Orthostatic challenge Flashcards
What is the orthostatic challenge?
Standing up translocates around 0.5L of blood from the wider circulation into the feet and calves. There is a risk of hypoperfusion of the brain.
What happens to stop us passing out when we stand up?
Arterial baroreflex
What happens to cardiac output when you stand?
Decrease in return to the heart - decreased cardiac output
Describe the baroreflex
Baroreceptors signal decreased input to the nucleus tracts solitarius through reduced firing rate vagal outflow to increase HR
Increase peripheral resistance, to try and increase mean arterial pressure.
What happens to CO/SV when baroreflex enganged?
CO and SV often remains depressed. (CO does increase)
What leads to activation of the sympathetic component of the arterial baroreflex (5)?
Vasoactive drugs e.g. organic nitrates
Blood loss
Neck suction
Change in posture
Lower body negative pressure
What does gravity do to venous pressure?
Gravity increases transmural pressure in veins
Why does gravity lead to venous pooling?
Gravity increases transmural pressure in veins
Resulting venous distention increases blood volume in lower body.
What does CVP change from in supine and orthostasis?
5mmHg supine
0mmHg standing
What happens to SV in orthostasis?
Reduces (reduced preload)
What happens to cerebral circulation in sub-atmospheric pressure?
The cerebral circulation becomes an inverted U tube siphon the effect of gravity on the carotid and vertebral arteries is offset by its effect on the jugular and vertebral veins and CSF.
(implies that no work is done on blood to increase its gravitational potential energy because the pressure gradients are equal and opposite in direction in the ascending arterial and the descending venous limbs of the loop)
How can standing up affect the cerebral perfusion?
Standing up can cause transient cerebral hypo perfusion and dizziness.
What happens to the internal jugular vein in orthostasis?
Internal jugular vein collapses above heart level during orthostasis, as gravity reduces its internal pressure to below atmospheric pressure.
What is a risk when cerebral venous circulation is subatmospheric?
Air embolism
When in orthostasis what happens to the veins above the heart?
Veins above heart level experience negative hydrostatic pressure and will be collapsed as the surrounding tissue pressure exceeds vein pressure, with a trickle of blood flowing through them.
In orthostasis what happens to the venous sinuses?
Note, the dural sinuses have rigid walls and cannot collapse, and so the pressure within them is subatmospheric.
What does high pressure in capillaries below the heart lead to?
High hydrostatic pressure leads to an increase transcapillary filtration of fluid into the interstitial spaces
What are dependent capillaries?
Where blood can pool?
Vasovasal compression in supine pregnant women
When a pregnant woman lies on her back, the abdominal aorta and inferior vena cava can become compressed by the uterus, can cause hypotension that can result in loss of consciousness.
Leads to less venous returns reduced cardiac output. Can be very dangerous to the foetus.
Name three important cardiovascular variables that are altered when a human is passively tilted from the horizontal to the vertical position, and indicate the direction of change.
Pooling of blood in lower extremities reduces venous return
This decreases cardiac output
Arterial pressure in the head falls
What two reflex mechanisms are triggered in orthostasis
Baroreflex
Cardio-pulmonary reflex
What is the cardio-pulmonary reflex?
Changes in aortic filling pressure lead activate low pressure baroreceptors
Two sites of baroreceptor
Carotid sinus/Aortic arch
Afferent neural pathway associated with carotid sinus?
Glossopharyngeal
Afferent neural pathway associated with aortic arch?
Vagus
Site of integration of baroreflex?
NTS
What happens to the discharge of arterial baroreceptors when a human is passively
tilted from the horizontal to the vertical?
Rapidly decreases
Sudden change from the supine to the upright posture is normally associated with
Fall in central venous pressure and arterial blood pressure in the head .
Sympathetic outflow is primarily mediated by the pre ganglionic release of … and the postsynaptic release of …
ACh, NA
A sudden change of posture going from the standing to the supine position would lead acutely to
A rise in CVP
An infusion of furosemide would lead to
A fall in CVP
A short period of sustained exercise such as running for two minutes would lead acutely to
A rise in CVP
A transfusion (venous infusion) of 1 litre of plasma would lead acutely to
A rise in CVP
Infusion of an inotrope would lead acutely to
A fall in CVP
