exam 2 lecture 19 and 20 Flashcards
1
Q
1.The A-a gradient
A
The difference between the amount of O2 in the alveolar gas versus the arterial blood tells you if the lung is sick, or there’s not enough breathing going on
2
Q
PaO2
A
pressure of oxygen gas dissolved in the arterial blood
mmHg
3
Q
PaCO2
A
pressure of carbon dioxide dissolved in the arterial blood
mmHg
4
Q
5 causes of hypoxemia
A
- Low inspired oxygen (decreased PiO2)•High altitude
- Hypoventilation•Not breathing enough (hold your breath)
- Diffusion limitation•Blood-gas barrier is too thick
- Shunt•Some blood is bypassing the lungs•“Hole in heart”
- Ventilation-perfusion (V/Q) inequality•The amount of ventilation isn’t matching the blood flow in the lungs
5
Q
what causes low inspired oxygen?
A
decreased PiO2
high altitude
6
Q
what causes hypoventilation
A
not breathing enough/holding your breath
7
Q
what is a diffusion limitation
A
blood gas barrier is too thick (Fick’s law)
8
Q
What causes a shunt
A
some blood is bypassing the lungs
hole in the heart
9
Q
what causes V/Q mixmatch
A
the amount of ventilation isn’t matching the blood flow in the lungs
10
Q
what causes of hypoxemia are problems with the lungs?
A
- Diffusion limitation•Blood-gas barrier is too thick
- Shunt•Some blood is bypassing the lungs•“Hole in heart”
- Ventilation-perfusion (V/Q) inequality•The amount of ventilation isn’t matching the blood flow in the lungs
11
Q
what causes of hypoxemia are causes by problems not the lungs
A
- low inspired oxygen, decreased PiO2, high altitude
- Hypoventilation•Not breathing enough (hold your breath)
12
Q
which one, PAO2 or PaO2, has to be bigger ?
A
PAO2 needs to be bigger to get air to move by diffusion from the alveoli to the arterial blood
13
Q
what is the normal A-a gradient
A
5-10 mmHg
14
Q
A
showing how O2 is lost from the atmosphere to the tissues
A-a gradient =5-10 mmHg
15
Q
how to calculate PAO2
A
respiratory quotient (RQ) = relationship between O2 and CO2
mixed diet = 0.8
PAO2= FiO2(Patm-47) - (PaCO2/0.8)
= O2 going in and O2 going out
16
Q
= is the amount of oxygen that enters the alveoli from the airways (PiO2) MINUS the amount oxygen leaving the alveoli into the capillary blood
A
PAO2
partial pressure of O2 in the alveolar
usually 100mmHg
17
Q
PH20
A
47
pressure of H20 dissolved in the airways cause the body is warmer then the environment
18
Q
___= [(PATM – PH2O) x FiO2] – (PACO2 / R)
A
PAO2
19
Q
CO2 is so highly soluble that alveolar CO2 (PACO2) and arterial CO2 (PaCO2) are basically ___
A
the same.
20
Q
what causes high A-a gradient?
A
lung problem
•If hypoxemia is caused by a problem with the lungs (pulmonary hypoxemia), then oxygen won’t transfer well from the alveolus to the blood, so the A-a gradient will be HIGH
- Diffusion limitation•Blood-gas barrier is too thick
- Shunt•Some blood is bypassing the lungs•“Hole in heart”
- Ventilation-perfusion (V/Q) inequality•The amount of ventilation isn’t matching the blood flow in the lungs
21
Q
what causes of hypoxemia do not effect the A-a gradient?
A
- Low inspired oxygen (decreased PiO2)•High altitude
- HypoventilationNot breathing enough (hold your breath)
22
Q
•If hypoxemia is caused by a problem with the lungs (pulmonary hypoxemia), then oxygen won’t transfer well from the alveolus to the blood, so the A-a gradient will be ___
A
high
- Diffusion limitation•Blood-gas barrier is too thick
- Shunt•Some blood is bypassing the lungs•“Hole in heart”
- Ventilation-perfusion (V/Q) inequality•The amount of ventilation isn’t matching the blood flow in the lungs
23
Q
•If hypoxemia is just caused by decreased breathing effort (hypoventilation; don’t forget to breathe!) or the PiO2 is lower (high altitude) but the A-a gradient will be ___
A
normal
- Low inspired oxygen (decreased PiO2)•High altitude
- Hypoventilation•Not breathing enough (hold your breath)
24
Q
why is A-a gradient normal with low PiO2
A
PiO2= FiO2(Patm-PH20)
at high altitude both PAO2 and PaO2 are decreased so the A-a gradient is normal
25
what is a common response to low PaO2
hyperventilation
decreased PaCO2
26
often ___ (hyperventilation in response to low PaO2)
decreased PaCO2
27
what effects CO2 in your blood
ventilation
28
PACO2=VCO2/VA
amount of CO2 directly effected by ventilation
the faster you breathe the less CO2
the slower you breath the more CO2
29
\_\_\_ FiO2 alleviates hypoxemia because it leads to an increase in alveolar PO2
increasing
PAO2 = **FiO2** (PATM – PH2O) – (PACO2/R)
PiO2= **FiO2 (PATM – PH2O)**
30
what happens to PCO2 with hypoventilation?
