(X.) Electrocardiography
Dep. of Physiology, Fac. of Med., MU, 2015 © M. Bébarová, T. Stračina
Electrocardiography (ECG)
Definition:
Recording of changes of electric potential differences arisen from cardiac
electric activity by electrodes placed on patient´s body surface.
An electric potential is the amount of electric potential energy that a unitary
point electric charge would have if located at any point in space, and is equal to
the work done by an electric field in carrying a unit positive charge from infinity
to that point
Voltage is electric potential difference - the difference in electric potential energy
between two points per unit electric charge. It is measured in units of volts (V)
Conduction system of the heart
Cardiac muscle automaticity
Ganong´s Review of Medical Physiology, 23rd edition
ECG signal
• Summation of electric potential changes
• Cardiac vector
ECG lead
Standard 12-leads ECG
ECG lead - conductive connection of 2 electrodes; record of electric
potential differences between 2 electrodes in course of time
Classification of ECG leads:
• According to the position (and in agreement with historical development)
• Standard limb leads: I, II, III
• Augmented limb leads: aVR, aVL, aVF
• Chest leads: V1-V6
• According to polarity
• Unipolar: aVR, aVL, aVF, V1-V6
• Bipolar: I, II, III
• Standard 12-leads ECG
• Modified position of electrodes
• 24-hours ECG monitoring
• = ECG Holter monitoring
• ECG telemetric card
• Implantable ECG monitor
Possibilities of ECG recording
• Convention of electrodes position
• Examined person lie on his/her
back, breathe freely
Rules for ECG recording
Electrocardiogram
Time [s]
• Voltage changes in course of
time
• Y axis: voltage [mV]
• X axis: time [s]
• Speed of paper feeding:
• 25 mm/s: 1 mm = 0.04 s = 40 ms
• 50 mm/s: 1 mm = 0.02 s = 20 ms
Evaluation of ECG
1. Rhythm
• Regularity: regular/irregular
• Origin: sinus (SA node) / other
2. Heart rate – according to RR interval duration
3. Duration of (in ms) P wave, PQ interval, QRS complex
4. Changes in ST: isoelectric / elevation / depression
5. QT interval length (in ms)
• Bazett correction: QTc = QT / RR
6. Transitional zone (rS to Rs) in leads V1-V6
7. Electrical axis of the heart (cardiac vector)
Electrical axis of the heart
R L
F
Application of ECG in diagnostics of
arrhythmias
A B C D E
Application of ECG in diagnostics of
myocardial infarction (ischemia of the heart)
Application of ECG in diagnostics of
hyperkalemia