Download 1. ECG - Ping Pong - Karolinska Institutet

LABORATORY HANDBOOK
ECG
For the course:
Human Physiology
Karolinska Institutet • Department of Physiology and Pharmacology • Physiology Education
Postal address: 171 77 Stockholm
Visiting address: Von Eulers väg 4a, plan 2
Tel exp: 08-524 872 29 • Tel vx: 08-524 864 00
ECG LABORATORY
I
GOALS
You will know what a normal ECG looks like and what it means. You will learn the principles
of recording an ECG. You will know the difference between unipolar and bipolar recordings.
You will know the common recording sites, how to place electrodes on these points, and
connect them correctly to an ECG recorder.
You will know how to obtain an ECG recording after a quick glance through the operation
manual or a verbal briefing using whatever ECG recorder there is available.
You will understand the different parts/waves of a recorded ECG waveform and what
corresponding electrophysiological activity in the heart they represent.
You will understand why the different leads e.g. II, aVR, V1, V5. look the way they do.
You will be able to approximate the heart’s electrical axis on a routine ECG.
II
Definition
Electrocardiogram (ECG): the graphic recording of the small extracellular electrical activity
produced by the movement of action potentials through cardiac myocytes.
III
Basic Facts
At rest, there is a potential difference across the cardiac muscle cell membrane of about –80
mV, with the inside negative with respect to the outside. Stimulation of the heart muscle cell
gives rise to an action potential (AP, Fig 1) with a relatively long duration of 200 to 800 ms.
-80
Fig 1 AP causes the inside of the cell to become positive with respect to the outside i.e. depolarisation.
This is then succeeded by a restoration of the ionic gradient i.e. repolarisation.
Human Physiology rev Fall16
ECG
1
If a strip of myocardium is depolarised, a wave of depolarisation spreads along the strip. This
depolarisation gives rise to a so called di-pole in the heart, areas of different charges (positive
and negative). If an electrode is placed at each end, the electrode towards which the
depolarisation is spreading (positive pole) will record a positive deflection. The electrode
from which the impulse is coming will record a negative deflection (Fig 2a).
Depolarization:
Di-pole (neg ➔ pos)
Upon repolarisation, the opposite happens. The electrode from which the depolarisation starts
records a positive deflection and vice versa (Fig 2b).
Figure 2a
Repolarization:
Di-pole (pos ➔ neg)
Figure 2b
The situation in the intact heart is more complicated because the simultaneous depolarisation
comes from different directions. These changes in potential, when added together, produce a
single resultant waveform – the ECG, which is recorded.
Human Physiology
ECG
2
Genesis of the ECG waveform
1. The impulse starts from the sinoatrial (SA) node and depolarisation
spreads radially over the atria. A positive wave, known as the P
wave, is recorded.
2. Excitation moves from the atria to the ventricles via the
atrioventricular (AV) node and the bundle of His. Conduction
through the AV node is slow which results in a delay before the
ventricle depolarises which shows up as the isoelectric section
between the P wave and the coming Q wave.
3. The septum of the ventricle depolarises from left to right. This gives
the transitory, negative Q wave.
4. Depolarisation of the ventricular walls spreads from the endocardial
surface to the epicardial surface. The thicker walls of the left
ventricle have a greater muscle mass than the right ventricle. Thus
the electrical resultant is dominated by the left ventricle and is
recorded as a large, positive wave, the R wave.
5. The last parts of the ventricular walls to be activated are those which
lie around the aorta and the pulmonary vein. The resultant electrical
activity is due chiefly to the right side and one sees a small negative
wave, the S wave.
6. After depolarisation of the ventricular walls, there is a short period
before repolarisation during which there are no major changes in
potential, which shows up as the isoelectric ST segment.
7. Since the action potential is longer in the muscle cells located on the
inner side of the ventricle than those located on the outer surface,
