Trystan Casey
Electrical alternans is an electrocardiographic phenomenon where there is a beat-to-beat variation in the height (amplitude), duration (length of the interval), or direction (up or down) of any of the EKG complexes or intervals. It is a broad term that describes the alternate-beat variation in the electrocardiograph (ECG) waveform. Electrical alternans is not a distinct entity but rather an ECG sign of an underlying cardiac pathology. It is estimated to be encountered in about 1 to 6 out of 10,000 ECGs. The two most common causes of electrical alternans are cardiac tamponade and pericardial effusion.
| Characteristics | Values |
|---|---|
| Definition | Electrical alternans is an electrocardiographic phenomenon that describes the beat-to-beat alternation in the height (amplitude), duration (length of the interval), or direction (up or down) of any of the EKG complexes or intervals. |
| Common Causes | Cardiac tamponade, pericardial effusion, hypertrophic cardiomyopathy, and bidirectional ventricular tachycardia from digoxin toxicity. |
| Types | P wave, PR interval, QRS complex, R-R interval, ST segment, T wave, and U wave forms of alternans. |
| Incidence | Electrical alternans is estimated to be encountered in about 1 to 6 out of 10,000 ECGs. |
| Prognosis | The finding of electrical alternans during supraventricular tachycardia does not change the prognosis. However, some forms, such as obvious "macro-T wave" alternans, indicate impending ventricular tachycardia and fibrillation leading to cardiac arrest. |
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What You'll Learn
QRS complex amplitude
Electrical alternans is an electrocardiographic phenomenon characterised by the alternation of QRS complex amplitude or axis between beats, with a possible wandering baseline. The QRS complex is the combination of three graphical deflections (Q-wave, R-wave, and S-wave) observed on a typical electrocardiogram (ECG). It is usually the most visually prominent part of the tracing. The duration, amplitude, and morphology of the QRS complex are useful in diagnosing cardiac arrhythmias, conduction abnormalities, ventricular hypertrophy, myocardial infarction, electrolyte derangements, and other disease states.
The amplitude of the QRS complex is measured using the PR segment as the baseline. The PR segment is the flat line between the end of the P-wave and the onset of the QRS complex, reflecting slow impulse conduction through the atrioventricular node. The QRS complex amplitude is important in detecting electrical alternans, which can be observed in conditions such as cardiac tamponade and severe pericardial effusion. In these cases, the heart's pendulum-like motion and rotation from beat to beat are detected as varying amplitudes on the ECG electrodes, resulting in the classic ECG rhythm.
The amplitude of the QRS complex is also significant in the detection of arrhythmias. Accurate R-wave peak detection is crucial for heart rate measurement and arrhythmia detection. The R-wave amplitude in leads I, II, and III should typically be less than or equal to 20 mm, and the normal progression of the R-wave implies a gradual increase in amplitude from V1 to V5, followed by a decrease from V5 to V6. Poor R-wave progression can be attributed to anterior myocardial infarction, left bundle branch block, Wolff-Parkinson-White syndrome, or ventricular hypertrophy.
Additionally, the QRS complex amplitude plays a role in assessing ventricular depolarization and repolarization. The QT duration, measured from the onset of the QRS complex to the end of the T-wave, reflects the total duration of these processes. The QT duration is inversely related to heart rate, and the heart rate-adjusted QT interval is referred to as the corrected QT interval (QTc interval). A prolonged QTc interval increases the risk of ventricular arrhythmias.
In summary, the QRS complex amplitude is a critical aspect of ECG interpretation and has important implications for diagnosing various cardiac conditions, including electrical alternans, arrhythmias, and ventricular abnormalities. The measurement of the QRS complex amplitude from the PR segment baseline provides valuable information for clinical evaluation and decision-making.
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T wave alternans
T-wave alternans (TWA) is a beat-to-beat variation in the amplitude and/or morphology of the T-wave component of the ECG. The TWA test uses an ECG to measure the heart's electrical conduction by attaching electrodes to the torso. The test looks for the presence of repolarization alternans (T-wave alternans), which is a variation in the vector and amplitude of the T wave. This variation is small, in the range of microvolts, so sensitive digital signal processing techniques are required to detect TWA.
Microvolt T-wave alternans (MTWA) is a variant of TWA that can detect T-wave alternans signals as small as one-millionth of a volt. It is defined as an alternation in the morphology of the T wave in an every-other-beat or AB-AB pattern. Research conducted in the early 1980s by Dr Richard Cohen and colleagues at MIT established a link between visually imperceptible alternans at the microvolt level and susceptibility to arrhythmias. Dr Cohen, along with Dr Joseph Smith, also developed the Spectral Method, which allowed for the measurement of alternans at the level of one microvolt.
MTWA acts as a risk stratifier between patients who need implantable cardiac defibrillators (ICDs) and those who do not. Patients who test negative for MTWA are less likely to require an ICD than those who test positive. TWA has been shown to be closely associated with the risk of sudden cardiac death (SCD) across a wide range of clinical settings. It is important to note that TWA is a rate-dependent phenomenon, and once induced, it is stable and persistent.
