From the Guidelines
Fainting due to long standing occurs through a mechanism called orthostatic hypotension, where blood pools in the lower extremities due to gravity, reducing venous return to the heart and subsequently lowering blood pressure. This phenomenon is supported by the study published in the European Heart Journal 1, which explains that upon moving from a supine to an erect posture, a significant amount of blood shifts away from the chest to the venous capacitance system below the diaphragm, estimated to be around one half to one liter of thoracic blood. Additionally, prolonged standing leads to a decrease in plasma volume due to filtration of protein-free fluid into the interstitial spaces, further reducing venous return.
The body attempts to compensate for this reduction in blood volume and pressure through vasoconstriction of the resistance and capacitance vessels, as well as increases in heart rate, mediated by the autonomic nervous system and reinforced by local reflex mechanisms like the venoarteriolar reflex 1. However, in some cases, these compensatory mechanisms can fail, leading to a decrease in cardiac output and a drop in mean arterial pressure. The study highlights that the rapid short-term adjustments to orthostatic stress are mediated exclusively by the neural pathways of the autonomic nervous system, while additional adjustments during prolonged orthostatic stress are mediated by the humoral limb of the neuroendocrine system.
Key factors contributing to the failure of these compensatory mechanisms include dehydration, warm environments, emotional stress, and certain medications that affect blood pressure regulation. To prevent fainting episodes due to prolonged standing, individuals should:
- Avoid prolonged motionless standing
- Stay well-hydrated
- Flex leg muscles periodically to promote venous return
- Consider compression stockings if prone to fainting episodes
It is also important to note that treatment of syncope due to orthostatic hypotension is recommended in all patients, and may entail modification of drug treatment for concomitant conditions, as suggested by the guidelines published in the European Heart Journal 1.
From the Research
Mechanism of Fainting due to Long Standing
The mechanism that causes fainting due to long standing is complex and involves multiple factors.
- Orthostatic hypotension, a fall in blood pressure on assuming an upright position, is a common problem that can lead to fainting 2.
- When an individual stands for a long time, a significant amount of blood pools in the lower extremities, estimated to be around 500 mL 2.
- This pooling of blood leads to a decrease in venous return, resulting in a concomitant decrease in cardiac output and hypoperfusion of the cerebral circulation 2.
- The decrease in cerebral blood flow can cause syncope or various other symptoms, including lightheadedness and dizziness 2, 3.
- Cerebral vasoconstriction may also occur during orthostatic hypotension, compromising cerebral autoregulation and contributing to the loss of consciousness 4.
- The autonomic nervous system plays a crucial role in regulating blood pressure and cerebral blood flow during standing, and dysfunction of this system can contribute to fainting 2, 3.
Key Factors Contributing to Fainting
- Prolonged standing can lead to a decrease in cardiac output and cerebral blood flow, increasing the risk of fainting 2, 3.
- Orthostatic hypotension is more prominent in elderly patients due to the increased intake of vasoactive medications and concomitant decrease in physiologic function, such as baroreceptor sensitivity 2.
- Reflex (vasovagal) responses, cardiac arrhythmias, and autonomic failure are common causes of syncope 3, 5.
- Muscle activity, including the "muscle pump," is an important defense against a critical reduction in central blood volume and can help prevent fainting 3.