Interpreting ECG

An ECG is printed on paper covered with a grid of squares. Notice that five small squares on the paper form a larger square. The width of a single small square on ECG paper represents 0.04 seconds. To successfully interpret ECGs, you must have this value committed to memory. Do this now. If each small square represents 0.04 seconds, then a second will be 25 small squares across. If you print out a minute's worth of your heart's electrical activity, the paper would be 1500 small squares wide. If something on an ECG is, let's say, 12 small squares in width, that means that it lasted 12 x 0.04, or almost half a second. A common length of an ECG printout is 6 seconds; this is known as a "six second strip."

The first little hump is known as the P wave. It occurs when the atria depolarize (i.e. trigger).

The next three waves constitute the QRS complex. They represent the ventricles depolarizing. These three are lumped together because a normal rhythm may not have all three. Many times, you'll only see a R and an S. This is not abnormal. If there are less than three, how do we know which one is which? Well, the R wave is the first wave ABOVE the isoelectric line. You then name the waves in relation to the R wave. If it falls before the R wave, it is called the Q wave; after the R wave is the S wave.


Each one of the figures represents an ECG pattern displaying three types of abnormal rhythms: Tachycardia, Bradycardia, and Arrhymthmia. Identify each.