Electrocardiography is the process of non-invasive medical recording, showing the electrical activity of the heart over a period of time using electrodes placed on the patient's body. These electrodes detect electrical changes on the skin that arise from the heart muscle depolarizing during each heartbeat.
The heart contraction takes place due to an electrical stimulus. This normally derives from the sinoatrial node, and spreads to heart muscle cells through a special electrical conduction system. ECG is able to measure this with electrodes placed on the patient's limbs and surface of the chest. In this way the recorded electrical signals will draw the ECG wave, which is a regular curve with unique characteristics.
ECG -called also Einthoven-type diversion - was invented by Willem Einthoven (1860-1927), a Dutch physiologist. He was awarded with the Nobel Prize in Medicine for his discovery in 1924. His discovery and observation is correct even up to this day.
Twelve-lead ECG of a 26-year-old male
The purpose of ECG
The purpose of an ECG recording is to examine the spread of the heart’s electrical potential. The following parameters can be analyzed by ECG:
- heart rhythm
- the starting point of stimulus
- velocity of electrical conduction
- thickness of myocardium
- possible disorder in the circulation of myocardium
During ECG recording (you can learn more here about recording with WIWE) we can draw conclusions regarding the status of the heart by examining its electrical activity. Electrical signs are gathered together by the electrodes placed on the limbs and precodial area. In case of the 12 lead ECG there are 4 limb electrodes and 6 precodial electrodes. The ECG produces 6 leads (I, II, III, aVF, aVR, aVL) with the information coming from the 4 limb electrodes, while the 6 precardial electrodes are equivalent to the other 6 leads (V1, V2, V3, V4, V5, V6). The purpose of different leads is to show the heart from different sides. The 12 leads provide us a reasonable number of directions, which lets us have an overall picture of our heart. For research purposes applying 100 or even more leads can be reasonable.
The waves recorded by ECG, namely P; Q; R; S; T; and U are integrated international notations. Each wave defines the depolarization or repolarization of the different parts of the heart during its functioning.
- P-wave (atrial wave): The p-wave represents depolarization of the atria. It has positive amplitude of 1-2 mm. Its duration is 0,06-0,11 sec.
P-R Interval: This interval reflects the time the electrical impulse takes to get from the sinus node through the AV node. Its duration is 0,04-0,1 sec.
QRS complex: The QRS complex represents the rapid depolarization of the right and left ventricles. The complex consists of a negative Q-wave, which is not always recognizable, a high positive R-wave, which is the electrical polarization of muscle’s main mass (its amplitude is 10 mm) and a negative S-wave. Depolarization of Ventricles’s whole muscle takes place during the running of QRS complex. Its total duration is 0,06-0,1 sec of which 0,03 is the depolarization is interventricularis septum, 0,055 sec is the depolarization of the Right Ventrical and 0,068 sec is the depolarization of the Left Ventricle.
ST interval:it represents the period during which the ventricles are repolarized.
T-wave: a long-drawn wave of middle amplitude, shows the total repolarization of ventricles, its duration is 0,2 sec.
Q-T interval: the total duration of depolarization and repolarization in the ventricle’s muscle.
U-wave: The U wave is hypothesized to be caused by the repolarization of the interventricular septum or the ventricles. Its duration is 0,1-0,2 sec.
Heart rhythm, sinus rhythm
We need to examine the number of heart beats in case of a chosen lead to be able to identify the heart beats securely. This is usually the II. lead (WIWE uses lead I, you can learn more here). To discover arrhythmias we need at least 12 or more sequential cardiac cycles.
Normal heart rate is called sinus rhythm, during which the sinusnode acts as a natural pacemaker.
The parameters of sinus rhythm
- 60-100 heart beats per minute
- P -wave has a positive amplitude in the II. lead, and negative in the VR
- QRS complex comes after the P –wave every time