How do you differentiate between old and new ischemia on an electrocardiogram (ECG)?

Differentiating Old from New Ischemia on ECG

The most reliable way to differentiate between old and new ischemia on ECG is to identify dynamic changes in ST segments and T waves, with new ischemia showing acute ST-segment elevation or depression, while old ischemia typically demonstrates pathological Q waves without active ST-T changes.

Key ECG Findings in New (Acute) Ischemia

ST-Segment Changes

• ST-segment elevation:

  • ≥2.0 mm (0.2 mV) in men ≥40 years in leads V2-V3
  • ≥2.5 mm (0.25 mV) in men <40 years in leads V2-V3
  • ≥1.5 mm (0.15 mV) in women in leads V2-V3
  • ≥1.0 mm (0.1 mV) in other contiguous leads 1

• ST-segment depression:

  • New horizontal or downsloping ST depression ≥0.05 mV (0.5 mm) in two contiguous leads 1
  • ST depression in leads V1-V3 may indicate posterior wall ischemia, especially when accompanied by upright T waves 2

T-Wave Changes

• New T-wave inversion ≥0.1 mV (1 mm) in two contiguous leads with prominent R wave or R/S ratio >1 1
• Hyperacute T waves (tall, symmetrical, and peaked) may be the earliest sign of acute ischemia, appearing before ST elevation 1
• Deeply inverted T waves (>0.5 mV) in leads V2-V4, often with QT prolongation, may indicate severe stenosis of the proximal left anterior descending coronary artery 1

Dynamic Changes

• Serial ECG recordings showing evolving changes in the ST-T waveforms strongly suggest acute ischemia 1
• The appearance of new changes or worsening of existing changes during symptoms is highly indicative of acute ischemia

Key ECG Findings in Old (Prior) Ischemia

Q Waves

• Pathological Q waves without active ST-T changes are the hallmark of prior myocardial infarction 1
• Q wave criteria for prior MI:
  • Q wave ≥0.03 sec and ≥0.1 mV deep or QS complex in leads I, II, aVL, aVF or V1-V6 in any two contiguous leads 1
  • Any Q wave in leads V2-V3 ≥0.02 sec or QS complex in leads V2 and V3 1
  • R wave ≥0.04 sec in V1-V2 and R/S ≥1 with a concordant positive T wave in absence of conduction defect 1

Persistent T-Wave Inversion

• T waves may remain inverted in leads with previous ST-segment elevation for varying periods (days to permanently) after an ischemic event 1
• These persistent T-wave inversions without dynamic ST changes suggest old ischemia

Distinguishing Features

Temporal Evolution

• New ischemia: Dynamic changes over minutes to hours
• Old ischemia: Stable findings on serial ECGs

Associated Findings

• New ischemia: Often accompanied by symptoms (chest pain, dyspnea), elevated cardiac biomarkers
• Old ischemia: Usually without acute symptoms, normal cardiac biomarkers (unless there's reinfarction)

Serial ECG Comparison

• Comparison with previous ECGs is crucial - the most reliable method to distinguish new from old changes 1
• New changes from a prior baseline ECG strongly suggest acute ischemia

Special Considerations

Posterior Wall Ischemia

• ST depression in leads V1-V3 with upright T waves may represent posterior wall ischemia 2
• Consider recording posterior leads (V7-V9) where ST elevation ≥0.5 mm confirms posterior involvement 2

Left Bundle Branch Block (LBBB)

• Ischemia diagnosis is challenging with LBBB
• Concordant ST elevation (in leads with positive QRS) is one of the best indicators of acute ischemia with occluded artery 1

Common Pitfalls

1. Failing to obtain serial ECGs: A single ECG may miss evolving changes

   • Solution: Obtain serial ECGs at 15-30 minute intervals in symptomatic patients with initially non-diagnostic ECGs 1

2. Misinterpreting ST depression:

   • Upsloping ST depression is generally considered "equivocal" and not specifically diagnostic for ischemia 1
   • Horizontal or downsloping ST depression is more specific for ischemia

3. Overlooking reciprocal changes:

   • ST elevation in one territory may cause reciprocal ST depression elsewhere
   • These reciprocal changes can help confirm true ischemia versus false positives

4. Confusing ST elevation in Q-wave leads:

   • ST elevation in leads with pre-existing Q waves may represent either:
     • New ischemia in the peri-infarct zone
     • Ventricular wall motion abnormality (dyskinesis/akinesis) 1

5. Missing global ischemia pattern:

   • Widespread ST depression with inverted T waves (maximal in V4-V5) plus ST elevation in aVR suggests global ischemia, often due to left main or severe multivessel disease 3

Remember that the ECG is just one component of diagnosis. Clinical context, cardiac biomarkers, and imaging studies should be integrated for accurate assessment of ischemia timing.