Electrocardiography (ECG) is a non-invasive medical method of recording and interpreting the electrical activity of the heart over time using a device called an 'electrocardiograph'. Its purpose is to examine the over-all rhythm of the heart and detect any abnormalities or weaknesses it may have so that these can promptly addressed. 

The output of a typical electrocardiograph is a moving paper/stylus print-out of the amplitudes of the heart's electrical impulses (millivolts) on the y-axis and time (seconds) on the x-axis. The ECG paper plot usually runs at about 25 mm per second, although higher paper speeds are also used. An ECG that is calibrated at 10 mm/mV means that a standard electrical signal amplitude of 1 mV must cause the stylus of the printer to move vertically by 1 cm.

The sinus node (also known as the sinoatrial node or the heart's 'primary pacemaker') of the right atrium of the heart produces electrical impulses that trigger cardiac contractions (systole) at a regular beat, and thus provide the natural rhythm of the beating of the heart.  ECG operates on the principle that these electrical impulses can be measured by placing electrodes at selected points on the skin of the patient, and can therefore be monitored and analyzed to reveal any problems with the heart being observed.

Electrodes are placed on different sides of the heart so that the movements of different heart muscle areas can be observed.  The electric potential differences or voltages across different pairs of electrodes are shown on the ECG display, allowing the doctor to analyze the heart muscle activity at various points.

ECG is the best known method for diagnosing abnormal heart rhythms, especially those that are caused by damage to the electrically conductive tissues. Incorrect levels (too high or too low) of dissolved electrolytes are also a common cause of abnormal heart rhythms detected by ECG.

    

Figure 1.  A photo of an electrocardiograph (left) and an example of a simple ECG plot (right).

     

During a myocardial infarction wherein heart tissue destruction results from an obstruction of a blood vessel to the heart, ECG can be used to identify the damaged heart muscle.  Note, however, that ECG can only do this for certain heart muscles, so it can't be used to rule out the occurrence of damage in heart muscles that it can not evaluate. Also, ECG can not precisely assess the pumping ability of the heart. For this purpose, echocardiography or nuclear medicine tests are preferred to ECG.

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