Acute Posterior Myocardial
Infarction
Case
presentation
A 69-year-old man
presented to a rural emergency department (ED) with a 1-hour history of severe
epigastric pain that seemed to radiate between his shoulder blades. His arms
felt tingly. The electrocardiogram shown below was obtained by the ED nurse
soon after he arrived at the hospital.
The electrocardiogram
(ECG) shows marked ST depression anteriorly, along with nonspecific ST
depression and T-wave inversion laterally. These changes could be due to
anterior ischemia or acute posterior myocardial infarction (MI).
A 15-lead ECG was
obtained by moving the V4 lead from the left precordium to the same position on
the right anterior chest (V4R), the V5 lead to just below the tip of the
scapula on the left back (V8), and the V6 lead to a point halfway between V8
and the spine (V9), as shown in Fig. 1.
Fig.
1. Transverse
view of the thorax, showing positions of the chest leads relative to the heart
chambers (RV = right ventricle, LV = left ventricle).
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Figure 2 shows the QRS
complexes in V8 and V9. There was a 1-mm ST segment elevation in both leads,
which is direct evidence of acute posterior MI. The ST depression seen
anteriorly (V1 to V4) was therefore reciprocal ST depression, given that these
leads face the opposite side of the heart from the current of injury that
caused ST elevation in the two posterior leads (Fig. 2).
Fig.
2.Tracings
from leads V8 and V9. ST elevation is clearly visible (arrows).
Acute MI must be
diagnosed immediately by rural physicians, because the goal is to start
thrombolysis within 30 to 60 minutes from the time the patient enters the
hospital.[1] Acute posterior and far right MI are difficult to diagnose from a
12-lead ECG, because the 12 leads do not face those areas of the heart.
A 15-lead ECG is
indicated when a standard 12-lead ECG fails to give good evidence for acute MI
in cases in which posterior or right ventricular MI is suspected, and when the
extent of damage in the right ventricle must be determined in cases of inferior
MI. ST elevation in lead V4R would be evidence for right ventricular infarction
and in leads V8 and V9 for posterior MI. The presence of the anterior
reciprocal ST depression significantly improves the positive predictive value
of the ECG for acute MI.
There have been no randomized trials showing any benefit of
thrombolysis in acute posterior MI, however use of thrombolysis in posterior
infarction under 12 hr is recommended in most guidelines. In this case, the ECGs were sent immediately
by fax to a cardiologist, who agreed with the diagnosis and recommended
immediate thrombolysis during a telephone consultation. He documented his
support in a note faxed back to the rural physician. The patient was given tissue
plasminogen activator without incident. The creatine kinase level peaked at 500
U/L, and the patient has done well since.
1.
A standard 12 Lead ECG is performed.
2.
The V4 Lead is moved to the right anterior chest opposite to the corresponding
Left chest placement.
3.
The V5 Lead is moved under the midscapular Line (becomes V8).
4.
The V6 lead is moved to the Left paraspinal border (becomes V9).
5.
The ECG is repeated.
6.
The V4, V5, and V6 Leads on the ECG should be manually relabeled V4R, V8, and
V9.
1.
Thompson J. Country cardiograms case 4: acute posterior myocardial
infarction. Can J Rural Med 1997;2 (2):76
2.
Podrid
P. ECG tutorial: myocardial
infartion. UpToDate 2006.
3.
Menon
V, Harrington RA, Hochman JS, et al. Thrombolysis and adjunctive therapy in acute
myocardial infarction: the Seventh ACCP Conference on Antithrombotic and
Thrombolytic Therapy. Chest 2004 Sep;126(3
Suppl):549S-575S.