DEPARTMENT OF BIOPHYSICS

Charles University in Prague, 2nd Faculty of Medicine

Project 2
ELECTROCARDIOGRAPHY

PRACTICAL TASKS
Task 1. Electrocardiography
Measure and record the 12-lead ECG (for one member of the group).
According to the instruction, measure and evaluate the recorded signal and draw the
electric axis of the heart.
Task 2. Heart rate variability
Examine and evaluate the heart rate variability.

THEORETICAL INTRODUCTION

ELECTROCARDIOGRAPHY
Electrocardiography (ECG) is the standard noninvasive method of functional
examination of the electrical activity of the myocardium. In contrast with the CNS our heart
displays a much greater synchronicity and periodicity. The signal propagates from the
myocardium relatively freely in all directions without being substantially weakened. The ECG
signal can thus be registered at a relatively high amplitude (units or tens of mV) at practically
any place of the body surface.
Origin and course of the ECG signal. The impulse for contraction of the myocardium
originates in the sinoatrial (SA) node in the area of the right auricle whence it propagates
farther. The primary signal is so weak that in a normal ECG it would hardly be registered. The
first wave of the ECG record that can be seen is the P-wave which reflects depolarization of
the auricles, i.e. their beginning contraction. Repolarization of the auricles cannot be
recognized in the ECG record as the corresponding signal is screened off by a much greater
signal proceeding from the depolarization of ventricles; this signal is characterized by a
complex of QRS waves. The next T-wave reflects the subsequent repolarization of the
ventricles. It is beyond the curriculum of the first year of biophysics to study in detail the
interpretation, physiology or pathophysiology of ECG and hence we restrict ourselves to a
brief description.

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 ECG waves, intervals and segments.
QRS complex form by QRS waves
PR interval: from the beginning of the P wave
to the beginning of the QRS complex.
PR segment: part of the isoelectric line from the
beginning of the P wave to the beginning of the
QRS complex
ST segment: part of the isoelectric line from the
end of the QRS complex to the beginning of the
T wave
QT interval: from the beginning of the QRS
complex to the end of the T wave
length of the heart period (heart rate) T:
the distance between two subsequent sharp R
waves – R-R interval

Einthoven’s (bipolar) leads. Electrocardiography as a clinical method was introduced

in 1906 by the Dutch physician Einthoven. He recorded the ECG signal of a patient by a
string galvanometer between the arms because of the ease of attaching the electrodes on the
wrists. The measured signal then corresponds to the potential difference between the two
electrodes, hence we have to do with a bipolar connection. In the right hand it is labeled R
(usually red), in the left hand L (usually yellow) and then the L-R signal is described as the
1st Einthoven lead. Later on another electrode was attached near the left ankle (F, green) so
that the F-R potential difference could be read (2nd Einthoven lead) as well as the F-L one
(3rd Einthoven lead). The N electrode (neutral, black) is not included in the scanning and
serves merely for grounding. However, it should not be omitted as then the measurement
could be affected by various disturbances and the sensitive input amplifiers could be
damaged.

Einthoven's leads.
I. = L – R
II. = F – R
III. = F – L

Heart-axis vector. The summation potential of all myocardium cells forms a kind of
electric dipole in space which changes its size and direction during a heartbeat. This virtual
vector is called the electric heart axis vector. As it changes its size and direction will differ at
moments when different waves of the ECG record reach their maximum. The largest and most
important of these is the direction of the electric heart axis vector R.

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 Heart-axis vector (mean electrical vector).

Einthoven’s triangle. If we now depict the bipolarly connected three Einthoven leads as
sides of an equilateral (so-called Einthoven) triangle with electrodes R, L and F at its apexes,
we obtain a coordinate system of three axes at 60° angles to each other into which the heart
axis vector is projected. Depending on the polarity and size of the individual waves of the
ECG record in the individual leads we can compute or estimate the rotation of the heart axis
vector. Thus for example, if the R-wave is highest in the 2nd lead we can estimate that the
electric heart axis vector will lie approximately in the direction of the side of the Einthoven
triangle which represents the 2nd lead, i.e. in the direction right-down (when looking at the
patient). This is the normal usual slope of the electric heart axis. The direction horizontal-right
indicates 0° and the angular degrees are measured from this direction clockwise so that the
direction of the 2nd channel corresponds to an inclination of the heart axis of +60°.
Deviations from the norm are called rotation of the heart axis to the right or to the left.

Einthoven's triangle.

