Cardiovascular: 
• Outflow Obstruction :
     • Aortic Stenosis
     • HOCM (Hypertrophic Cardiomyopathy)
     • Pulmonary Embolism
     • Cardiac Tamponade
     • Congenital Heart Disease
     • Other Valve Abnormalities
• Decreased Cardiac Output:
     • Myocardial Infarction
     • Aortic Dissection
     • Arrhythmias
          • Look for AV blocks, Prolonged QT, Wolf-Parkinson-White, Brugada, ARVD

Neurovascular: * Important to distinguish in history syncope vs. seizure*
• SAH
• TIA/Stroke
• Vasovagal (fear, pain, emotion, etc.)
• Situational (carotid sensitivity, post micturition, coughing, etc.)

Decreased Intravascular Volume:
• Bleeding
• Ruptured AAA
• GI Bleed
• Ruptured Ectopic Pregnancy
• Dehydration
• Others (ex: sepsis)

Others:
• Environmental (ex: CO poisoning)
• Medications/ Toxicologic
• Hypoglycemia
• Hypercapnia