CONTENTS

• Low voltage
  • General causes of low voltage
  • Limb lead low voltage
    • DCM (dilated cardiomyopathy)
  • Precordial lead low voltage
• Pericardial effusion
  • Electrical alternans
• Hypothyroidism

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low voltage

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diagnostic criteria for low voltage

• The entire QRS complex is <5 mm in all limb leads (more common).
• The entire QRS complex is <10 mm in all precordial leads (more specific for pathology). (28495230)

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general causes of low voltage

insulation between the heart and electrodes 

• Fluid:
  • 🚨 Pericardial effusion.
  • Pleural effusion.
• Obesity.
• Air:
  • Emphysema.
  • Subcutaneous emphysema.
  • Left-sided pneumothorax.
  • Pneumomediastinum.

weak heart

• 🚨 Acute reduction in voltage:
  • Myocarditis.
  • Takotsubo cardiomyopathy.
  • Acute myocardial stunning due to MI.
• Dilated cardiomyopathy (often reduced voltages in the frontal plane, but not the horizontal plane). (de Luna 2022)
  • Chronic ischemic cardiomyopathy.
  • Nonischemic dilated cardiomyopathy (may tend to have larger voltages in V3, as compared to ischemic cardiomyopathy). (de Luna 2022)
• Infiltrative cardiomyopathy:
  • Amyloidosis.
  • Sarcoidosis.
  • Hemochromatosis.
• Endocrine:
  • Adrenal insufficiency.
  • Hypothyroidism (usually with bradycardia).

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low voltage in limb leads

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causes of isolated low limb voltage may include: 

• Pericardial effusion.
• Pleural effusion.
• Obesity.
• Lung hyperinflation pattern. 📖
• Infiltrative cardiomyopathy.
• Dilated cardiomyopathy (ischemic or nonischemic). ⚡️ (18353352)
• Prior myocardial infarction.

Comment: One cause of isolated low limb voltage is disease processes that cause the depolarization vector to be in the anterior/posterior direction (orthogonal to the limb leads). Examples of this include lung hyperinflation and dilated cardiomyopathy.  

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DCM (dilated cardiomyopathy)

Goldberger's triad for DCM

• Goldberg's triad consists roughly of the following:
  • [1] Low voltage in the limb leads (<8 mV).
  • [2] LVH that is driven by large precordial voltages (e.g., modified Sokolow-Lyon criteria).
  • [3] Delayed R-wave progression (with S>R in V4).  Slow R-wave progression may be followed by an abrupt transition from a deep S-wave to a tall R-wave.
• The crux of the triad is LVH, with high precordial voltages and low limb lead voltages. This creates a left/right dichotomy when looking grossly at the ECG, with voltage low on the left and high on the right.
• The performance of this triad is controversial. If encountered, it is suggestive of dilated cardiomyopathy.
• The basis of the triad is that LV dilation causes a shift in the horizontal plane vector so that it becomes orthogonal to the frontal plane. (34157938)

other potential findings in DCM

• V3 voltage may help differentiate between nonischemic and ischemic cardiomyopathy.  In ischemic cardiomyopathy, the QRS complex in V3 usually has low voltage, whereas the voltage may be relatively high in nonischemic cardiomyopathy. (de Luna 2022)
• R-V6 > R-V5 (may reflect dilation and greater contact with the left chest wall).
• Left atrial abnormality.

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low voltage in precordial leads

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causes of low voltage in the precordial leads (but not limb leads): 

• Pericardial effusion.
• Obesity.
• Infiltrative cardiomyopathies.
• Coronary disease with extensive infarction. (28495230, O'Keefe 2021)

causes of low voltage in left precordial leads:

• [1] Abnormal body habitus (e.g., adiposity).
• [2] Air or fluid issues:
  • Left-sided pneumothorax.
  • Pneumomediastinum.
  • Subcutaneous emphysema.
  • Lung hyperinflation pattern (e.g., emphysema). 📖
• [3] Anterior myocardial infarction (either remote or subacute).
• [4] Dilated cardiomyopathy (see section above).
• [5] Dextrocardia (may cause weird reverse R-wave transitions).

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pericardial effusion

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traditional ECG triad in pericardial effusion

• [1] Low voltage:
  • Frequently low precordial voltage (with normal limb-lead voltage).
  • It may vary depending on where fluid accumulates.
  • The new onset of low voltage is strongly suggestive.
• [2] Tachycardia.
• [3] Electrical alternans:
  • Rarely seen.
  • Total electrical alternans (involving P, QRS, and T-waves) is more specific for effusion.
  • The differential diagnosis of electrical alternans is discussed in the section below.

other features may include

• PR depression may be seen (especially in effusion due to viral pericarditis, malignancy, or connective tissue disorders).

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electrical alternans

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Electrical alternans is defined as a beat-to-beat variation in at least one component of the ECG signal. This may occur with every other beat or in a recurring ratio (e.g., 3:1 or 4:1).

QRS alternans 

• Large pericardial effusion:
  • Total electrical alternans: If alternans involve P, QRS, and T waves, then effusion is usually present.
  • Effusion is less tightly associated with electrical alternans than is commonly thought. Most patients with pericardial effusion won't have electrical alternans. Alternatively, most ECGs displaying electrical alternans will not be due to pericardial effusion.
• Tachyarrhythmia:
  • [1] Supraventricular tachycardias, including:
    • AF or AFlutter (may relate to subtle variations in conduction due to irregular R-R intervals).
    • AVNRT.
    • AVRT with an accessory pathway (may cause more dramatic alternans).
  • [2] VT (especially bidirectional ventricular tachycardia 📖).
  • ⚠️ Be sure to differentiate QRS alternans from fusion/capture beats (which reveal AV dissociation and generally indicate ventricular tachycardia).
• Alternating bundle branch block.
• Relative left ventricular dysfunction:
  • Severe ventricular dysfunction & cardiomegaly.
  • Hypertension. (O'Keefe 2021)
• Acute PE.
• Hypothermia.
• Respiratory variation (benign):
  • It may occur in III and aVF (due to proximity to the diaphragm).
  • The timing includes a shift every couple of beats (timed with respiratory motion), rather than every other beat.

ST segment & T-wave alternans

• Ischemia may cause ST segment alternans (which may predict an increased risk of malignant arrhythmia). This usually reflects severe and hyperacute ischemia, with the involvement of a sizeable ischemic area. (de Luna 2022)

T-wave alternans

• ⚠️ T-wave alternans is often a harbinger of malignant arrhythmias.
• [1] QT prolongation may cause T-wave alternans (varying T-wave morphology, associated with Torsade de pointes).
  • (a) May occur in a patient with congenital long QT syndrome.
  • (b) May occur in acquired long QT. (de Luna 2022)
• [2] Acute myocardial ischemia.
• [3] Cardiomyopathy (either ischemic or nonischemic). (Berberian 2021)

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hypothyroidism

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common findings in hypothyroidism

• [1] Sinus bradycardia.
• [2] Low-voltage:
  • P-waves may be small and unrecognizable.
  • QRS complexes often have low voltage (which may be partially due to an associated pericardial effusion).
• [3] T-wave flattening or even inversion. (de Luna 2022)
• [4] QT prolongation (potentially leading to Torsade de pointes).

differential diagnosis

• Adrenal insufficiency can also cause sinus bradycardia, low voltage in limb leads, and low or inverted T-waves.
• The entire differential diagnosis of low voltage is listed in the section above. ⚡️

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questions & discussion

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