A posterior electrocardiogram (ECG) is a specialized diagnostic test that extends the electrical view of the heart beyond the standard 12-lead acquisition. This procedure involves applying additional electrodes to the patient’s back to create the V7, V8, and V9 leads, often resulting in a 15-lead or 18-lead recording. The primary function of this extended view is to visualize the posterior wall of the left ventricle, which is an area often obscured in a routine ECG recording. This technique provides information necessary to detect myocardial injury in a region that can otherwise be a “blind spot” during initial cardiac assessment.
Clinical Justification for Posterior Leads
The standard 12-lead ECG is frequently insufficient for definitively diagnosing a Posterior Wall Myocardial Infarction (PWMI). None of the routine precordial leads are positioned to look directly at the rear surface of the heart, meaning they can miss acute coronary occlusions. This blind spot is clinically important because PWMI may account for a substantial percentage of all heart attacks.
A posterior infarction is often first suggested by “reciprocal changes” seen on the standard ECG in leads V1 through V3. These reciprocal changes include horizontal ST-segment depression, tall R waves, and upright T waves in those anterior leads. Failure to confirm this suspected injury with V7, V8, and V9 can result in the patient being misclassified, leading to delays in time-sensitive reperfusion therapy and potentially worse health outcomes.
Essential Equipment and Patient Preparation
The procedure requires a standard ECG machine capable of recording the additional leads, along with a set of electrodes and connecting cables. Typically, the machine’s existing V4, V5, and V6 cables are temporarily repurposed for the posterior leads, or separate dedicated cables may be used for a simultaneous 15-lead acquisition.
Patient comfort is a high priority, so the patient is often placed in a semi-recumbent position or rolled slightly onto their right side into a left lateral decubitus position. This positioning helps to expose the posterior chest wall and allows for easier access for electrode placement.
Before application, meticulous skin preparation is crucial to ensure a high-quality electrical signal and minimize interference from artifact. The skin must be cleaned to remove any oils, lotions, or dead skin cells that can impede electrical conduction. This cleaning is usually performed with soap and water or a non-alcohol wipe, followed by vigorous drying of the area. If excessive body hair is present, it should be clipped or shaved to allow for optimal electrode adhesion and skin contact.
Step-by-Step Electrode Placement
The placement of the V7, V8, and V9 electrodes is precise and relies on specific anatomical landmarks on the left side of the back. All three posterior leads must be aligned horizontally with the standard V6 position, which is typically found at the level of the fifth intercostal space.
The first electrode, V7, is applied along the left posterior axillary line (the back edge of the armpit). Moving inward, the second electrode, V8, is placed on the left mid-scapular line, often corresponding to the area just below the tip of the left shoulder blade. The final electrode, V9, is positioned on the left paravertebral line, immediately next to the spine.
To facilitate V9 placement, the patient’s left arm should be gently moved forward across the chest, which pulls the scapula out of the way. Once the electrodes are correctly positioned, the repurposed V4, V5, and V6 cables are attached to the new V7, V8, and V9 sites, and the machine is set to record the extended tracing.
Interpreting V7, V8, and V9
The interpretation of the posterior leads centers on identifying ST-segment elevation, the direct sign of acute injury. A positive finding for an acute PWMI is defined by an ST-segment elevation of 0.5 millimeter (mm) or greater in any of the V7, V8, or V9 leads. This threshold is lower than the criteria used for anterior leads because the posterior electrodes are physically farther from the heart muscle, resulting in a smaller electrical signal.
This finding provides the definitive confirmation of the problem suggested by the reciprocal ST-segment depression in V1 through V3 on the initial recording. The presence of ST elevation in these posterior leads means the patient is experiencing an ST-Elevation Myocardial Infarction (STEMI) equivalent. This clear evidence of acute injury is a direct trigger for emergency reperfusion strategies, such as percutaneous coronary intervention, to salvage the affected heart muscle.