What is a low voltage complex on an electrocardiogram (ECG)?

Low Voltage Complex on ECG: Definition and Clinical Significance

Low voltage on ECG is defined as QRS amplitude <0.5 mV (5 mm) in all limb leads AND/OR <1.0 mV (10 mm) in all precordial leads, and represents a critical warning sign demanding immediate echocardiography to identify life-threatening conditions including cardiac amyloidosis, pericardial effusion with tamponade, and dilated cardiomyopathy. 1, 2

Diagnostic Criteria

Low voltage can be assessed using three distinct methods, each with different prevalence rates and clinical implications:

• Limb lead criteria (L-LQRSV): QRS amplitude ≤0.5 mV in all limb leads—this is the most common pattern, accounting for 81.4% of low voltage cases 3
• Precordial lead criteria (P-LQRSV): QRS amplitude ≤1.0 mV in all precordial leads—accounts for 12.3% of cases 3
• Combined criteria (C-LQRSV): Both limb and precordial criteria met simultaneously—the rarest pattern at 6.4% but carries the highest risk with 12.41% in-hospital mortality 3
• Sokolow index: ≤1.5 mV is independently associated with adverse outcomes (HR 1.690, p=0.0246) and has the highest prevalence among cardiac amyloidosis patients at 60% 4

Life-Threatening Differential Diagnosis

Cardiac Amyloidosis

Low voltage despite increased left ventricular wall thickness on echocardiography is pathognomonic for cardiac amyloidosis. 5, 1 This voltage-to-mass discordance is the key diagnostic finding:

• Reduced QRS voltage occurs in approximately 50% of cardiac amyloidosis patients despite ventricular wall thickening 5
• The ratio of QRS voltage to LV wall thickness is diagnostically valuable 1
• Cardiac amyloidosis shows 49.4% prevalence of low voltage, the highest among all cardiomyopathies 3
• Low voltage is a relatively late finding and may not be useful for early identification 4

Pericardial Effusion/Tamponade

• Pericardial effusion can present with low voltage as the initial ECG finding and requires immediate echocardiography to assess for hemodynamic compromise 1, 6
• The mechanism involves increased distance between the heart and recording electrodes due to fluid accumulation 7

Dilated Cardiomyopathy

• Among patients with low voltage isolated to limb leads (voltage discordance), 63% have dilated ventricles with average ejection fraction of 33% 2
• Low voltage in this context reflects reduced myocardial mass or altered electrical properties 2

Other Critical Causes

• Arrhythmogenic cardiomyopathy: 31.2% prevalence of low voltage 3
• Takotsubo syndrome: 25.7% prevalence of low voltage 3
• Myocarditis and sarcoidosis: Can present with low voltage and conduction abnormalities 1

Extracardiac Causes

Low voltage can result from influences of the passive body volume conductor rather than cardiac pathology:

• Chronic obstructive pulmonary disease: Low diaphragm position causes V3 and V4 to be located above ventricular boundaries, recording negative deflections 5
• Obesity: Increased tissue impedance attenuates QRS voltage 5
• Peripheral edema: Any etiology induces reversible low voltage and reduces P wave and T wave amplitudes 7
• Pleural effusion: Increases distance between heart and chest wall electrodes 7

Technical Causes (Must Exclude First)

Electrode misplacement is the most frequent cause of apparent low voltage and must be excluded before pursuing extensive workup. 8, 9

• Superior displacement of V1 and V2 electrodes reduces R-wave amplitude by approximately 0.1 mV per interspace 5, 9
• Inferior placement of V5 and V6 in the sixth intercostal space alters amplitudes used in ventricular hypertrophy diagnosis 5
• Precordial lead misplacement explains considerable variability in amplitude measurements between serial tracings 5
• Placement of precordial electrodes in women with large breasts remains problematic and affects voltage measurements 5

Mandatory Evaluation Algorithm

Step 1: Verify Technical Accuracy

• Repeat ECG with meticulous attention to electrode placement, ensuring V1 and V2 are in the fourth intercostal space at sternal borders 5, 8
• Ensure V5 and V6 follow the horizontal plane of V4 rather than the fifth intercostal space 5

Step 2: Immediate Echocardiography

Echocardiography is the mandatory first test and must be obtained urgently. 1

• Assess LV systolic function, wall thickness, and presence of pericardial effusion 1
• Calculate wall thickness-to-voltage ratio: Low voltage with increased wall thickness strongly suggests amyloidosis 1
• Evaluate for dilated cardiomyopathy, particularly if voltage discordance (low limb voltage with normal precordial voltage) is present 2

Step 3: Advanced Imaging Based on Initial Findings

• Cardiac MRI with late gadolinium enhancement: Recommended if echocardiogram shows structural abnormalities, especially for tissue characterization in suspected infiltrative disease 1
• Subendocardial late gadolinium enhancement globally suggests cardiac amyloid involvement 5
• Combined low voltage (C-LQRSV) patients show higher prevalence of myocardial fibrosis on CMR compared to isolated patterns 3

Step 4: Additional Testing

• 24-48 hour ambulatory ECG monitoring if conduction abnormalities are present 1
• BNP levels: Elevated in 93% sensitivity and 90% specificity for cardiac amyloidosis involvement, even without clinical heart failure 5
• Consider endomyocardial biopsy if diagnosis remains uncertain after extensive non-invasive testing 5

Prognostic Significance

Combined low voltage (C-LQRSV) confers the highest risk with an odds ratio of 8.63 for in-hospital death or discharge against medical advice. 3

• Overall LQRSV is an independent predictor of adverse outcomes (OR 3.27,95% CI 2.94-3.64) 3
• Sokolow index ≤1.5 mV independently predicts hospitalization, transplant, and death (HR 1.690) 4
• C-LQRSV patients exhibit worse echocardiographic parameters compared to isolated patterns or normal voltage 3

Critical Pitfalls to Avoid

• Never dismiss low voltage as benign without echocardiography: Even in asymptomatic patients, underlying structural disease may be present 1, 6
• Do not rely on low voltage alone for diagnosis: Only 50% of cardiac amyloidosis patients demonstrate this finding 5
• Recognize voltage discordance: Low voltage isolated to limb leads correlates with dilated cardiomyopathy in >60% of cases without other associated conditions 2
• Consider clinical context: Age, cardiac risk factors, symptoms, and family history modify interpretation 9
• Peripheral edema alters measurements: Any edema etiology induces reversible low voltage with enormous clinical implications 7