T wave alternans (TWA) is a periodic beat-to-beat variation in the amplitude or shape of the T wave in an electrocardiogram (ECG or EKG) TWA was first described in 1908. At that time, only large variations ("macroscopic" TWA) could be detected. Those large TWAs were associated with increased susceptibility to lethal ventricular tachycardias.
Most modern references to TWA refer to microvolt T wave alternans (MTWA), a non-invasive heart test that can identify patients who are at increased risk of sudden cardiac death. [1] [2] It is most often used in patients who have had myocardial infarctions (heart attacks) or other heart damage to see if they are at high risk of developing a potentially lethal cardiac arrhythmia. Those who are found to be at high risk would therefore benefit from the placement of a defibrillator device which can stop an arrhythmia and save the patient's life.
The TWA test uses an electrocardiogram (ECG) measurement of the heart's electrical conduction using electrodes attached to one's torso. It takes approximately a half-hour to perform on an outpatient basis. The test looks for the presence of repolarization alternans (T-wave alternans), which is variation in the vector and amplitude of the T wave component of the EKG. The amount of variation is small, on the order of microvolts, so sensitive digital signal processing techniques are required to detect TWA. See also wikidoc article on TWA.
Microvolt T wave alternans is a variant of T wave alternans that detects T wave alternans signals as small as one-millionth of a volt. Microvolt T wave alternans is defined as an alternation in the morphology of the T wave in an every other beat or AB-AB pattern. It has long been associated with ventricular arrhythmias and sudden death. First recognized nearly a century ago, visually discernible alternans were linked to the rapid onset of ventricular tachyarrhythmias.[ citation needed ]
Research conducted in the early 1980s by Dr. Richard Cohen and his colleagues at MIT explored the idea that visually indiscernible alternans may be equally significant. These efforts established a link between visually imperceptible alternans at the microvolt level and susceptibility to arrhythmias and showed alternans to be a heart rate dependent phenomenon. In addition, Dr. Joseph Smith, working with Dr. Cohen developed a methodology known as the Spectral Method which allowed measurement of alternans at the level of one microvolt.[ citation needed ]
Microvolt T wave alternans testing acts as a risk stratifier between patients who need implantable cardiac defibrillators (ICDs) and those who do not. [3] Patients who test negative for MTWA are less likely to require an ICD than those who test positive. In some cases, the test is equivalent to a more invasive electrophysiology (EP) study. [4]
Multiple prospective clinical trials indicate that patients from broad groups of at risk populations who test MTWA negative will likely live ventricular event-free for 12 to 24 months after their initial MTWA test.[ citation needed ]
MTWA results are given as positive, negative, or indeterminate. Those with indeterminate results can be tested again. Otherwise, positive and indeterminate results are often lumped together when making clinical judgments about the likelihood of sudden cardiac death. In patients who have a negative (normal) MTWA test the risk of sudden cardiac death is very low. The Negative Predictive Value of MTWA testing has been shown to be 98% accurate for follow-up periods of 12–24 months in various clinical studies. Negative patients should be retested every 12 months as cardiac function can change over time. [4]
Patients who test MTWA positive or indeterminate for heart rate or dense ectopy (abnormal) should be referred to an electrophysiologist for further evaluation.[ citation needed ]
Patients who have an indeterminate test should be retested immediately. Studies indicate that over 50% of patients who initially test as indeterminate, become determinate if retested during the same session.[ citation needed ]
Two methods are currently FDA-cleared to perform MTWA testing in the U.S., namely, the Spectral Method, which was developed by Cohen and Smith at M.I.T. and was commercialized by Cambridge Heart, and the Modified Moving Average (MMA) method, which was developed by Nearing and Verrier at Harvard Medical School, Beth Israel Deaconess Medical Center, and is commercialized by GE Healthcare. The Spectral Method requires a specialized exercise protocol and proprietary electrodes and washout of beta-adrenergic blocking agents to allow the patient to achieve a target heart rate of 105-110 beats/min. The MMA method uses routine, symptom-limited exercise stress testing or ambulatory ECG monitoring and standard electrodes and requires that chronic medications be retained. Both methods achieve 1-microvolt resolution. Interpretation of Spectral Method test results is described above. With the MMA method, risk is defined by the peak MTWA level, with cutpoints of 47μV and 60 μV for abnormal and severely abnormal risk, respectively. Quantification of TWA levels allows physicians to track patients' responses to medications and cardiac rehabilitation.[ citation needed ]
Over 8000 subjects have been enrolled in Spectral Method studies that predicted outcomes, including the ALPHA study [5] and the ABCD study. [6] An additional 3145 (28% of total) subjects were enrolled in Spectral Method studies that did not predict outcomes, including the SCD-HeFT TWA substudy, [7] the MASTER study, [8] and the CARISMA study. [9] Over 5000 patients have been enrolled in MMA studies, with >3500 in the FINCAVAS exercise-based series. [10] The remaining 1500 patients were studied during ambulatory ECG monitoring. All MMA-based TWA studies have predicted outcomes. A head-to-head comparison of the Spectral and MMA methods revealed similar hazard ratios, kappa statistics, and areas under the receiver-operator characteristic curve. [11] The MTWA consensus guideline, authored by 11 experts in both methods, described both methods, their history, and their utility. [12] A trial of MTWA-guided ICD implantation, REFINE-ICD (NCT00673842), is underway. [13] MTWA testing has been recommended for ventricular arrhythmia risk assessment by the American College of Cardiology, American Heart Association and European Society of Cardiology [14] and by CMS in National Coverage Analysis for Implantable Cardioverter Defibrillators (CAG-00157N).
