Download Part 1: EKG

Lab 8.1
Lab 8 – EKG/Blood Pressure
EKG
OBJECTIVES - after completing this lab you should be able to:
1) describe the function of the conducting system of the heart including the pacemaker and
pacemaker potentials
2) discuss the theoretical basis for the EKG
3) describe a normal lead II trace and the events that cause each part of the recording (P wave,
QRS complex, T wave, PR segment, PR interval, ST segment, TP interval)
4) determine heart rate, duration and amplitude of each wave or complex from a recording
5) explain the effects of exercise on each component of the EKG
6) recognize EKG recordings and be able to explain the cause of heart block and extrasystole
BACKGROUND
The heart has the ability to produce and conduct an electrical signal that coordinates the
individual cells’ contractions to accomplish effective pumping action. This signal is called the
cardiac impulse. It originates in the pacemaker and travels over a specific pathway through the
heart.
1. The electrical activity is intrinsic--it does not come from the nervous system.
Specialized cells in the heart called autorhythmic cells depolarize spontaneously in a rhythmic
manner to generate the cardiac impulse. There are three sets of autorhythmic cells in the heart:
SA node
AV node
Purkinje fibers
inherent rate
100 bpm
40-60 bpm
20-40 bpm
with parasympathetic (vagal) tone
70-80 bpm
n/a
n/a
They generate the cardiac impulse by producing a “pacemaker potential”--the resting potential is
not stable but drifts closer and closer to the threshold until the cell depolarizes. Because the SA
node has the fastest rate it usually sets the rate for the entire heart. It is the message from the SA
node that normally controls the timing of contraction of the rest of the heart.
2. The intrinsic rate of depolarization of the pacemaker can be altered by signals from the
nervous system, hormones, drugs, and ion imbalances.
Although the heart sets its own intrinsic rate, that rate can be altered by extrinsic factors.
Normally it is slowed by parasympathetic impulses (parasympathetic tone) to conserve energy. It
can be speeded by direct sympathetic stimulation or by catecholamines from the adrenal gland. It
is also affected by several drugs and ion imbalances.
Lab 8.2
3. Electrical activity is conducted over a pathway of specialized cardiac cells called the
conduction system so that contraction of the heart is optimal for pumping blood.
The conduction system consists of cardiac cells that are specialized to conduct electrical impulses
rather than to contract: SA node, AV node, AV bundle, Purkinje fibers. They conduct the
cardiac impulse at different rates, and some fibers in the conduction pathway are specially
designed to slow the cardiac impulse down between the atria and the ventricles (AV delay). The
conduction system must accomplish two goals:
a. The atria must complete their contraction before contraction begins in the ventricles.
b. All contractile cells of each chamber should contract at about the same time so that maximal
strength can be developed to pump the blood.
To accomplish the first goal, the cardiac impulse is deliberately slowed down while it travels
through the AV node. This delay (AV delay) lasts about 0.1 second, and is just long enough to
allow the atria to finish contracting and pumping blood into the ventricles before the ventricles
begin their contraction.
The second goal is accomplished by the direct transmission of the cardiac impulse from one cell
to another via the gap junctions. The cells of the atria contract at about the same time. However,
the interventricular septal cells contract before the other cells in the ventricular myocardium.
This allows the cells in the outer walls of the ventricles to create even greater pressure by
pushing the blood against the septum.
4. Electrical activity (depolarization) precedes mechanical activity (contraction).
Before a muscle cell can contract, its membrane must be depolarized. Only then can Ca++ be
released into the cytosol to combine with troponin. The signal travels from the pacemaker cells
to the contractile cells, and from one contractile cell to another, via gap junctions in the
intercalated disks. The membrane of each cell must depolarize before it can contract. You can
consider these events as waves that travel over the myocardium from the SA node to the
ventricular myocardium: first depolarization, then contractions.
5. The electrical activity can be detected by electrodes placed on the body’s surface because it is
conducted through the body fluids
Electrodes placed on the skin can detect electrical activity not only in the heart but also in the
brain and in skeletal muscles. The recording of electrical activity obtained this way is called an
electrocardiogram (EKG). Because the electrodes are on the body’s surface, they are not an
accurate record of the electrical activity in the myocardium. The EKG waves represent the
combined activity of many myocardial cells at once, the sum of the potentials from all heart cells
at any point in time. Because the atria depolarize first, atrial activity appears first in the EKG,
with ventricular activity following it.
