Interpretation of the Low-Voltage ECG

The low-voltage electrocardiogram (ECG) is associated with various cardiac and noncardiac conditions as well as lead wire reversals and other electronic equipment problems. Low-voltage QRS in the ECG creates problems with continuous cardiac monitoring, such as false lethal arrhythmia alarms at central monitoring stations, and general false alarms, thus contributing to alarm fatigue. The ECG challenge for this issue is to examine the physiological, anatomical, and electrical equipment problems of low-voltage, or low-amplitude, ECG and to suggest methods for troubleshooting the low-voltage ECG to ensure reliable cardiac monitoring.QRS amplitude varies through the lifespan, tends to be greater in males than in females, and is subject to a wide range of individual variations.1 QRS voltage is measured from the nadir of the QRS complex to its peak. Low-voltage ECG is usually defined as a QRS amplitude of 5 mm (0.5 mV) or less in all of the frontal plane leads and 10 mm (1.0 mV) or less in all of the precordial leads.1 The lower normal limits of R-wave amplitude as described by Conover2 are listed in Table 1. Lead V4 usually has the tallest R-wave amplitude. Low voltage often occurs in the frontal plane without low voltage in the precordial leads (voltage discordance). Voltage discordance is defined as a QRS amplitude of 5 mm or less in the frontal plane leads, while at least 2 contiguous precordial leads have voltages greater than 10 mm.3The amplitude of the QRS complex has an inverse relationship to the distance of the recording electrode from the heart, and it is affected by conditions that decrease either the generation of electrical signals from the heart to recording electrodes or the transmission of those signals.3 Low voltage is a standard diagnosis for ECG interpretation, as confirmed by an expert panel in 2007, and is usually recognized in computerized ECG interpretations.4 Low voltage can be present on the initial ECG, or it can develop progressively over time due to changing conditions such as pericardial effusion or bleeding: comparison of serial ECGs is useful for the diagnosis of such conditions.In the absence of a pathological cause, low-voltage QRS in the ECG can be a normal variant, it can be caused by electrical malfunction of the recorder or cable connections, or it can be related to electrode misplacement and cable transposition.5,6 The right leg cable is actually inactive in generation of the frontal plane leads; therefore, it can be placed anywhere on the body and will not affect the QRS amplitude.5 However, transpositions of the active extremity lead wires right arm, left arm, and left leg result in low QRS amplitude and a flat line reading in any of leads I, II, or III, depending on the altered configuration.5 Arm and leg electrode transpositions may not affect precordial lead QRS amplitude, because the central terminal may not be affected, as when switching left arm and right arm lead cables.6 Cable switches also may occur when using continuous monitoring equipment; quality monitoring and regular checks of electrode placement and cable designations alleviate these problems.Equipment malfunction, such as loss of continuous connection of the ECG acquisition module, and loose or dry electrodes also can create signal loss that can generate an artificial low-amplitude QRS. Biomedical engineering specialists can repair 12-lead ECG acquisition equipment, and continuous monitoring electrode problems can be remedied through vigilant skin preparation and regular electrode replacement. In cases in which electrical problems are not present, and no medical explanations are present for low voltage, the condition is considered a normal variant.7The pathological conditions related to low QRS amplitude are divided into situations in which voltage generation is low and those in which normal voltage transmission is affected (Table 2).3 Some situations or conditions involve a combination of transmission and generation problems; in cardiac tamponade, the myocardial tissue is compressed while the fluid surrounding the heart attenuates the QRS voltage read at the surface ECG.8Conditions of low-voltage generation include those in which active myocardial muscle mass is decreased (dilated cardiomyopathy) or in which scar tissue alters waveforms and the voltage measured at the body surface is attenuated (multiple myocardial infarctions).7 Pathological deposition of protein fibers in myocardial tissue also affects QRS voltage, as is the case with the infiltrative disease cardiac amyloidosis.9,10 Low QRS voltage also can be a sign of advanced hemochromatosis, an iron storage disease that causes systolic dysfunction.10 The low voltage seen in myocarditis also is considered to be partially caused by pathological processes within the myocardium.11The 12-lead ECG is measured from electrodes placed on the body surface; therefore, conditions that change the conductivity of tissue between the electrode and the voltage generator (the heart), or the distance of an electrode from the heart, can affect the voltage recorded at the surface electrodes. Many pathological conditions can interfere with conduction, including cardiac tamponade, chronic obstructive