Download Electrocardiogram (EKG, ECG) Lab

Electrocardiogram (EKG, ECG) Lab
An electrocardiogram, or EKG, is a graphical recording of the electrical events occurring within the
heart. A typical EKG tracing consists of five identifiable waves. Each wave is noted by one of the letters
P, Q, R, S, or T.
Figure 1
The P wave represents the wave of depolarization that spreads from the SA node throughout the atria, and
is usually 0.08 to 0.1 seconds (80-100 ms) in duration. The brief isoelectric (zero voltage) period after the
P wave represents the time in which the impulse is traveling within the AV node where the conduction
velocity is greatly retarded.
The period of time from the onset of the P wave to the beginning of the QRS complex is termed the P-R
interval, which normally ranges from 0.12 to 0.20 seconds in duration. This interval represents the time
between the onset of atrial depolarization and the onset of ventricular depolarization. If the P-R interval is
>0.2 sec, a conduction defect (usually within the AV node) is present (first-degree heart block).
The QRS complex represents ventricular depolarization. The duration of the QRS complex is normally
0.06 to 0.1 seconds. This relatively short duration indicates that ventricular depolarization normally
occurs very rapidly. If the QRS complex is prolonged (> 0.1 sec), conduction is impaired within the
ventricles. This can occur with bundle branch blocks or whenever a ventricular foci (abnormal pacemaker
site) becomes the pacemaker driving the ventricle. Such an ectopic foci nearly always results in impulses
being conducted over slower pathways within the heart, thereby increasing the time for depolarization and
the duration of the QRS complex.
The isoelectric period (ST segment) following the QRS is the time at which the entire ventricle is
depolarized and roughly corresponds to the plateau phase of the ventricular action potential. The ST
segment is important in the diagnosis of ventricular ischemia or hypoxia because under those conditions,
the ST segment can become either depressed or elevated.
The T wave represents ventricular repolarization and is longer in duration than depolarization.
The Q-T interval represents the time for both ventricular depolarization and repolarization to occur, and
therefore roughly estimates the duration of an average ventricular action potential. This interval can range
from 0.2 to 0.4 seconds depending upon heart rate. At high heart rates, ventricular action potentials
shorten in duration, which decreases the Q-T interval. Because prolonged Q-T intervals can be diagnostic
for susceptibility to certain types of tachyarrhythmias, it is important to determine if a given Q-T interval
is excessively long. In practice, the Q-T interval is expressed as a "corrected Q-T (Q-Tc)" by taking the
Q-T interval and dividing it by the square root of the R-R interval (interval between ventricular
depolarizations). This allows an assessment of the Q-T interval that is independent of heart rate. Normal
corrected Q-Tc intervals are less than 0.44 seconds.
There is no distinctly visible wave representing atrial repolarization in the ECG because it occurs during
ventricular depolarization. Because the wave of atrial repolarization is relatively small in amplitude (i.e.,
has low voltage), it is masked by the much larger ventricular-generated QRS complex.
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 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 medicines.
Some abnormal EKG’s:
Irregular heart rhythms :
1. heart block - located in AV node (no pacemaker keeping time)
autorhythmic cells in ventricles go at own pace
contractions are out of sync
2. atrial fibrillation - atria not contracting uniformly ; essentially contractions stop, efficiency is reduced
3. ventricular fibrillation - circulatory failure ; need immediate defibrillation (clear!)
see also: http://www.kumc.edu/kumcpeds/cardiology/pedelectrocardiograms.html
http://members.evansville.net/ict/ekg.htm
*** In class, be sure to play the electrocardiogram game at:
http://nobelprize.org/educational_games/medicine/ecg/
In this experiment, you will use the EKG sensor to make a five second graphical recording of your heart’s
electrical activity. You will identify the different components of the waveforms and use them to determine
your heart rate.
PROCEDURE
Part I Standard limb lead EKG
1. Connect the EKG Sensor to the Vernier computer interface. Open the file “12 Analyzing Heart EKG”
from the Human Physiology with Vernier folder.
2. Attach three electrode tabs to your arms, as shown in
Figure 2. Place a single patch on the inside of the right
wrist, on the inside of the right upper forearm (distal to the
elbow), and on the inside of the left upper forearm (distal
to elbow).
3. Connect the EKG clips to the electrode tabs as shown in
Figure 2. Sit in a relaxed position in a chair, with your
forearms resting on your legs or on the arms of the chair.
When you are properly positioned, have someone click
to begin data collection.
4. Once data collection is finished, click and drag to
highlight each interval listed in Table 1. Use Figure 3 as
your guide when determining these intervals. Enter the x
value of each highlighted area to the nearest 0.01 s in
Table 1. This value can be found in the lower left corner of
the graph.
5. Calculate the heart rate in beats/min using the EKG data.
Record the heart rate to the nearest whole number in Table
1.
6. Store this run by choosing Store Latest Run from the Experiment menu.
Figure 3
Figure 2


P-R interval:
QRS complex:
time from the beginning of P wave to the start of the QRS complex
time from Q deflection to S deflection

Q-T interval:
time from Q deflection to the end of the T
DATA:
Table 1
Interval
Time (s)
P–R
QRS
Q–T
R–R
Heart Rate (bpm)
Table 2
Standard Resting Electrocardiogram Interval Times
P–R interval
0.12 to 0.20 s
QRS interval
less than 0.12 s
Q–T interval
0.30 to 0.40 s
total time for 1 beat
.80 sec
DATA ANALYSIS:
1. 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 3 (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.
Table 3
P Wave
ECG
a
Abs.
PR Interval
Nml.
Abs./Abn.
QRS Interval
Abs./Abn.
Pres.
1
X
X
X
X
X
2
X
X
X
X
X
3
X
X
Nml.
Abn.
T Wave Shape
Beat
X
Nml.
QRS
Shape
X
Nml.
Abs./Abn.
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
Discussion Questions:
1. Describe the flow of electrical activity in the heart during the cardiac cycle (SA node,
A-V node, A-V bundle, Purkinje fibers).
2. Why can’t an EKG be used by a cardiologist to diagnose all heart problems? Which
problems is it most effective for?
http://www.medicinenet.com/Electrocardiogram_ECG_or_EKG/article.htm
3. Label the following on your graph:
P wave
QRS wave
T wave
Atrial systole; atrial diastole
Ventricular systole; ventricular diastole
First heart sound (lub); 2nd heart sound (dub)
A-V valves closed; semilunar valves closed