Electrocardiography

Electrocardiography (ECG) is the basic examination method in cardiology. Its principle is the monitoring of the heart's electrical activity and electrocardiogram (ECG waveform curves) allows its evaluation. ECG examinations are mostly non-invasive. Using electrodes placed on the skin, but also on the esophagus wall or right in the heart, we measure the voltage difference as a manifestation of myocardial action potential spreading.

Since the electrical activity of the heart is substantial for its mechanical activity, ECG has an important diagnostic role in a number of heart diseases. In addition, we can detect disturbances of extra-cardiac causes (eg. thyroid dysfunction, ionic dysbalance). Despite that, the ECG examination itself can't be used individually as a diagnostic method and it is always important to assess the overall clinical picture.

Indications
ECG is indicated if there is suspicion on:
 * heat disease: heart attack, arrhythmias, conduction disturbances, myocardial ischemia;
 * metabolic disease: hypocalcemia or hypercalcemia, hypokalemia or hyperkalemia;
 * endocrine disease: diseases of the thyroid gland (hypothyroidism and hyperthyroidism).

Principle
During the proliferation of myocardial action potential, local electricity currents arise in areas of different potential interfaces, resulting in an electromagnetic field. Body fluids work as good conductors which allows us to detect changes in the cardiac potential even on the surface of the body.

W. Einthoven (1860-1927)

The capture of these potentials was introduced by the Dutch physiologist Willem Einthoven in the early 20th century. It was three standard bipolar limbs (I, II, III), used still today, forming the so-called Einthoven triangle, in which the heart lies in the imaginary centroid. The principle of these leads is the connection of two active electrodes whose polarity is predetermined. The leads then record the potential difference between the electrodes and indicate the resulting amplitude. The vector sum of all three amplitudes of these limbs is equal to zero (Einthoven's law).

Today ECG measurement is improved by adding additional leads. They are unipolar leads, which are created by connecting the active electrode with an indifferent electrode - a Wilson's central terminal, which when connected to the resistors should have a permanently zero value. This is how we record the actual magnitude of the potential. Thus, we obtain additional limb (VR, VL, VF) and thoracic (V1-V6) ducts. Recording from limb unipolar leads can be enhanced by disconnecting the active electrode from the zero terminal, then measuring the potential between the disconnected and the two remaining electrodes. This way we get so-called augmented semi-unipolar Goldberger limb leads (aVR, aVL, aVF). Therefore the most common ECG paper has currently 12-lead. Under special circumstances, other leads may be used.



ECG curve
The spread of myocardial depolarization is different, depending on which lead is sensing the potential, that is, in which direction and in which plane it is located relative to the heart. The path of propagation of the potential in the heart has a typical character and produces typical deflections - waves, oscillations and lines that correspond to a certain phase of the electrical heart cycle. Myocardial action potential normally arises from spontaneous depolarization in the sinoatrial node (SA), from where it spreads to the atrial muscles. The instantaneous final vector has a relatively small amplitude due to the thin wall of the atrium, and is pointing left down. In the ECG record it appears to be the P wave. Slow conduction in the atrioventricular node (AV) slows down the propagation of depolarization from the atria to the ventricles to separate the atrial systole from the ventricular systole. In the ECG graph it is visible as an isoelectric line. Total depolarisation spreading from the atria to the ventricles signifies the PQ interval.
 * P wave - depolarization of the atrium,
 * PQ interval - depolarisation spreading from the atria to the chambers,
 * QRS complex - ventricular depolarization,
 * Q = negative wave preceeding the R wave,
 * R = any positive wave of the QRS complex,
 * S = negative wave following the R wave,
 * T wave - repolarization of chambers,
 * U wave - not constant, its origin is not clear.

This is followed by depolarization of the chambers, forming the QRS complex in the ECG record. The impuls progresses through Hiss bundle and Tawar's arm to the muscles of the interventral septum, where depolarization spreads from left to right, thus the instantaneous vector directs right down. In the ECG is either a negative Q, or a positive R wave, depending on the lead. In the next step the excitation spreads to the heart apex, the instantaneous vector points to the left down direction, forming the central part of the QRS complex, in most leads it is the R wave. From there, depolarization spreads through the Purkyne fibers to the working myocardium of the two ventricles, from the endocard to the epicard. In the instantaneous vector the left ventricle's thick muscularity plays a role, so the vector points to the left. And in the end it points in the up and left direction when depolarizing the basal portion of the left ventricle, while completing the ventricular QRS complex.

After the depolarization is complete, the electrical heart activity is zero for a while, the muscular fibers are in the plateau phase, when no electrical currents flow through the myocard. In the ECG record will be shown as an isoelectric line - ST segment. After this moment, ventricular repolarization begins, which goes backwards from the epicardium to the endocardium. However, the resulting final vector is the same as for depolarization, because repolarization is an electrically opposite plot. Thus, a wave T is formed. Sometimes a U wave can be recorded on the ECG record, the origin of which is not known precisely, it is probably the manifestation of papillary muscles repolarization.

Electrocardiogram
The ECG is captured on millimeter paper. To correctly subtract the values, we must have: The ECG record evaluation consists of ten points: Evaluation of ECG records allows us to detect arrhythmias, as a sign of malfunctions of electric impluse creation or propagation. It also plays an important role in the detection of ischemic changes, localization and stage of myocardial infarction. Changes in the ECG are found either in all leads, or in just one, or the group of leads that are related to the anatomy of the heart. For example the locus of myocardial infarction (IM) can be determined by the knowledge of its vascular supply and the projection of the heart sections on the surface of the body in a relation to the direction of recording of individual leads.
 * reference puls 1mV (corresponding to amplitude), usually = 10 mm,
 * paper feed rate of either 50 mm / s (1 mm → 0.02 s) or 25 mm/s (1 mm → 0.04 s)12leadECG.jpg
 * 1) cardiac action
 * 2) heart rhythm
 * 3) heart rate
 * 4) wave P
 * 5) PQ interval
 * 6) QRS complex
 * 7) ST segment
 * 8) wave T
 * 9) QT interval
 * 10) axis of the heart

Related Articles

 * Heart Autorhythmicity
 * ECG Abnormalities