What does a flat T wave on an electrocardiogram (ECG) indicate?

Flat T Waves on ECG: Clinical Significance and Evaluation

Flat T waves on ECG indicate potential cardiac abnormalities including electrolyte disturbances, ischemia, or early cardiomyopathy, and warrant careful clinical evaluation especially when present in leads I, II, aVL, and V4-V6. 1, 2

Definition and Normal Parameters

• Flat T waves are defined as T waves with peak amplitude between -0.1 and +0.1 mV in leads I, II, aVL, and V4-V6 1
• Normal T waves are typically upright in leads I, II and chest leads V3-V6, and may be inverted or upright in leads aVL, III, and V1 1
• T-wave amplitude is normally most positive in lead V2 or V3, with upper normal thresholds of 1.0-1.4 mV in men and 0.7-1.0 mV in women 1
• A T wave may also be considered "low" when its amplitude is less than 10% of the R-wave amplitude in the same lead 1

Clinical Significance

• Flat T waves are associated with increased risk of adverse cardiovascular outcomes, with a relative risk of 1.4 (95% CI = 1.1 to 1.9) for 30-day cardiovascular events compared to normal T waves 3
• Flat T waves in lateral leads are independently associated with increased risk of sudden cardiac death (hazard ratio 1.81; 95% CI 1.13-2.91) even after adjusting for multiple factors 4
• Flat T waves may represent the initial phenotypic expression of underlying cardiomyopathy, even before detectable structural changes appear on cardiac imaging 2
• Patients with flat T waves tend to be older and have more cardiovascular comorbidities compared to those with normal T waves 4

Differential Diagnosis

• Electrolyte abnormalities, particularly hypokalemia 1, 2
• Early ischemic changes in acute coronary syndrome 2, 3
• Cardiomyopathies (hypertrophic, dilated, arrhythmogenic right ventricular) 2, 5
• Medication effects (quinidine-like drugs, tricyclic antidepressants, phenothiazines) 1, 5
• Systemic hypertension 5
• Left ventricular non-compaction 5
• Aortic valve disease 5

Evaluation Algorithm

1. Initial Assessment:

   • Evaluate for cardiac symptoms (chest pain, dyspnea, palpitations, syncope) 2
   • Assess cardiovascular risk factors (age, hypertension, diabetes, smoking, family history) 2
   • Check for medication use that may affect T waves 5

2. Laboratory Testing:

   • Obtain electrolytes, particularly potassium levels 1, 2
   • Measure cardiac biomarkers (troponin) to rule out acute myocardial injury 2

3. Imaging:

   • Perform echocardiography to exclude structural heart disease, even if initial evaluation is normal 2, 5
   • Consider advanced imaging such as cardiac MRI with gadolinium to detect subtle myocardial abnormalities or fibrosis if echocardiography is non-diagnostic 2, 5

4. Additional Testing Based on Initial Findings:

   • Consider stress testing to evaluate for inducible ischemia 6
   • Consider coronary evaluation (CT angiography or invasive coronary angiography) if suspicion for coronary artery disease is high 6
   • Obtain Holter monitoring to detect ventricular arrhythmias if cardiomyopathy is suspected 5

Clinical Pitfalls and Considerations

• Interpreting isolated T-wave abnormalities is difficult and often leads to ambiguous and inaccurate conclusions 1
• Inappropriate diagnoses of myocardial ischemia and infarction are common errors when evaluating T-wave abnormalities 1
• A single normal echocardiogram does not exclude the possibility of developing cardiomyopathy in the future 6
• Do not dismiss flat T waves as a normal variant without proper evaluation, especially when present in lateral leads (I, aVL, V5-V6) 5
• T-wave abnormalities that are secondary to abnormalities in ventricular conduction should be labeled as such 1

Follow-up Recommendations

• Perform serial ECGs and echocardiography to monitor for development of structural heart disease, even if initial evaluation is normal 2, 5
• Consider cardiology consultation for ongoing management, especially if flat T waves are present in lateral leads or accompanied by other concerning findings 2
• Implement risk factor modification based on findings 6
• Long-term follow-up is essential as T-wave abnormalities may precede structural heart disease by months or years 6