increased
31
how to fix increased PCO2?
ventilation (needs to breathe faster)
increased CO2 is from hypoventilation
32
causes of hypoventilation
–Central respiratory depression (CNS disease, drugs)
–Abnormal respiratory muscle function
–Thoracic cage abnormalities
–Pleural space disease
–Upper airway obstruction
33
what happens to PO2 and PCO2 during hypoventilation and how do you fix it?
O2 decreases
CO2 increases
O2 fixed by giving more oxygen (increase FiO2)
PaO2= **FiO2 (PATM – PH2O) -** (PACO2/R)
CO2 fixed by ventilating patient and increasing breathe
34
explain diffusion limitation and how is causes hypoxemia
hemoglobin requires 0.25 seconds to get all O2, it normal takes hemoglobin 0.75 seconds to get across- plenty of time
if the walls become thickened (ficks law) it is harder for O2 to diffuse and it takes longer for hemoglobin to get full.
CO2 not effected cause it is 20x more soluble then O2
35
what happens to A-a gradient with diffusion limitation
high
lots of O2 in the alveoli but none of it can get to the blood so big difference between A and a
36
•Impaired diffusion generally results from an ___ – due to fluid or fibrosis in the interstitium (not in the alveolar lumen)
**increase in the thickness of the blood gas barrier**
37
what happens to PaCO2 during diffusion limitation
usually normal
•PaCO2 does not increase until lesion is severe because it diffuses 20X more readily
38
where is A-a gradient changed for diffusion limiations?
39
where is A-a gradient increased for a shunt
40
\_\_\_ is when deoxygenated venous blood added to arterial circulation that has bypassed ventilated areas of lung
shunt
41
what are two physiologic shunts?
bronchial circulation
coronary venous blood
42
what are two pathologic shunts?
right to left cardiac shunt (VSD)
intrapulmonary- •Deoxygenated blood passes through normal pulmonary vasculature without contacting ventilated alveoli
43
what cause of hypoxemia responds poorly to oxygen?
shunt
big shunt will skip lungs, doesn't matter if you put a bunch of O2 into the lungs, the O2 will not get into the blood
44
show how supplemental O2 will effect PaO2 with a shunt
big shunt= O2 does NOT help
little shunt = O2 helps
45
CO2 is so soluble, as long as a small amount of blood gets to the lungs CO2 will diffuse easily
46
V/Q mismatch will cause an A-a gradient to be \_\_\_
high
47
\_\_\_ responsible for most of the defective gas exchange (& resultant hypoxemia) in pulmonary disease
•V/Q mismatch
48
gas flow
ventilation (L/min)
49
blood flow
ml/min
50
dead space ventilation
V/Q \> 1
ventilation excessive relative to blood flow
breathing to fast or blood to slow
51
venous admixture
perfusion (pulmonary capillary blood flow) excessive compared to ventilation
**V/Q \< 1**
blood to fast, venous deoxygenated blood will mix with oxygenated blood cause it moved to quickly to gain O2 from alveoli
52
what has V/Q \<1
venous admixture
blood to fast to get enough oxygen
deoxygenated blood will mix with oxygenated blood
53
what has V/Q \>1
dead space ventilation
breathing normal but blood too slow
54
what is the V/Q from a blocked airway
less then 1
too much Q not enough V
**venous admixture**
55
what is the V/Q from a blood clot
greater than 1
dead space ventilation
too much V not enough Q
56
what are some causes of venous admixture
V/Q less then 1
•*Decreased* V/Q (\<1) = “intrapulmonary shunt”
–Occlusion of small airways (asthma, bronchitis)
–fluid filled alveoli (pulmonary edema)
–Collapsed alveoli (atelectasis)
57
what are some causes of dead space ventilation
V/Q greater than 1
–emphysema (destruction of capillary alveolar interface
–reduced blood flow (emboli (blood clot), pulmonary hypertension)
58
normal value for PACO2
38-44 mmHg
59
normal value for PaO2
90-100 mmHg
60
term for increase for CO2
hypercarbia
hypercapnia
61
calculate PAO2
if room air and PACO2=48.3
if 100% O2
PAO2= FiO2(Patm-47) - (PACO2/0.8)
0. 21(760-47) -(48.3/0.8)
149. 73- 60.38
**89.36 mmHg**
1(760-47) -(48.3/0.8)
713-60.38
**652.62**
62
what is the A-a gradient if
room air: PAO2 is 90, PaO2 is 59.8
100% O2= PAO2 653, PaO2 59.8
what does the A-a tell us?
