The diagnosis of LQTS is not easy since 2.5% of the healthy population has prolonged QT interval, and 10–15% of LQTS patients have a normal QT interval. A commonly used criterion to diagnose LQTS is the LQTS "diagnostic score", calculated by assigning different points to various criteria (listed below). With four or more points, the probability is high for LQTS; with one point or less, the probability is low. A score of two or three points indicates intermediate probability.

- QTc (Defined as QT interval / square root of RR interval)

- ≥ 480 ms - 3 points

- 460-470 ms - 2 points

- 450 ms and male gender - 1 point

- "Torsades de pointes" ventricular tachycardia - 2 points

- T wave alternans - 1 point

- Notched T wave in at least 3 leads - 1 point

- Low heart rate for age (children) - 0.5 points

- Syncope (one cannot receive points both for syncope and "torsades de pointes")

- With stress - 2 points

- Without stress - 1 point

- Congenital deafness - 0.5 points

- Family history (the same family member cannot be counted for LQTS and sudden death)

- Other family members with definite LQTS - 1 point

- Sudden death in immediate family members (before age 30) - 0.5 points

Recent diagnostic criteria have been published out of the Arrhythmia Research Laboratory at the University of Ottawa Heart Institute from Drs. Michael H Gollob and Jason D Roberts.

The Short QT Syndrome diagnostic criterion is based on a point system as follows:

QTc in milliseconds

Jpoint-Tpeak interval

Clinical History

Family History

Genotype

Patients are deemed high-probability (> or equal to 4 points), intermediate probability (3 points) or low probability (2 or less points).

The risk for untreated LQTS patients having events (syncopes or cardiac arrest) can be predicted from their genotype (LQT1-8), gender, and corrected QT interval.

- High risk (> 50%) - QTc > 500 ms, LQT1, LQT2, and LQT3 (males)

- Intermediate risk (30-50%) - QTc > 500 ms, LQT3 (females) or QTc < 500 ms, LQT2 (females) and LQT3

- Low risk (< 30%) - QTc < 500 ms, LQT1 and LQT2 (males)

A 1992 study reported that mortality for symptomatic, untreated patients was 20% within the first year and 50% within the first 10 years after the initial syncope.

Genetic testing for Brugada syndrome is clinically available and may help confirm a diagnosis, as well as differentiate between relatives who are at risk for the disease and those who are not. Some symptoms when pinpointing this disease include fainting, irregular heartbeats, and chaotic heartbeats. However, just detecting the irregular heartbeat may be a sign of another disease, so the doctor must detect another symptom as well.

In terms of the diagnosis of Romano–Ward syndrome the following is done to ascertain the condition(the "Schwartz Score" helps in so doing):

- Exercise test

- ECG

- Family history

In some cases, the disease can be detected by observing characteristic patterns on an electrocardiogram. These patterns may be present all the time, they might be elicited by the administration of particular drugs (e.g., Class IA, such as ajmaline or procainamide, or class 1C, such as flecainide or pilsicainide, antiarrhythmic drugs that block sodium channels and cause appearance of ECG abnormalities), or they might resurface spontaneously due to as-yet unclarified triggers.

Brugada syndrome has three different ECG patterns:

- Type 1 has a coved type ST elevation with at least 2 mm (0.2 mV) J-point elevation and a gradually descending ST segment followed by a negative T-wave.

- Type 2 has a saddle-back pattern with a least 2 mm J-point elevation and at least 1 mm ST elevation with a positive or biphasic T-wave. Type 2 pattern can occasionally be seen in healthy subjects.

- Type 3 has either a coved (type 1 like) or a saddle-back (type 2 like) pattern, with less than 2 mm J-point elevation and less than 1 mm ST elevation. Type 3 pattern is not rare in healthy subjects.

The pattern seen on the ECG is persistent ST elevations in the electrocardiographic leads V-V with a right bundle branch block (RBBB) appearance, with or without the terminal S waves in the lateral leads that are associated with a typical RBBB. A prolongation of the PR interval (a conduction disturbance in the heart) is also frequently seen. The ECG can fluctuate over time, depending on the autonomic balance and the administration of antiarrhythmic drugs. Adrenergic stimulation decreases the ST segment elevation, while vagal stimulation worsens it. (There is a case report of a patient who died while shaving, presumed due to the vagal stimulation of the carotid sinus massage.)

The administration of class Ia, Ic, and III drugs increases the ST segment elevation, as does fever. Exercise decreases ST segment elevation in some people, but increases it in others (after exercise, when the body temperature has risen). The changes in heart rate induced by atrial pacing are accompanied by changes in the degree of ST segment elevation. When the heart rate decreases, the ST segment elevation increases, and when the heart rate increases, the ST segment elevation decreases. However, the contrary can also be observed.

