Orthostatic hypotension and venous pooling
Orthostatic hypotension is a drop in blood pressure on standing that lasts longer than normal: blood pools in the leg veins and the reflexes that should correct this are weak.
Remember
On standing, blood pools in the legs, so the heart fills with less blood before each beat; if the reflexes that should correct this are slow, light-headedness and falls follow.
Memory aid
stand, pool, fill less, pump less
Say it in order to recall the chain: standing makes blood pool, so the heart fills less and then pumps less.
When a person stands, gravity pulls blood into the leg veins, and about 500 to 800 ml collects there. Less blood then returns to the heart, which is called a fall in venous return.
With less blood returning, the ventricle fills less. The volume it holds at the end of filling is the end-diastolic volume, and it is lower.
The Frank-Starling law says that heart muscle contracts less forcefully when it is stretched less, so the amount ejected per beat, the stroke volume, is smaller. Cardiac output (the blood pumped per minute) falls, and blood pressure falls with it.
This lasts only moments. Baroreceptors are pressure sensors, and in the baroreceptor reflex they sense the fall, so the heart speeds up and the arteries tighten.
When the reflex is weak, the dip lasts longer, which is orthostatic hypotension, and the brain is under-supplied, causing light-headedness, near-fainting or a fall. The receptors become less sensitive with increasing age and hypertension. A beta-blocker, a medicine that blunts the heart's response to adrenaline-type signals, can also impair the response. Dehydration is another cause, because a low blood volume makes the pooling matter more.
Key facts
- Pooling
- 500 to 800 ml of blood collects in the leg veins on standing.
- Chain of events
- Less venous return, lower end-diastolic volume, smaller stroke volume, then lower cardiac output.
- Definition
- A systolic fall of 20 mmHg or more, or a diastolic fall of 10 mmHg or more, within 3 minutes of standing.
- Who is vulnerable
- Older age, hypertension and beta-blockers weaken the compensation (the body's response that normally restores blood pressure).
- Another cause
- Dehydration, because low blood volume gives the pooling a larger impact.
- Classic clue
- Dizziness or near-fainting after standing, easing when the person lies down.
Exam traps
- A fall in resistance to blood flow does not explain the initial drop in pressure on standing, because the baroreceptors trigger vasoconstriction, so resistance rises rather than falls.
- Extra vagal output (the nerve supply that slows the heart) does not explain the fall in pressure on standing, because the heart rate normally rises on standing.
- A faster heart rate does not cause the fall in cardiac output on standing, because it is part of the compensation that helps to maintain output when stroke volume falls.
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