Which initial investigations should be performed in a patient with narrow complex tachycardia to assess for underlying causes?

Initial investigations for a patient presenting with narrow complex tachycardia should include a 12-lead electrocardiogram (ECG) to identify the arrhythmia and any associated conduction abnormalities or features suggestive of underlying heart disease.

Blood tests should be arranged, including full blood count, urea and electrolytes, thyroid function tests, liver function tests, and HbA1c, to identify systemic causes such as thyrotoxicosis or electrolyte imbalances.

A cardiovascular risk assessment should be performed to evaluate for risk factors that may contribute to arrhythmia.

If structural heart disease is suspected clinically or on ECG (e.g., presence of murmurs, left bundle branch block, left ventricular hypertrophy, or Q waves), an echocardiogram should be requested to assess cardiac structure and function.

Ambulatory ECG monitoring (such as Holter or event recorders) may be considered if the arrhythmia is paroxysmal and not captured on initial ECG, with the choice of monitor depending on symptom frequency and duration.

Referral to cardiology is indicated if there is syncope, family history of sudden cardiac death, exercise-induced palpitations, or evidence of structural heart disease or significant ECG abnormalities.