Electrical Activity of the Heart
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ECG
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Intrinsic Conduction System
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Pathway of Depolarization
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Heart Pacemaker
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Summary
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Introduction to Electrocardiography
An electrocardiogram (ECG or EKG) is a graphical recording of the electrical events occurring
within the heart. In a healthy heart there is a natural pacemaker in the right atrium (the sinoatrial
node) which initiates an electrical sequence. This impulse then passes down natural conduction
pathways between the atria to the atrioventricular node and from there to both ventricles. The
natural conduction pathways facilitate orderly spread of the impulse and coordinated contraction
of first the atria and then the ventricles. The electrical journey creates unique deflections in the
EKG that tell a story about heart function and health (Figure 1). Even more information is
obtained by looking at the story from different angles, which is accomplished by placing
electrodes in various positions on the chest and extremities. A positive deflection in an EKG
tracing represents electrical activity moving toward the active lead (the green lead in this
experiment).
Five components of a single beat are traditionally recognized and labeled P, Q, R, S, and T. The P
wave represents the start of the electrical journey as the impulse spreads from the sinoatrial node
downward from the atria through the atrioventricular node and to the ventricles. Ventricular
activation is represented by the QRS complex. The T wave results from ventricular repolarization,
which is a recovery of the ventricular muscle tissue to its resting state. By looking at several beats
you can also calculate the rate for each component.
Figure 1
Doctors and other trained personnel can look at an EKG tracing and see evidence for disorders of
the heart such as abnormal slowing, speeding, irregular rhythms, injury to muscle tissue (angina),
and death of muscle tissue (myocardial infarction). The length of an interval indicates whether an
impulse is following its normal pathway. A long interval reveals that an impulse has been slowed
or has taken a longer route. A short interval reflects an impulse which followed a shorter route. If
a complex is absent, the electrical impulse did not rise normally, or was blocked at that part of the
heart. Lack of normal depolarization of the atria leads to an absent P wave. An absent QRS
Human Physiology Experiments
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1
Introduction to Electrocardiography
complex after a normal P wave indicates the electrical impulse was blocked before it reached the
ventricles. Abnormally shaped complexes result from abnormal spread of the impulse through the
muscle tissue, such as in myocardial infarction where the impulse cannot follow its normal
pathway because of tissue death or injury. Electrical patterns may also be changed by metabolic
abnormalities and by various medications.
Figure 2
Figure 3
• P-R interval:
time from the beginning of P wave to the start of the QRS complex
• QRS complex:
time from Q deflection to S deflection
• Q-T interval:
time from Q deflection to the end of the T
OBJECTIVES
• Learn to recognize the different wave forms seen in an EKG and associate these wave forms
with activity of the heart.
2
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Human Physiology Experiments
Introduction to Electrocardiography
DATA ANALYSIS
Health-care professionals ask the following questions when interpreting an EKG:
• Can all components be identified in each beat?
• Are the intervals between each component and each complex consistent?
• Are there clear abnormalities of any of the wave components?
Using these questions as guides, analyze each of the following three-beat EKG tracings and record
your conclusions in Table 1 (indicate presence or absence of the P wave, and whether other
intervals and/or shapes are normal or abnormal). The first analysis (a) is done for you.
a.
b.
c.
d.
e.
f.
g.
h.
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Introduction to Electrocardiography
Table 1
P Wave
PR Interval
ECG Beat Pres. Abs. Nml. Abs./Abn.
a
QRS Interval
Nml.
QRS
Shape
T Wave
Abs./Abn. Nml. Abn. Nml. Abs./Abn.
1
X
X
X
X
X
2
X
X
X
X
X
3
X
X
X
X
X
1
b
2
3
1
c
2
3
1
d
2
3
1
e
2
3
1
f
2
3
1
g
2
3
1
h
2
3
4
©Vernier Software & Technology
Human Physiology Experiments
Introduction to Electrocardiography
a)
b)
c)
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Introduction to Electrocardiography
d)
e)
f)
6
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Introduction to Electrocardiography
g)
h)
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Vernier ECG’s:
a)
Second degree heart block
b) Premature atrial contraction:
c) Atrial fibrillation:
d) Premature ventricular contraction:
e) Ischemia:
f)
Myocardial infarction:
g) First degree block:
h) Complete heart block:
The main function of the human heart is to pump blood to various body parts and organs.
