Many people with long QT syndrome have no signs or symptoms.

Some people may experience the following symptoms:

- Fainting (or syncope). This may occur when the patient is emotionally or physically stressed. It is unusual in QT syndrome to have any signs before the person actually faints.

- Seizures

- Sudden death. If there is sudden death, and doctors suspect long QT syndrome as the cause, they may recommend that the family members of the deceased get tested for the disease.

No specific set of criteria has been developed for diagnosis of pacemaker syndrome. Most of the signs and symptoms of pacemaker syndrome are nonspecific, and many are prevalent in the elderly population at baseline. In the lab, pacemaker interrogation plays a crucial role in determining if the pacemaker mode had any contribution to symptoms.

Symptoms commonly documented in patients history, classified according to cause:

- Neurological - Dizziness, near syncope, and confusion.

- Heart failure - Dyspnea, orthopnea, paroxysmal nocturnal dyspnea, and edema.

- Hypotension - Seizure, mental status change, diaphoresis, and signs of orthostatic hypotension and shock.

- Low cardiac output - Fatigue, weakness, dyspnea on exertion, lethargy, and lightheadedness.

- Hemodynamic - Pulsation in the neck and abdomen, choking sensation, jaw pain, right upper quadrant (RUQ) pain, chest colds, and headache.

- Heart rate related - Palpitations associated with arrhythmias

In particular, the examiner should look for the following in the physical examination, as these are frequent findings at the time of admission:

- Vital signs may reveal hypotension, tachycardia, tachypnea, or low oxygen saturation.

- Pulse amplitude may vary, and blood pressure may fluctuate.

- Look for neck vein distension and cannon waves in the neck veins.

- Lungs may exhibit crackles.

- Cardiac examination may reveal regurgitant murmurs and variability of heart sounds.

- Liver may be pulsatile, and the RUQ may be tender to palpation. Ascites may be present in severe cases.

- The lower extremities may be edematous.

- Neurologic examination may reveal confusion, dizziness, or altered mental status.

Even though many types of sick sinus syndrome produce no symptoms, a person may present with one or more of the following signs and symptoms:

- Stokes-Adams attacks – fainting due to asystole or ventricular fibrillation

- Dizziness or light-headedness

- Palpitations

- Chest pain or angina

- Shortness of breath

- Fatigue

- Headache

- Nausea

Familial atrial fibrillation is an autosomal dominant heart condition that causes disruptions in the heart's normal rhythm. This condition is characterized by uncoordinated electrical activity in the heart's upper chambers (the atria), which causes the heartbeat to become fast and irregular.

The most common symptom is dizziness or syncope which often occurs during exercise or as a response to emotional stress.

CPVT typically start manifesting during the first or second decade of life. The majority of events occur during childhood with more than 60% of affected individuals having their first episode of syncope or cardiac arrest by age 12-20.

Up to 80% of individuals with ARVD present have symptoms like syncope and dyspnea.The remainder frequently present with palpitations or other symptoms due to right ventricular outflow tract (RVOT) tachycardia (a type of monomorphic ventricular tachycardia).

Symptoms are usually exercise-related. In populations where hypertrophic cardiomyopathy is screened out prior to involvement in competitive athletics, it is a common cause of sudden cardiac death.

The first clinical signs of ARVD are usually during adolescence. However, signs of ARVD have been demonstrated in infants.

Brugada syndrome (BrS) is a genetic condition that results in abnormal electrical activity within the heart, increasing the risk of sudden cardiac death. Those affected may have episodes of passing out. Typically this occurs when a person is at rest.

It is often inherited from a person's parent with about a quarter of people having a family history. Some cases may be due to a new mutation or certain medications. The abnormal heart rhythms can be triggered by a fever or increased vagal tone. Diagnosis is typically by electrocardiogram (ECG), however, the abnormalities may not be consistently present.

Treatment may be with an implantable cardioverter defibrillator (ICD). Isoproterenol may be used in those who are acutely unstable. In those without symptoms the risk of death is much lower, and how to treat this group is unclear. Testing people's family members may be recommended.

