Lesson 5
ELECTROCARDIOGRAPHY I
Components of the ECG
Computer # 29
Joe Smith
Nick Jones
Jill Johnson
Kate Anderson
Tuesday Lab Section
September 20, 2007
Hypothesis
The heart rate will be lowest when the subject is supine. The heart rate will
increase when the subject sits up compared to supine. Post-exercise, the heart rate will be
highest compared to seated and supine. In looking at the effect of respiration, the heart
rate will increase during inhalation and the heart rate will decrease during expiration.
This effect will be greater during deep breathing compared to regular breathing.
Specific Aims
We will learn how to record a three-lead electrocardiogram (ECG). We will
record the ECG while our subject is supine, seated, and after exercise. We will compare
the ECG trace of our subject during deep breathing relative to regular breathing,
emphasizing the influence of inhalation and exhalation on heart rate. We will analyze the
ECG traces to gain an understanding of the relationship between the electrical and
mechanical events of the cardiac cycle.
Background
The heart consists of four chambers, two atria and two ventricles. [Briefly
describe blood flow path in heart] The mechanical pumping of the heart depends on
electrical stimulation that causes the cardiac muscles to contract.
The electrical activity of the heart follows a regular pattern that defines the
cardiac cycle. The electrical signal originates in the sinoatrial (SA) node [Describe the
path that the electrical signal follows from the SA node to the Purkinje fibers] As action
potentials move through the cells, the cells depolarize and contract. Once the cells have
contracted, a repolarization signal follows the same pathway and causes the cells to relax
(Widmaier 2006).
The frequency of firing from the SA node, corresponding to a change in heart
rate, can be influenced by several factors. [Briefly describe the effect of autonomic
nervous system and the resting respiratory cycle on heart rate]
An electrocardiogram (ECG) is used to record the electrical activity of the heart.
In a three-lead ECG, electrodes are placed on the wrist and ankle to record the electrical
signal. Echoes of the depolarization and repolarization of the heart are sent through the
rest of the body, which can be detected by placing a pair of very sensitive receivers
(electrodes) on other parts of the body (Pflanzer 2006).
An ECG trace has a distinct waveform pattern that correlates to particular
electrical events in the heart. The P wave results from atrial depolarization [Describe the
electrical event that corresponds to each wave].
Methods
The experiment was performed as described in Lesson 5 of the Biopac Student
Lab Manual (Pflanzer 2006). The three-lead electrode set (SS2L), three disposable
electrodes, and the lab mat were used in this experiment. Our subject was [give the
subject profile information]. We placed the white lead on the electrode on the right
anterior forearm at the wrist, the red lead was placed on the electrode just above the left
inner ankle, and the black lead was placed on the electrode on the medial surface of the
right leg, just above the ankle as shown in Figure 1.
right forearm
WHITE lead
right leg
BLACK lead
(ground)
left leg
RED lead
Figure 1: Electrode lead configuration for lead II ECG. Reproduced from
Pflanzer 2006 without permission.
Our subject was supine on the lab mat and relaxed for 5 minutes prior to data
collection. After calibration of the system, we began our experiment. There were 5 data
conditions tested. The first four conditions each had an ECG recording of 20 seconds.
The first condition tested was supine with regular breathing, a marker was inserted for
each inhalation and exhalation. The second condition tested was [describe the other test
conditions]. The subject did 30 push-ups in the lab room to elevate her heart rate. We
then recorded the ECG of the subject for one minute post-exercise.
Results
For the first test condition of supine with regular breathing, we analyzed the
components of the ECG waveform as given in Table 1. The QT interval, corresponding
to ventricular systole, and the T-R interval, corresponding to ventricular diastole, are
given in Table 2. The heart rate for supine with regular breathing was measured in terms
of duration and beats per minute (BPM) as given in Table 3, with an average heart rate of
77 BPM. For supine, deep breathing, the heart rate was found to increase to 84 BPM
during inhalation and decreased to 73 BPM as shown in Table 4. The subject’s heart rate
increased after sitting up from a supine position. The average heart rate for seated regular
breathing was found to be 82 BPM (Table 5). When the subject breathed deeply, the
heart rate increased to 87 BPM during inspiration and decreased to 80 BPM (Table 6).