A-a= 30.2
A-a= 593.2
abnormal= issue with lungs (shunt, V/Q or diffusion limitation)
63
The effects of gravity mean that the more dependent (lower down) alveoli are ____ than the less-dependent alveoli
smaller and less ventilated
64
explain
gravity causes alveoli at the bottom to be smaller (slinky)
gravity causes blood to pool at the bottom
leads to V/Q mixmatch
65
what is hypoxic vasoconstriction?
when V/Q messed up the pulmonary arteries will become smaller in response to low alveolar O2, this reduction in blood flow(Q) will result in an increase in V/Q
not enough ventilation, blood flow will decrease to match
66
explain pulmonary reflex to V/Q less then 1
decreased V will trigger pulmonary arteries to constrict so that V/Q increases
67
explain pulmonary reflex to V/Q greater then 1
too much ventilation will cause **pulmonary arteries to constrict t**o match V
68
When the blood vessels sense a low oxygen tension (from hypoventilated alveoli, low V) THEY ____ thus reducing Q, which corrects the V/Q mismatch
vasocontrict
69
explain hypocapnic bronchoconstriction
too little CO2
airways will close/constrict to allow CO2 to try to normalize
The airways bronchoconstrict in response to low alveolar CO2 causing decreased alveolar ventilation (V) → decrease V/Q ratio
70
what can cause hypocapnia?
hyperventilating → breath off CO2
pulmonary embolus→ blood can't get to lungs, CO2 can't get out??
71
how to correct local V/Q mixmatch?
72
anesthesia will do what to the compensatory mechanisms for V/Q mixmatch?
turn them off!
73
what is V/Q for the top and bottom?
top = greater than 1= dead space ventilation
bottom = less then 1 = venous admixture
74
why does supplemental O2 help V/Q mixmatch for ventilated patient?
cancels out abnormal A-a, there is so much extra O2 that the V/Q will balance out
75
what is a right to left shunt?
blood from deoxygenated (right side) is mixing with oxygenated (left side) without becoming oxygenated
•It means that blood from the de-oxygenated right side of the heart bypasses the lung and is shunted directly into the left side of the heart, so it dilutes the oxygenated blood with deoxygenated
76
normal= hypoventilation or altitude
abnormal= shunt, V/Q or diffusion limitation
77
responsive= low V/Q or diffusion limitation
non responsive= shunt
78
what happens to PaCO2, A-a gradient and PaO2 on oxygen for all 5 causes of hypoxia?
79
calculate A-a gradient if
PAO2= 0.3(760-47) - (45/0.8)
214- 56.25
158
A-a= 158-67= 91 abnormal
issue with lungs (shunt, v/q or diffusion)
her PaCO2 is also elevated→ hypoventilation
**two things are going wrong**
80
hypoventilation does what to CO2?
what can cause this?
increases CO2
–Brain problem (anesthesia, brain disease)
–Diaphragmatic problem (exhaustion, injury)
–Lung problem? (has to be really REALLY severe)
81
hyperventilation does what to CO2
what causes this?
**decreases**
–Fear or pain (increased resp rate)
–Hypoxia (low oxygen increases resp. rate)
–Acidemia (if the blood is too acidic from other problems, the patient will “blow off” the acidic CO2 to normalize blood pH)
82
calculate A-a
what does this mean?
PAO2= FiO2(760-47) -(PaCO2/0.8)
0. 21(760-47)- (60/0.8)
149. 73-73
76. 73
A-a= 76.73-70= 6.73
A-a normal and CO2 increased= hypoventilation issue
83
PaCO2 when hypoventilating or hyperventilating
hyper= less then 40
hypo = greater than 40
PACO2= VCO2/VA
**CO2 is so highly soluble that alveolar CO2 (PACO2) and arterial CO2 (PaCO2) are basically the same, so clinically they are used interchangeably**
