How do you differentiate between ordering a right-sided (electrocardiogram) EKG and a posterior (electrocardiogram) EKG based on a standard view 12-lead (electrocardiogram) EKG?

Differentiating Between Right-Sided and Posterior EKG Based on Standard 12-Lead EKG

Right-sided EKG should be ordered when ST elevation is seen in leads II, III, and aVF (with greater elevation in III than II), while posterior EKG should be ordered when ST depression is observed in leads V1-V3 without inferior lead changes. 1

Right-Sided EKG Indications

Right-sided EKG leads (V3R-V6R, especially V4R) should be obtained when:

• ST elevation is present in inferior leads (II, III, aVF) suggesting inferior wall MI 2
• ST elevation is greater in lead III than in lead II (suggesting RCA occlusion) 1
• ST depression is present in leads I and aVL (reciprocal changes of inferior MI) 1

The American Heart Association and American College of Cardiology specifically recommend recording right-sided precordial leads V3R and V4R when ST elevation greater than 0.1 mV occurs in leads II, III, and aVF 2. This is crucial because:

• Right ventricular involvement significantly impacts morbidity and mortality 1
• These patients are preload-dependent and require careful fluid management 1
• Nitrates and other preload-reducing medications should be avoided 1
• ST elevation in right-sided leads disappears more rapidly than other ST changes, making early acquisition essential 2, 1

Posterior EKG Indications

Posterior EKG leads (V7-V9) should be obtained when:

• ST depression is present in precordial leads V1-V3 without inferior lead changes 2, 1
• There is suspicion of posterior wall infarction but the standard 12-lead ECG is non-diagnostic 3
• Distinguishing between posterior MI and anterior ischemia is necessary 2

The American College of Cardiology recommends posterior leads in settings where treatment will depend on documentation of ST elevation during infarction or other acute coronary syndrome 2.

Key Differences in Interpretation

1. Right-sided EKG:

   • ST elevation ≥0.1 mV in one or more right precordial leads (especially V4R) is diagnostic for right ventricular injury 2, 4
   • Most commonly associated with inferior MI (RCA occlusion) 1
   • Requires immediate recording as ST changes in right-sided leads disappear rapidly 2

2. Posterior EKG:

   • ST elevation ≥0.5 mm (not 1 mm as in standard leads) in V7-V9 confirms posterior MI 1, 5
   • Often associated with ST depression in V1-V3 on standard ECG 2, 1
   • May be the only site of ST elevation in some cases of posterior infarction 2
   • Often associated with left circumflex artery occlusion 3

Important Clinical Considerations

• Right ventricular involvement occurs in approximately 30-50% of inferior MIs and significantly increases morbidity and mortality 4
• Posterior MI is often missed on standard 12-lead ECG because no leads directly face the posterior wall 3
• Using a lower threshold (0.5 mm vs 1 mm) for ST elevation in posterior leads significantly improves sensitivity for detecting posterior ischemia 5
• Some cases of posterior MI show ST elevation only in posterior leads without reciprocal anterior ST depression 5

Common Pitfalls

• Failing to recognize that ST depression in V1-V3 may represent posterior MI requiring STEMI management protocols 1
• Delaying acquisition of right-sided leads, as ST elevation in these leads disappears more quickly than other ST changes 2, 1
• Using the standard 1 mm ST elevation criterion for posterior leads instead of the more appropriate 0.5 mm threshold 5
• Overlooking left circumflex occlusion, which is often missed on standard 12-lead ECG 1

By following these guidelines, clinicians can appropriately determine when to order right-sided versus posterior ECG leads based on findings from the standard 12-lead ECG, potentially improving diagnosis and management of acute coronary syndromes.