Orthostatic challenge

Question 1

What is the orthostatic challenge?

Answer 1

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.

Question 2

What happens to stop us passing out when we stand up?

Answer 2

Arterial baroreflex

Question 3

What happens to cardiac output when you stand?

Answer 3

Decrease in return to the heart - decreased cardiac output

Question 4

Describe the baroreflex

Answer 4

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.

Question 5

What happens to CO/SV when baroreflex enganged?

Answer 5

CO and SV often remains depressed. (CO does increase)

Question 6

What leads to activation of the sympathetic component of the arterial baroreflex (5)?

Answer 6

Vasoactive drugs e.g. organic nitrates

Blood loss

Neck suction

Change in posture

Lower body negative pressure

Question 7

What does gravity do to venous pressure?

Answer 7

Gravity increases transmural pressure in veins

Question 8

Why does gravity lead to venous pooling?

Answer 8

Gravity increases transmural pressure in veins

Resulting venous distention increases blood volume in lower body.

Question 9

What does CVP change from in supine and orthostasis?

Answer 9

5mmHg supine

0mmHg standing

Question 10

What happens to SV in orthostasis?

Answer 10

Reduces (reduced preload)

Question 11

What happens to cerebral circulation in sub-atmospheric pressure?

Answer 11

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)

Question 12

How can standing up affect the cerebral perfusion?

Answer 12

Standing up can cause transient cerebral hypo perfusion and dizziness.

Question 13

What happens to the internal jugular vein in orthostasis?

Answer 13

Internal jugular vein collapses above heart level during orthostasis, as gravity reduces its internal pressure to below atmospheric pressure.

Question 14

What is a risk when cerebral venous circulation is subatmospheric?

Answer 14

Air embolism

Question 15

When in orthostasis what happens to the veins above the heart?

Answer 15

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.

Question 16

In orthostasis what happens to the venous sinuses?

Answer 16

Note, the dural sinuses have rigid walls and cannot collapse, and so the pressure within them is subatmospheric.

Question 17

What does high pressure in capillaries below the heart lead to?

Answer 17

High hydrostatic pressure leads to an increase transcapillary filtration of fluid into the interstitial spaces

Question 18

What are dependent capillaries?

Answer 18

Where blood can pool?

Question 19

Vasovasal compression in supine pregnant women

Answer 19

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.

Question 20

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.

Answer 20

Pooling of blood in lower extremities reduces venous return

This decreases cardiac output

Arterial pressure in the head falls

Question 21

What two reflex mechanisms are triggered in orthostasis

Answer 21

Baroreflex

Cardio-pulmonary reflex

Question 22

What is the cardio-pulmonary reflex?

Answer 22

Changes in aortic filling pressure lead activate low pressure baroreceptors

Question 23

Two sites of baroreceptor

Answer 23

Carotid sinus/Aortic arch

Question 24

Afferent neural pathway associated with carotid sinus?

Answer 24

Glossopharyngeal

Question 25

Afferent neural pathway associated with aortic arch?

Answer 25

Vagus

Question 26

Site of integration of baroreflex?

Answer 26

NTS

Question 27

What happens to the discharge of arterial baroreceptors when a human is passively
tilted from the horizontal to the vertical?

Answer 27

Rapidly decreases

Question 28

Sudden change from the supine to the upright posture is normally associated with

Answer 28

Fall in central venous pressure and arterial blood pressure in the head .

Question 29

Sympathetic outflow is primarily mediated by the pre ganglionic release of … and the postsynaptic release of …

Answer 29

ACh, NA

Question 30

A sudden change of posture going from the standing to the supine position would lead acutely to

Answer 30

A rise in CVP

Question 31

An infusion of furosemide would lead to

Answer 31

A fall in CVP

Question 32

A short period of sustained exercise such as running for two minutes would lead acutely to

Answer 32

A rise in CVP

Question 33

A transfusion (venous infusion) of 1 litre of plasma would lead acutely to

Answer 33

A rise in CVP

Question 34

Infusion of an inotrope would lead acutely to

Answer 34

A fall in CVP