

The human heart is a vital organ that functions to keep blood circulating within the body. It has

a complex structure and acts as a specialized pump for blood circulation. It can be thought of as two

separate pumpsone pumping blood to the lungs and the other through the peripheral organs

working harmoniously for the bodys survival (Guyton, 2011). The heart has four distinct chambers: the

left and right atria and the left and right ventricles. The right half of the heart is involved in pulmonary

circulation while the left half is involved in systemic

circulation. Deoxygenated blood flows from thebody into the heart via the inferior and superior

vena cava and then enters the right atrium. Blood

then flows to the right ventricle and is pumped and

flows into the pulmonary artery and continues to

the lungs where it becomes oxygenated. The

oxygenated blood then enters the heart once more

through the pulmonary veins and enters the left

atrium. From the left atrium, blood flows into the

left ventricle and is pumped into the aorta which

then carries the blood throughout the whole bodythrough series of arteries and capillaries. The blood

then returns to the heart via the veins and the

cycle repeats. Valves in the heart function to

prevent backflow of blood.

This circulation is made possible by rhythmic beating or contractions of the heart muscles or

cardiac muscles. Cardiac contraction is initiated by an action potential from the impulse conducting

system of the heart. The impulse conducting system

consists of specialized cells that initiate heartbeat

and electrically coordinate contractions of the heart

chambers. The sinoatrial (SA) is a small mass of

specialized cardiac muscle fibers in the wall of the

right atrium, to the right of the superior vena cava

entrance and normally initiates the electrical

impulse for contraction. Another node lies beneath

the endocardium in the inferoposterior part of the

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interarterial septum and is called the arterioventricular (AV) node. The SA and AV nodes are considered

as the pacemaker of the heart. These certain heart cells do not require external provocation to initiate

action potential. Rather, these pacemaker cells have the capability of self-initiated depolarization in a

rhythmic fashion or a property known as automaticity. Other parts of the impulse conducting system of

the heart include the bundle of His perforating the interventricular septum posteriorly then bifurcating

into the left and right bundle branches. The right bundle branch innervates the right ventricle and the

left bundle branch innervates the left ventricle. These thin innervations in both right and left ventricles

are called Purkinje fibers (Lilly, n.d.).

Unless provoked, the cardiac muscle remains stable at its resting membrane potential

(approximately -90mV). This resting stage prior to depolarization of the membrane is termed as phase 4.

Phase 0 or depolarization follows. A transient current of repolarization returns the membrane potential

to approximately 0mV (Phase 1). This is followed by Phase2. During this phase, a 0mV voltage is

maintained for a prolonged period known as the plateau and is followed by Phase 3 which is the final

period of repolarization that returns

the membrane potential back to theresting potential. This return to

Phase 4 prepares the cell for the

next stimulus for depolarization.

However, pacemaker cells have a

different Phase 4 unlike cardiac

muscle cells. Phase 4 of the

pacemaker cell action potential is

not flat but has an upward slope

representing spontaneous gradual

depolarization.

Cardiac contraction relies on the organized flow of electrical impulses through the heart. When

the cardiac impulse passes through the heart, electrical current also spreads from the heart into the

adjacent tissues surrounding the heart. A small portion of the current spreads all the way to the surface

of the body. If electrodes are placed on the skin on opposite sides of the heart, electrical potentials

generated by the current can be recorded. The electrocardiogram (ECG) is an easily obtained recording

of the hearts activity and provides information about cardiac structure and function. The ECG

recordings are presented as line graphs of electrical measurements of voltage changes as the heart

contracts. Through this, diagnosis of heart diseases could be inferred.

Methodology

Samples of normal and abnormal polygraph recordings of ECG were obtained and analyzed. The

heart rates were determined and normal and abnormal readings were compared. Using the Einthovens

triangle and law, the overall direction and magnitude of the electrical impulses conducted over the heart

or the cardiac electrical axis.

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Results and Discussions

Components of the Electrocardiogram

Shown in the following figure is an obtained sample of an ECG recording of a normal heart

rhythm.

The normal heart ECG shows a series of waves representing the changes of the net potential differences

through time as the heart contracts. These waves, in correct sequence, are the P wave, the QRS complex,

and the T wave. The QRS complex is often, but not always, in 3 distinct waves (Q, R, and S waves). The

waves in the ECG represent events in the rhythmic heartbeat.

The P wave is caused by electrical potentials generated when the atria depolarize before atrial

contraction begins. The QRS complex is caused by potentials generated when the ventricles depolarize

before contraction (as the depolarization waves spreads through the ventricles). The first two waves are

both depolarization waves while the T wave represents a repolarization wave. It is caused by the

potentials generated as the ventricles recover from the state of being depolarized.