Between 1 and 30 per 10,000 people are affected. Onset of symptoms is usually in adulthood. It is more common in people of Asian descent. Males are more commonly affected than females. It is named after the Spanish cardiologists Pedro and Josep Brugada who described the condition in 1992. Their brother Ramon Brugada described the underlying genetics in 1998.

People with WPW are usually asymptomatic when not having a fast heart rate. However, individuals may experience palpitations, dizziness, shortness of breath, or infrequently syncope (fainting or near fainting) during episodes of supraventricular tachycardia. The telltale "delta wave" may sometimes be seen on an electrocardiogram (ECG/EKG).

Lown–Ganong–Levine syndrome (LGL) is a pre-excitation syndrome of the heart due to abnormal electrical communication between the atria and the ventricles. Once thought to involve an accessory conduction pathway, it is grouped with Wolff–Parkinson–White syndrome as an atrioventricular re-entrant tachycardia (AVRT). Individuals with LGL syndrome have a short PR interval with normal QRS complexes and paroxysms of clinically-significant tachycardia. The syndrome is named after Bernard Lown, William Francis Ganong, Jr., and Samuel A. Levine.

Individuals with a short PR interval found incidentally on EKG were once thought to have LGL syndrome. However, subsequent studies have shown that a short PR interval in the absence of symptomatic tachycardia is simply a benign EKG variant.

LGL syndrome is diagnosed on the basis of the surface EKG in a symptomatic individual with a PR interval less than or equal to 0.12 second (120 ms) with normal QRS complex configuration and duration. It can be distinguished from WPW syndrome because the delta waves seen in WPW syndrome are not seen in LGL syndrome. It is a clinical diagnosis that came about before the advent of electrophysiology studies. Be aware, however, that not all WPW EKG's have a delta wave; the absence of a delta wave does not conclusively rule out WPW.

Clinically, it is often asymptomatic by itself and considered benign in nature.

Arrhythmogenic right ventricular dysplasia (ARVD) is an inherited heart disease.

ARVD is caused by genetic defects of the parts of heart muscle (also called "myocardium" or "cardiac muscle") known as desmosomes, areas on the surface of heart muscle cells which link the cells together. The desmosomes are composed of several proteins, and many of those proteins can have harmful mutations.

The disease is a type of nonischemic cardiomyopathy that involves primarily the right ventricle. It is characterized by hypokinetic areas involving the free wall of the right ventricle, with fibrofatty replacement of the right ventricular myocardium, with associated arrhythmias originating in the right ventricle.

ARVD can be found in association with diffuse palmoplantar keratoderma, and woolly hair, in an autosomal recessive condition called Naxos disease, because this genetic abnormality can also affect the integrity of the superficial layers of the skin most exposed to pressure stress.

ARVC/D is an important cause of ventricular arrhythmias in children and young adults. It is seen predominantly in males, and 30–50% of cases have a familial distribution.

Long QT syndrome (LQTS) is a condition which affects repolarization of the heart after a heartbeat. This results in an increased risk of an irregular heartbeat which can result in fainting, drowning, or sudden death. These episodes can be triggered by exercise or stress. Other associated symptoms may include hearing loss.

Long QT syndrome may be present at birth or develop later in life. The inherited form may occur by itself or as part of larger genetic disorder. Onset later in life may result from certain medications, low blood potassium, low blood calcium, or heart failure. Medications that are implicated include certain antiarrhythmic, antibiotics, and antipsychotics. Diagnosis is based on an electrocardiogram (EKG) finding a corrected QT interval of greater than 440 to 500 milliseconds together with clinical findings.

Management may include avoiding strenuous exercise, getting sufficient potassium in the diet, the use of beta blockers, or a implantable cardiac defibrillator. Without treatment there is a 50%, 10 year risk of death, for the inherited versions. With treatment this decreases to less than 1% over 20 years.

Long QT syndrome is estimated to affect 1 in 7,000 people. Females are affected more often than males. Most people with the condition develop symptoms before they are 40 years old. It is a relatively common cause of sudden death along with Brugada syndrome and arrhythmogenic right ventricular dysplasia. In the United States it results in about 3,500 deaths a year. The condition was first clearly described in 1957.

