• A diagnostic method recording the electrical activity of the heart (especially in cardiology)
• Allows to determine:
  • arrhythmia,
  • ischemic changes in the myocardium,
  • control of the effect of cardio-pharmaceuticals, etc.
• The recording is made by an electrocardiograph.
• The resulting waveform is called an electrocardiogram.

Heart 

• It consist of four parts:
  • right atrium and ventricle,
  • left atrium and ventricle.
• It is formed by striated muscle fibers.
• It is not controlled by will.
• Sophisticated double-acting pump

• Contractions of the atria and then the ventricles gradually occur.
• Unoxygenated blood travels to the right atrium and then from the chamber to the lungs, where it is oxygenated and returns to the left atrium, from where it is pumped throughout the body.
• The correct function is ensured by the heart's transmission system.
• It is formed by specially adapted cells of the myocardium.
• Sinoatrial node 
• Atrioventricular node
• His bundle
• Tawara branches 
• Purkinje fibers
• These cells also periodically create irritation.

Electrocardiographic leads

• The electric field created in the heart spreads through the conductive environment of the entire body and can therefore be sensed even from its surface.
• A twelve-lead system is used in clinical practice.
• We divide leads into three groups:
  • Einthoven bipolar limbs (I, II, III)
  • Goldberger unipolar limbs (aVR, aVL, aVF)
  • Wilson unipolar thoracic (V1, V2, … , V6)
• These leads are already standardized and have a prescribed exact location on the body.
• Ag/AgCl electrodes are commonly used. 

Bipolar limb leads

• Called Einthoven's (I, II, III)
• There are three electrodes, located on the limbs:
  • right arm (marked RA),
  • left arm (marked LA),
  • left leg (marked LL).
• Each lead is marked with Roman numerals and corresponds to the voltage between two electrodes.
  • LA – RA lead I,
  • LL – RA lead II,
  • LL – LA lead III.

• Called Goldberger's (aVR, aVL, aVF)
• There are three electrodes, placed on the limbs as in the standard Einthoven connection.
• Each lead is connected by a positive terminal to the corresponding electrode.
• aVR (positive terminal connected to RA),
• aVL (positive terminal connected to RL),
• aVF (positive terminal connected to LL).
• The remaining electrodes are connected through a 5kΩ resistor to the reference terminal

Originally these leads were designed by Wilson, who used the connection of all three leads as a reference terminal, Goldberger modified this connection by disconnecting the lead being measured and got an increased voltage (aV).

• Electrode positions can be changed
• In intensive care, sensing electrodes are placed on the chest
  • Skin conductivity is sufficient, minimal deviations between positions (limbs vs. chest) 
  • The advantage is:
    • minimizing motion artifacts
    • Free limbs for NIBP and IV access

Unipolar thoracic leads

• Called Wilson's (V1, V2, V3, V4, V5, V6)
• There are six electrodes, located on the chest.
• Each lead is connected by a positive terminal to the corresponding electrode.
• The reference terminal (Wilson central terminal) is created by connecting the limb electrodes across the resistor.
• The placement of the electrodes is governed by the position of the sternum, where two electrodes are placed. Others are then placed in the intercostal space and go around the chest to the mid-axial line. 

• Due to the position of the limb electrodes, the heart can be idealized and considered as an elementary electrical dipole.

• The isoelectric line is expressed as a straight line on the electrocardiogram, occurring between cardiac cycles and also within individual cycles between their elements.
• Circular deviation above the isoelectric line, or below it, is called positive,
  or negative wave.
• Steep deviation above or below the isoelectric line is called oscillation.
• Several consecutive oscillations are denoted as a complex.
• An interval is the time between two elements usually involving one of them.
• A segment indicates the time between two elements…

• Oscillations 
  • Q,R and S indicate ventricular depolarization, usually assessed as a whole – complex. 
  • In the ECG we have only one complex and that is the QRS.
• Waves 
  • P indicates atrial depolarization, it is positive in most leads. It manifests itself most prominently in the II. It is negative only in aVR. It reaches 0.1 – 0.15 mV and lasts < 100ms.
  • T is one of the most unstable parts of the ECG. It has a deviation equal to the main QRS deviation and reaches 1/8 to 2/3 of the R amplitude. It represents repolarization of the ventricles. 
  • U reaches a very small amplitude. It corresponds to the repolarization of some parts of the transmission system.
• Intervals
  • In the ECG, we often observe the PQ, QT a RR (PP) intervals
  • The PQ interval will reveal any problems with conduction between the atria and the ventricles.
  • The QT interval includes the Q oscillation as well as the T wave, therefore its change is caused by a number of physiological and pathological influences.
  • The RR interval is the most frequently measured, as it corresponds to the heart rate.
• Segments
  • We most often measure the ST and PQ segments.
  • The ST segment normally corresponds to the isoelectric line, e.g. a myocardial infarction is indicated by a significant deviation.

Electrocardiograph

• Exclusively a 12-lead ECG recording is used for diagnostic purposes.
• For simple basic monitoring in clinical practice, only limb sensing - lead reduction is often sufficient.
• The output parameters of the ECG device are also standardized:
  • one mV at the input corresponds to 10mm on paper,
  • paper advance is 25mm/s or 50mm/s.
• Frequency ranges:
  • for diagnostic purposes: 0.01 – 100/150 Hz,
  • for monitoring purposes: 0.5 – 30/50 Hz.

• With the correct position of the electrodes (A), the monitor recognizes the R oscillation based on the amplitude (local maximum)
• In the case of a non-standard position of the ECG electrodes (B), the signal is distorted - here the emphasis is on the R amplitude
  • The algorithm detects a new local maximum, the monitor displays 2 times the heart rate (HR) than it is in reality

Regular inspection of the ECG 


During regular ECG inspection, it is necessary:

• Visual check of the ECG cable
  • If the insulation is damaged, it must be replaced
• Measure leakage currents within the device, or through the attachment part (ECG leads)
  • Danger of electric shock
• Use of an ECG simulator
• The inspection is conducted min. 1x/year by a technician/biomedical engineer