Lab 8.3
The best locations for monitoring the heart’s electrical output are on the limbs and on the anterior
chest (textbook p. 319). We will only use 3 or 4 electrodes: one on each limb. The record
obtained from each bipolar lead (I, II, III) represents the difference in potential between two of
the electrodes. For example, lead II compares the right arm to the left leg. The unipolar leads
(aVL, aVR, aVF) record only the potential that reaches an individual electrode. In a clinical
setting, 6 additional unipolar leads would be placed in an arc across the chest
PROCEDURES
Equipment and materials:
computer and Labscribe software/hardware
snap-on EKG leads, EKG cable
adhesive electrodes
1. Open Labscribe and follow instructions
2. Prepare the subject:
A. Remove jewelry (except earrings) and put in a safe place
B. Attach electrodes on the anterior arm just proximal to right and left wrists and
To the left ankle (put it about 2 inches above the medial malleolus)
3. Attach lead wires to electrodes using labels on lead plug (LL = left leg, etc.)
4. The subject must sit still (no talking or moving) and should not be allowed to watch the
screen while the EKG is recorded
5. Type “resting”, click “START” button on screen, then the “Enter” key
If there is a lot of electronic noise in the recording stop the recording, go to Edit, choose
Preferences, then change the sampling speed from 200 to 50 and then repeat step 5; call the
instructor if this doesn’t help
6. Record for about 2 minutes, then click the “STOP” button on the screen
7. Have the subject exercise (with leads attached if possible) until winded and hyperventilating then have her sit down again
8. Type “exercise”, click “START” button on screen, then the “Enter” key
9. Record for about 2 minutes, then click the “STOP” button on the screen
10. Remove leads from electrodes, remove and discard electrodes
11. Collect and record data: duration is T2-T1 in seconds
Lab 8.4
resting
exercising
duration of RR interval
heart rate (calculated)
duration of PR interval
duration of TP interval (end of T to beginning of next P)
Questions
1. Using the baseline below, draw and label a typical EKG including P wave, PR segment, QRS
complex, ST segment, and T wave (p. 319). Use brackets to label the waves-the beginning and
end of each component must be clearly marked.
2. During which part of the EKG does each event occur (be as specific as possible):
atrial depolarization
AV nodal delay
ventricular depolarization
ventricles are contracting and
emptying
ventricular repolarization
ventricles are relaxed and filling
Lab 8.5
3. What is the function of AV nodal delay?
4. Fill in this chart summarizing how the pacemaker potential works (p. 310-311):
major ion
involved?
direction of ion
movement with
respect to cell
membrane?
does it cause
depolarization or
repolarization?
-60 mV to threshold
threshold to 0
0 back down to -60 mV
5. Which part of the heart would take over if
In the case of:
a. a myocardial infarction
damages the SA node
b. complete heart block
occurs (the cardiac impulse is
generated in the SA node but
cannot be transmitted to the
AV node
c. an ectopic focus occurs and
depolarizes at a rate of 120
bpm
which part of the conduction
system would control the
atria?
which part of the conduction
system would control the
ventricles?
Lab 8.6
6. Refer to the diagram below of an EKG showing third (complete) degree heart block.
a. What causes heart block?
b. What is happening during a complete heart block?
c. In heart block, the atrial rate is (normal, high or low?) ______________ but the ventricular
rate is (normal, high or low?) __________________.
Lab 8.7
Blood Pressure
Objectives - after completing this lab you should be able to:
1) explain how arterial blood pressure is measured
2) define and explain the relationship between systolic, diastolic, pulse, and mean
arterial pressure
3) compare pressures in the systemic vs pulmonary arteries
4) compare pressure in systemic arteries, arterioles ,capillaries and veins
5) relate changes in systolic pressure to the cardiac cycle
6) describe the effect of exercise on blood pressure
7) analyze data from blood pressure measurements
BACKGROUND:
Blood pressure refers to the pressure exerted by the blood against the walls
of the blood vessels. Blood pressure is highest in the arteries and decreases steadily through the vascular system
until it returns to the heart. The entry of blood into the arteries each time the heart beats causes the pressure within
them to rise. The peak pressure obtained during each cardiac cycle is called the systolic pressure and the lowest
pressure is the diastolic pressure. Blood pressure is recorded (in mm Hg) as systolic over diastolic. The difference
between systolic and diastolic pressures equals the pulse pressure. Mean arterial pressure (MAP) equals diastolic
pressure plus 1/3 pulse pressure. [MAP is the pressure that drives blood through the vascular system.]