repolarisation occurs first on the outer surface and then on the inner
surface of the ventricles. This inverses the di-pole as to presented
previously (see Fig 2b). As a result, the repolarisation wave shows
up as a positive wave instead of a negative. Thus, despite the T
wave, which represents repolarisation of the ventricles, being
positive it marks a repolarisation not a depolarisation.
8. Sometimes an additional wave, the U wave, can be seen whose
origin is disputed. Many attribute it to repolarisation of the papillary
muscles.
Human Physiology
ECG
3
Different types of recording
In order to study the complex electrical activity of the heart it is not enough to only use 2
electrodes as seen in fig. 2. We have to be able to see the heart from different sides/angles and
therefore, in a typical clinical setting we use 9 measuring electrodes. These 9 electrodes make
up for 12 leads (a 12 lead ECG). Each lead consists of a positive and a negative
electrode/reference point and together they create a measuring axis for that lead. Using 12
leads means we can, for example, distinguish the location of a pathological process in the
heart, such as a heart attack, thus treat it correspondingly.
Exploring
electrode
ECG recorder
and printer
Figure 3
Wilson’s electrode (V) acting
as a reference electrode
When discussing the electrodes in the ECG one differentiates between unipolar and bipolar
recordings.
Unipolar recording is done between one exploring electrode and a neutral or reference
electrode. The most common reference electrode is called Wilson’s electrode made by
coupling the three limb leads together (Fig 3) designated by the abbreviation V.
Bipolar recordings are made by recording the potential difference between any two
electrodes. If these electrodes sit at the same distance from the heart, the potential difference
recorded by the electrodes will depend on their placement with respect to the spread of
depolarisation (position of the di-pole).
The limb leads I, II and III were the first recordings made at the end of the 19th century (Fig
4). At that time there were no amplifiers in routine medical use and to reduce the cutaneous
resistance, subjects had to place their arm and legs in tubs of salt solution.
Human Physiology
ECG
4
Lead I records between the left and right arms with the left arm as the positive electrode.
Lead II records between the right arm and the left leg with the leg as the positive electrode.
Lead III records between the left arm and left leg with the leg as the positive electrode.
Here, positive electrode means that a wave of depolarisation directed towards the electrode
gives a positive wave on the recording.
Figure 4
Augmented unipolar leads: aVR, aVL, aVF
The three augmented unipolar limb leads compare one limb electrode to the average of the
other two. The exploring electrode sits on the right arm in aVR recording, on the left arm in
aVL recording and on the left ankle aVF recording (Fig 5). The reference electrode is
achieved by connecting the remaining corners in the triangle (R arm, L arm, L leg) via a
resistance. For further discussion of this, consult your lab mentor or course textbook.
Figure 5
Human Physiology
ECG
5
Chest leads
The most common types of chest electrodes are unipolar and have a Wilson electrode as the
reference. They are labelled V1 to V6 and represent the positive electrodes, placed on the
chest. In the clinical ECG exam the Wilson electrode is made up by the electrodes placed on
the limbs (frontal leads) and the reference point (negative electrode) is calculated
automatically by the ECG machine. When recording the ECG during exercise, the limb
electrodes are often placed on the shoulders and the thighs, which helps to reduce artefacts
due to movement and muscle activity.
Placement of the electrodes on the chest (Fig 6)
V1: Fourth intercostal space, right of sternum
V2: Fourth intercostal space, left of sternum
V4: Mid clavicular line, fifth intercostal space
V3: Halfway between V2 and V4
V5: Anterior axillary line, same level as V4
V6: Mid axillary line, same level as V4
Sometimes V7 is used at the edge of the
axillary line, same level as V4 and V4R
(corresponding to V4 but on the right side).
Figure 6
ECG recording theory
The leads that are most often used clinically are:

