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SKILLS – ECG Reading & Interpretation Background 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. Normal Values Feature Description Duration RR The interval between an R wave and the next R wave is the inverse of the 0.6 to 1.2 sec interval heart rate. Normal resting heart rate is between 50 and 100 bpm 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 PR impulse takes to travel from the sinus node through the AV node and 0.12 to 0.20 s interval entering the ventricles. The PR interval is therefore a good estimate of AV node function. The QRS complex reflects the rapid depolarization of the right and left QRS ventricles. They have a large muscle mass compared to the atria and so the 0.08 to 0.12 s complex QRS complex usually has a much larger amplitude than the P-wave. ST The ST interval is measured from the J point to the end of the T wave. 0.32 s interval The QT interval is measured from the beginning of the QRS complex to the QT end of the T wave. A prolonged QT interval is a risk factor for ventricular 0.30 to 0.43 s interval tachyarrhythmias and sudden death. It varies with heart rate and for clinical relevance requires a correction for this, giving the QTc. EXAMPLE Calculating the time for some of the intervals helps medical professionals interpret possible cardiac abnormalities. In the diagram to the left calculate the following: PR Interval 4 ¼ boxes = 4.25 x 0.04 sec =0.17 sec QRS Interval 2 ¼ boxes = 2.25 x 0.04 sec = 0.09 sec QT Interval 10 ¼ boxes = 10.25 x 0.04 sec = 0.41 sec ST Interval 8 boxes = 8 x 0.04 sec = 0.32 sec Activity Calculate the R-R, PR, QRS, QT, and ST intervals for the following ECG strips. Show your work! A R-R = __________________________ PR = __________________________ QRS = _________________________ QT = __________________________ ST = ___________________________ B R-R = __________________________ PR = __________________________ QRS = _________________________ QT = __________________________ ST = ___________________________ C R-R = __________________________ PR = __________________________ QRS = _________________________ QT = __________________________ ST = ___________________________ D R-R = __________________________ PR = __________________________ QRS = _________________________ QT = __________________________ ST = ___________________________ E R-R = __________________________ PR = __________________________ QRS = _________________________ QT = __________________________ ST = ___________________________ F R-R = __________________________ PR = __________________________ QRS = _________________________ QT = __________________________ ST = ___________________________ G R-R = __________________________ PR = __________________________ QRS = _________________________ QT = __________________________ ST = ___________________________