IX. Myocardial
Infarction
Topics for study:
- Introduction
(Read this first)
- Inferior
Q-Wave MI Family
- Anterior
Q-Wave MI Family
- MI
+ Bundle Branch Block
- Non
Q-Wave MI
- The
Pseudoinfarctions
- Miscellaneous
QRS Abnormalities
1. Introduction to ECG Recognition of Myocardial Infarction
When myocardial blood supply is abruptly reduced or cut off to
a region of the heart, a sequence of injurious events occur beginning with
subendocardial or transmural ischemia, followed by necrosis, and eventual
fibrosis (scarring) if the blood supply isn't restored in an appropriate
period of time. Rupture of an atherosclerotic plaque followed by acute
coronary thrombosis is the usual mechanism of acute MI. The ECG changes
reflecting this sequence usually follow a well-known pattern depending on the
location and size of the MI. MI's resulting from total coronary occlusion
result in more homogeneous tissue damage and are usually reflected by a
Q-wave MI pattern on the ECG. MI's resulting from subtotal occlusion
result in more heterogeneous damage, which may be evidenced by a non
Q-wave MI pattern on the ECG. Two-thirds of MI's presenting to
emergency rooms evolve to non-Q wave MI's, most having ST segment depression
or T wave inversion.
Most
MI's are located in the left ventricle. In the setting of a proximal
right coronary artery occlusion, however, up to 50% may also have a component
of right
ventricular infarction as well. Right-sided chest leads are necessary to
recognize RV MI.
In
general, the more leads of the 12-lead ECG with MI changes (Q waves and ST
elevation), the larger the infarct size and the worse the prognosis.
Additional leads on the back, V7-9 (horizontal to V6), may be used to improve
the recognition of true posterior MI.
The
left anterior descending coronary artery (LAD) and it's branches usually
supply the anterior and anterolateral walls of the left ventricle and the
anterior two-thirds of the septum. The left circumflex coronary artery (LCX)
and its branches usually supply the posterolateral wall of the left ventricle.
The right coronary artery (RCA) supplies the right ventricle, the inferior
(diaphragmatic) and true posterior walls of the left ventricle, and the
posterior third of the septum. The RCA also gives off the AV nodal coronary
artery in 85-90% of individuals; in the remaining 10-15%, this artery is a
branch of the LCX.
Usual
ECG evolution of a Q-wave MI; not all of the following patterns may be seen;
the time from onset of MI to the final pattern is quite variable and related
to the size of MI, the rapidity of reperfusion (if any), and the location of
the MI.
A.
Normal ECG prior to MI
B.
Hyperacute T wave changes - increased T wave amplitude and width; may also
see ST elevation
C.
Marked ST elevation with hyperacute T wave changes (transmural
injury)
D. Pathologic Q waves, less ST elevation, terminal T wave
inversion (necrosis)
(Pathologic Q waves are usually defined as duration
>0.04 s or >25% of R-wave amplitude)
E. Pathologic Q waves, T wave inversion (necrosis and
fibrosis)
F.
Pathologic Q waves, upright T waves (fibrosis)
click here
to view
2. Inferior MI Family of Q-wave MI's
(includes inferior, true posterior, and right ventricular MI's)
Inferior MI
Pathologic Q waves and evolving ST-T changes in leads II,
III, aVF
Q
waves usually largest in lead III, next largest in lead aVF, and smallest in
lead II
Example #1: frontal plane leads with fully evolved inferior
MI (note Q-waves, residual ST elevation, and T inversion in II, III, aVF)
click
here to view
Example #2: Old inferior MI (note largest Q in lead III, next
largest in aVF, and smallest in lead II)
click
here to view
True
posterior MI
ECG
changes are seen in anterior precordial leads V1-3, but are the mirror
image of an anteroseptal MI:
Increased R wave amplitude and duration (i.e., a
"pathologic R wave" is a mirror image of a pathologic Q)
R/S ratio in V1 or V2 >1 (i.e., prominent anterior
forces)
Hyperacute ST-T wave changes: i.e., ST depression and
large, inverted T waves in V1-3
Late normalization of ST-T with symmetrical upright T waves
in V1-3
Often seen with inferior MI (i.e., "inferoposterior MI")
Example #1: Acute inferoposterior MI (note tall R
waves V1-3, marked ST depression V1-3, ST elevation in II, III, aVF)
click
here to view
Example #2: Old inferoposterior MI (note tall R in V1-3,
upright T waves and inferior Q waves)
click
here to view
Example #3: Old posterolateral MI (precordial leads):
note tall R waves and upright T's in V1-3, and loss of R in V6
click
here to view
Right
Ventricular MI (only seen with proximal right coronary occlusion; i.e., with
inferior family MI's)
ECG
findings usually require additional leads on right chest (V1R to V6R,
analogous to the left chest leads)
ST
elevation, >1mm, in right chest leads, especially V4R (see below)
click
here to view
3. Anterior Family of Q-wave MI's
Anteroseptal MI
Q,
QS, or qrS complexes in leads V1-V3 (V4)
Evolving ST-T changes
Example: Fully evolved anteroseptal MI (note QS waves in
V1-2, qrS complex in V3, plus ST-T wave changes)
click
here to view
Anterior MI (similar changes, but usually V1 is spared; if V4-6
involved call it "anterolateral")
Example: Acute anterior or anterolateral MI (note Q's
V2-6 plus hyperacute ST-T changes)
click
here to view
High Lateral MI (typical MI features seen in leads I and/or
aVL)
Example: note Q-wave, slight ST elevation, and T
inversion in lead aVL
click
here to view
(Note also the slight U-wave inversion
in leads II, III, aVF, V4-6, a strong marker for coronary disease)
4. MI with Bundle Branch Block
MI + Right Bundle Branch Block
Usually easy to recognize because Q waves and ST-T changes are
not altered by the RBBB
Example #1: Inferior MI + RBBB (note Q's in II, III, aVF
and rSR' in lead V1)
click
here to view
Example #2: Anteroseptal MI with RBBB (note Q's in
leads V1-V3, terminal R wave in V1, fat S wave in V6)
click
here to view
MI + Left Bundle Branch Block
Often a difficult ECG diagnosis because in LBBB the right
ventricle is activated first and left ventricular infarct Q waves
may not appear at the beginning of the QRS complex (unless the septum is
involved).