Currently, some individuals with short QT syndrome have had implantation of an implantable cardioverter-defibrillator (ICD) as a preventive action, although it has not been demonstrated that heart problems have occurred before deciding to implant an ICD.

A recent study has suggested the use of certain antiarrhythmic agents, particularly quinidine, may be of benefit in individuals with short QT syndrome due to their effects on prolonging the action potential and by their action on the I channels. Some trials are currently under way but do not show a longer QT statistically.

Treatment for Romano–Ward syndrome can "deal with" the imbalance between the right and left sides of the sympathetic nervous system which may play a role in the cause of this syndrome. The imbalance can be temporarily abolished with a left stellate ganglion block, which shorten the QT interval. If this is successful, surgical ganglionectomy can be performed as a permanent treatment.Ventricular dysrhythmia may be managed by beta-adrenergic blockade (propranolol)

Syndactyly and other deformities are typically observed and diagnosed at birth. Long QT syndrome sometimes presents itself as a complication due to surgery to correct syndactyly. Other times, children collapse spontaneously while playing. In all cases it is confirmed with ECG measurements. Sequencing of the CACNA1C gene further confirms the diagnosis.

JLNS patients with "KCNQ1" mutations are particularly prone to pathological lengthening of the QT interval, which predisposes them to episodes of "torsades de pointes" and sudden cardiac death. In this context, if the patient has had syncopal episodes or history of cardiac arrest, an implantable cardiac defibrillator should be used in addition to a beta blocker such as propranolol.

In general, the minimal evaluation of atrial fibrillation should be performed in all individuals with AF. The goal of this evaluation is to determine the general treatment regimen for the individual. If results of the general evaluation warrant it, further studies may then be performed.

Limited studies have suggested that screening for atrial fibrillation in those 65 years and older increases the number of cases of atrial fibrillation detected.

The ECG tracing in torsades demonstrates a "polymorphic ventricular tachycardia" with a characteristic illusion of a twisting of the QRS complex around the isoelectric baseline (peaks, which are at first pointing up, appear to be pointing down for subsequent "beats" when looking at ECG traces of the "heartbeat"). It is hemodynamically unstable and causes a sudden drop in arterial blood pressure, leading to dizziness and fainting. Depending on their cause, most individual episodes of torsades de pointes revert to normal sinus rhythm within a few seconds; however, episodes may also persist and possibly degenerate into ventricular fibrillation, leading to sudden death in the absence of prompt medical intervention. Torsades de pointes is associated with long QT syndrome, a condition whereby prolonged QT intervals are visible on an ECG. Long QT intervals predispose the patient to an , wherein the R-wave, representing ventricular depolarization, occurs during the relative refractory period at the end of repolarization (represented by the latter half of the T-wave). An R-on-T can initiate torsades. Sometimes, pathologic T-U waves may be seen in the ECG before the initiation of torsades.

A "short-coupled variant of torsade de pointes", which presents without long QT syndrome, was also described in 1994 as having the following characteristics:

- Drastic rotation of the heart's electrical axis

- Prolonged QT interval (LQTS) - may not be present in the short-coupled variant of torsade de pointes

- Preceded by long and short RR-intervals - not present in the short-coupled variant of torsade de pointes

- Triggered by a premature ventricular contraction (R-on-T PVC)

Surgery is typically used to correct structural heart defects and syndactyly. Propanolol or beta-adrenergic blockers are often prescribed as well as insertion of a pacemaker to maintain proper heart rhythm. With the characterization of Timothy syndrome mutations indicating that they cause defects in calcium currents, it has been suggested that calcium channel blockers may be effective as a therapeutic agent.

Myofibre break-up, abbreviated MFB, is associated with ventricular fibrillation leading to death. Histomorphologically, MFB is characterized by fractures of the cardiac myofibres perpendicular to their long axis, with squaring of the myofibre nuclei.

The diagnosis of ventricular tachycardia is made based on the rhythm seen on either a 12-lead ECG or a telemetry rhythm strip. It may be very difficult to differentiate between ventricular tachycardia and a wide-complex supraventricular tachycardia in some cases. In particular, supraventricular tachycardias with aberrant conduction from a pre-existing bundle branch block are commonly misdiagnosed as ventricular tachycardia. Other rarer phenomena include ashman beats and antedromic atrioventricular re-entry tachycardias.

