An abnormal (i.e., pathologic) Q wave indicates the presence of dead myocardial tissue.

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An abnormal (i.e., pathologic) Q wave is more than 0.04 second in duration or more than one third the height of the following R wave in that lead. Myocardial infarction is one possible cause of abnormal Q waves. In the early hours of infarction, an abnormal Q wave may not have developed to its full width or amplitude. Therefore a single ECG tracing might not identify an abnormal Q wave. In a patient with a suspected myocardial infarction, be sure to look at Q waves closely. Even if the initial ECG tracings do not show Q waves that are more than 0.04 second in duration or equal to or more than one third of the amplitude of the QRS complex, pathology must be considered if the Q waves become wider or deeper in each subsequent tracing.

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