In electrocardiography, the T wave represents the repolarization of the ventricles. The interval from the beginning of the QRS complex to the apex of the T wave is referred to as the absolute refractory period. The last half of the T wave is referred to as the relative refractory period or vulnerable period. The T wave contains more information than the QT interval. The T wave can be described by its symmetry, skewness, slope of ascending and descending limbs, amplitude and subintervals like the Tpeak–Tend interval. In most leads, the T wave is positive. This is due to the repolarization of the membrane. During ventricle contraction (QRS complex), the heart depolarizes. Repolarization of the ventricle happens in the opposite direction of depolarization and is negative current, signifying the relaxation of the cardiac muscle of the ventricles. But this negative flow causes a positive T wave; although the cell becomes more negatively charged, the net effect is in the positive direction, and the ECG reports this as a positive spike. However, a negative T wave is normal in lead aVR. Lead V1 generally have a negative T wave. In addition, it is not uncommon to have a negative T wave in lead III, aVL, or aVF. A periodic beat-to-beat variation in the amplitude or shape of the T wave may be termed T wave alternans.
Cardiac physiology The refractory period of cardiac muscle is distinct from that of skeletal muscle. Nerves that innervate skeletal muscle have an extremely short refractory period after being subjected to an action potential (of the order of 1 ms). This can lead to sustained or tetanic contraction. In the heart, contractions must be spaced to maintain a rhythm. Unlike in muscle, repolarization occurs at a slow rate (100 ms). This prevents the heart from undergoing sustained contractions because it forces the refractory period and cardiac action potential firing to be of the same length of time. Repolarization depends on the charges of ions and their flow across membranes. In skeletal muscle cells, repolarization is simple. First, sodium ions flow into the cell to depolarize it and cause skeletal muscle contraction. Once the action potential is over, potassium ions flow out of the cell due to increased cell membrane permeability to those ions. This high permeability contributes to the rapid repolarization of the membrane potential. This repolarization occurs quickly enough that another action potential can cause depolarization before the last action potential has dissipated. Cardiac muscle differs in that there are more calcium channels that counteract the potassium channels. While potassium quickly flows out of the cell, calcium slowly flows into the cell. This causes the repolarization to occur more slowly, making the refractory period as long as the action potential, preventing sustained contractions. The T wave is representative of the repolarization of the membrane. In an EKG reading, the T wave is notable because it must be present before the next depolarization. An absent or strangely shaped T wave may signify disruption in repolarization or another segment of the heartbeat.
Normal T wave Normally, T waves are upright in all leads, except aVR and V1 leads. Highest amplitude of T wave is found at V2 and V3 leads. The shape of the T wave is usually asymmetrical with a rounded peak. T wave inversions from V2 to V4 leads are frequently found and normal in children. In normal adults, T wave inversions from V2 to V3 are less commonly found but can be normal. The depth of the T wave also becomes progressively shallow from one to the next lead. The height of the T wave should not exceed 5 mm in limb leads and more than 10 mm in precordial leads.
Abnormalities Both the abnormalities of the ST segment and T wave represents the abnormalities of the ventricular repolarization or secondary to abnormalities in ventricular depolarisation.
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