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.