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When a cardiac cell is at its resting membrane potential, the cell is polarized. This means that

the extracellular side of the cell is completely positive with respect to the intracellular side. This

equilibrium is disturbed when the cell is stimulated by an action potential. During action potential,

cations rush into the cell and the polarity in the stimulated region transiently reverses. This makes the

extracellular side negatively-charged with respect to the inside of the cell. This is depolarization. During

depolarization, a potential difference is created on the cell surface between the depolarized area and

the still polarized parts of the cell. In the figure below, A represents this phenomenon. Since by

convention, the direction of electrical current flows from the negatively to the positively charged areas,

the current in the example flows toward the positive terminal of the voltmeter, rendering an upward

deflection in the graph (Lilly). It is important to note that once depolarization has reached the halfwaymark, the maximum possible potential difference is recorded. From this point, there will be more

negatively-charged areas, and the graph rendered would be a decreasing slope. Once the cell is fully

depolarized, the charges on the surface of the cell is homogenous, rendering a 0 potential difference as

indicated by a flat line seen in B on the figure below. Once repolarization begins, a potential difference

is once more generated on the outside of the cell. However, this time, the current is directed towards

the negative electrode and thus the voltmeter deflects towards the negative. Once repolarization

reaches halfway, the maximum negative potential difference is recorded and from this point the

potential difference returns to zero (when repolarization is complete).

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It is shown that repolarization renders a wave opposite that of the depolarization wave but isnot the case in the normal T wave. This is because in the human heart, repolarization proceeds in a

direction opposite that of depolarization. Therefore, deflections of the voltmeter would be in the same

direction that is in the example, towards the positive. However, it is notable that the repolarization

wave is of lower amplitude and more prolonged than that of depolarization.

The figure below represents the action potential of a single ventricular fiber and its

corresponding ECG recording. This helps in distinguishing the effect of ventricular contraction on the

ECG recording.

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This shows that ventricular contraction is largely responsible for the QRS complex and the T

wave. The QRS complex is generated once the ventricles undergo depolarization. It is important to note

that when the muscle is fully depolarized or fully polarized, the ECG recording will render a flat line as

there is no potential difference on the surface.

Heart Rhythm and Heart Rate

The standard ECG paper speed is 25 mm/s. This means that 5 big boxes on the paper represent a

second and every small box (1 mm) represents 0.04 s. And under correct calibration, each 1mm box is

equivalent to 0.2 mV. This means that a big box is equivalent to 1 mV (Guyton, 2011). However, some

are calibrated in a way wherein 10mm is equivalent to 1 mV (Lilly).

Under normal conditions, the following are observed:

ECG Component Duration (s) Voltage (mV)

P wave 0.10 0.2

QRS complex 0.080.12 1

T wave 0.160.27 0.3

PR interval 0.130.16 0

QT segment 0.300.34 0

PR segment 0.030.06 0

ST segment 0.08 0

The normal heart rate for adults is 6080 beats per minute (bpm) and for children, 100 bpm.

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The heart rate can be calculated from the ECG reading by using the formula:

()

Or more simply,

()

where, the number of small boxes is the distance between 2 successive QRS complexes.

In the samples above, the heart rates for I and II would be 83.33 bpm and 78.95 bpm

respectively.

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Abnormalities in heart rate can be seen in the figure above. Here the differences are seen on the

intervals between the t waves and the next p wave. A longer distance would mean a slower heartbeat

while a shorter distance would mean a faster heartbeat. The figure below shows abnormalities in the

wave components of the ECG. Various medical conditions related to the heart can be deduced from the

ECG.

ECG Lead Reference System and the Mean QRS Axis

In normal heart ventricles, current flows from negative to positive primarily from the base of the

heart toward the apex for the period of almost the entire cycle of depolarization, except at the very end.

And if a meter is connected to electrodes on the surface of the body as shown in the figure above, the

electrode nearer the base will be negative, whereas the electrode nearer the apex will be positive, and

the recording meter will show positive recording in the electrocardiogram.

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Depolarization spreads rapidly through the heart by means of the cardiac impulse system and

electrical forces are generated by each cell. The sum of these forces is recorded by the ECG. The

direction and magnitude of deflections on the EKG recording depend on how these electrical forces are

aligned to a set of specific reference axes known as EKG leads.

There are three kinds of leads used in electrocardiography. These are the unipolar limb leads,bipolar limb leads, and the chest or precordial leads. However, the paper is limited on the bipolar limb

leads.

Electrical connections between a patients limbs and the electrocardiograph for recording

electrocardiograms from the so-called standard bipolar limb leads are shown in the figure below. The

term bipolar denotes that the electrocardiogram is recorded from two electrodes located on different

sides of the heart, in this case, on the limbs. Thus, a lead is not just a single wire connecting from the

body but a combination of two wires and their electrodes to generate a complete circuit between the

body and the electrocardiograph. The electrocardiograph in each case is represented by an electrical

meter in the diagram, but note that the actual electrocardiograph is a high-speed recording meter with

a moving paper.

