Tachyarrhythmia is defined as a heart rate above 100 beats per minute. It is classified based on its origin: supraventricular tachycardias (SVTs) typically present with a narrow QRS complex (less than 120 ms) and are regular, while ventricular tachycardias have a wide QRS complex (greater than 120 ms) and can be irregular. The etiology is broad and includes any heart problem such as heart failure, coronary artery disease (including myocardial infarction and unstable angina), cardiomyopathy, myocarditis, pericarditis, infective endocarditis, valvular heart disease, and congenital heart disease like ASD. Other causes extend to pulmonary embolism, cor pulmonale, diabetes, hypertension, hypoxia, acidosis, fever, sepsis, anemia, postoperative states (especially after cardiac surgery), and electrolyte disturbances (involving potassium, sodium, calcium, and magnesium). Hyperthyroidism, hypothyroidism, obesity, sleep apnea, certain medications (e.g., antidepressants, antibiotics, antifungals, antipsychotics, bronchodilators), alcohol, smoking, caffeine, drug abuse, genetic conditions like long QT syndrome, and stress or anxiety can also be contributing factors.

Clinically, patients may be asymptomatic or present with symptoms like palpitations, dyspnea, congestive heart failure, chest pain, syncope, presyncope, or even cardiac arrest. Those with atrial fibrillation (AF) may present with embolic events like stroke, limb ischemia, or mesenteric vascular occlusion. Signs include a rapid and weak pulse (regular in regular tachyarrhythmias or irregularly irregular with a pulse deficit in AF), low or normal blood pressure, and variable intensity of heart sounds. Examination of the neck veins may reveal a cannon 'a' wave, a large positive venous pulse that occurs when an atrium contracts against a closed tricuspid valve during AV dissociation (seen in nodal or ventricular tachycardia), or an absent 'a' wave in atrial fibrillation.

The approach to a patient with tachyarrhythmia involves a thorough history and physical examination, a 12-lead ECG, and further investigations such as a complete blood count (CBC), serum electrolytes, thyroid function tests, echocardiography, and Holter monitoring (24-48 hour ECG). Investigation for myocardial ischemia is important in patients with risk factors for coronary artery disease, and an electrophysiological study should be considered for a definitive diagnosis or when catheter ablation is a potential treatment.