pulmonary disease (especially emphysema), pneumothorax, plural effusion, peripheral edema, and subcutaneous emphysema.7 Obesity, hypoalbuminemia, and increased hematocrit levels can also cause low QRS voltage. In addition, hypovolemia (as a result of the Brody effect) also can lead to intermittent low voltage. Decreased blood volume in the heart leads to a decrease in conduction of electrical activity to the surface electrodes.In obese individuals, low-voltage QRS is caused by increased chest wall fat and possibly epicardial fat.12 One study found a significantly higher rate of low QRS voltage in 100 obese individuals when compared with 100 lean individuals. Moreover, the frequency of QRS voltage reduction decreased after patients in the study had weight loss surgery.12Subcutaneous emphysema, usually related to chest surgery, can cause low-voltage QRS by increasing the amount of air between the electrode and the heart. Chronic obstructive pulmonary disease in the form of emphysema also increases both the amount of air and the distance between the heart and the electrodes (increased chest diameter).7 A suggestion has been made that pulmonary edema can cause low voltage, but limited research suggests that voltage change is actually related to concomitant peripheral edema.13 The body serves as a volume conductor, and excess volume in the form of peripheral edema leads to decreased electrical impedance and voltage attenuation.14 Fluid accumulations in large pleural effusions (Figure 1) attenuate QRS voltage by increasing impedance in the chest, which serves as a conductor of electricity.7Continuous cardiac monitoring for patients with low-voltage ECG is challenging. Most modern 5-cable monitoring systems automatically and simultaneously scan a combination of the 6 frontal plane leads and 1 precordial lead. The single chest electrode is placed at one of the standard precordial positions (V1–V6), and the rhythm strip is manually marked with the lead description. Typically, decisions about placement of this electrode are made on the basis of the purpose of cardiac monitoring for the individual. The 2004 American Heart Association standards15 for cardiac monitoring in hospitalized patients advise that monitoring should focus on arrhythmia interpretation, ST-segment monitoring, and QT-interval monitoring, and the precordial electrode positions for each of these will vary. Lead V1 or V6 is used for arrhythmia monitoring, lead V3 or V5 is used for ST-segment monitoring, and the lead with the widest QT interval is used for QT monitoring.When all of the frontal plane leads exhibit low QRS voltage, as shown in the ECG in Figure 1, monitoring systems may be unable to sense the QRS in the frontal plane. When voltage is also low in most of the precordial leads, the system may not be able to sense the preferred lead. In such cases the best strategy is to choose the precordial lead with the tallest QRS as the second monitoring lead.A systematic approach is used to troubleshoot low voltage; first check electrode placement and ensure that the frontal plane electrodes are not transposed. Next, to rule out mechanical equipment or electrical problems, look at the 12-lead ECG and confirm that the patient has low-voltage QRS waveforms. After examining the ECG, select the precordial lead with the highest amplitude on the 12-lead ECG and place the chest monitoring electrode in that position. Figure 1 represents the ECG of a patient with massive right-sided pleural effusion and edema of the lower extremities. Note the low amplitude in all of the frontal plane leads and the first 4 precordial leads. When the patient was monitored in leads II and V1, the monitoring equipment was unable to read the QRS. The chest electrode was moved to the V6 position, which had enough voltage to allow the equipment’s arrhythmia algorithms to work properly.Note that simply increasing the signal gain at the central monitoring station will not solve the problem of monitoring patients with low voltage. In Figure 2a, a morbidly obese patient’s QRS voltage is too low for monitoring in the selected leads, and the system incorrectly identifies the rhythm as ventricular fibrillation when it is actually an atrial dysrhythmia with bundle branch block. In Figure 2b, the gain was raised to the maximum at the central station. Note that the rhythm now appears to be ventricular tachycardia in the second monitored lead, and the computer continues to categorize the rhythm as lethal. The best practice in these situations is to check the equipment and electrode placement and to customize chest electrode placement for maximum QRS amplitude.Low-voltage QRS can be related to equipment malfunction, pathological conditions, and electrode misplacement, or it can be a normal variant. Critical and acute care nurses may have frequent contact with patients with low voltage. Low voltage creates challenges for continuous cardiac monitoring in the form of false alarms. Alarm fatigue develops when frequent false alarms desensitize staff members to critical clinical alarms. Quality control over equipment and the patient-system interface, along with customized electrode placement, can prevent and alleviate false alarms related to low QRS voltage.