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Fundamentals of ECG Reading

by Ana

The electrocardiogram (ECG) is one of the oldest and most used cardiac investigation methods to date. Also known as the EKG, it is a picture that depicts the electrical conduction throughout the heart’s cycle. In comparison to the normal ECG reading, physicians will be able to determine a variety of cardiac diseases from noticeable changes in an ECG strip.

Proper placement of chest leads

To begin the procedure, it is important for EMTs and paramedics to know the exact placement of each ECG lead wires on the patient. The standard ECG has 12 leads, which consist of three standard limb leads, three augmented limb leads, and six precordial leads that are attached to ECG cables. Once the placement of these electrodes is incorrect, the results may show negative changes on the ECG or a wrong diagnosis.


Understanding the normal ECG

An ECG can be read in a strip of paper that is basically a grid of small and large squares, with each large square containing five small squares. A small square equals to 40 milliseconds time, which means that a large square represents 200 ms. This system makes it easy to measure cardiac timing intervals as well as the heart rate.

A normal ECG reading starts from the P wave, the first electrical signal, which originates from the heart’s atria. Then comes a short interval that represents a slowing down of the electrical depolarization that happens from the atrioventricular node to the ventricles. This is called the PR interval, which shows a straight horizontal line that is seen as a brief period of zero electrical activity.

When the depolarization of the ventricles occur, the ECG paper then shows the QRS complex: the Q wave being the first downward deflection; the R wave being the next positive or upward deflection; and the S wave being the next negative or downward deflection. Going back to the ventricles, a repolarization of the myocardium then occurs, reflecting the ST segment, which is normally isoelectric, and the upward T wave.


Normal ECG intervals

With the help of ECG lead wires, clinicians are able to produce a recording of the ECG on paper that depicts the phases of electrical depolarization in millisecond intervals. Normally, the PR interval ranges from 120 to 20 ms, composing three to five small squares on an ECG grid. Meanwhile, a QRS duration ranges up to 120 ms or three small squares. When it comes to the normal QT interval, the range is up to 440 ms.

Any minor or significant change from the normal ECG reading could mean a certain cardiac condition, may it be an abnormal heart rhythm or a bundle block. ECG cables and equipment can also help diagnose acute myocardial infarction or ischemic attacks, which can be interpreted from either anterior or inferior ST elevations.

Determining the heart rate through the ECG

Different methods work for every medical practitioner when it comes to working out the heart rate through the ECG paper. One method is to take note of the sequence: 300, 150, 75, 60, 50. This works according to the number of big blocks in the strip. If there’s only one big block, the heart rate is 300. If two, the rate is 150, and it goes on.

One thing to remember though is that the heart is made up of two chambers, the atria and the ventricles, which are producing separate heart rates of their own. Normally, the two heart rates should be the same, as in a 1:1 ratio, but this isn’t so in case of a disease.

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