Q waves

Q wave
File:ECG Q wave.png
Example of a Q wave on an ECG
Specialty	Cardiology

A Q wave is a specific deflection on an electrocardiogram (ECG) that represents the initial negative deflection of the QRS complex. It is an important feature in the interpretation of ECGs, particularly in the diagnosis of myocardial infarction (heart attack).

Characteristics

A Q wave is defined as the first downward deflection after the P wave and before the R wave in the QRS complex. It is important to note that not all Q waves are pathological. Normal Q waves can be seen in certain leads and are usually small and narrow.

Normal Q Waves

Normal Q waves are typically seen in the left-sided leads, such as Lead I, Lead aVL, Lead V5, and Lead V6. These Q waves are usually less than 0.04 seconds in duration and less than 25% of the height of the subsequent R wave.

Pathological Q Waves

Pathological Q waves are indicative of myocardial necrosis and are a key marker of a previous myocardial infarction. These Q waves are usually wider (greater than 0.04 seconds) and deeper (greater than 25% of the height of the R wave) than normal Q waves. They are often seen in leads that correspond to the area of the heart affected by the infarction.

Clinical Significance

The presence of pathological Q waves on an ECG is a significant finding. It suggests that there has been a loss of viable myocardium in the region of the heart corresponding to the leads showing the Q waves. This is often due to a previous myocardial infarction.

Diagnosis of Myocardial Infarction

In the context of a suspected myocardial infarction, the presence of new Q waves can help confirm the diagnosis. The World Health Organization criteria for myocardial infarction include the presence of pathological Q waves on the ECG.

Other Conditions

While Q waves are most commonly associated with myocardial infarction, they can also be seen in other conditions such as hypertrophic cardiomyopathy and ventricular aneurysm.

Interpretation

Interpreting Q waves requires careful analysis of the ECG. It is important to consider the clinical context and correlate ECG findings with the patient's symptoms and history. Not all Q waves are pathological, and their presence should be interpreted in conjunction with other ECG changes and clinical findings.

Also see

• Electrocardiogram
• Myocardial infarction
• Cardiac conduction system
• Hypertrophic cardiomyopathy

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