Human heart, computer artwork. Human heart with an electrocardiogram (ECG) trace in the background. The heart is a hollow muscle that pumps blood around the body. Its main blood vessels (blue and red) can be seen at upper centre. An ECG trace represents the pumping of the heart as a series of waves (left). These waves correspond to the electrical activity that occurs in the heart muscle before and during contraction. An ECG is an important heart monitoring technique that can be used to diagnose cardiac disorders. When a person suffers brain-death, electrical activity in the body ceases and the ECG produces a flatline (right).