The "T" wave on an EKG is a very unstable wave influenced by many factors and artefacts. The T wave represents the repolarization or resetting of the electrical cells in the bottom chambers of the heart. Reasons for inverted T waves are numerous and may include ischemia, heart attack, hyperventilation, anxiety, certain medications, infections around the heart, pulmonary embolus (blood clot in the lung), electrolyte disturbance (abnormalities in sodium, potassium levels) etc.
An EKG has to supported with and inconjuction with other wave forms as well as clinical evidence and supporting symptom before there can be a diagnosis.