Heart defect (AV block). Electrocardiogram (ECG) of electrical activity (yellow traces) in the heart in a case of an atrioventricular (AV) block in a man who lost consciousness after strenuous physical effort. Two of the six ECG traces are shown here. An AV block is a condition where the heart's electrical system malfunctions, and the initial electrical impulse that starts a heartbeat does not spread over the entire heart. This can lead to reduced pumping of blood, light- headedness, and palpitations. The exact form of the block can be diagnosed by examining the patient's ECG. An ECG is obtained by using electrodes to detect the electrical activity of the heart.
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