Goldberg’s (unipolar) leads. For better differentiation of nuances the Einthoven leads
were later supplemented with other directions: Connecting the limb electrodes over resistors
of the same size generated a virtual center (the so-called Wilson clamp) into which reference
inputs from three other differential amplifiers were connected. The vectors of the new
coordinate axes that were formed in this way can be visualized as arrows leading from the
center of the equilateral Einthoven triangle toward its apexes that represent the R, L and F
electrodes; the newly formed leads were named VR, VL and VF. At that time in history
electronic amplifiers were not yet used so that it was a drawback that the VR, VL and VF
medians were shorter than the triangle sides so that the signal obtained was low. An
improvement was obtained by a connection in which there awas no central point in the
triangle for all electrodes but for every reference point there was a point from two resistors
connecting the remaining electrodes. Geometrically this means that the vector arrows do not
start from the center (of gravity) of the triangle but from the centers of the opposite sides;

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these are then not medians but heights of the triangle; their direction is the same but their
length, and hence the size of the recorded signal is by 50 % higher – therefore, they are
marked with small a for augmented, i.e. lengthened. For this reason we label the
corresponding leads as aVR, aVL and aVF. They are called Goldberg’s leads and, in contrast
with the Einthoven bipolar leads, where every lead represents the potential difference between
two electrodes, we are dealing here with unipolar leads where each lead represents the
potential of a single electrode.

Godberg's leads.

Standard limb leads. By supplementing the Einthoven bipolar leads I, II and III with
Goldberg´s unipolar leads aVR, aVL and avF we obtain a total of six axes turned at 30°
angles, into which the vector of the electric heart axis can be projected. In view of the fact that
all six leads are derived from the potential of the three limb electrodes they are called six
standard limb leads. The plane in which the corresponding coordinate axes lie is roughly
parallel with the plane of the bed on which the examine patient lies on his back.

Hexaaxial system.

Chest leads. In the course of time the need arose to examine the movement of the
electric heart vector in space which means that the electrodes had to be placed in a plane
possibly vertical to this plane. This was achieved with the aid of six electrodes V1–V6 placed
on the patient´s chest so that electrodes V1 and V2 would be placed between the fourth and
fifth rib to the right and to the left of the sternum, farther to the left then electrode V3 and
further equidistantly placed electrodes V4, V5 and V6 between the fifth and sixth rib: V4 in a
line running through the center of left clavicle, V5 in a line passing through the frontal fold of
the arm-pit and finally V6 in a line below the center of the arm-pit.

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 Chest leads.

Standard 12-lead ECG consists of:

 3 bipolar limb Einthoven's leads I, II, III
 3 unipolar limb augmented leads aVR, aVL, aVF
 6 unipolar precordial leads

12-lead ECG.

HEART RATE VARIABILITY

Heart rate. Even simpler than the examination of the ECG curve in all twelve (six limb
and six chest) leads with the determination of the tilt of the heart axis and some other
parameters is the determination of heartbeat (frequency). We need a single lead on which the
QRS complexes will be easily discernible. The distance between spiked R-waves
corresponding to two subsequent heart pulses is labeled R-R interval. The heart frequency (in
Hz) is then the reciprocal value of the R-R interval. By multiplying with 60 we obtain the
number of pulses per minute.
HRV, baroreflex. The unused ECG channel can be used for recording some other
biosignal, such as breathing frequency. Careful examination of both concurrent biosignals
makes it possible to determine how the heartbeat changes in dependence on the respiration
phase of the patient. This dependence is physiological through periodic stimulation of
vegetative nerves that regulate the heart frequency during the respiratory cycle. Examination
of this dependence provides important information about its correct function. It has been
shown, for example, that blocking of this function during extreme prolonged exertion of
sportsmen (e.g. football players) may lead even to death which was difficult to identify until
recently. Regular examination of the heart variability of top sportsmen should now be rather
obligatory.

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PROCEDURE

Before starting the examination, fill in the report front-page – especially the part
characterizing examined person: among others a brief personal (OA) and familiar (RA)
anamnesis.

You will work with the BTL-08 ECG apparatus.

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Task 1. Electrocardiography
Measure and record the 12-lead ECG. (at least for one member of the group).
According to the instruction, measure and evaluate the recorded signal and draw
the electric axis of the heart.

Working procedure
1. Run the Ecg.exe file (e.g. in C:\BTL08Win directory).

