The heart is controlled by an electrical system in much the same way as your computer, TV or appliances. In a normal heart, the electrical signal starts at the SA node located in the atrium and then travels down toward the ventricles causing the heart to beat in a coordinated manner. After the electrical system discharges it must recharge in order to be able to coordinate the next beat.
Doctors are able to visualize electrical activity of the heart using an electrocardiogram (EKG or ECG). The t wave is part of the normal electrical sequence seen on the ECG and corresponds with the bottom half of the heart "recharging" in preparation for the next beat.
Sorry for commenting a post in the Expert Forum (Doctor's forum) but I couldn't resist answering this. You'll probably get a better answer from a doctor later.
Picture the heart like a table of domino bricks, all are raised. You poke one of them with your finger, and they start to fall. This "finger" in your heart is known as the sinus node, which initiates the depolarization (bricks falling) and the impulse spreads through your heart.
When the "bricks" in the upper heart are falling, this is known on EKG as a P wave, and when the "bricks" in the main pumping chambers are falling, this is known as the QRS complex.
After the bricks have fallen, the heart contracts and pumps blood. But as you know, the heart needs to recharge to be able to pump again. The bricks are raised again. This is seen on EKG as the T wave you are referring to.
This impressing process happens about once a second at rest, and three times a second when you exercise heavily.
Once now and then, a brick may fall without being "poked" by the sinus node, also causing all other bricks to fall. This may happen in the upper chambers, a so-called PAC, or in the main chambers, a PVC. Because the electrical current isn't conducted the way it's supposed to, both the QRS complex (with a PVC) and the T wave looks different from the normal beats, and a doctor or the EKG machine may recognize it as an abnormal beat. It happens in almost everyone now and then and this is felt as "palpitations".
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