Table 1: Components of the ECG waveform for supine, regular breathing
ECG
Duration (seconds)
Amplitude (mV)
Cycle
1
Cycle
2
Cycle
3
Cycle
1
Cycle 2 Cycle 3 Mean
Mean
Component
P wave
PR interval
PR segment
QRS complex
QT interval
ST segment
T wave
Table 2: Ventricular systole and diastole for Supine, Resting, Regular Breathing
∆ T (seconds)
Ventricular Readings
QT Interval
(corresponds to Ventricular Systole)
End of T wave to subsequent R wave
(corresponds to Ventricular Diastole)
Cycle 1
Cycle 2
Cycle 3
Mean
0.3
0.5
Table 3: Heart rate for Supine, Resting, Regular Breathing
Cardiac Cycle
Measurement
1
2
3
Mean
Range
∆T (seconds)
BPM
Table 4: Heart rate for Supine, Deep Breathing
Rhythm
Cycle 1 Cycle 2
Inspiration
Cycle 3
Mean
Cycle 3
Mean
Cycle 3
Mean
∆ T (seconds)
BPM
Expiration
∆ T (seconds)
BPM
Table 5: Heart rate for Seated, Regular Breathing
Heart Rate
Cycle 1 Cycle 2
∆ T (seconds)
BPM
Table 6: Heart rate for Seated, deep breathing
Rhythm
Cycle 1 Cycle 2
Inspiration
∆ T (seconds)
BPM
Expiration
∆ T (seconds)
BPM
Following exercise, the subject’s heart rate increased again to 125 BPM (Table 7.
Post-exercise, the QT interval, corresponding to ventricular systole, and the T-R interval,
corresponding to ventricular diastole, were measured and are given in Table 8. Both
intervals showed a decrease in duration compared to the supine condition (verify in
Tables 2 and 8). A summary of the conditions tested and the resulting heart rates is listed
in Table 9.
Table 7: Heart rate after exercise
Heart Rate
∆ T (seconds)
Cycle 1
Cycle 2
Cycle 3
Mean
BPM
Table 8: Ventricular systole and diastole after exercise
∆ T (seconds)
Ventricular Readings
QT Interval
(corresponds to Ventricular Systole)
Cycle 1
Cycle 2
Cycle 3
Mean
End of T wave to subsequent R wave
(corresponds to Ventricular Diastole)
Table 9 Data summary for heart rate
Condition
Mean
(BPM)
Range/ Std Dev
(BPM)
Supine, regular breathing
Supine, deep breathing, inhalation
Supine, deep breathing, exhalation
Seated, regular breathing
Seated, deep breathing, inhalation
Seated, deep breathing, exhalation
After exercise – start of recording
After exercise – end of recording
Discussion
In this section, you should:
(a) Discuss the results of your experiment. What do your results mean? Do they make sense
and why? There should be some physiological concepts addressed in this part of your
discussion. What would you expect to happen based on the experimental procedure? Here
you should also compare your data trends to those presented for a "normal, healthy
individual" in Vander/Widmaier.
For example: “Heart rate was found to increase after exercise. During exercise, there is an
increased demand for oxygen by the skeletal muscles. The flow of blood is increased to meet
the oxygen requirements by an increase in heart rate (Widmaier 2006).”
(b) You should answer ALL of the questions at the end of each lesson in the BIOPAC
manual. The questions should be answered in order and written in paragraph form. Do not
format as Q&A. Some questions will fit easily into the discussion of your results, and some
are an extension of the data collected and will need to be presented after you have discussed
your results. Even if you addressed some of these questions in Background, you need to
include them in your Discussion.
For example: “Each of our ECG traces had one P wave for every QRS complex. The P
waves were bell-shaped and had a lower amplitude compared to T wave. The T wave also
had a bell-shaped waveform.”
(c) What are some of the limitations of the study? What are some possible sources of
error/errata? The errors and limitations should be relevant to what you observed in the lab and
specific.
For example: “One source of error noted for the exercise data we collected was due to the fact
that the electrode began to peel off of the subject’s wrist after exercise. We had to delay
recording to re-adhere the electrode with medical tape, which may have reduced the increase
in heart rate we expected to observe. A limitation of our study was that we were only able to
test one subject. This did not give us a large sample of data from which to base our
conclusions.”
Conclusions
Our hypothesis was correct. We found that heart rate was lowest when the subject was
supine. The heart rate increased when the subject was seated compared to supine. Postexercise, the heart rate was highest compared to seated and supine. In looking at the
effect of respiration, the heart rate was increased during inhalation and decreased during
expiration. This effect was greater during deep breathing compared to regular breathing.
References
Iaizzo, P. (2005) “General Features of the Cardiovascular System.” From: Handbook of
Cardiac Anatomy, Physiology, and Devices. Edited by P.A. Iaizzo. Humana
Press Inc, Totowa, NJ. Pages 1-11.
Pflanzer, Richard, J.C. Uyehara, and William McMullen. (2006) “Lesson 5:
Electrocardiography I, Components of the ECG.” Biopac Student Lab Manual.
BIOPAC Systems, Inc., Santa Barbara, CA. p. 1-29.
Widmaier, E.P; H. Raff; and K.T. Strang. (2006) Vander’s Human Physiology: the
mechanisms of body function, Tenth Edition. McGraw-Hill, New York. p. 388411, 448-450