Posterior ECG Lead Placement: V7-V9 Position Guide for Posterior MI

Posterior ECG lead placement is the technique most clinicians know exists but rarely perform—until they miss a posterior MI that was hiding in plain sight. The standard 12-lead ECG is essentially blind to the posterior wall of the heart, and that blind spot can cost patients their lives.

Here's what most people don't realize: posterior MIs account for approximately 15-20% of all acute MIs, yet they're among the most frequently missed. The only clue on a standard 12-lead? Subtle ST depression in V1-V3—a finding that's easy to overlook or misinterpret.

This guide teaches you exactly how to perform posterior ECG lead placement using V7, V8, and V9. You'll learn the precise positions, when to use posterior leads on ECG, how to interpret the findings, and practical tips for patients who can't lean forward.

When to Use Posterior ECG Leads

Posterior leads aren't routine—they're indicated when specific ECG patterns suggest posterior wall involvement.

Primary Indication: ST Depression in V1-V3

The most important trigger for posterior leads ECG evaluation:

The "Mirror Image" Concept

Here's why posterior MI shows ST depression anteriorly:

The posterior wall is electrically "behind" the standard precordial leads. When the posterior wall has ST elevation (injury), leads V1-V3 see the opposite of that elevation—they see depression. It's like looking at something in a mirror.

According to American Heart Association guidelines, this ST depression in V1-V3 should prompt immediate evaluation with posterior leads to confirm or rule out posterior STEMI.

Clinical Scenarios Requiring Posterior Leads

Understanding Posterior MI: Why Standard ECG Misses It

To understand why placement of posterior ECG leads matters, you need to understand the anatomy.

The Posterior Wall's Location

The posterior wall of the left ventricle faces backward—toward the spine. The standard precordial leads (V1-V6) are all placed on the front and side of the chest. They simply can't "see" the posterior wall directly.

Think of it this way: if you're standing in front of someone, you can see their face clearly, but their back is invisible to you. That's exactly the relationship between standard leads and the posterior wall.

Blood Supply

The posterior wall is typically supplied by:

• Right coronary artery (RCA) — In right-dominant circulation (85% of people)
• Left circumflex artery (LCx) — In left-dominant circulation (15% of people)

This is why posterior MI frequently accompanies inferior MI (both supplied by RCA) or lateral MI (LCx territory).

Why Isolated Posterior MI Is Dangerous

When posterior MI occurs in isolation (without obvious inferior or lateral involvement), the standard 12-lead may show only subtle ST depression in V1-V3. This can be:

• Dismissed as "reciprocal changes"
• Attributed to "subendocardial ischemia"
• Completely overlooked

The result? A STEMI-equivalent that doesn't get treated as a STEMI. According to research published in clinical cardiology journals, isolated posterior MI has higher mortality partly because of delayed recognition and treatment.

Posterior ECG Leads Position: V7, V8, V9 Step-by-Step Placement

Here's exactly how to perform ECG posterior leads placement correctly.

V7 Position

Location: Posterior axillary line, same horizontal level as V6

To find V7:

1. Locate V6 (mid-axillary line, 5th intercostal space)
2. Note the horizontal level of V6
3. Move posteriorly to the posterior axillary line (the back edge of the armpit)
4. Place V7 at this point, maintaining the same horizontal level as V6

V8 Position

Location: Below the tip of the left scapula, same horizontal level as V6

To find V8:

1. Ask the patient to relax their shoulders
2. Locate the inferior angle (tip) of the left scapula
3. Place V8 just below this point, at the same horizontal level as V6 and V7

V9 Position

Location: Left paraspinal area, same horizontal level as V6

To find V9:

1. Find the left side of the spine
2. Measure approximately 2-3 cm lateral to the spinous processes
3. Place V9 at this point, maintaining the same horizontal level as V6, V7, and V8

Quick Reference Table: Posterior ECG Leads Position

The Critical Rule: Horizontal Alignment with V6

This is the most important principle of lead placement posterior ECG:

Why Alignment Matters

The posterior leads are designed to view the posterior wall from the same electrical "plane" as the lateral precordial leads. If V7-V9 are too high or too low, you're looking at a different part of the heart and may miss pathology or create false findings.

How to Ensure Alignment

My technique for maintaining horizontal alignment:

1. Mark V6's level: Use a skin marker or piece of tape to mark the exact horizontal level of V6 on the patient's side
2. Extend the line: Draw or visualize a horizontal line from V6 around to the back
3. Place V7-V9 on this line: Each electrode goes on this horizontal line at its respective landmark
4. Double-check: Step back and visually confirm all four electrodes (V6, V7, V8, V9) are truly horizontal

Posterior Lead Placement for Difficult Patients

Not every patient can sit forward comfortably for posterior lead placement. Here's how to handle common challenges.

Patient Cannot Sit Up

For patients who must remain supine:

1. Log-roll technique: With assistance, gently roll the patient onto their right side
2. Place V7, V8, V9 on the exposed left posterior chest
3. Connect leads and record quickly
4. Return patient to supine position

Alternative: Slide your hand under the patient's back and place electrodes "blind." This is less accurate but sometimes the only option.

Patient Cannot Lean Forward

If the patient is sitting but can't lean forward (respiratory distress, pain):

1. Have them rotate slightly to the left
2. Place electrodes from the side, reaching around to the back
3. Use the visual alignment technique—mark V6's level and estimate

Obese Patients

Finding landmarks on obese patients can be challenging:

• Scapula tip: Have the patient reach forward (if able) to protract the scapula and make it more prominent
• Posterior axillary line: Palpate the edge of the latissimus dorsi muscle
• Paraspinal area: Palpate the spinous processes directly

For more techniques with challenging patients, see our special ECG placement situations guide.

