ARTICLE DATA: Reviewer: Mian, Burhan Ahmed MBBS
Electrocardiography is a graphical recording of electrical potentials generated due to transmission of depolarization wave through heart and due to its spread into surrounding tissues and body surfaces.
Electrocardiogram is the graph obtained after performing the electrocardiography procedure.
MECHANISM OF SPREAD OF DEPOLARIZATION WAVE:
The ECG device detects and then amplifies the electrical changes occurring in the surrounding tissues and skin as the heart muscle depolarizes and repolarizes with each beat. At rest, the heart muscles have a positive potential outside the membrane and a negative potential inside the membrane. As the action potential spreads along the membrane, the charges reverse i.e. depolarization occurs which causes the heart muscles to contract. During each heart beat, the wave of depolarization travels in an orderly progression. At first, the SA node fires, the wave spreads to the atria, then passes through the intrinsic conduction pathways and then spreads all over the ventricles. This is detected by placing two electrodes on either side of the heart and the tiny rise and falls of the voltages can be visualized as a wavy line on a screen or a graph paper. This display can detect rhythm of the heart and any other weaknesses or abnormalities in different parts, if present.
PARTS OF ECG WAVE:
P wave: During normal atrial depolarization, the main electrical vector is directed from the SA node towards the AV node, and spreads from the right atrium to the left atrium. This turns into the P wave on the ECG. Its duration is 0.1seconds.
QRS complex: The QRS complex reflects the rapid depolarization of the right and left ventricles. They have a large muscle mass compared to the atria and so the QRS complex usually has a much larger amplitude than the P-wave. Its duration is 0.8-0.12seconds.
T wave: The T wave represents the repolarization (or recovery) of the ventricles. The interval from the beginning of the QRS complex to the apex of the T wave is referred to as the absolute refractory period. The last half of the T wave is referred to as the relative refractory period (or vulnerable period). Its duration is 0.27seconds.
U wave: The U wave is hypothesized to be caused by the repolarization of the interventricular septum. They normally have a low amplitude, and even more often completely absent. They always follow the T wave and also follow the same direction in amplitude. If they are too prominent we suspect hypokalemia, hypercalcemia or hyperthyroidism usually.
J wave: The J wave, elevated J-Point or Osborn Wave appears as a late delta wave following the QRS or as a small secondary R wave . It is considered pathognomonic of hypothermia or hypocalcaemia.
J-Point:The point at which the QRS complex finishes and the ST segment begins. Used to measure the degree of ST elevation or depression present.
An interval is the distance between two specific ECG events. For example:
R-R interval:The interval between an R wave and the next R wave . Normal resting heart rate is between 60 and 100 bpm. Its duration is 0.8seconds.
PR interval:The PR interval is measured from the beginning of the P wave to the beginning of the QRS complex. The PR interval reflects the time the electrical impulse takes to travel from the sinus node through the AV node and entering the ventricles. The PR interval is therefore a good estimate of AV node function. Its duration is 0.12-0.20seconds.
ST interval:The ST interval is measured from the J point to the end of the T wave. Its duration is 0.32seconds.
QT interval:The QT interval is measured from the beginning of the QRS complex to the end of the T wave. A prolonged QT interval is a risk factor for ventricular tachyarrhythmias and sudden death. It varies with heart rate and for clinical relevance requires a correction for this, giving the QTc. Its duration is <0.42seconds.
A segment is the length between two specific points on the ECG which are supposed to be at the baseline amplitude (not negative or positive). For example :
PR segment:The PR segment connects the P wave and the QRS complex. This coincides with the electrical conduction from the AV node to the bundle of His to the bundle branches and then to the Purkinje Fibers. This electrical activity does not produce a contraction directly and is merely traveling down towards the ventricles and this shows up flat on the ECG. The PR interval is more clinically relevant. Its duration is 0.5-0.12seconds.
ST segment:The ST segment connects the QRS complex and the T wave. The ST segment represents the period when the ventricles are depolarized. It is isoelectric. Its duration is 0.8-0.12seconds.
There are 12 leads on a standard ECG. Six of the leads of considered “limb leads” since they are placed on the arms and/or legs of the person. The other six leads are considered “precordial leads” since they are placed on the person’s chest (precordium).
RA:On the right arm, avoiding thick muscle.
LA:In the same location that RA was placed, but on the left arm.
RL:On the right leg, lateral calf muscle
LL:In the same location that RL was placed, but on the left leg.
V1:In the fourth intercostal space (between ribs 4 & 5) just to the right of the sternum (breastbone).
V2:In the fourth intercostal space (between ribs 4 & 5) just to the left of the sternum.
V3:Between leads V2 and V4.
V4:In the fifth intercostal space (between ribs 5 & 6) in the mid-clavicular line (the imaginary line that extends down from the midpoint of the clavicle (collarbone)).
V5:Horizontally even with V4, but in the anterior axillary line. (The anterior axillary line is the imaginary line that runs down from the point midway between the middle of the clavicle and the lateral end of the clavicle; the lateral end of the collarbone is the end closer to the arm.)
V6:Horizontally even with V4 and V5 in the midaxillary line. (The midaxillary line is the imaginary line that extends down from the middle of the patient’s armpit.)
- Lead I is the voltage between the (positive) left arm (LA) electrode and right arm (RA) electrode:
I = LA − RA.
- Lead II is the voltage between the (positive) left leg (LL) electrode and the right arm (RA) electrode:
II = LL − RA.
- Lead III is the voltage between the (positive) left leg (LL) electrode and the left arm (LA) electrode:
III = LL − LA.
In addition to the three bipolar limb leads described above, there are three augmented unipolar limb leads. These are termed unipolar leads because there is a single positive electrode that is referenced against a combination of the other limb electrodes. The positive electrodes for these augmented leads are located on the left arm (aVL), the right arm (aVR), and the left leg (aVF).In practice, these are the same electrodes used for leads I, II and III.
The Einthoven’s Triangle:
The Einthoven’s Triangle is drawn around the area of the heart. This illustrates that the two arms and the left leg form apices of the triangle surrounding the heart. The two arms form the upper part of the triangle and the lower apex is formed by the left leg.
The Einthoven’s Law states that if the electrical potentials of any two of the three bipolar limb leads are known at any given instant, the third one can be determined mathematically by simply summing the first two (but note that the positive and negative signs of the two different leads must be observed when making this summation).
Some pathological entities which can be seen on the ECG
Shortened QT interval: Hypercalcemia, some drugs, certain genetic abnormalities.
Prolonged QT interval: Hypocalcemia, some drugs, certain genetic abnormalities.
Flattened or inverted T waves: Coronary ischemia, hypokalemia, left ventricular hypertrophy, digoxin effect, some drugs.
Hyperacute T waves: Possibly the first manifestation of acute myocardial infarction, where T waves become more prominent, symmetrical, and pointed.
Prominent U waves: Hypokalemia.