Blood pressure is regulated by autonomic reflexes that have control centers
in the medulla oblongata. Input comes from baroreceptors located in the carotid artery and the aorta. Increased
stretch on the arterial walls stimulates the baroreceptors, which then send afferent signals to the cardiovascular
control centers. In response to these signals, the depressor center is activated and sends efferent signals via
parasympathetic nerves to the heart and blood vessels. The parasympathetic nerves, acting via the neurotransmitter
acetylcholine, cause decreased heart rate by reducing the rate of depolarization of the pacemaker cells. They also
allow most of the arteries to dilate, reducing peripheral resistance and thus reducing blood pressure.
The ausculatory method is an indirect method for determining arterial blood pressure. In this method, blood pressure
is determined by listening for sounds of turbulent blood in an artery which has been partially occluded. Laminar
blood flow in unoccluded arteries does not make sounds in the blood vessels. Also, if blood flow is completely
occluded in an artery, there is no sound.
Turbulent blood flow sounds (sounds of Korotkoff) are detected by the use of
a stethoscope held over the brachial artery. A sphygmomanometer (consisting of an
inflatable cuff and a mercury or aneroid manometer) is used to occlude the vessel
and to record the pressure. Both systolic and diastolic pressures are recorded from
this procedure.
Lab 8.8
PROCEDURES
A. Determining resting blood pressure - everyone in the class
1. Seat the subject. Palpate the left brachial artery at the elbow. Wrap the cuff firmly around his left arm just above
the elbow, making sure that the arrow is over the brachial artery.
2. Place the bell of the stethoscope over the brachial artery and the ear pieces in
your ears. (Clean stethoscope ear pieces with alcohol before you use them
and make sure they are pointed
forward.)
3. Close the needle valve (rotate knob on bulb clockwise) and rapidly inflate the cuff until the manometer reads
about 150 mm Hg (or until it is above the subject’s systolic pressure).
4. Open the valve a little (counterclockwise) so that the air leaks out slowly (about 1 mm/sec).
5. Note the pressure from the dial at which the first sound (thump) is heard. This is the systolic pressure.
6. Continue to allow the air to escape.
7. The sound will grow louder, then muffled, and then it will disappear. The pressure at which the sound becomes
muffled is the diastolic pressure.
8. Repeat this procedure three times or until you get agreement between consecutive readings. Allow the subject's
arm to rest for 2 minutes between readings.
9. Record your own resting BP here: ________________________
B. Effects of exercise on blood pressure - at least one person per group
1. Have each subject exercise for at least 3-4 minutes or until he/she is winded and has a noticeably elevated HR.
The target heart rate is determined by subtracting the subject’s age from 220 and then multiplying that by 0.75.
2. Determine his/her BP immediately and record in the data chart at 0 min post exercise.
3. Repeat measurement at 2, 4, 6 and 8 minutes post exercise and record in data chart.
4. Calculate pulse and mean arterial pressures.
7.9
Data Chart - S = systolic pressure, D = diastolic pressure, P = pulse pressure, and M = mean arterial pressure
Name
0 min
S
D
P
M
2 min
S
D
P
M
4 min
S
D
P
M
6 min
S
D
P
M
8 min
S
D
P
column average
Calculate the average pressure for each column. Graph each set of blood pressure data from the bottom line of the chart (systolic, diastolic, pulse, and mean
arterial) against time—your graph will have 4 more or less parallel lines that don’t cross.
M
7.10
Questions
1. Normal blood flow through the internal carotid artery is 400 mL/min. If this vessel’s radius is reduced by 50% of
its normal radius and the pressure does not change, what is the new blood flow?
2. Normal blood flow in the renal artery is 500 mL/min. Renal arterial pressure is 100 mmHg and renal venous
pressure is 10 mmHg. Calculate the vascular resistance of the renal vasculature.
3. Calculate the MAP for a person with a blood pressure of 140/90