Intensive Care / Ventilated Patients

For ICU patients who cannot be repositioned:

• Document "posterior leads unable to be obtained due to patient positioning"
• Use the anterior ST depression pattern as a surrogate indicator
• Consider when the patient is more stable or can be briefly repositioned

Comment
byu/roberthermanmd from discussion
inEKGs

Interpreting Posterior ECG Leads

Knowing how to place posterior ECG leads is only half the skill. You need to interpret what you see.

Positive Finding: What Confirms Posterior MI

ST elevation ≥0.5mm (0.05mV) in V7, V8, or V9 = Posterior STEMI

Note that the threshold is lower than standard leads (where we typically use 1mm). This is because:

• The posterior leads are farther from the heart
• Signal amplitude is naturally attenuated
• Even small ST elevation is clinically significant

Expected Normal Findings

In a normal posterior ECG:

• QRS complexes should be small (normal due to distance from heart)
• ST segments should be isoelectric or nearly so
• T waves typically small and upright

The Correlation with Anterior Leads

In posterior MI, you should see reciprocal relationship:

If posterior leads show ST elevation but V1-V3 don't show reciprocal depression, recheck electrode positioning—something may be off.

Combined Approach: Posterior + Right-Sided Leads

In many clinical scenarios, you'll need both posterior and right-sided leads. This is particularly true in inferior STEMI.

The Inferoposterior-RV MI Scenario

Inferior STEMI (II, III, aVF) can involve:

• Right ventricle (check V4R) — See our right-sided ECG lead placement guide
• Posterior wall (check V7-V9)
• Both

When you see inferior STEMI, the complete evaluation is:

1. Standard 12-lead: Confirm inferior STEMI
2. Right-sided leads (V4R minimum): Rule out RV involvement
3. Posterior leads (V7-V9): If ST depression in V1-V3, confirm posterior involvement

The 15-Lead ECG Approach

A 15-lead ECG combines standard 12-lead with V4R and V7-V9, giving you comprehensive cardiac coverage. For the complete technique, see our 15-lead ECG placement guide.

Practical Workflow

When time is critical, here's my approach:

1. Run standard 12-lead
2. If inferior STEMI → immediately add V4R
3. If V1-V3 show ST depression → add V7-V9
4. Total additional time: ~3 minutes for complete evaluation

Common Mistakes in Posterior ECG Lead Placement

For a comprehensive list of ECG placement errors, see our 10 common ECG placement mistakes guide. Here are the posterior-specific errors:

Mistake #1: V7-V9 Not Horizontally Aligned

The problem: Each electrode is at a different height, not matching V6.

The consequence: Inaccurate ST segment assessment—you might miss a posterior MI or diagnose one that isn't there.

The fix: Mark V6's level and extend that line around to the back. Use a measuring tape if needed.

Mistake #2: Forgetting to Check Posterior Leads

The problem: ST depression in V1-V3 is noted but posterior leads aren't obtained.

The consequence: Posterior STEMI misdiagnosed as "non-STEMI" or "reciprocal changes." Treatment delayed.

The fix: Make it automatic: ST depression V1-V3 = posterior leads. No exceptions.

Mistake #3: Electrodes Too Close to Spine (V9)

The problem: V9 placed directly on the spinous processes instead of paravertebrally.

The consequence: Poor electrode contact, excessive artifact, unreliable reading.

The fix: V9 should be 2-3 cm lateral to the spine, over the paraspinal muscles.

Mistake #4: Not Labeling the Tracing

The problem: Posterior leads recorded but not clearly marked.

The consequence: Interpreting physician doesn't know which leads are which.

The fix: Write "V7, V8, V9 - Posterior Leads" directly on the ECG. Some machines can label automatically.

Mistake #5: Using Wrong ST Elevation Threshold

The problem: Applying the standard 1mm threshold to posterior leads.

The consequence: Missing posterior MI that has "only" 0.5-0.75mm of elevation.

The fix: Remember the threshold for posterior leads is ≥0.5mm, not 1mm.

Frequently Asked Questions: Posterior Leads on ECG

Mastering Posterior ECG Lead Placement

Posterior ECG lead placement is an essential skill for any clinician managing acute coronary syndromes. The standard 12-lead ECG has a significant blind spot—the posterior wall—and V7, V8, V9 are the only way to see it directly.

Remember these key principles of posterior ECG lead placement:

• When: ST depression in V1-V3, inferior STEMI, or whenever posterior MI is suspected
• Where: V7 at posterior axillary line, V8 below scapula tip, V9 paraspinal—ALL at V6 level
• Why: Posterior MI represents 15-20% of acute MIs and is frequently missed on standard ECG
• Threshold: ST elevation ≥0.5mm confirms posterior STEMI
• Critical rule: Horizontal alignment with V6 is non-negotiable

The extra 2-3 minutes it takes to obtain posterior leads on ECG can catch a STEMI that would otherwise be missed. That's time well spent for every patient who presents with concerning chest pain.

Clinical Education Team

Our clinical education team includes cardiologists and emergency medicine specialists with extensive experience in acute MI management. This guide reflects current AHA/ACC guidelines combined with practical clinical techniques.