The underlying mechanism of TWA is thought to be related to cellular alternans and variation in action potential duration due to dysregulation of repolarization. If neighbouring cells repolarize in opposite phases, the resultant repolarization gradient and heterogeneity in cellular repolarization can lead to ventricular arrhythmias.
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ST segment alternans
In the context of variant angina, a form of chest pain caused by transient myocardial ischemic attacks, ST segment alternans is a significant indicator of the severity of ischemia and the likelihood of ventricular arrhythmias. It often precedes these arrhythmias, acting as an early warning sign. The presence of ST segment alternans during ischemic attacks is associated with greater ST elevation and a longer duration of the attack.
Additionally, ST segment alternans has been observed in cases of cocaine toxicity, where it is associated with profound myocardial ischemia and injury. This highlights the importance of recognizing ST segment alternans as a potential indicator of cardiac complications in toxicological emergencies.
Overall, ST segment alternans is a valuable ECG finding that provides insights into cardiac function and can guide clinical decision-making in various cardiac conditions, particularly those involving reduced blood flow to the heart muscle.
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P wave and PR interval
Electrical alternans is an electrocardiographic phenomenon characterised by the alternation of QRS complex amplitude or axis between beats and a possible wandering baseline. While the QRS complex is a key component of ECG interpretation, it is important to also understand the P wave and PR interval.
The P wave and PR interval are integral parts of an ECG. The P wave reflects atrial depolarisation (activation), while the PR interval is the distance between the onset of the P wave and the start of the QRS complex. This interval reflects conduction through the AV node and typically measures between 120-200ms. A PR interval of less than 120ms may indicate that electrical impulses are travelling too quickly between the atria and ventricles, as seen in Wolff-Parkinson-White syndrome. Conversely, a prolonged PR interval suggests a delay in the electrical signal as it passes through the AV node, known as a first-degree AV block.
The PR interval is assessed to determine whether impulse conduction from the atria to the ventricles is normal. The flat line between the end of the P wave and the beginning of the QRS complex is called the PR segment, which acts as the baseline for the ECG curve. The PR segment reflects slow impulse conduction through the atrioventricular node. Any abnormalities in the morphology or timing of the P wave and PR segment can indicate significant clinical pathology, making their interpretation crucial for healthcare professionals monitoring patients with cardiac abnormalities.
The interpretation of the P wave and PR interval is essential in ECG analysis. While a detailed understanding of their normal morphology is necessary, it is equally important to recognise common abnormalities. These deviations from the norm can indicate disease and lead to better patient management and treatment. For example, the characteristic features of Wolff-Parkinson-White syndrome include a short PR interval, a broad QRS complex, and a slurred upstroke known as the delta wave. Thus, the P wave and PR interval play a critical role in diagnosing and treating cardiac conditions.
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Electrical axis of the heart
The electrical axis of the heart is an integral part of interpreting an ECG. It reflects the average direction of ventricular depolarization during ventricular contraction, which is generally alongside the heart's longitudinal axis (to the left and downwards). The electrical axis is also known as the cardiac axis and is the sum of all depolarization vectors of the heart. Vector analysis determines the direction of the net flow of current through the heart.
The axis is described in degrees, with 0° indicating a horizontal direction towards the heart's left side. A normal cardiac vector runs from the base of the heart to the apex. The normal axis range is between -30° and +90°. If the axis is more positive than 90°, it is referred to as right axis deviation. If the axis is more negative than -30°, it is referred to as left axis deviation.
The ventricular axis is typically used in common clinical practice, although the atrial axis can be useful in certain clinical situations. The left ventricle makes up most of the heart muscle, so the normal cardiac axis is directed downward and slightly to the left. Abnormal axis deviation, indicating underlying pathology, is demonstrated by left axis deviation and right axis deviation.
Determining the electrical axis is a key step in interpreting an ECG, as it can provide insight into underlying disease states and help make certain diagnoses. It is also important for correctly identifying life-threatening arrhythmias that have implications for definitive management strategies.
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Frequently asked questions
Electrical alternans is an electrocardiographic phenomenon that describes the beat-to-beat variation in the height (amplitude), duration (length of the interval), or direction (up or down) of any of the EKG complexes or intervals.
While electrical alternans is usually associated with the QRS complex, it can also take the form of P wave, PR interval, QRS complex, R-R interval, ST segment, T wave, or U wave alternans.
Electrical alternans is caused by variations in the velocity of conduction through the anatomical structures of the heart. This can be due to changes in heart rate, autonomic tone, vagal tone, or drugs.
Electrical alternans is not a distinct entity but rather an indicator of an underlying cardiac pathology. For example, T-wave alternans is a marker of electrical instability and increased risk of cardiac events.
Electrical alternans is estimated to be found in about 1 to 6 out of 10,000 electrocardiograms (ECGs). QRS alternans is the most commonly reported type, while T-wave alternans is observed in about 45% of patients with congenital long QT syndrome.