Suggested ECG features, not all of which are specific for MI
include:
Q
waves of any size in two or more of leads I, aVL, V5, or V6 (See
below: one of the most reliable signs and probably indicates septal
infarction, because the septum is activated early from the right
ventricular side in LBBB)
click
here to view
Reversal of the usual R wave progression in precordial
leads (see above )
Notching of the downstroke of the S wave in precordial
leads to the right of the transition zone (i.e., before QRS changes from a
predominate S wave complex to a predominate R wave complex); this may be a
Q-wave equivalent.
Notching of the upstroke of the S wave in precordial leads
to the right of the transition zone (another Q-wave equivalent).
rSR' complex in leads I, V5 or V6 (the S is a Q-wave
equivalent occurring in the middle of the QRS complex)
RS complex in V5-6 rather than the usual monophasic R waves
seen in uncomplicated LBBB; (the S is a Q-wave equivalent).
"Primary" ST-T wave changes (i.e., ST-T changes in the same
direction as the QRS complex rather than the usual "secondary" ST-T
changes seen in uncomplicated LBBB); these changes may reflect an acute,
evolving MI.
5. Non-Q Wave MI
Recognized by evolving ST-T changes over time without the
formation of pathologic Q waves (in a patient with typical chest pain
symptoms and/or elevation in myocardial-specific enzymes)
Although it is tempting to localize the non-Q MI by the
particular leads showing ST-T changes, this is probably only valid for the
ST segment elevation pattern
Evolving ST-T changes may include any of the following
patterns:
Convex downward ST segment depression only (common)
Convex upwards or straight ST segment elevation only
(uncommon)
Symmetrical T wave inversion only (common)
Combinations of above changes
Example: Anterolateral ST-T wave changes
click
here to view
6. The Pseudoinfarcts
These are ECG conditions that mimic myocardial infarction
either by simulating pathologic Q or QS waves or mimicking the typical ST-T
changes of acute MI.
WPW preexcitation (negative delta wave may mimic
pathologic Q waves)
IHSS (septal hypertrophy may make normal septal Q waves
"fatter" thereby mimicking pathologic Q waves)
LVH (may have QS pattern or poor R wave progression in
leads V1-3)
RVH (tall R waves in V1 or V2 may mimic true posterior MI)
Complete or incomplete LBBB (QS waves or poor R wave
progression in leads V1-3)
Pneumothorax (loss of right precordial R waves)
Pulmonary emphysema and cor pulmonale (loss of R waves V1-3
and/or inferior Q waves with right axis deviation)
Left anterior fascicular block (may see small q-waves in
anterior chest leads)
Acute pericarditis (the ST segment elevation may mimic
acute transmural injury)
Central nervous system disease (may mimic non-Q wave MI by
causing diffuse ST-T wave changes)
7. Miscellaneous Abnormalities of the QRS Complex:
The differential diagnosis of these QRS abnormalities depend
on other ECG findings as well as clinical patient information
Poor R Wave Progression - defined as loss of, or no R waves
in leads V1-3 (R £2mm):
Normal variant (if the rest of the ECG is normal)
LVH (look for voltage criteria and ST-T changes of LV
"strain")
Complete or incomplete LBBB (increased QRS duration)
Left anterior fascicular block (should see LAD in frontal
plane)
Anterior
or anteroseptal MI
Emphysema and COPD (look for R/S ratio in V5-6 <1)
Diffuse infiltrative or myopathic processes
WPW preexcitation (look for delta waves, short PR)
Prominent Anterior Forces - defined as R/S ration >1 in V1
or V2
Normal variant (if rest of the ECG is normal)
True
posterior MI (look for evidence of inferior MI)
RVH (should see RAD in frontal plane and/or P-pulmonale)
Complete or incomplete RBBB (look for rSR' in V1)
WPW preexcitation (look for delta waves, short PR)
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