MI's

 

Q-Wave Myocardial Infarctions (MI): EKG’s are also used to look at old and new injuries to the heart’s muscle tissue.  When a patient presents with chest pain, you can use the EKG to discriminate between ischemia and infarct.  The EKG also gives the location of ischemia or injury, which then allows a physician to base treatment on that information and prevent ischemia from becoming permanent damage. 

Q waves become present on an EKG after the injury has occurred and the tissue has died.  It is important to assess Q waves for height, width, leads present in, and the clinical scenario.  A Q-wave MI is called an old (indeterminate age) MI, because all the changes both on the EKG and in the myocardium that would allow you to time or date the damage have already occurred.  The exception occurs when ST and T wave changes are still present and Q waves appear; then, you can determine the infarct is new.  The development of Q waves is close to the last step as discussed below.

Natural History of a Q-Wave MI: Four changes occur in an acute Q-wave MI

1.    Hyperacute or peaked T waves appear


            Peaked T wave

2.    Elevation of the ST segment


               ST elevation

3.    Appearance of clinically significant Q waves (see below for details)


             significant Q’s                          QS complex

4.    T wave inversion and the ST segment elevation begins to resolve


         Inverted T wave

Some Facts About Q Waves:

·        Narrow Q in I, aVL, aVF, V5, and V6 can be normal

·        QS or QR complexes may be seen normally in aVR

·        QS may be found normally in III and V1 (if even a small R wave is present by V3, the QS is most likely normal and not a sign of infarct); however, any Q in V2,V3 or V4 is abnormal unless the axis is rotated.

·        Significant Q waves: Q or QS waves are pathologic when they reach 0.04 sec in length (one small box) or at least 1/3 the size of the QRS complex

·        If ST segment elevation with a flat or convex (coved) appearance occurs with the Q waves, then the infarct is considered acute or recent (posterior infarct presents differently with ST depression in right precordial leads)

If ST elevation has not resolved two weeks after the acute infarct and Q waves are present, situation is suggestive of a ventricular aneurysm.

Where is the problem?

Location

Leads

Vessel

Anterior

V2-V4

LAD

Anteroseptal

V1-V3

 

Anterolateral

I, aVL, V4-V6

 

Large Anterior

V1-V5

Left Main

Lateral

I, aVL

Circumflex

Inferior

II, III, aVF

RCA

Posterior

V1-V2 (tall R waves with ST segment depression

RCA, left circumflex, or marginal

Method for Detecting a Q-Wave MI: Finally, the steps to how it’s done.

·        Look for significant (see above) Q waves in all 11 leads ignoring aVR

·        List these leads and the heart regions they correspond to

·        Check R wave progression and see that there is a normal transition around V3-V4 or note if, instead, there is a tall R or rSr´ in V1 to suggest posterior infarct (see below)

·        Check for any ST segment elevation

·        Check for reciprocal changes: ST segment depression in other lead groups

·        Scan the EKG for flat or inverted T waves

Remember:

·        T wave inversion can be seen normally in III, and aVF; also if the QRS is down going the inverted T wave is probably normal.

·        The shape of the ST segment elevation is important.  More concave, or smiley, elevation is less concerning as it could be from early repolarization.  Regardless, if the patient has chest pain, an elevation must be considered abnormal.

Non-Q Wave MI: Not all MI’s have Q waves and up to 1/3 never develop Q waves or they develop and resolve.  Two reasons for this: either the infarct was not complete (or transmural) or because the infarct occurred in an electrically “silent” area of the heart, where an EKG cannot record the injury.  This has led some to call a non-Q wave MI a “silent MI” however; this term is used in several clinical scenarios.  Confusion may arise as physicians also refer to a “silent MI” as one in which the patient presents with no chest pain, but maybe dyspnea, mental status changes, feeling “not quite right”, or no symptoms at all. 

A second form of a non-Q wave MI is obviously an acute infarct that has not had the time to resolve and eventually Q waves will appear.  At the time the patient presents there are no Q’s, but rather ST segment coved elevation with or without inverted T waves.  Typically, one also finds reciprocal changes in other lead groupings that consist of horizontal or down-sloping ST depression.  These changes occur, again, with or without T wave inversion.  You must be careful when assessing reciprocal changes as they could also represent an area of ischemia and a new region of myocardium at risk.

ST Segment Patterns:

Ischemia: Horizontal or down-sloping ST segment depression with or without T wave inversion, but without ST elevation in any other leads; the segment depression is typically symmetric (“V” shaped)

Ventricular “Strain”: Asymmetric depression that may be seen with LVH (lateral leads) or RVH (right precordial leads)

Digoxin: Strain-like pattern, wide scooped ST depression, or nothing at all

Non-Specific ST Changes: ST flattening or slight depression with significance unclear

Posterior Infarct: A Special Situation

Detecting a posterior MI requires a different way of viewing an EKG since no lead gives a direct view of the posterior wall of the left ventricle.  Several arteries can cause a posterior MI, but it is most often seen with an inferior MI caused by an occlusion of the RCA.  Viewing damage in this area involves an indirect means.

·        Look for a tall R wave in V1 and V2- remembering that R waves should be small in these leads and get taller as you move to the left precordial leads

·        These R waves must be present in the absence of RVH and therefore, the patient must be over 30-35 years of age (since that is when the criteria for RVH and LVH become reliable)

·        R waves must be at least 0.04sec or longer; or R/S ratio of at least 1:1

·        If tall R waves are present look for ST depression (if ST depression existed alone it would be difficult to tell if the changes were simply reciprocal changes for an inferior MI or an acute posterior infarct)

Mirror Test: Although the above description does work, the easiest way to detect a posterior MI is to use the Mirror Test.  Once you have determined an inferior (or other) MI has occurred, you begin looking for reciprocal changes.  If you see ST depression in V1, V2, and V3, flip the EKG over and hold it up to the light.  Now read those leads flipped over.  Are there significant Q waves?  Is the ST segment elevated with a coved appearance?  Are the T waves inverted?  Answering yes tells you, there is a posterior infarct as well.

Points to Consider:

·        With an occlusion of the RCA, you often see conduction abnormalities as it supplies arteries to SA and AV nodes

·        Left main occlusions=sudden death

·        LAD occlusions often result in BBB’s or Mobitz II

·        In 10% of cases, the circumflex artery also supplies the inferior/posterior wall of the LV and not just the lateral wall

·        Most common cause of an acute MI- sudden occlusion of the artery

·        Tall R waves in right precordial leads can also be due to RBBB (will see “rabbit ears” or “M” formation), WPW (look for delta wave), or RVH (check for RAD)