Get familiar with the working of an electrocardiograph

Hyderabad: You are probably familiar with this scene from a typical hospital television show: A patient is hooked up to a monitoring machine that shows voltage traces on a screen and makes the sound “… pip… pip… pip….peeeeeeeeeeeeeeeeeeeeee” as the patient goes into cardiac arrest. This type of machine (electro-cardiograph) is used to obtain an […]

Hyderabad: You are probably familiar with this scene from a typical hospital television show: A patient is hooked up to a monitoring machine that shows voltage traces on a screen and makes the sound “… pip… pip… pip….peeeeeeeeeeeeeeeeeeeeee” as the patient goes into cardiac arrest.

This type of machine (electro-cardiograph) is used to obtain an electrocardiogram (ECG). ECG is a graphical representation of the electrical activity of the heart during a cardiac cycle.

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To obtain a standard ECG, a patient is connected to the machine with three electrical leads (one to each wrist and to the left ankle) that continuously monitor the heart activity.

For a detailed evaluation of the heart’s function, multiple leads are attached to the chest region.

Here, we will talk only about a standard ECG. Each peak in the ECG is identified with a letter from P to T that corresponds to a specific electrical activity of the heart.

* The P-wave represents the electrical excitation (or depolarisation) of the atria, which leads to the contraction of both the atria.

* The QRS complex represents the depolarisation of the ventricles, which initiates the ventricular contraction.

* The contraction starts shortly after Q and marks the beginning of the systole.

* The T-wave represents the return of the ventricles from excited to normal state (repolarisation). The end of the T-wave marks the end of systole.

Obviously, by counting the number of QRS complexes that occur in a given time period, one can determine the heart beat rate of an individual.

Since the ECGs obtained from different individuals have roughly the same shape for a given lead configuration, any deviation from this shape indicates a possible abnormality or disease. Hence, it is of a great clinical significance.

To be continued…