The pumping activity results in blood exerting force on walls of multiple veins and arteries
adapted to transport blood (DeMers & Wachs, 2020). The pumping of blood results in blood
pressure, which could be high or low depending on the amount of pressure exerted. Blood
pressure should be maintained at a normal level rather than at a high or low level. High blood is
critical because it may rupture the blood vessel or cause blood clotting, while low blood vessel
results in vital organs receiving insufficient nutrients and oxygen supply.
Blood pressure is a physiological activity that involves systolic and diastolic pressure.
Systolic blood pressure is the force in the arterial system caused by the contraction of the heart,
forcing blood into the aorta (DeMers & Wachs, 2020). Diastolic blood pressure is the force on
the arteries when the heart is at rest. Similarly, Blood pressure can be summarized as a product of
two processes, including cardiac output and peripheral resistance. Cardiac output is a result of
heart rate multiplied by stroke volume. Therefore, blood pressure is a product of three processes,
heart rate, peripheral resistance, and stroke volume.
Blood pressure is measured by using a device called a sphygmomanometer. The
measurements are expressed in systolic and diastolic blood pressure (DeMers & Wachs, 2020).
The systolic and diastolic blood pressure for a normal adult is 120 and 80 mmHg in a resting
position. Measurements for pre-hypertensive systolic and diastolic pressure are 120 -139 and
80-89mmHG. Similarly, hypertensive systolic and diastolic pressure readings are above 140 and
90 mmHg.
The systolic and diastolic pressure is maintained by the expansion and contraction in the
arterial system. The systolic and diastolic blood pressure of a person in different positions is
similar because the positioning does not influence the heart rate, peripheral resistance, or stroke
volume. Based on the stated hypothesis, blood pressure is determined by the independent
variable, including the heart rate, the stroke volume, or the peripheral resistance will be the
determinants of blood pressure. Dependent variables, such as the body position, can be
manipulated and alter the blood pressure.
Overview of how I did the experiment
Explaining the establishment of baseline blood pressure and heart rates
Baseline of blood pressure
Normal blood pressure is vital for human survival. Blood pressure along the blood
vessels is directly proportional to blood flow. The blood pressure is also indirectly proportional
to resistance determined by blood viscosity (Homan, Bordes & Cichowski, 2020). The
establishment of blood pressure requires the application of pressure in the brachial artery, which
results in the blocking of the vessel. Passage of pressure from the sphygmomanometer results in
blood flow through the vessel and also causes noise. Continuous pressure release smoothens the
blood flow. When blood flow through the vessel is completed, the cuff equates to the blood
pressure.
Heart rate
Systolic blood pressure and diastolic pressure values are used to calcite mean arterial
pressure. Mean arterial pressure level is significant in determining the supply of tissues with
sufficient oxygen. However, a high MAP is dangerous because it is an indication of
cardiovascular stress, which increases heart rate and blood pressure.
General overview of how the methods and baseline data to study the effects of
changes in body position on blood pressure and heart rate.
Experimental methods,
The experimental method of measuring pressure and heart rate requires one to have an
Automatic sphygmomanometer, Stethoscope, and Yoga mat or towel. An individual is asked to
rest to allow a cuff to be attached to the right hand. The cuff is started to allow measurement of
blood pressure and heart rate (Seltman, 2015). The blood pressure and the heart rate are read
twice, and the average for the values is calculated. The systolic and diastolic blood pressure
should be averaged separately. Measurement of blood pressure is repeated on the left arm while
the person is in a recumbent position. The cuff is started to allow measurement of the blood
pressure and heart rate reading twice. The measurement procedure is repeated twice on the left
arm. Data collected is recorded in a table as shown below.
Table 2: Calculations (mmHg)
Condition
Left arm
(seated)
Right arm
(seated)
Right arm
(laying down)
Left-arm
(laying down)
The left-arm
(post-exercise)
Pulse Pressure
MAP
Reference
DeMers, D., & Wachs, D. (2020). Physiology, mean arterial pressure.