Various diagnostic criteria have been developed to determine whether a wide complex tachycardia is ventricular tachycardia or a more benign rhythm. In addition to these diagnostic criteria, if the individual has a past history of a myocardial infarction, congestive heart failure, or recent angina, the wide complex tachycardia is much more likely to be ventricular tachycardia.

The proper diagnosis is important, as the misdiagnosis of supraventricular tachycardia when ventricular tachycardia is present is associated with worse prognosis. This is particularly true if calcium channel blockers, such as verapamil, are used to attempt to terminate a presumed supraventricular tachycardia. Therefore, it is wisest to assume that all wide complex tachycardia is VT until proven otherwise.

Treatment is directed towards the withdrawal of the offending agent, infusion of magnesium sulfate, antiarrhythmic drugs, and electrical therapy, such as a temporary pacemaker, as needed.

Because of the polymorphic nature of torsades de pointes, synchronized cardioversion may not be possible, and the patient may require an unsynchronized shock (or defibrillation).

Jervell and Lange-Nielsen syndrome (JLNS) is a type of long QT syndrome associated with severe, bilateral sensorineural hearing loss. Long QT syndrome causes the cardiac muscle to take longer than usual to recharge between beats. If untreated, the irregular heartbeats, called arrhythmias, can lead to fainting, seizures, or sudden death. It was first described by Anton Jervell and Fred Lange-Nielsen in 1957.

Defibrillation is the definitive treatment of ventricular fibrillation, whereby an electrical current is applied to the ventricular mass either directly or externally through pads or paddles, with the aim of depolarising enough of the myocardium for co-ordinated contractions to occur again. The use of this is often dictated around the world by Advanced Cardiac Life Support or Advanced Life Support algorithms, which is taught to medical practitioners including doctors, nurses and paramedics and also advocates the use of drugs, predominantly epinephrine, after every second unsuccessful attempt at defibrillation, as well as cardiopulmonary resuscitation (CPR) in between defibrillation attempts. Though ALS/ACLS algorithms encourage the use of drugs, they state first and foremost that defibrillation should not be delayed for any other intervention and that adequate cardiopulmonary resuscitation be delivered with minimal interruption.

The precordial thump is a manoeuver promoted as a mechanical alternative to defibrillation. Some advanced life support algorithms advocate its use once and only in the case of witnessed and monitored V-fib arrests as the likelihood of it successfully cardioverting a patient are small and this diminishes quickly in the first minute of onset.

Patients who survive a 'V-fib arrest' and who make a good recovery from this are often considered for implantation of an implantable cardioverter-defibrillator, which can quickly deliver this same life-saving defibrillation should another episode of ventricular fibrillation occur outside a hospital environment.

Ventricular tachycardia can be classified based on its "morphology":

- Monomorphic ventricular tachycardia means that the appearance of all the beats match each other in each lead of a surface electrocardiogram (ECG).

- Scar-related monomorphic ventricular tachycardia is the most common type and a frequent cause of death in patients having survived a heart attack or myocardial infarction, especially if they have a weak heart muscle.

- RVOT tachycardia is a type of monomorphic ventricular tachycardia originating in the right ventricular outflow tract. RVOT morphology refers to the characteristic pattern of this type of tachycardia on an ECG.

- The source of the re-entry circuit can be identified by evaluating the morphology of the QRS complex in the V1 lead of a surface ECG. If the R wave is dominant (consistent with a right bundle branch block morphology), this indicates the origin of the VT is the left ventricle. Conversely, if the S wave is dominant (consistent with a left bundle branch block morphology, this is consistent with VT originating from the right ventricle or interventricular septum.

- Polymorphic ventricular tachycardia, on the other hand, has beat-to-beat variations in morphology. This may appear as a cyclical progressive change in cardiac axis, previously referred to by its French name "torsades de pointes" ("twisting of the spikes"). However, at the current time, the term torsades de pointes is reserved for polymorphic VT occurring in the context of a prolonged resting QT interval.

Another way to classify ventricular tachycardias is the "duration of the episodes": Three or more beats in a row on an ECG that originate from the ventricle at a rate of more than 100 beats per minute constitute a ventricular tachycardia.

- If the fast rhythm self-terminates within 30 seconds, it is considered a non-sustained ventricular tachycardia.

- If the rhythm lasts more than 30 seconds, it is known as a sustained ventricular tachycardia (even if it terminates on its own after 30 seconds).

