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

Flattened T Wave Indicates

Flattened T waves most commonly indicate hypokalemia (low potassium), but can also represent non-specific findings related to myocardial ischemia, electrolyte disturbances, autonomic tone changes, or early stages of various cardiac conditions.

Primary Clinical Significance

Hypokalemia (Most Common Cause)

• Hypokalemia is the classic cause of T-wave flattening, typically accompanied by ST depression and prominent U waves, with complete reversal upon potassium repletion 1
• When serum potassium falls between 3.0-3.5 mEq/L, T-wave flattening begins to appear; at levels below 2.5 mEq/L, all three findings (sagging ST segments, low T-waves, and prominent U-waves) are commonly present 2
• At the lowest potassium levels, the T-wave may become merely a notch on the upstroke of a giant U-wave, which can be mistaken for ST-segment depression and long QT interval of myocardial ischemia 2

Myocardial Ischemia

• T-wave flattening in lateral leads (V5-V6) is clinically particularly important and concerning for pathological cardiac causes 1
• The American College of Cardiology warns that interpreting isolated T-wave abnormalities is difficult and often leads to inappropriate diagnoses of myocardial ischemia, emphasizing the importance of considering reversible causes such as electrolyte abnormalities first 1
• In patients with potential acute coronary syndrome, T-wave flattening in two or more leads is associated with higher rates of 30-day cardiovascular events (8.2% vs. 5.7% in those without T-wave abnormalities) 3

Diagnostic Approach Algorithm

Step 1: Check Electrolytes Immediately

• Obtain serum potassium, magnesium, and calcium levels as the first diagnostic step, as these are the most common reversible causes 1
• Hypomagnesemia often coexists with hypokalemia and can affect T-wave morphology 4

Step 2: Assess Clinical Context

• Determine if the patient has chest pain, shortness of breath, or other ischemic symptoms requiring immediate ACS evaluation 1
• Compare with prior ECGs if available to identify new changes versus chronic findings 1
• Consider medication history (tricyclic antidepressants, phenothiazines can cause T-wave changes) 1

Step 3: Lead-Specific Analysis

• T-wave flattening in V5-V6 (lateral leads) warrants more aggressive cardiac workup 1
• Diffuse T-wave flattening with prominent U waves strongly suggests hypokalemia 2
• Isolated T-wave flattening in V1-V3 may represent normal variant in certain populations 1

Step 4: Risk Stratification

• If symptomatic with chest pain/dyspnea: Treat as potential ACS with serial ECGs, cardiac biomarkers, and continuous monitoring 1
• If asymptomatic with electrolyte abnormality: Correct electrolytes and repeat ECG to confirm resolution 1, 2
• If persistent after electrolyte correction: Consider stress testing or advanced cardiac imaging 1

Critical Pitfalls to Avoid

• Do not assume ischemia without checking electrolytes first - hypokalemia is a common, completely reversible cause that mimics ischemic changes 1, 2
• The giant U-wave with T-wave notching in severe hypokalemia can be mistaken for ST depression and prolonged QT interval, leading to unnecessary cardiac catheterization 2
• Multiple concurrent electrolyte abnormalities (hyperkalemia with hypocalcemia and hypomagnesemia) can create unusual ECG patterns that obscure individual findings 5
• The specificity of T-wave flattening for any single cause is low, requiring clinical correlation 1

When to Escalate Care

• New T-wave flattening with chest pain or ischemic symptoms requires immediate emergency department evaluation for ACS 1
• T-wave flattening accompanied by other ECG changes (ST depression >0.5mm, new Q waves, arrhythmias) warrants urgent cardiology consultation 1
• Severe hypokalemia (<2.5 mEq/L) with T-wave flattening requires cardiac monitoring due to arrhythmia risk 2