Homan, T. D., Bordes, S., & Cichowski, E. (2020). Physiology, pulse pressure. StatPearls
Seltman, H. J. (2015). Experimental design and analysis.
Introduction to Electrocardiography
Introduction to Electrocardiography
An electrocardiogram (ECG or EKG) is a graphical recording of the electrical events occurring
within the heart. In a healthy heart there is a natural pacemaker in the right atrium (the sinoatrial
node) which initiates an electrical sequence. This impulse then passes down natural conduction
pathways between the atria to the atrioventricular node and from there to both ventricles. The
natural conduction pathways facilitate orderly spread of the impulse and coordinated contraction
of first the atria and then the ventricles. The electrical journey creates unique deflections in the
EKG that tell a story about heart function and health (Figure 1). Even more information is
obtained by looking at the story from different angles, which is accomplished by placing
electrodes in various positions on the chest and extremities. A positive deflection in an EKG
tracing represents electrical activity moving toward the active lead (the green lead in this
experiment).
Five components of a single beat are traditionally recognized and labeled P, Q, R, S, and T. The P
wave represents the start of the electrical journey as the impulse spreads from the sinoatrial node
downward from the atria through the atrioventricular node and to the ventricles. Ventricular
activation is represented by the QRS complex. The T wave results from ventricular repolarization,
which is a recovery of the ventricular muscle tissue to its resting state. By looking at several beats
you can also calculate the rate for each component.
Figure 1
Doctors and other trained personnel can look at an EKG tracing and see evidence for disorders of
the heart such as abnormal slowing, speeding, irregular rhythms, injury to muscle tissue (angina),
and death of muscle tissue (myocardial infarction). The length of an interval indicates whether an
impulse is following its normal pathway. A long interval reveals that an impulse has been slowed
2
©Vernier Software & Technology
Human Physiology Experiments
Introduction to Electrocardiography
or has taken a longer route. A short interval reflects an impulse which followed a shorter route. If
a complex is absent, the electrical impulse did not rise normally, or was blocked at that part of the
heart. Lack of normal depolarization of the atria leads to an absent P wave. An absent QRS
complex after a normal P wave indicates the electrical impulse was blocked before it reached the
ventricles. Abnormally shaped complexes result from abnormal spread of the impulse through the
muscle tissue, such as in myocardial infarction where the impulse cannot follow its normal
pathway because of tissue death or injury. Electrical patterns may also be changed by metabolic
abnormalities and by various medications.
Figure 2
Figure 3
● P-R interval:
time from the beginning of P wave to the start of the QRS complex
● QRS complex:
time from Q deflection to S deflection
● Q-T interval:
time from Q deflection to the end of the T
OBJECTIVES
● Learn to recognize the different wave forms seen in an EKG and associate these wave forms
with activity of the heart.
Human Physiology Experiments
©Vernier Software & Technology
3
Introduction to Electrocardiography
DATA ANALYSIS
Health-care professionals ask the following questions when interpreting an EKG:
● Can all components be identified in each beat?
● Are the intervals between each component and each complex consistent?
● Are there clear abnormalities of any of the wave components?
Using these questions as guides, analyze each of the following three-beat EKG tracings and record
your conclusions in Table 1 (indicate presence or absence of the P wave, and whether other
intervals and/or shapes are normal or abnormal). The first analysis (a) is done for you.
a.
b.
c.
d.
e.
f.
g.
h.
4
©Vernier Software & Technology
Human Physiology Experiments
Introduction to Electrocardiography
Table 1
P Wave
PR Interval
ECG Beat Pres. Abs. Nml. Abs./Abn.
a
b
c
e
f
g
h
Nml.
QRS
Shape
T Wave
Abs./Abn. Nml. Abn. Nml. Abs./Abn.