This test is significant for insurance companies because it can potentially save thousands of dollars per patient by reducing the implantation of unnecessary implantable cardiac defibrillators (ICD's). [15] Use of the spectral method for measuring Microvolt T-wave Alternans has been approved for reimbursement by Medicare, as well as major insurers such as Aetna, Cigna, and Humana. The Spectral Method was FDA approved in 2001 and approved in 2006 for federal reimbursement by the Center for Medicare and Medicaid Services (CMS), (CAG-00293N). The MMA method received federal approval for local CMS contractor reimbursement in 2015 (CAG-00293R2). Both methods use CPT code 93025.[ citation needed ]
In 2004 & 2005, NASA's Glenn Research Center and Cleveland's MetroHealth Medical Center, Case Western Reserve University, teamed to investigate the value of MTWA testing for astronauts both pre- and during space flight. Experiments were performed both on the ground and on NASA's KC-135 aircraft. [16] [17]
Cardiac arrest is a sudden loss of blood flow resulting from the failure of the heart to pump effectively. Signs include loss of consciousness and abnormal or absent breathing. Some individuals may experience chest pain, shortness of breath, or nausea immediately before entering cardiac arrest. Radiating pain to one arm is a common symptom, as is long term malaise and "general weakness of heart". If not treated within minutes, it typically leads to death.
A cardiac pacemaker, is a medical device that generates electrical impulses delivered by electrodes to cause the heart muscle chambers to contract and therefore pump blood; by doing so this device replaces and/or regulates the function of the electrical conduction system of the heart.
Defibrillation is a treatment for life-threatening cardiac dysrhythmias, specifically ventricular fibrillation (VF) and non-perfusing ventricular tachycardia (VT). A defibrillator delivers a dose of electric current to the heart. Although not fully understood, this process depolarizes a large amount of the heart muscle, ending the dysrhythmia. Subsequently, the body's natural pacemaker in the sinoatrial node of the heart is able to re-establish normal sinus rhythm. A heart which is in asystole (flatline) cannot be restarted by a defibrillator, but would be treated by cardiopulmonary resuscitation (CPR).
Brugada syndrome (BrS) is a genetic disorder in which the electrical activity within the heart is abnormal. It increases the risk of abnormal heart rhythms and sudden cardiac death. Those affected may have episodes of passing out. The abnormal heart rhythms seen in those with Brugada syndrome often occur at rest. They may be triggered by a fever.
Long QT syndrome (LQTS) is a condition in which repolarization of the heart after a heartbeat is affected. It results in an increased risk of an irregular heartbeat which can result in fainting, drowning, seizures, or sudden death. These episodes can be triggered by exercise or stress. Some rare forms of LQTS are associated with other symptoms and signs including deafness and periods of muscle weakness.
An implantable cardioverter-defibrillator (ICD) or automated implantable cardioverter defibrillator (AICD) is a device implantable inside the body, able to perform cardioversion, defibrillation, and pacing of the heart. The device is therefore capable of correcting most life-threatening cardiac arrhythmias. The ICD is the first-line treatment and prophylactic therapy for patients at risk for sudden cardiac death due to ventricular fibrillation and ventricular tachycardia. Current devices can be programmed to detect abnormal heart rhythms and deliver therapy via programmable antitachycardia pacing in addition to low-energy and high-energy shocks.
Arrhythmogenic cardiomyopathy (ACM), arrhythmogenic right ventricular dysplasia (ARVD), or arrhythmogenic right ventricular cardiomyopathy (ARVC), is an inherited heart disease.
Short QT syndrome (SQT) is a very rare genetic disease of the electrical system of the heart, and is associated with an increased risk of abnormal heart rhythms and sudden cardiac death. The syndrome gets its name from a characteristic feature seen on an electrocardiogram (ECG) – a shortening of the QT interval. It is caused by mutations in genes encoding ion channels that shorten the cardiac action potential, and appears to be inherited in an autosomal dominant pattern. The condition is diagnosed using a 12-lead ECG. Short QT syndrome can be treated using an implantable cardioverter-defibrillator or medications including quinidine. Short QT syndrome was first described in 2000, and the first genetic mutation associated with the condition was identified in 2004.
