Low QRS Voltage in ECG
Low QRS Voltage in ECG
🔹 Definition
Low QRS Voltage is defined when the amplitude of the QRS complexes is abnormally small:
- Limb leads: QRS amplitude < 5 mm (0.5 mV) in all leads.
- Precordial leads: QRS amplitude < 10 mm (1.0 mV).
🔹 Mechanism (Why it happens?)
Low voltage occurs when:
- Electrical impulse generation is reduced → weak depolarization.
- Electrical impulse transmission is dampened → intervening tissue/fluid/fat reduces signal reaching electrodes.
- Myocardial mass reduction → less depolarizing tissue.
🔹 Causes
1. Pericardial & Extracardiac causes
- Pericardial effusion (classic cause)
- Pleural effusion
- Obesity
- COPD (hyperinflated lungs reduce signal)
2. Myocardial causes
- Myxedema (severe hypothyroidism)
- Infiltrative cardiomyopathies (amyloidosis, sarcoidosis, hemochromatosis)
- Myocardial infarction with scarring
- Dilated cardiomyopathy
3. Other systemic causes
- Severe cachexia/malnutrition
- End-stage renal disease (fluid overload)
🔹 Clinical Associations
- Electrical alternans + Low QRS voltage → highly suggestive of large pericardial effusion / cardiac tamponade.
- Low voltage in hypothyroidism often accompanies bradycardia.
- In amyloidosis, “low voltage + thickened ventricular walls on echo” is a diagnostic clue.
🔹 ECG Examples
- Limb leads: all QRS < 5 mm
- Chest leads: all QRS < 10 mm
- Sometimes combined with electrical alternans.
🔹 Quick Mnemonic for Causes: “BIG FAT LAD”
- B – Big effusion (pericardial)
- I – Infiltrative disease (amyloid, sarcoid)
- G – Generalized obesity
- F – Fluid (pleural effusion, anasarca)
- A – Air (emphysema/COPD)
- T – Thyroid (myxedema)
- L – Low muscle mass (cachexia, MI, dilated cardiomyopathy)
- A – Amyloid
- D – Dilated heart
✅ Key Exam Clue:
Low QRS voltage + tachycardia + electrical alternans → Cardiac tamponade until proven otherwise.
1. Low QRS voltage in limb leads is defined as QRS amplitude less than:
Correct: Low QRS voltage is defined as <5 mm amplitude in all limb leads.
2. Low QRS voltage in precordial leads is defined as QRS amplitude less than:
Correct: In precordial leads, low QRS voltage is <10 mm.
3. The classical ECG finding of pericardial effusion is:
Correct: Large pericardial effusion produces low voltage and electrical alternans.
4. Which systemic condition is most commonly associated with low QRS voltage?
Correct: Myxedema (severe hypothyroidism) is a classic cause of low QRS voltage.
5. Electrical alternans refers to:
Correct: Electrical alternans = beat-to-beat alternation in QRS amplitude, often with pericardial effusion.
6. Which infiltrative disease is characteristically associated with low QRS voltage despite increased wall thickness on echo?
Correct: Amyloidosis produces thick myocardium on echo but low QRS voltage on ECG.
7. Which pulmonary condition commonly produces low QRS voltage?
Correct: Hyperinflated lungs in COPD dampen ECG voltages.
8. The mnemonic “BIG FAT LAD” is used for remembering causes of:
Correct: “BIG FAT LAD” lists causes of low QRS voltage.
9. Low QRS voltage in combination with tachycardia and electrical alternans is strongly suggestive of:
Correct: Triad = Low voltage + tachycardia + electrical alternans → cardiac tamponade.
10. Which of the following is NOT a typical cause of low QRS voltage?
Correct: Hyperthyroidism does not usually cause low QRS voltage.
11. In low QRS voltage, which is TRUE?
Correct: Low QRS voltage has multiple etiologies and needs clinical context.
12. Low QRS voltage is most frequently seen in which nutritional/metabolic state?
Correct: Severe malnutrition/cachexia reduces myocardial mass and produces low voltage.
13. In hypothyroidism, the ECG classically shows:
Correct: Myxedema heart → low QRS, sinus bradycardia.
14. Which echocardiographic finding correlates with low QRS voltage in amyloidosis?
Correct: Echo shows thick myocardium with granular sparkling, yet ECG shows low QRS voltage in amyloidosis.
15. Pleural effusion causes low QRS voltage primarily due to:
Correct: Pleural/pericardial fluid dampens voltage reaching chest wall electrodes.
16. Low QRS voltage in all leads is most likely seen in:
Correct: Large pericardial effusion is a classic cause of low voltage in all leads.
17. Which of the following combinations is diagnostic of cardiac tamponade?
Correct: Triad strongly suggests tamponade.
18. Which is NOT a mechanism of low QRS voltage?
Correct: Low voltage results from reduced signal generation/transmission, not faster conduction.
19. Which lead criteria best describe low QRS voltage?
Correct: Standard definition is <5 mm limb leads, <10 mm precordial.
20. Which condition combines thickened ventricles on echo but paradoxically low QRS voltage on ECG?
Correct: Amyloidosis shows the paradox of thick myocardium + low QRS voltage.
