Rapid heartbeat (tachycardia)

Tachycardia is a resting heart rate above 100 beats per minute. Most tachycardia is sinus tachycardia — the heart's natural pacemaker (sinoatrial node) firing faster in response to a physiological demand (exercise, fever, dehydration) or a circulating substance (caffeine, alcohol withdrawal, hyperthyroidism, stress hormones). The bloodwork search asks: is this driven by something the heart can't control? Common reversible drivers: anaemia (heart pumps faster to deliver oxygen with less haemoglobin), hyperthyroidism (excess thyroid hormone is a direct chronotrope), low potassium / magnesium (predispose to arrhythmias), dehydration, fever, drug effects. Pathological non-sinus tachycardias (atrial fibrillation, supraventricular tachycardia, ventricular tachycardia) have specific ECG signatures and dedicated workups — they are red flags.

• Sinus tachycardia — physiological response (exercise, fever, anxiety, pain, dehydration) or pathological (anaemia, hyperthyroidism, drug effect).
• Anaemia — iron-deficiency, B12, chronic disease; haemoglobin <10 g/dL.
• Hyperthyroidism — fine tremor, weight loss, heat intolerance, palpitations.
• Dehydration / volume depletion — vomiting, diarrhoea, diuretic excess.
• Fever, infection — sepsis is a red flag.
• Atrial fibrillation — irregularly irregular rhythm, requires ECG, anticoagulation if persistent.
• Supraventricular tachycardia — sudden 150-220 bpm episodes; vagal manoeuvres / adenosine.
• Stimulants — caffeine, alcohol withdrawal, cocaine, amphetamines, nicotine, decongestants.
• Pulmonary embolism — tachycardia + sudden breathlessness + chest pain; emergency.
• Heart failure, myocardial ischaemia — chronic or acute; specific cardiac workup.
• Pheochromocytoma — paroxysmal tachycardia + hypertension + sweats + headache.

First-line panel: TSH and free T4, CBC + ferritin (anaemia screen), electrolytes (potassium, magnesium, calcium, sodium), creatinine. Add D-dimer if pulmonary embolism plausible (clinical risk score gates this); add troponin if chest pain or dyspnoea suggests ischaemia. Mediora.AI is best at the (low haemoglobin + low ferritin + low TSH) cluster — the easy reversible causes. ECG is the diagnostic essential for non-sinus tachycardias.