15. Prolonged Arterial Occlusion: Q Waves and Equivalents

Headshot of Chris Anderson, MD · Clinical Education Director, Pediatrics
Chris Anderson
MD · Clinical Education Director, Pediatrics
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Headshot of Kevin Hodges, Vice Chair, Emergency Medicine
Kevin Hodges
Vice Chair, Emergency Medicine
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Table of Contents
  • Define and Identify the following:
    • Start of the QRS.
    • Normal direction of the start of the QRS.
  • Define and identify an abnormal Q wave.
  • Define and identify an abnormal Q wave equivalent.
  • Describe an approach to the timing of Q waves as acute, recent, or old.
  • Define and identify Q wave infarction by location.
  • Identify septal infarction in the presence of RBBB.
  • Identify septal infarction as a cause of LBBB.
  • Define and identify poor R wave progression.
  • Define nonspecific Q waves.

The three parts of the QRS

In Chapter 10, the overall direction of the entire QRS was used to diagnose hemiblock. In Chapters 11 and 12, visualization of the very end of the QRS provided the diagnosis of right versus left bundle branch block. Finally, the direction of the first 0.04 seconds (that is, the very beginning of the QRS) is used to diagnose Q wave infarction. This must be evaluated on every EKG unless LBBB is present. The initial part of the QRS complex is one little box, or 0.04 seconds wide. Possibilities for this first part of the QRS are shown in Figure 15.1a to Figure 15.1c. Looking at only the first 0.04 seconds (or the first little box) of the QRS, Figure 15.1a is positive, Figure 15.1b is both positive and negative, and Figure 15.1c is negative.

In addition, if this is the only EKG, the First Rule of the T Waves still applies. These T waves may be a separate event, represent ischemia or infarction, and may be new or old.

The QRS: Beginning normally

This initial part of the QRS normally distributes itself evenly and equally through all parts of the left ventricle. This can be visualized as pointing to the patient’s left side and inferiorly.

The QRS: Beginning abnormally

Complete occlusion of an epicardial artery by a ruptured plaque and superimposed thrombus causes extreme ischemia and ST segment elevation pointing toward the involved area of the LV (see Chapter 14). When the obstruction is not relieved, death of a wide thickness of myocardial cells STEMI or (transmural infarction) results.

The diagnostic hallmark of this transmural infarction on the EKG is an abnormality of the initial part of the QRS, which points away from the infarcted area. This dead tissue generates no electrical contribution to the QRS. The surviving parts of the ventricle produce a QRS that rotates away from the dead zone. This QRS beginning can be visualized as pointing away from an area of electrically dead myocardial cells. This negative beginning to the QRS is called a Q wave.

Criterion for significant Q waves

The criterion for an abnormal beginning to the QRS is that the first 0.04 seconds is on average negative. Figure 15.5 demonstrates four QRS complexes, each with a Q wave.

The first QRS (Letter A) in Figure 15.5 demonstrates the obvious case of an abnormal Q wave. The whole QRS is negative in this example. There is no question then that the first 0.04 seconds is negative and, therefore, forms a significant Q wave.

The second QRS (Letter B) demonstrates a QRS that is positive and negative. Since the first 0.04 seconds though is entirely negative, this is also a significant Q wave.

The third example in Figure 15.5 (Letter C) is the case that presents a problem clinically for the EKG reader. The first 0.04 seconds of the QRS is partly negative and partly positive. A useful terminology for this borderline case is nonspecific Q wave.

The last QRS (Letter D) demonstrates a clearly positive initial 0.04 seconds even though it started with a Q wave. This is a normal Q wave.

Timing of Q wave infarction

Timing of the infarction can be suggested but not proven on a single EKG. If an abnormal Q wave pattern is present, the timing can be characterized according to Table 15.1. The presence of ST segment elevation always suggests acuteness (hours) of the event, particularly if it is the first EKG obtained (Figure 15.6a). Rarely, untreated ST segment elevation persists indefinitely, and indicates formation of a left ventricular aneurysm.

  • After a few hours or days, ST segment elevation can end in a partially inverted, or biphasic T wave, as in Figure 15.6b.
  • After a period of days to weeks, the ST elevation usually resolves into T wave inversion, as in Figure 15.6c.
  • After a month or so, the T wave may completely normalize as in Figure 15.6d, and the infarction is termed either “old” or “age indeterminate.”

Table 15.1

T wave/ST pattern Timing estimate
ST elevation
Acute (hours)
Biphasic ST-T wave
Recent (hours to days)
T wave inversion
Recent (days to weeks)
Normal T and ST segments
Old or age indeterminate

memorize this

Table 15.2 Summary of criteria for Q wave infarction
Q wave pattern Location of infarction
II, III, AVF
Inferior
I, AVL
Lateral
V1, V2
Septal
V3, V4
Anterior
V5, V6
Lateral
Tall wide R wave in V2
Posterior