The Q Wave

The Q wave is the short initial downward stroke of the QRS complex formed during the beginning of ventricular depolarization ¹.

Normal Q wave

Normal Q Wave in Limb Leads

In limb leads, the Q wave is present in one or more of the inferior leads (leads II, III, aVF) in more than 50% of normal adults and in leads I and aVL in about 50% ².

The duration of the Q wave is of considerable importance in the diagnosis of myocardial infarction. With the exception of leads III and aVR, the Q waves in the limb leads normally do not exceed 0.03 s in duration (a little more than half a small square) ².

The depth of the Q wave is less than 25% of the following R wave or less than 0.4 mV (4 small squares) in all limb leads, but lead III is the exception ².

In lead III, Q wave duration is occasionally as long as 0.04 s but rarely is it 0.05 s. This lead accounts for most of the erroneous diagnoses of myocardial infarction ².

In leads III, aVF, and aVL the initial R wave may be absent, and a QS or QR deflection may represent a normal variant ^2 3.

The QS pattern in lead III is more prevalent in obese and the QS pattern is more prevalent in lead aVL in slender subjects ².

In lead aVR the Q wave is usually is part of the QS morphology.

Normal Q Wave in Precordial Leads

Small Q waves may be present in left precordial leads in more than 75 percent of normal subjects ².

They are seen most frequently in lead V6, less frequently in leads V5 and V4, and rarely in V3.

Q waves in these leads are present more often in young subjects than in subjects older than 40 years.

The duration of the Q waves is 0.03 s or less. The amplitude usually is less than 0.2 mV, although it may reach 0.4 mV (4 small squares) ².

An amplitude of 0.4 mV or more may be encountered in teenagers.

In the posterior leads V7–V9, Q wave duration of 0.03 s may be observed in 20% of normal male subjects ².

Abnormal Q Wave

On the electrocardiogram, an abnormal Q wave is usually defined in adults as one that has a duration of 0.04 s (a small square) or more ².

Some authors consider the amplitude of the Q wave a criterion of abnormality, when the amplitude of the Q wave exceeds 25% of the following R wave. Such a finding, however, has low specificity and should not be used as a reliable marker of myocardial infarction in practice ².

The definition of a wide Q wave does not apply to leads aVR and V1, which may normally lack the initial R wave.

A Q wave is also considered abnormal, even if it is less than 30 ms in duration, when it is present in leads that normally display an initial R wave (leads V2 and V3) ².

Myocardial Infarction with Abnormal Q Wave

During an acute myocardial infarction, the development of new Q waves indicates myocardial necrosis, which starts 6 to 14 hours after the onset of symptoms ^2 4.

Most commonly the Q waves develop while the ST-segment is still elevated and persist for a variable number of days, weeks, months, or years and often indefinitely ².

When the abnormal Q waves are associated with ST-segment deviations or T wave changes in the same leads, the likelihood of myocardial infarction is increased ⁴.

In the present era of thrombolytic therapy and primary angioplasty for STEMI, the reversal of abnormal Q waves occurs more frequently.

Transient Q waves may be observed during an episode of acute ischemia or (rarely) during acute myocardial infarction with successful reperfusion.

In asymptomatic patients, the presence of abnormal Q waves on an electrocardiogram indicates a high probability of ischemic heart disease ⁴.

The development of Q waves during STEMI increases the prognostic risk ⁴.

The presence of Q waves on the EKG should not necessarily change the reperfusion strategy decision ⁵.

Other causes of Q waves

Improper leads placement, QRS complex abnormalities, or technical errors (e.g. lead reversal) may result in the appearance of new Q waves or QS complexes, as compared with a prior tracing ⁴.

In infants and children with an an anomalous left main coronary artery originating from the pulmonary artery, abnormal Q waves with a duration greater than 0.03 s may be observed in leads I and aVL in most cases ².

Q waves may also occur in cardiomyopathies due to myocardial fibrosis in the absence of ischemic heart disease ².

New Q waves similar in morphology to those observed in acute myocardial infarction are also common after acute stroke, reported in approximately 10 percent of patients with acute ischemic or hemorrhagic stroke ⁶.