1
X
X
X
X
X
2
X
X
X
X
X
3
X
1
X
2
X
X
X
X
X
X
X
X
X
X
X
X
X
X
3
X
X
X
X
1
X
X
X
X
X
2
X
X
X
X
X
3
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
X
1
d
QRS Interval
X
2
X
3
X
X
1
X
X
X
X
X
2
X
X
X
X
X
3
X
X
X
X
X
1
X
X
X
X
X
2
X
X
X
X
X
3
X
X
X
X
X
1
X
2
X
X
X
X
X
X
X
X
X
3
X
X
X
X
X
1
X
X
X
X
X
2
X
X
X
X
X
3
X
X
X
X
X
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Introduction to Electrocardiography
a)
b)
6
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Introduction to Electrocardiography
c)
d)
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Introduction to Electrocardiography
e)
f)
8
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Introduction to Electrocardiography
g)
h)
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Running head: PRELAB HYPOTHESIS
Hypothesis in a Blood Pressure Experiment
Name of student
Institution
Date
2
PRELAB HYPOTHESIS
Hypothesis in a Blood Pressure Experiment
Introduction
As the heart pumps blood, the blood exerts pressure/force on the wall of the blood vessels
resulting in blood pressure. Blood pressure must be kept normal through checks and balances for
optimal physiological mechanisms. If the force exerted on blood vessels is too high, the vessels
may rupture leading to blood clotting or bleeding. On the other hand, too low pressure means
vital organs like the brain, kidney, or heart are endangered because they require an immediate
supply of oxygen and nutrients (Homan, 2020; DeMers, 2020).
Blood pressure is a measure of systolic and diastolic pressure. The difference between the
two results in pulse pressure. Systolic blood pressure (systemic arterial blood pressure) is the
force that occurs in the arterial system when the heart contracts to force blood into the aorta. On
the other hand, diastolic pressure is the force exerted on arteries when the blood is resting after
ejecting blood (Homan, 2020; DeMers, 2020).
The measurements are recordable using a sphygmomanometer and are expressed in
systolic/diastolic. For a normal adult, the average systolic and diastolic blood pressure is 120 and
80 mmHg in a resting state. During ventricular contraction, blood is pushed into the aorta full of
blood. The aorta expands to accommodate the increase and recoils back due to the ability of its
walls to stretch. The expansion/distention and recoiling occur throughout the arterial system and
are responsible for maintaining systolic and diastolic pressure respectively (Homan, 2020;
DeMers, 2020). According to the American Heart Association normal blood pressure reading is
120/80, pre-hypertensive 120-139/80-89, and hypertensive as readings above 140/90 mmHg.
PRELAB HYPOTHESIS
3
Blood pressure is the result of cardiac output (CO) and peripheral resistance (PR).
Similarly, cardiac output is a product of stroke volume and heart rate. Thus blood pressure (BP)
is the product of stroke volume (SV), heart rate (HR), and peripheral resistance (PR). Any
change in the above factors always alters blood pressure but the body has significant
compensatory mechanisms to correct any change up to some levels (Homan, 2020; DeMers,
2020).
The write-up will examine factors that affect venous return which in effect affect cardiac
output. The factors include; the position of the body, skeletal muscle, and respiratory pump.
Venous returns play a major role in determining cardiac output. Essentially there are other
factors involved in the return of blood through the veins back to the heart. Gravity helps in
returning blood from the head and neck when a person is standing or sitting and offers less
resistance when one lies flat. The presence of the valves in veins prevents the backflow of blood
when standing/sitting (Homan, 2020; DeMers, 2020).
The Hypothesis
If we take blood pressure readings of a person in different positions (recumbent, supine,
or prone), then we expect similar reading because the positioning does not affect heart rate,
stroke volume, or peripheral resistance.
Based on the above hypothesis, the dependent variable will be the determinants of blood
pressure (heart rate, peripheral resistance, or stroke volume). They will not be manipulated in the
experiment. The independent variable will include different positions the readings will be taken.
They are changed and can affect blood pressure. Factors that will be kept constant
PRELAB HYPOTHESIS
(Constant/control variables) will include maintaining a resting state, using the same
sphygmomanometer and same person.
Reference
Homan, T. D., Bordes, S., & Cichowski, E. (2020). Physiology, pulse pressure. StatPearls
Seltman, H. J. (2015). Experimental design and analysis.
DeMers, D., & Wachs, D. (2020). Physiology, mean arterial pressure. StatPearls
4
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