2. Open Folders menu ("Kartotéka") by clicking it in the menu or using F3 key.

3. Open the folder of your study group.

4. Click on New patient icon ("Nový pacient") and fill in the data of examined person in
particular group directory.

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 In the field ID the birth number should be entered. In case we don't want to state the
number and the control algorithm reports the invalidity, we can switch off this item.
Or enter a not controllable birth number e.g.. 010101/999.
In the field Comment fill in basic information that can influence heart examination:
smoking, alcohol, sport, cholesterol, family anamnesis, BMI, medication, RA: Familiar
anamnesis OA: Personal anamnesis.

5. Disentangle electrode cables and place them sidewise and connect the banana plug and
electrodes.
 limb electrodes R, F, L with tongs
 chest electrodes C1–C6 with bulbs

6. Examined person lies down. Prepare the electrodes and place/attach them on the patient.
Attach limb electrodes on moistened limbs:
 R – red - right arm (wrist)
 L – yellow - left arm (wrist)
 F – green - left leg (ankle)

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  N – black - right leg (ankle)

7. Chest electrodes V1–V6 (labeled C1– C6) with bulbs should be placed according to
Figure. Handle the electrodes carefully!
 V1 – red - 4th intercostal to the right of sternum
 V2 – yellow - 4th intercostal space to the left of sternum
 V3 – green - between V2 and V4
 V4 – brown - 5th intercostal in MDCL line
 V5 – black - at height of V4 in the front axillary line, evenly between V4 and V6
 V6 – violet - at height of V4 in the middle axillary line

 check regular electrode placement along a smooth line and, if necessary, correct the
electrode position
8. If not using chest leads, banana plugs are not connected with electrodes. Cross-connect
them and connect to ground electrode (N).
9. Check that all the electrodes are attached and no connectors are left unplugged.

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 Turn on the ECG module (on/off button).

In case we have forgotten to start the unit, notification External Unit is NOT connected
will appear.
In case the unit is on, there is a cabling error. We do not re-connect cables, we call for
the assistant.
For not connected unit, the notification disappears and the DEMO program is
automatically started. We can confuse it with the patient examination.

10. Using F6 key or mouse start the New standard ECG examination (Nový standardní
EKG náběr). Follow the ECG signal on the screen. Wait until the signal stabilizes.

11. Check (or set) parameters using mouse or keys in the right part of the window:
 F1: Help
 F3: Number and apportionment of leads
 1×12: 12 leads consecutively
 2× 6: 12 leads, limb on the right hand side, chest the left hand side
(most sutable)
 1× 6. 6 leads (limb or chest)

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  F4: Time base (posuv) = speed usually 25 mm/s, can be increased to 50 mm/s
 F5: Amplitude (zesílení) = amplitude measure: usually 10 mm/mV, d can be set up and
down.
 F6 – Filter (filtr) – 50 Hz
 F7 – Time const. (časová konstanta) = time constant: choose as long as possible (3.2
s)
 In case of „floating" record set down.
12. After signal stabilizing, save 10s period by pressing the Enter key. After a while repeat
the signal recording two more times.
13. Terminate signal recording by Esc key.
14. Take a brief view of recorded ECG signals, in case of necessity repeat the measurement.

15. Switch off the unit by on/off button (press longer).

16. Remove the electrodes, "disconnect" the examined person; if the same person will be
examined for heart rate variability - task 2, remove only the chest electrodes V1–V6 and
interconnect their connectors to one node
17. Print out the three recorded signals, different 10 s record will be processed by each team
member .

 use arrow keys to move within the records, individual recorded signals are separated
by thick vertical lines
 the record position is given on the below (3.41 s (2 – 00:17 means, that the beginning
of the particular curve lies 3,41 s from the second 10s examination and the record
was started 0 min 15 s after the first one

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  Printing: key F2, parameter setting:
 Gird (mřížka): normal
 Time base (posuv): 25 mm/s
 Amplitude (amplituda): set appropriately (large enough not overlaping
curves), usually 10 mm/mV
 Print leads (tisknout svody): all recorded (usualyy all)
 Wave length (délka záznamu): 5s
 Diagnosis (diagnóza): automatic diagnosis (intervals); approved
diagnosos (nothing – empty)
 Image export: not important (no record will be exporteg)
 Description of Diagnosis and Detail expect medical evaluation.
Otherwise, the Automatic diagnosis is implausible – do not use.
 Print preview: F2: (magnification can be checked)
 Printing: Enter

Each group member prints own record and works autonomly (repeat item 17).