Lead I. To record limb lead I, the negative terminal of the electrocardiograph is connected to the right

arm and the positive terminal to the left arm. Thus, when the point where the right arm connects to the

chest is electronegative with respect to the point where the left arm connects, the electrocardiograph

records positively, that is, above the zero voltage line in the electrocardiogram. When the opposite is

true, the electrocardiograph records below the line.

Lead II.To record limb lead II, the negative terminal of the electrocardiograph is connected to the right

arm and the positive terminal to the left leg. Therefore, the electrocardiograph records positively when

the right arm is negative with respect to the left leg.

Lead III.To record limb lead III, the negative terminal of the electrocardiograph is connected to the left

arm and the positive terminal to the left leg. Therefore, the electrocardiograph records positively when

the left arm is negative with respect to the left leg.

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Einthovens Triangle. This is drawn around the area of the heart which illustrates that the two arms and

the left leg form apices of a triangle surrounding the heart. The two apices at the upper part of the

triangle represent the points at which the two arms connect electrically with the fluids around the heart,

and the lower apex is the point at which the left leg connects with the fluids.

Einthovens Law. This law states that if the electrical potentials of any two of the three bipolar limb

electrocardiographic leads are known at any given instant, the third one can be determined

mathematically by simply summing the first two (but note that the positive and negative signs of the

different leads must be observed when making this summation).

For example, as noted in the figure above, the right arm is -0.2 mV (negative) with respect to the

average potential in the body, the left arm is + 0.3 mV (positive), and the left leg is +1.0 mV (positive). As

seen in the meters in the figure, it can be observed that lead I records a positive potential of +0.5

millivolt because this is the difference between the -0.2 mV on the right arm and the +0.3 mV on the leftarm. Then, lead III records a positive potential of +0.7 mV, and lead II records a positive potential of +1.2

mV because these are the instantaneous potential differences between the respective pairs of limbs.

Note that the sum of the voltages in leads I and III equals the voltage in lead II; that is, 0.5 plus 0.7

equals 1.2. Thus, mathematically, this principle, called Einthovens law, holds true at any given instant

while the three standard bipolar electrocardiograms are being recorded.

Actual normal electrocardiograms for all three bipolar leads are shown in the figure below.

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It is obviously seen that the electrocardiograms in these three leads are similar to one another

because they all record positive P waves and positive T waves, and the major portion of the QRS

complex is also positive in each electrocardiogram. On analysis of the three electrocardiograms, with

careful measurements and proper observance of polarities, it can be deduced that the sum of the

potentials in leads I and III equals the potential in lead II, thus validating Einthovens law.

When the leads are put together, they create the axial reference system that can help in

determining the mean QRS Axis.

Each of the bipolar lead is actually a pair of electrodes connected to the body on opposite sidesof the heart. The direction from negative electrode to positive electrode is called the axisof the lead.

Lead I is recorded from two electrodes placed respectively on the two arms. Because the

electrodes lies exactly in the horizontal direction, with the positive electrode to the left, the axis of lead I

is 0 degrees.

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In recording lead II, electrodes are placed on the right arm and left leg. The right arm connects

to the torso in the upper right-hand corner and the left leg connects in the lower left-hand corner.

Therefore, the direction of this lead is about +60 degrees.

With the same analysis, it can be seen that lead III has an axis of about +120 degrees.

The axes direction of all these leads when placed in a coordinate plane is known as the

hexagonal or axial reference system. The polarities of the electrodes are indicated by the plus and minus

signs in the figure. These axes and their polarities, particularly for the bipolar limb leads I, II, and III are

very essential to clearly understand the vectorial analysis of electrocardiogram.

Vectorial analysis can give us the mean QRS electrical axis. This axis represents the average of

the instantaneous forces generated during the sequence of ventricular depolarization. This is expressed

in degrees and its normal value falls between -30 and +90. The axis can be determined accurately by

plotting the magnitude of the QRS complexes of leads I and III on their corresponding axes in the axial

reference diagram and drawing perpendicular lines from the lip of the vectors. The point of intersection

is the tip of the QRS axis. An example is shown below. Here, the mean axis is normal.

There are cases however when the resultant vector direction is more negative than -30

implying left axis deviation or greater than +90 implying a right axis deviation. The axis represents the

direction of the net forces acting. A left axis deviation suggests greater force in the left side. A left axis

deviation could be the result of left ventricular hypertrophy. A right axis deviation suggests greater force

on the right side of the heart and might be caused by right ventricular hypertrophy.

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However, an easier way would be just to look at leads 1 and 2. The following table shows the

corresponding interpretations.

Conclusions and Recommendations

The electrocardiogram provides important information regarding the structure and integrity of

the heart and remains one of the simplest but most important diagnostic tool in detecting heart

ailments. As the heart is a vital organ in our body,

References

Guyton; Hall, J. (2011). Textbook of medical physiology (12thed.)

Lilly, L. (n.d.) Pathophysiology of heart disease (2nd

ed.). Massachusetts:Williams & Wilkins

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Lopez, M. LEC 14: Diagnostics in cardiology IIadult ECG. Manila: UPCM