The bipolar limb leads: I, II, and III.

The unipolar augment limb leads: aVR, aVL, and aVF.

The unipolar chest leads V1, V2, V3, V4, V5, and V6.
The appearance of the ECG depends upon where the exploring electrode is placed. This
means that the electrical activity stays the same but the recorded wave varies depending on
what lead you look at. This could be compared to a “spotlight lamp”, whereas the light
intensity one could see would differ depending on where you stand, e.g. in front of the lamp,
on the side or perpendicular to it, where no light would be seen.
Human Physiology
ECG
6
For example, the same electrical activity recorded from different points:
Figure 7
In order to facilitate the recording of a routine ECG, an internationally recognised colour
coding of the cables is used.
Red = right arm
Yellow = left arm
Green = left leg
Black = right leg (earth on older machines, active lead on newer machines)
For the chest leads, one uses the colours red, yellow, green, brown, black and blue together
with the letters a to f or numbers 1 to 6.
Electrical axis of the heart
The heart’s electrical axis is routinely determined in the frontal plane. The electrical axis is a
function of the heart’s position in the chest cavity, the absolute thickness of the muscle in the
two ventricles and the condition of the ventricles. The calculations are based on
approximations and it is only obvious changes in the electrical axis that are interesting. These
include when the axis is outside normal ranges or sudden changes over time (e.g when.
comparing two ECG recordings).
When calculating the heart axis one assumes that:

the heart, which is the centre of electrical activity, is located in the centre of the chest
cavity

the chest is round

the leads which are used in the calculations (leads I, II and II) are located equidistant from
the heart

