12-lead ECG and long rhythm strip during the arrhythmia and after resolution (heart rate, delta wave, AV conduction abnormality, bundle branch block, Q waves, evidence of LV hypertrophy, QT interval, ST/T abnormalities)

Electrolytes (if on diuretic, include magnesium) and creatinine

Blood glucose

Thyroid function (for later analysis)

Plasma digoxin level if taking digoxin (at least 8h after digoxin last taken)

Plasma troponin

Chest X-ray (heart size, evidence of raised left atrial pressure, coexistent pathology, for example pneumonia?)

Echocardiogram (for LV function, RV function, valve disease, pericardial effusion) if there is VT. Can be done the next day for other primary arrhythmias. May not be indicated if AF or flutter complicates pneumonia and the patient has no historical symptoms or signs of cardiac disease.