Suspected diagnosis	Clinical clues	Electrocardiogram	Suggested diagnostic testing
Neurocardiogenic syncope	Noxious stimulus, prolonged upright position	Normal	Exercise testing
Supraventricular tachyarrhythmias	Palpitations, response to carotid sinus pressure	Pre-excitation	Electrophysiologic study and definitive therapy
Hypertrophic cardiomyopathy	Grade III/VI systolic murmur (louder on Valsalva maneuver), family history of sudden death	Normal, pseudoinfarction pattern, left ventricular hypertrophy with strain	ECG with Doppler ultrasonography
Myocarditis, pericarditis	Previous upper respiratory tract infection, pneumonia; shortness of breath; recreational drug use	Simulation of a myocardial infarction with ectopy	Viral studies, echocardiography, drug screening
Aortic stenosis	Exertional syncope, grade III/VI harsh systolic crescendo-decrescendo murmur	Left ventricular hypertrophy	ECG with Doppler ultrasonography
Mitral valve prolapse	“Thumping heart,” midsystolic click with or without a murmur	QT interval may be prolonged	Echocardiography with Doppler ultrasonography
Prolonged QT syndrome	Recurrent syncope with family history of sudden death	Prolonged corrected QT interval (> 0.44)	Family history, exercise stress test with ECG after exercise
Coronary anomalies	Usually asymptomatic, near sudden death event, family history of sudden death	Normal resting ECG	Coronary angiography; cardiac MRI
Acquired coronary artery diseases	Chest pain syndrome; family history of sudden death	Ischemia, may be normal	Exercise testing with or without perfusion or contractile imaging, lipid studies
Right ventricular dysplasia	Asymptomatic until syncope, tachyarrhythmias, family history of sudden death	T wave inversion V1-V3 PVCs with LBBB configuration	Echocardiography/Doppler study, electrocardiography
Exertional hyponatremia	Prolonged endurance event, altered consciousness with normal temperature	Nonspecific changes, may be normal	Serum electrolytes, urine and serum osmolality
Hyperthermia, heat stroke	Prolonged endurance event, altered consciousness with elevated temperature	Nonspecific changes, may be normal	Rectal temperature, electrolyte levels, CK, LFTs, CBC count, urine myoglobin, sickle cell screen
Seizure	Incontinence, prolonged postictal state	Nonspecific changes, may be normal	Electroencephalogram, cranial MRI or CT