the body’s tissues conduct electricity equally well
The body is considered to be an equilateral triangle = Einthoven’s triangle (Fig. 8). Each side
of the triangle represents a frontal lead (I, II and III). Additionally, each side is “used” to
calculate the augmented unipolar limb leads (aVR, aVL and aVR) with the positive electrode
towards the perpendicular side (left/right shoulder and left foot).
Human Physiology
ECG
7
Figure 8
In Sweden the limb leads in the Einthoven’s triangle are printed out in a particular sequence
on a six-channel ECG recorder (the most commonly found in hospitals here) known as the
Cabrera system. In the Cabrera system one imagines that all frontal leads (their measurement
axes) are brought together to cross the same central point (the heart).
In Figure 9, the heart is shown with exploring/positive electrodes arranged according to the
Cabrera system. Limb lead I corresponds to 0º, aVL to -30º, II to +60º, aVF to +90º and III to
+120º. In order to have the different electrodes with 30º between them, the Cabrera system
inverts aVR to –aVR which then corresponds to +30º.
Figure 9
Note that in the figure, the large black arrow is the vector representing depolarisation of
ventricles (QRS-complex). This shows the largest sum of electrical activity during the heart
cycle which also corresponds to the heart axis. By noting in which lead the sum of the positive
and negative parts of the QRS wave is greatest (Tip! look for the highest R-wave), one
immediately sees roughly where the electrical axis lies. Read more about the heart axis in the
next section.
Human Physiology
ECG
8
Calculate the heart axis
In order to calculate the mean vector, one measures the sum of the heights of the Q, R and S
waves in each of the leads I, II and III. The negative deflections are subtracted from the
positive deflections and the resultant sum is drawn as a vector on the respective side of a
triangle (the base of the vector is placed at the centre of the side). One draws a perpendicular
from the head of the arrows and where they meet, this indicates the head of the mean vector.
The electrical axis can be:
Normal axis: -30º >angle < +110º
Right axis deviation: angle > +110º
Left axis deviation: -30º <angle > -90º
Figure 10
Example from the figure above
I:
III:
Human Physiology
ECG
9
Procedure
A computer with an amplifier and data-collection software is used for the ECG recording.
1. Appoint a subject for the lab.
Bipolar limb leads.
In this section you will move the electrodes that you can see lead I, II and III (in that order) on
the computer screen. Examine how the recording is affected if you reverse the polarity of the
connection or if the subject moves.
2. Open the file “EKG avledning I II III” in the folder “EKG” on the computer desktop.
3. Apply some electrode gel to the metal of three electrode clamps. Attach one of them to
the right ankle. Connect the ground cable to this.
4. Combine the other two clamps with the “plus” and “minus”- cables in channel 1 and
attach them to achieve limb lead I. Use Fig. 4. Press START to initiate the recording.
If there is a lot of “noise” (disturbances in the recording): check the cables, shave
away hair on the points of contact, apply additional electrode gel.
Insert a comment that describes the recording by writing “lead I” and pressing
[ENTER].
Record 10-20 seconds of data without noise. Press “STOP” to end the recording.
5. Reverse the polarity of the connection by switching the cables at the electrode clamps.
Press “START”. Insert a comment that describes the recording. How does changing
the polarity of the connection affect the recording.
_______________________
Let the subject contract and relax the muscles in the arms, trunk and legs. Insert a
comment that describes the recording. How and why does the recording change?
_______________________
_______________________
6. Repeat number 4 with limb leads II and III. Note! Before you do it, discuss within
the group how you would expect the ECG reading to change in lead II and III. Use
the board to plot your assumptions and then compare.
Chest leads
You will examine the different chest leads using a Wilson electrode as a reference electrode.
The exploring chest lead will be a suction electrode that does not require electrode gel. The
exploring electrode is denounced “+” and the reference electrode “-“
7. Mark the chest leads on the subject with a pen (see Fig. 6). You can use a different
subject for this part.
Human Physiology
ECG
10
8. Connect left arm, right arm and left leg, which constitute the Wilson electrode (see
Fig. 3). Continue by connecting to the cable that is attached to the negative pole on the
bridge. Attach the suction electrode to the positive pole and place it on a chest lead
that gives a typical right ventricular lead (e.g. V1).
9. Press “START”. Insert a comment.
Record for 10-20 seconds without noise. Press “STOP”.
10. Move the chest lead to a typical left ventricular lead. Repeat #9.
11. Disconnect the subject.
12. Save your data by choosing “Save as” in the “Arkiv”-menu. Name the file with your
group number and date.
Analysis:
You should have data from lead I, II, and III, and from a right and left ventricular lead. The
next step is to analyze and interpret the collected information.
13. Scroll through the recording. Compare the right and left ventricular leads. Is there a
difference between the recordings? If yes, in which way and why?
_______________________
_______________________
_______________________
14. Compare lead I, II, and III in the same way.
_______________________
_______________________
_______________________
15. Mark two ECG-cycles in lead II and press “Zoom” (the small magnifying glass in the
toolbar). A new window containing the selected information will open. Identify the
following (and discuss the electrophysiological genesis):
P-wave
QRS-complex
T-wave
Human Physiology
ECG
11
Figure 11
16. To determine the amplitude of a wave the “Cross” (X) is placed on the top or bottom
of a wave. The time interval is achieved by using the marker (M) which is located in
the lower left corner. Place the marker on the recording. The time between the M and
X is measured and can be seen in the upper part of the window. You can for example
measure the duration of the P-wave by placing the M in the beginning of the wave and
the X where it ends.
Determine amplitudes and durations and fill in the table below:
Table 1.
Component
Duration (seconds)
Which information does the duration give?
P-wave
PQ-interval
(from the beginning of
P to the beginning of
Q)
QRS-complex
(from the beginning of
Q to the end of S)
QT-interval
(from the beginning of
Q to the end of T)
T-wave
Human Physiology
ECG
12
Zoom
Chart Window
17. Press “Chart Window” to leave the “Zoom” window. Double-click in the marker box
to reset the marker (see Using the computer software). Next, calculate the heart rate by
measuring the time between heart beats. Mark four ECG cycles and press “Zoom”.
Measure the time between two adjacent R-waves and enter in table 2. Repeat for two
other R-R intervals. Calculate the heart rate according to this formula:
60
Heart rate (beats per minut) 
R - R interval (sec)
Table 2
R-R interval (seconds)
Heart rate (beats per minute)
1
2
3
18. Is it possible to identify a P-wave, QRS-complex and T-wave in all leads?
________________________________________________________________
19. Press “STOP”. Disconnect the subject.
20. Discuss
21. Estimate the hearts electrical axis by comparing the different leads.
22. How do you calculate the electrical axis more accurately?
________________________________________________________________
23. Clean and wipe down the ECG-clamps.
24. Your group will receive a demonstration of a 12-lead ECG.
Human Physiology
ECG
13
EXAMPLE OF AN ECG OBTAINED WITH A SIX CHANNEL ECG RECORDER
Human Physiology
ECG
14