Ventricular tachycardia is a type of regular, fast heart rate that arises from improper electrical activity in the ventricles of the heart. Although a few seconds may not result in problems, longer periods are dangerous; and multiple episodes over a short period of time is referred to as an Electrical Storm. Short periods may occur without symptoms, or present with lightheadedness, palpitations, or chest pain. Ventricular tachycardia may result in ventricular fibrillation and turn into cardiac arrest. It is found initially in about 7% of people in cardiac arrest.
Disopyramide is an antiarrhythmic medication used in the treatment of ventricular tachycardia. It is a sodium channel blocker and therefore classified as a Class 1a anti-arrhythmic agent. Disopyramide has a negative inotropic effect on the ventricular myocardium, significantly decreasing the contractility. Disopyramide also has an anticholinergic effect on the heart which accounts for many adverse side effects. Disopyramide is available in both oral and intravenous forms, and has a low degree of toxicity.
Clinical cardiac electrophysiology, is a branch of the medical specialty of cardiology and is concerned with the study and treatment of rhythm disorders of the heart. Cardiologists with expertise in this area are usually referred to as electrophysiologists. Electrophysiologists are trained in the mechanism, function, and performance of the electrical activities of the heart. Electrophysiologists work closely with other cardiologists and cardiac surgeons to assist or guide therapy for heart rhythm disturbances (arrhythmias). They are trained to perform interventional and surgical procedures to treat cardiac arrhythmia.
Catecholaminergic polymorphic ventricular tachycardia (CPVT) is an inherited genetic disorder that predisposes those affected to potentially life-threatening abnormal heart rhythms or arrhythmias. The arrhythmias seen in CPVT typically occur during exercise or at times of emotional stress, and classically take the form of bidirectional ventricular tachycardia or ventricular fibrillation. Those affected may be asymptomatic, but they may also experience blackouts or even sudden cardiac death.
Cardiac resynchronisation therapy is the insertion of electrodes in the left and right ventricles of the heart, as well as on occasion the right atrium, to treat heart failure by coordinating the function of the left and right ventricles via a pacemaker, a small device inserted into the interior chest wall.
Management of heart failure requires a multimodal approach. It involves a combination of lifestyle modifications, medications, and possibly the use of devices or surgery.
A wearable cardioverter defibrillator (WCD) is a device worn by patients who are at risk for sudden cardiac arrest (SCA). A WCD allows physicians time to assess for their patient's arrhythmic risk and make appropriate plans.
Boxer cardiomyopathy is a disease of the myocardium primarily affecting Boxer dogs. It is characterized by the development of ventricular tachyarrhythmias, resulting in syncope and sudden cardiac death. Myocardial failure and congestive heart failure are uncommon manifestations of the disease.
Cameron Health was a medical device developer based in San Clemente, California, USA. Cameron Health had its European office, Cameron Health BV, in Arnhem, The Netherlands. The privately held company's focus was on a new generation of minimally invasive implantable cardioverter-defibrillator (ICD) which they called a Subcutaneous Implantable Defibrillator (S-ICD). Cameron Health's approach avoided implanting transvenous leads into the heart, which had been the usual procedure for cardiac devices. Instead, the Cameron ICD was entirely implanted outside the thoracic wall.
Cardiac Contractility Modulation is a therapy which is intended for the treatment of patients with moderate to severe heart failure with symptoms despite optimal medical therapy who can benefit from an improvement in cardiac output. The short- and long-term use of this therapy enhances the strength of ventricular contraction and therefore the heart's pumping capacity by modulating (adjusting) the myocardial contractility. This is provided by a pacemaker-like device that applies non-excitatory electrical signals adjusted to and synchronized with the electrical action in the cardiac cycle.
Yaariv Khaykin is a Canadian cardiologist and a clinical researcher in the area of electrophysiology. He is the director of the Newmarket Electrophysiology Research Group at the Southlake Regional Health Centre. He has published research into complex ablation and pioneered cardiac ablation methods.
Defibrillation threshold indicates the minimum amount of energy needed to return normal rhythm to a heart that is beating in a cardiac dysrhythmia. Typical examples are the minimum amount of energy, expressed in joules, delivered by external defibrillator paddles or pads, required to break atrial fibrillation and restore normal sinus rhythm. Other common scenarios are restoring normal rhythm from atrial flutter, ventricular tachycardia or ventricular fibrillation. The defibrillation threshold ranking in these settings, from lowest to highest, would be, in order, ventricular tachycardia, atrial flutter, atrial fibrillation, ventricular fibrillation. The highest amount of energy that an external defibrillator can deliver at the present time is 360 joules biphasic. In clinical practice, the real threshold can be approximated but not exactly established, since the defibrillating shock can be delivered only once. Certain medications, in particular sotalol, tend to lower such threshold, while others, such as amiodarone, may increase it.