There is little to no compensatory increase in heart rate or blood pressure when standing for up to 10 minutes. This is often due to an underlying disorder or medication use and is accompanied by other hypoadrenergic signs. Central nervous system ischemia
The central ischemic response is triggered by an inadequate supply of oxygenated blood in the brain. Common examples include strokes and transient ischemic attacks. While these conditions often impair consciousness they rarely meet the medical definition of syncope. Vertebrobasilar transient ischemic attacks may produce true syncope as a symptom.The respiratory system may compensate for dropping oxygen levels through hyperventilation, though a sudden ischemic episode may also proceed faster than the respiratory system can respond. These processes cause the typical symptoms of fainting: pale skin, rapid breathing, nausea, and weakness of the limbs, particularly of the legs. If the ischemia is intense or prolonged, limb weakness progresses to collapse. The weakness of the legs causes most people to sit or lie down if there is time to do so. This may avert a complete collapse, but whether the patient sits down or falls down, the result of an ischaemic episode is a posture in which less blood pressure is required to achieve adequate blood flow. An individual with very little skin pigmentation may appear to have all color drained from his or her face at the onset of an episode. This effect combined with the following collapse can make a strong and dramatic impression on bystanders.
Symptoms include deep and fast respiration, weak and slow pulse, and respiratory pauses that may last for 60 seconds. Subclavian steal syndrome arises from retrograde (reversed) flow of blood in the vertebral artery or the internal thoracic artery, due to a proximal stenosis (narrowing) and/or occlusion of the subclavian artery. Symptoms such as syncope, lightheadedness, and paresthesias occur while exercising the arm on the affected side (most commonly the left). Aortic dissection (a tear in the aorta) and cardiomyopathy can also result in syncope.Various medications, such as beta blockers, may cause bradycardia induced syncope.A pulmonary embolism can cause obstructed blood vessels and is the cause of syncope in less than 1% of people who present to the emergency department. Blood pressure
Orthostatic (postural) hypotensive syncope is caused primarily by an excessive drop in blood pressure when standing up from a previous position of lying or sitting down. When the head is elevated above the feet the pull of gravity causes blood pressure in the head to drop. This is sensed by stretch receptors in the walls of vessels in the carotid sinus and aortic arch. These receptors then trigger a sympathetic nervous response to compensate and redistribute blood back into the brain. The sympathetic response causes peripheral vasoconstriction and increased heart rate. These together act to raise blood pressure back to baseline. Apparently healthy individuals may experience minor symptoms ("lightheadedness", "greying-out") as they stand up if blood pressure is slow to respond to the stress of upright posture.