A posterior electrocardiogram (ECG) is a specialized diagnostic test that supplements the standard 12-lead ECG, providing a view of the heart’s electrical activity from the back of the chest. This procedure involves placing three additional leads, labeled V7, V8, and V9, onto the patient’s posterior torso. The primary purpose of this extended recording is to confirm or rule out an acute posterior wall myocardial infarction (MI), which the standard 12-lead ECG often misses entirely. The standard twelve leads, placed on the limbs and front of the chest, create a blind spot for injury in the posterior wall of the left ventricle. Performing a posterior ECG allows healthcare providers to identify patients who require immediate, life-saving reperfusion therapy.
Clinical Necessity for Posterior Leads
The need for a posterior ECG arises when a patient presents with symptoms of a heart attack, such as chest pain, but the standard 12-lead tracing does not show the classic ST-segment elevation. A posterior MI, which accounts for up to 7% of all ST-elevation MIs (STEMIs), produces a characteristic “mirror image” pattern on the anterior leads, V1 through V3. This reciprocal change is the most important trigger to perform the extended test.
Instead of the ST-segment elevation that defines an acute MI, the standard ECG in a posterior infarct will show horizontal ST-segment depression in leads V1 to V3. This depression is the electrical opposite of the true elevation occurring on the posterior wall. Other suspicious changes include tall R waves and upright T waves in the same anterior leads, which are the reciprocal of posterior Q waves and inverted T waves.
These indirect signs are suggestive, but the definitive diagnosis of a posterior STEMI requires direct visualization of the injury current. Failing to recognize this pattern and perform the posterior leads can lead to a misdiagnosis, classifying the patient as having a less severe non-STEMI or unstable angina. Delayed recognition of a true posterior STEMI prevents timely intervention, such as angioplasty or clot-busting drugs, which is associated with worse patient outcomes.
Step-by-Step Lead Placement (V7, V8, V9)
Accurate placement of the posterior leads is paramount for obtaining a diagnostic tracing. The three electrodes, V7, V8, and V9, must all be positioned on the left side of the back and aligned horizontally with the V6 electrode from the standard tracing. This horizontal level corresponds to the fifth intercostal space.
To perform the procedure, the patient must be positioned to allow access to their back, typically by sitting them upright or having them roll onto their right side. The first step is to locate the V6 electrode site on the standard 12-lead placement, which is at the mid-axillary line.
The V7 electrode is placed first on the posterior axillary line, which is the vertical line descending from the back edge of the armpit. The V8 electrode is then placed at the mid-scapular line, which is an imaginary vertical line running down the middle of the left shoulder blade. For the V8 placement, it is often helpful to locate the inferior angle, or tip, of the left scapula.
Finally, the V9 electrode is positioned at the left paraspinal border, which is the vertical line next to the spine on the left side. All three of these leads must remain on the same horizontal plane as the V6 site. To record these three new leads, the leads used for V4, V5, and V6 on the standard tracing are commonly moved to the V7, V8, and V9 positions, respectively, if the machine does not have dedicated additional cables. The resulting tracing is often referred to as a 15-lead ECG.
Interpreting Findings in Posterior ECG
The posterior ECG is considered diagnostic for acute posterior MI when ST-segment elevation is observed in one or more of the V7, V8, or V9 leads. The required degree of elevation is modest compared to the criteria for anterior MIs, reflecting the smaller electrical signal that reaches the posterior chest wall. A finding of ST-segment elevation of \(0.5\) millimeters (or \(0.05\) millivolts) or greater in any single posterior lead is sufficient to diagnose a posterior STEMI.
For men under 40 years of age, a slightly higher threshold of \(1.0\) millimeter (\(0.10\) millivolts) is often used to ensure diagnostic specificity. In addition to the acute changes, the presence of new Q waves in V7 through V9 indicates that the infarct is established and the tissue is necrotic.
Healthcare providers must correlate these findings with the patient’s overall clinical picture, including symptoms and cardiac biomarker levels, though the ECG finding itself often mandates immediate treatment. The timely identification of ST elevation in these posterior leads qualifies the patient for urgent reperfusion therapy, which significantly limits the extent of myocardial damage and improves survival. The posterior ECG unmasks a life-threatening condition that was merely suggested by the anterior leads, ensuring the patient receives appropriate, time-sensitive intervention.