How to read a normal ECG(Electrocardiogram)?
When the electric cardiac impulse passes through the heart, electrical current also spreads from the heart into the adjacent tissues surrounding the heart. A small portion of the current spreads all the way to the surface of the body. If the electrodes are placed on the skin on opposite sides of the heart, electrical potentials generated by the current can be recorded and this recording is known as electrocardiogram(ECG). Before we start, the reader must know about some of the terms: anatomy of the heart, depolarization, repolarization and ECG graphing paper. Here is a brief information about these.
The human heart has 4 chambers. Right/Left Atria and Right/Left Ventricles. Atria are involved in collection of blood and ventricles are involved in pumping blood out of the heart. In the right atrium there is SA node or Sinoatrial node which generates electrical impulses which start the heart beat, hence called as pacemaker of a normal heart. Heart is made up of cardiac muscles. The muscles of the atria and ventricles contract and relax independently.
A conductive cell like nerve cell or muscle fiber generally has negativity inside the cell and positivity outside the cell. When these cells are stimulated, the negativity of the the cell is lost and starts to become positive. This process is called depolarization. And repolarization simply means to return to the original polarity of the cell that is negative inside and positive outside. It must be noted that muscle contraction occurs after depolarization and muscle relaxes after repolarization.
ECG graphing paper is a simple graphing paper which has its horizontal axis marked with time and vertical axis marked with millivolts. Each second is denoted by 25 small squares or 25 mm in horizontal axis. And Each millivolt is denoted by 10 small squares or 10 mm vertically.
Now we are ready to know what are the elevations and depressions that are commonly seen in a normal ECG.
Well, the normal electrocardiogram is composed of a P wave, QRS complex and a T wave. QRS complex is often but not always three separate waves: the Q wave, R wave and the S wave. Now lets see what these waves in ECG mean. (It should be noted that, when we talk about ECG, we mostly talk about Left side of the heart which is related to collecting oxygenated blood from lungs and pumping it to the different parts of body.)
The P wave is caused by electrical potentials generated when the atria depolarize. Meaning, the atria contracts after the P wave is generated. During this the atria forces its blood to the ventricles. QRS complex is caused by potentials generated when the ventricles depolarize followed by its contraction forcing its blood to the systemic circulation to nourish the other body tissues. Therefore both the P wave and QRS complex are depolarization waves. However T wave is caused by potentials generated as the ventricles recover from the state of depolarization. T wave is known as repolarization wave. T wave is followed by the ventricle relaxation. During this state, the atria are filled with blood again to start the cycle again. The things that should be noted while examining an ECG are the presence of these waves, their position, rate, rhythm, voltage and intervals.
The voltage of QRS complex usually is 1 to 1.5 millivolts from the top of the R wave to the bottom of the S wave. The voltage of the P wave is 0.1 to 0.3 millivolts and of T wave is 0.2 to 0.3 millivolts. P-Q or P-R interval is about 4 small squares of 0.16 seconds, Q-T interval is about 8 to 10 small squares or 0.35 seconds.
There are so called Electrocardiographic leads that med techs put on our body while doing ECG. There are 3 bipolar limb leads also called standard bipolar limb leads: lead I, lead II and lead III. The highest elevations or QRS complexes are seen in lead II. It should be noted that, all the waves in all these three leads are upright. There are additional leads called Precordial leads or Chest leads which are 6 in number named from V1 to V6. It should be noted that V1 and V2 has inverted QRS complex. Another additional leads are called augmented unipolar limb leads namely aVR placed in right arm, aVL placed in left arm and aVF placed in left leg. It should be noted that aVR has inverted QRS complex just as in the case for V1 and V2.
These are all the things about a normal electrocardiograph. If there is missing waves, abnormally long or tall waves, non rhythmical waves or inverted waves in other leads than in aforementioned leads, then the ECG is not normal and the heart may be pathologic.