Electrocardiogram (ECG or EKG) is a non-invasive test to gauge the heart's condition. It records and shows the electrical signal generated by a heart when it pumps. The electrical signal synchronizes all the pumping acivities during a heart beat. ECG is useful for detecting certain types of conditions and because of its low cost, it is the most common test for screening heart diseases.

ecg pqrst waves

A heart consists of two pumps (right and left) and each pump has two chambers (upper and lower). The upper chamber is call atrium and the lower chamber is called ventricle. The right pump circulates blood from other parts of the body to the lung and the left pump circulates blood from the lung to the rest of the body.

Each ECG cycle has 5 waves - PQRST. The P wave corresponds to the atrial depolalrization and the pumping of blood from the atrium to the ventricle. The QRS complex corresponds to the depolarization of blood and the pumping of blood out from the ventricle to the body and lung. The T wave corresponds to the repolarization of the ventricle and the recovery of the ventricle for next cycle.

The following diagram is from Electrocardiogram and shows the electrical originated from upper left artium (sinoatrial or SA node), travels down to the boundary between the atrium and ventricle (atrioventricular or AV node) and then to the entire venticle.

ECG and heart beat

From this simple description, one can infer that:

  • The P wave is generated by the atrium and any abnormal P wave (inverted, too high, too long, missing) is normally associated with the atrium. Missing P wave is a telltale sign for atrial fibrillation.
  • Each QRS complex comes after a P wave for the atrium and ventricle to work synchronously.
  • There is a short delay between the P wave and the QRS complex to allow time to fill the ventricle with blood and get ready to pump. If the delay is too long (>200ms), there would be nodal block of electrical signal from the atrium to the ventricle. If the delay is too short (<120ms), the signal may find a shortcut to go from the SA node to the AV node.
  • The distance between each QRS complex corresponds to the time for one heart beat. This distance should be stable between beats. Situations, such as tachycardia, bradycardia, bigeminy and skipped beat can be identified easily.
  • If there is no distinct PQRST waves and looks like a sine wave with changing amplitude, it may be an indication of ventricle fibrillation or flutter. This happens when the ventricle heart muscle contracts in an asynchroneous manner.