Circulation 8 Flashcards

1
Q

Brain mass vs. blood demand

A

2% total body weight, but 15% of resting cardiac output

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2
Q

What is the brain’s main energy source?

A

Aerobic metabolism of glucose (needs a relatively large amount of oxygen and glucose)

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3
Q

What distance of capillaries does the brain contain?

A

400 miles

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4
Q

Blood brain barrier is due to what four things?

A

Endothelial cell tight junctions, basement membranes, neuroglial processes, metabolic enzymes

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5
Q

What types of substances can cross the BBB?

A

Lipid soluble substances such as oxygen, carbon dioxide, ethanol, and steroids, as well as those with transporters

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6
Q

What is the molecular weight cutoff for the BBB?

A

> 500 daltons (which includes most drugs)

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7
Q

Five factors affecting cerebral blood flow

A
  1. Autoregulation
  2. Tissue pressure (Monro-Kellie doctrine)
  3. Metabolism
  4. Autonomic nervous system
  5. Cushing’s response
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8
Q

Autoregulation of brain bloodflow

A

Maintains a consistent blood flow to the brain, but flow rates in differing areas of the brain will be dependent upon which part of the brain is active

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9
Q

CPP during normal states

A

80-100 mmHg

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10
Q

What happens with fluctuations in CPP?

A

If it falls - cerebral vasodilation

If it rises - cerebral vasoconstriction

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11
Q

Consequences of cranial rigidity on cerebral blood flow

A

Elevations in intracranial pressure will cause vascular compression, resulting in ischemia

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12
Q

Calculation of CPP

A

MAP minus intracranial venous pressure

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13
Q

Monro-Kellie doctrine equation

A

Brain volume + cerebral vascular volume + cerbrospinal fluid volume = constant

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14
Q

Monro-Kellie Doctrine

A

When the volume of one compartment increases, there must be a corresponding compensatory decrease in the volume of the other compartments

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15
Q

To what metabolite is cerebral blood flow very sensitive?

A

Arterial PCO2 (indirectly, pH)

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16
Q

Metabolic pH changes and blood flow

A

Decreased pH causes vasodilation and an increase in blood flow
Increased pH causes vasoconstriction and a decrease in blood flow

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17
Q

What effect does change in blood pH have on cerebral circulation?

A

Very little; H+ does not easily cross the BBB

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18
Q

Indirect effect of change of blood pH on cerebral blood flow

A

pH will change the amount of CO2 in the blood, which CAN cross the BBB and will rapidly change the amount of blood flow in the brain

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19
Q

How are patients with high intracranial pressures treated differently?

A

They are artificially ventilated at high rates to lower PCO2 levels, which vasoconstricts cerebral blood vessels and thereby decreases cerebral edema

20
Q

For what is O2 in cerebral flow mostly used?

A

It may have some play in cerebral vasodilation at decreased levels, but it mostly helps with metabolism changes during fMRIs

21
Q

When is adenosine formed?

A

In response to ischemia, hypoxia, hypotension, electrical stimulation of the brain, and seizures

22
Q

Timeframe of adenosine intervention

A

In situations that either reduce O2 supply or increase O2 demand, adenosine is formed within 5 seconds and remains elevated throughout event

23
Q

Mechanism of action of adenosine

A

Acts on purinergic receptors on vascular smooth muscle cells to activate ATP-sensitive potassium current, causing hyperpolarization, turning off calcium current and lowering intracellular calcium

24
Q

Mechanism of action of potassium ions

A

EC potassium concentrations increase in response to electrical stimulation of the brain and during seizures, hyperpolarizing and vasodilating vessels by stimulating Na/K-ATPase pump

25
Parasympathetic cerebral control
A division of the facial nerve carries parasympathetic innervation to some cerebral vessels to cause vasodilation
26
Sympathetic cerebral control
Exerts minimal vasoconstriction to increase cerebral vascular resistance
27
Cushing response
An elevation of intracranial pressure can cause a decrease in cerebral perfusion
28
What does cerebral ischmia stimulate?
Vasomotor centers in the medulla that increase sympathetic nerve activity, elevating systemic blood pressure
29
Percentage of blood received: pulmonary vs bronchial circulation
100% - pulmonary | 1% - bronchial
30
Compliance in pulmonary vessels
7 times more compliant than systemic arteries due to minimal smooth muscle
31
Mean pressure gradient in the lungs
6 mmHg
32
What is different about the capillaries in the lungs than in the systemic circuit?
They have a major influence on vascular resistance: represent about 40% of resistance
33
How is flow regulated in the pulmonary circuit?
Recruitment of new vessels; they do NOT autoregulate
34
Effect of inspiration on the extra-alveolar vessels
Negative intrapleural pressure distends the vessels, decreasing resistance
35
Effect of inspiration on the alveolar microvessels
Inflation of alveoli compresses and elongates the vessels, increasing resistance
36
Net effect o inspiration
Slight increase or no change in pulmonary peripheral resistance
37
Hydrostatic pressure in the lungs
Gravity and anatomy's influence on distribution of pulmonary blood flow
38
A change in 1 cm of height is equivalent to a change in hydrostatic pressure of __ mmHg.
0.74
39
Vascular "waterfall" effect
In an upright person, height divides the pulmonary vascular system and blood flow into 3 "zones"
40
Zone 1
Alveolar pressure exceeds both arterial and venous pressures, causing capillaries to collapse DOES NOT EXIST UNDER NORMAL CONDITIONS
41
Conditions under which zone 1 can be met
During hypotension or positive pressure mechanical ventilation
42
Zone 2
Alveolar pressure exceeds venous pressure, but not arterial pressure, causing partial collapse of capillaries
43
Conditions of zone 2
Upper third of the lung in a normal person
44
Zone 3
Both arterial and venous pressures are greater than the alveolar pressure; flow determined by that gradient
45
Conditions of zone 3
Primarily in the bottom third of the lung