A third way to classify ventricular tachycardia is on the basis of its "symptoms": Pulseless VT is associated with no effective cardiac output, hence, no effective pulse, and is a cause of cardiac arrest. In this circumstance, it is best treated the same way as ventricular fibrillation (VF), and is recognized as one of the shockable rhythms on the cardiac arrest protocol. Some VT is associated with reasonable cardiac output and may even be asymptomatic. The heart usually tolerates this rhythm poorly in the medium to long term, and patients may certainly deteriorate to pulseless VT or to VF.

Less common is ventricular tachycardia that occurs in individuals with structurally normal hearts. This is known as idiopathic ventricular tachycardia and in the monomorphic form coincides with little or no increased risk of sudden cardiac death. In general, idiopathic ventricular tachycardia occurs in younger individuals diagnosed with VT. While the causes of idiopathic VT are not known, in general it is presumed to be congenital, and can be brought on by any number of diverse factors.

Andersen–Tawil syndrome, also called Andersen syndrome and Long QT syndrome 7, is a form of long QT syndrome. It is a rare genetic disorder, and is inherited in an autosomal dominant pattern and predisposes patients to cardiac arrhythmias. Jervell and Lange-Nielsen Syndrome is a similar disorder which is also associated with sensorineural hearing loss. It was first described by Ellen Damgaard Andersen.

Despite the grave initial presentation in some of the patients, most of the patients survive the initial acute event, with a very low rate of in-hospital mortality or complications. Once a patient has recovered from the acute stage of the syndrome, they can expect a favorable outcome and the long-term prognosis is excellent. Even when ventricular systolic function is heavily compromised at presentation, it typically improves within the first few days and normalises within the first few months. Although infrequent, recurrence of the syndrome has been reported and seems to be associated with the nature of the trigger.

This condition is incredibly rare, with only 100 cases reported worldwide, however there are thought to be many cases that have been left undiagnosed. It is either inherited from at least one parent containing the mutated gene. or it can be gained through the mutation of the KCNJ2 gene.

Afterdepolarizations are abnormal depolarizations of cardiac myocytes that interrupt phase 2, phase 3, or phase 4 of the cardiac action potential in the electrical conduction system of the heart. Afterdepolarizations may lead to cardiac arrhythmias.

Transient apical ballooning syndrome or Takotsubo cardiomyopathy is found in 1.7–2.2% of patients presenting with acute coronary syndrome. While the original case studies reported on individuals in Japan, Takotsubo cardiomyopathy has been noted more recently in the United States and Western Europe. It is likely that the syndrome previously went undiagnosed before it was described in detail in the Japanese literature. Evaluation of individuals with Takotsubo cardiomyopathy typically includes a coronary angiogram to rule out occlusion of the left anterior descending artery, which will not reveal any significant blockages that would cause the left ventricular dysfunction. Provided that the individual survives their initial presentation, the left ventricular function improves within two months.

The diagnosis of Takotsubo cardiomyopathy may be difficult upon presentation. The ECG findings often are confused with those found during an acute anterior wall myocardial infarction. It classically mimics ST-segment elevation myocardial infarction, and is characterised by acute onset of transient ventricular apical wall motion abnormalities (ballooning) accompanied by chest pain, shortness of breath, ST-segment elevation, T-wave inversion or QT-interval prolongation on ECG. Cardiac enzymes are usually negative and are moderate at worst, and cardiac catheterization usually shows absence of significant coronary artery disease.

The diagnosis is made by the pathognomonic wall motion abnormalities, in which the base of the left ventricle is contracting normally or is hyperkinetic while the remainder of the left ventricle is akinetic or dyskinetic. This is accompanied by the lack of significant coronary artery disease that would explain the wall motion abnormalities. Although apical ballooning has been described classically as the angiographic manifestation of takotsubo, it has been shown that left ventricular dysfunction in this syndrome includes not only the classic apical ballooning, but also different angiographic morphologies such as mid-ventricular ballooning and, rarely, local ballooning of other segments.

The ballooning patterns were classified by Shimizu et al. as Takotsubo type for apical akinesia and basal hyperkinesia, reverse Takotsubo for basal akinesia and apical hyperkinesia, mid-ventricular type for mid-ventricular ballooning accompanied by basal and apical hyperkinesia, and localised type for any other segmental left ventricular ballooning with clinical characteristics of Takotsubo-like left ventricular dysfunction.

In short, the main criteria for the diagnosis of Takotsubo cardiomyopathy are: the patient must have experienced a stressor before the symptoms began to arise; the patient’s ECG reading must show abnormalities from a normal heart; the patient must not show signs of coronary blockage or other common causes of heart troubles; the levels of cardiac enzymes in the heart must be elevated or irregular; and the patient must recover complete contraction and be functioning normally in a short amount of time.