Sick sinus syndrome (SSS), also called sinus dysfunction, or sinoatrial node disease ("SND"), is a group of abnormal heart rhythms (arrhythmias) presumably caused by a malfunction of the sinus node, the heart's primary pacemaker. Tachycardia-bradycardia syndrome is a variant of sick sinus syndrome in which the arrhythmia alternates between slow and fast heart rates. Tachycardia-bradycardia syndrome is often associated with ischemic heart disease and heart valve disease.

Although there are many signs and symptoms associated with PVCs, PVCs may have no symptoms at all. An isolated PVC is hard to catch without the use of a Holter monitor. PVCs may be perceived as a skipped heart beat, a strong beat, or a feeling of suction in the chest. They may also cause chest pain, a faint feeling, fatigue, or hyperventilation after exercise. Several PVCs in a row becomes a form of ventricular tachycardia (VT), which is a potentially fatal abnormal heart rhythm. Overall it has been seen that the symptom felt most by patients experiencing a PVC is the mere perception of a skipped heartbeat. The more frequently these contractions occur, the more likely there are to be symptoms, despite the fact that these beats have little effect of the pumping action of the heart and therefore cause minimal if any symptoms.

Some other possible signs and symptoms of PVCs:

- Abnormal ECG

- Irregular heart beat

- Dyspnea

- Dizziness

- Feeling your heart beat (palpitations)

- Feeling of occasional, forceful beats

- Increased awareness of your heart beat

- Perception of a skipped heartbeat

Wolff–Parkinson–White syndrome (WPW) is a disorder due to a specific type of problem with the electrical system of the heart which has resulted in symptoms. About 40% of people with the electrical problem never develop symptoms. Symptoms can include an abnormally fast heartbeat, palpitations, shortness of breath, lightheadedness, or syncope. Rarely cardiac arrest may occur. The most common type of irregular heartbeat that occurs is known as paroxysmal supraventricular tachycardia.

The cause of WPW is typically unknown. A small number of cases are due to a mutation of the PRKAG2 gene which may be inherited from a person's parents in an autosomal dominant fashion. The underlying mechanism involves an accessory electrical conduction pathway between the atria and the ventricles. It is associated with other conditions such as Ebstein anomaly and hypokalemic periodic paralysis. Diagnosis is typically when an electrocardiogram (ECG) show a short PR interval and a delta wave. It is a type of pre-excitation syndromes.

WPW syndrome is treated with either medications or radiofrequency catheter ablation. It affects between 0.1 and 0.3% in the population. The risk of death in those without symptoms is about 0.5% per year in children and 0.1% per year in adults. In those without symptoms ongoing observation may be reasonable. In those with WPW complicated by atrial fibrillation, cardioversion or the medication procainamide may be used. The condition is named after Louis Wolff, John Parkinson, and Paul Dudley White who described the ECG findings in 1930.

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.

Pacemaker syndrome is a disease that represents the clinical consequences of suboptimal atrioventricular (AV) synchrony or AV dyssynchrony, regardless of the pacing mode, after pacemaker implantation.

It is an iatrogenic disease—an adverse effect resulting from medical treatment—that is often underdiagnosed. In general, the symptoms of the syndrome are a combination of decreased cardiac output, loss of atrial contribution to ventricular filling, loss of total peripheral resistance response, and nonphysiologic pressure waves.

Individuals with a low heart rate prior to pacemaker implantation are more at risk of developing pacemaker syndrome. Normally the first chamber of the heart (atrium) contracts as the second chamber (ventricle) is relaxed, allowing the ventricle to fill before it contracts and pumps blood out of the heart. When the timing between the two chambers goes out of synchronization, less blood is delivered on each beat. Patients who develop pacemaker syndrome may require adjustment of the pacemaker, or fitting of another lead to better coordinate the timing of atrial and ventricular contraction.

If untreated, this abnormal heart rhythm can lead to dizziness, chest pain, a sensation of fluttering or pounding in the chest (palpitations), shortness of breath, or fainting (syncope). Atrial fibrillation also increases the risk of stroke. Complications of familial atrial fibrillation can occur at any age, although some people with this heart condition never experience any health problems associated with the disorder.