Report elaboration (after finishing task 2)

1. Choose one lead (best readable one) for further evaluation
2. Measure individual requested intervals and amplitudes in the printed record, fill in the
appropriate report section; please respect given units
3. Calculate average R-R interval and heart-rate (frequency) in both required units
4. Draw the lengths of amplitudes of QRS vectors to the scheme of Einthoven triangle;
using these projections in individual leads restore the vector of the electric axis of the
heart (example given in Figure below).
5. Evaluate amplitude [mV] and slope [º] of the axis vector from the drawing, note them
down.
6. Compare the results to those of automatic evaluation and discuss the possible
differences and the method itself.
7. Conclusion should be brief and accurate.

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 Example of electric axis of the heart restoration.

Task 2: Heart rate variability

Examine and evaluate the heart rate variability.

Working procedure
regulated breathing-frequency method:

1. Follow instruction of the task 1 – items 1–11 with slight modifications:

o Run the Ecg.exe file (e.g. in C:\BTL08Win directory).
The program can be started from Admin profile only (not from the student one). If
the computer not running, ask the teacher to do it.
o Open Folders menu ("Kartotéka") by clicking it in the menu or using F3 key.
o Open the folder of your study group.
o Click on New patient icon ("Nový pacient") and fill in the data of examined person
in particular group directory.
o Examined person lies down. Prepare the electrodes and place/attach them on the
patient. Attach limb electrodes on moistened limbs.
 R – red - right arm (wrist)
 L – yellow - left arm (wrist)
 F – green - left leg (ankle)
 N – black - right leg (ankle)
Chest electrodes are not used in this experiment.
o Check that all the electrodes are attached and no connectors are left unplugged.
Turn on the ECG module (on/off button)
2. Using F6 key or mouse double-click start the New standard ECG examination (Nový
standardní EKG náběr) in order to check the patients ECG. Wait until the signal
stabilizes. Check signals in individual leads and choose the one with most pronounced
R-waves. Note the chosen lead down as the "Einthoven lead used".
Terminate signal recording by Esc key.
3. Using F6 key or double-click on New Long ECG examination (Nový long EKG náběr)
start the "long" ECG signal recording.

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4. Use (check in) only the chosen lead for "long record". Set the recording period to 3
minutes and click OK. In the right screen section, parameters of the recording can be set.

5. Examined person stays in quiet, breathes deeply and slowly. The aim of "guided
breathing" is to be subsequently able to assign individual parts of ECG record to
individual phases of respiration cycle.

 Supervise the signal while recording, try to avoid any noises or other interferences.
 If signal is stabilized, start the recording:

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 Examined person breathes regularly in 5 seconds intervals – starting with 5 s
inhalation simultaneously with starting of the long ECG recording (Enter key),
then exhales for 5 s etc. Throughout the 3 minutes of recording the ECG signal 5
seconds lasting inhalation is followed by 5 s exhalation. Breathing can be guided
either by examiners or by examined person him/herself (checking the wall clock).
Remaining time of recording is displayed in lower right part of the screen, the
heart symbol is red.
6. Examined person breathes deeply, in case of thickness interrup the examination and
inform the teacher. Do not examine epileptic people.
7. Terminate signal recording by Esc key.
8. Switch off the unit by on/off button (press longer) and remove the electrodes –
disconnect the examined person.
9. Take a brief view of recorded ECG signals, in case of necessity repeat the measurement.
10. Print out the three recorded signals, different 10 s record will be processed by each team
member.

 Use arrow keys to move within the records, individual recorded signals are separated
by thick vertical lines.
 Printing parameters are similar with task 1.
 Print out the record minute by minute (one minute recording can be printed on one
paper sheet), thus, each team member will process different part of the recording.

Report elaboration
Elaborates one minute of the record.
1. Fill in the requested information in the report according to the obtained ECG record.
2. Calculate average heart-rate (frequency) from the R-R period in the processed part of
the record (1 min).
3. Correlate the ECG record with known respiration history
4. For each 5 s lasting inhalation and exhalation interval (section) measure the distance of
boundary (most distant) R-waves in the interval (section), count the number of R-R
periods in the examined interval (section), calculate average length of R-R interval in
each section and then evaluate average frequency during inhalation and exhalation
phase.
5. Compare the calculated results with the automatic evaluation.
6. Evaluate and discuss the results and the method itself, conclude.

References

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Amler E. et al. Chapters from Biophysics. Published by the Institute of Biophysics,
Charles University in Prague, 2nd Faculty of Medicine, Praha, 2006

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