1.1 Basic Function of the Heart The heart has 4 chambers that act together to pump blood throughout the body. The 2 smaller upper chambers are called atria, and the larger lower chambers are called ventricles. The right atrium receives oxygen-depleted blood coming back from the body via 2 large veins: the superior vena cava and inferior vena cava. The right atrium pumps this blood into the right ventricle, which then pumps the blood into the lungs, which is oxygenated. The blood then comes back into the left atrium, which is then pumped into the left ventricle. The left ventricle then pumps the blood back to the circulatory system via the aorta, the largest artery in the body. The pressure that the left ventricle exerts to keep the blood moving throughout the whole body is the blood pressure. 1.2 Conduction System of the Heart The chambers of the heart pump with the automatic discharge of electricity from the sinoatrial (SA) node, a group of specialized cells in the right atrium. On average, there are 60 to 100 times discharges per minute. When the SA node discharges, both atria contract, and the electrical impulse is relayed to the atrioventricular (AV) node that is between the 2 ventricles. The electrical wave that is propagated at the AV node causes both ventricles to contract and pump blood. The normal delay between the atrium and ventricle contractions is 0.12 to 0.20 seconds. By studying the electrical activity that results when heart muscle cells contract, we gain insight to the health and workings of the heart. These electrical activities can be detected, recorded and studied with an electrocardiograph, for example ReadMyHeart or InstantCheck. 1.3 Common Causes of Abnormal ECG Tracings Abnormal ECG tracings may be caused by many different factors, including: • Coronary artery disease • Electrolyte imbalances in your blood (such as sodium or potassium) • Changes in your heart muscle • Injury from a heart attack • Healing process after heart surgery Please consult your physician if you are concerned about your ECG recording and symptoms to get more information about your condition.

1.4 Normal ECG Waveform Below figure is a presentation of a normal ECG waveform from Lead I.      HR: The number of heartbeat per unit time, usually in minutes.     QRS Interval : Ventricular depolarization. Regular duration is no longer than 0.1 seconds.     ST Segment : ST segment represents the period from the end of ventricular depolarizationto the                        beginning of ventricular repolarization.