Atrial fibrillation is the most common type of sustained abnormal heart rhythm (arrhythmia), affecting more than 3 million people in the United States. The risk of developing this irregular heart rhythm increases with age. The incidence of the familial form of atrial fibrillation is unknown; however, recent studies suggest that up to 30 percent of all people with atrial fibrillation may have a history of the condition in their family.

Ventricular fibrillation is a cause of cardiac arrest and sudden cardiac death. The ventricular muscle twitches randomly rather than contracting in a co-ordinated fashion (from the apex of the heart to the outflow of the ventricles), and so the ventricles fail to pump blood around the body - because of this, it is classified as a cardiac arrest rhythm, and patients in V-fib should be treated with cardiopulmonary resuscitation and prompt defibrillation. Left untreated, ventricular fibrillation is rapidly fatal as the vital organs of the body, including the heart, are starved of oxygen, and as a result patients in this rhythm will not be conscious or responsive to stimuli. Prior to cardiac arrest, patients may complain of varying symptoms depending on the underlying cause. Patients may exhibit signs of agonal breathing, which to the layperson can look like normal spontaneous breathing, but it is in fact a sign of hypoperfusion of the brainstem.

It has an appearance on electrocardiography of irregular electrical activity with no discernable pattern. It may be described as 'coarse' or 'fine' depending on its amplitude, or as progressing from coarse to fine V-fib. Coarse V-fib may be more responsive to defibrillation, while fine V-fib can mimic the appearance of asystole on a defibrillator or cardiac monitor set to a low gain. Some clinicians may attempt to defibrillate fine V-fib in the hope that it can be reverted to a cardiac rhythm compatible with life, whereas others will deliver CPR and sometimes drugs as described in the advanced cardiac life support protocols in an attempt to increase its amplitude and the odds of successful defibrillation.

It occurs because the duration of the refractory period of the myocardium is proportional to the R-R interval of the preceding cycle. A short R-R interval is associated with a shorter duration of action potential and vice versa. A long R-R cycle will prolong the ensuing refractory period, and if a shorter cycle follows, the beat terminating the cycle is likely to be conducted aberrantly. Because the refractory period of the right bundle branch is longer than the left, the right bundle will still be in the refractory period when the supraventricular impulse reaches the His-Purkinje system, resulting in a complex with right bundle branch morphology.

Boxer cardiomyopathy is an adult-onset disease with three distinct clinical presentations:

The concealed form is characterized by an asymptomatic dog with premature ventricular contractions (PVCs).

The overt form is characterized by ventricular tachyarrhythmias and syncope. Dogs with overt disease may also have episodic weakness and exercise intolerance, but syncope is the predominant manifestation.

The third form, which is recognized much less frequently, is characterized by myocardial systolic dysfunction. This may result in left-sided, right-sided, or bi-ventricular congestive heart failure. It is not known if this form represents a separate clinical entity, or whether it is part of the continuum of disease.

It can be divided by the origin:

- supraventricular tachycardia

- ventricular tachycardia

People with TIC most often present with symptoms of congestive heart failure and/or symptoms related to their irregular heart rhythm. Symptoms of congestive heart failure can include shortness of breath, ankle swelling, fatigue, and weight gain. Symptoms of an irregular heart rhythm can include palpitations and chest discomfort.

The timecourse of TIC is most well-studied in experiments on animals. Researchers have found that animals began to exhibit abnormal changes in blood flow after just one day of an artificially generated fast heart rate (designed to simulate a tachyarrythmia). As their TIC progresses, these animals will have worsening heart function (e.g.: reduced cardiac output and reduced ejection fraction) for 3–5 weeks. The worsened heart function then persists at a stable state until the heart rate is returned to normal. With normal heart rates, these animals begin to demonstrate improving heart function at 1–2 days, and even complete recovery of ejection fraction at 1 month.

Human studies of the timecourse of TIC are not as robust as animal studies, though current studies suggest that the majority of people with TIC will recover a significant degree of heart function over months to years.