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Pulmonary Week 2 > ICL 1.5: Arterial Blood Gases > Flashcards

ICL 1.5: Arterial Blood Gases Flashcards

1
Q

what is hypoxia?

A

a condition where either all (generalized hypoxia) or a specific part of the body (regional hypoxia) does not receive or is not able to use adequate oxygen for aerobic metabolism

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2
Q

what is hypoxemia?

A

a reduction in the concentration of oxygen in arterial blood

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3
Q

how do we know the partial pressure of O2, CO2, etc. in the blood?

A

puncture the artery and get an arterial blood gas!

the radial artery is the best one to get it from and you heparinize the blood so that it doesn’t clot

brachia and femoral arteries can also be used

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4
Q

what are the normal ranges for pH, CO2, pO2, HCO3 and O2 saturation in the arterial blood?

A

pH = 7.35-7.45

CO2 = 35-45 mmHg

pO2 = 80-100 mmHg

HCO3 = 22-26

O2 saturation = 95-100%

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5
Q

what is the primary function of the lung?

A

alveolar gas exchange

inhaled oxygen enters the lungs and reaches the alveoli – oxygen passes through the alveolar-blood barrier into the blood in the capillaries

similarly, CO2 passes from the blood into the alveoli and is then exhaled

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6
Q

what is the alveolar-arterial gradient?

A

PiO2 is the pressure of O2 in the inspired gas and 21% of inspired air is made of oxygen

barometric pressure at sea level is 760 mmHg

water vapor in the inspired air has a pressure of 47 mmHg

so (760-47)x0.21 = 150 mmHg is the pressure of oxygen when it reaches the lungs!

there’s also CO2 in the alveoli and it’s about 40 mmHg but then 40/.8 which is the normal RQ, pCO2 = 50 mmHg

so PAO2 = 150-50 = 100 mmHg in the alveoli

then the arterial pressure of oxygen is PaO2 = 80-100 which means the alveolar-arterial gradient of oxygen is anywhere from 5-20 normally (100-80)

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7
Q

what is the alveolar gas equation?

A

PAO2 = FiO2(Patm-PH2O) - PaCO2/RQ

FiO2 = 0.21
Patm = 760
PH2O = 47
RQ = 0.8 normally
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8
Q

what happens to FiO2 if a patient is on a ventilator or on oxygen?

A

if a patient is on 100% oxygen then FiO2 goes from .21 to 1!

so the PAO2 would increase if a patient is on oxygen

PAO2 = FiO2(Patm-PH2O) - PaCO2/RQ

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9
Q

what 5 things can cause hypoxemia?

A

hypoxemia is a reduction in the concentration of oxygen in arterial blood

  1. decreased inspired oxygen tension (high altitude)
  2. hypoventilation (CO2 > 40-50 means PAO2 will decrease)
  3. ventilation/perfusion mismatch
  4. right to left shunt
  5. impaired diffusion
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10
Q

what does the alveolar-arterial gradient tell us about the hypoxemia patient?

A

it the A-a gradient is normal, the cause of hypoxemia must be either:
1. hypoventilation (i.e. high Paco2)

2 low Pl (i.e. extreme elevation)

if the A-a gradient is elevated, the cause of hypoxemia must be either:
1. V/Q Mismatch

  1. shunt
  2. impaired diffusion
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11
Q

what is a normal alveolar-arterial gradient?

A

5-15 mmHg

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12
Q

what are the PaO2, PaCO2, A-a and PaO2 with 100% O2 in the case of hypoventilation?

A

PaO2 = low

PaCO2 = high

A-a = normal

PaO2 with 100% O2 = >550

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13
Q

what are the PaO2, PaCO2, A-a and PaO2 with 100% O2 in the case of poor diffusion?

A

PaO2 = low

PaCO2 = normal-low

A-a = high

PaO2 with 100% O2 = >550

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14
Q

what are the PaO2, PaCO2, A-a and PaO2 with 100% O2 in the case of a right-left shunt?

A

PaO2 = low

PaCO2 = normal-low

A-a = high

PaO2 with 100% O2 = <550

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15
Q

what are the PaO2, PaCO2, A-a and PaO2 with 100% O2 in the case of V/A imbalance?

A

PaO2 = low

PaCO2 = normal-low

A-a = high

PaO2 with 100% O2 = >550

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16
Q

An 56-year-old man with a history of CAD, HTN, and 60 pack years of smoking, presents to the ER with a productive cough and dyspnea x 3 days. On exam, his RR= 28 and O2 sat = 81% on room air. His breaths are shallow and with pursed lips.

pH = 7.31
PaCO2 = 60
PaO2 = 57

diagnosis?
A-a gradient?

A

COPD exacerbation

PAO2 = FiO2(Ptot-PH2O) - PaCO2/RQ

PAO2 = .21(760-47) - (60/.8) = 75

A-a = PAO2-PaO2 = 75-57 = 18 mmHg

so he has an elevated alveolar-arterial gradient! so there’s probably hypoventilation plus something like V/Q mismatch, decreased diffusion, etc.

COPD = emphysema + chronic bronchitis

emphysema = hyperinflation so when you exhale, you don’t exhale all the CO2 so you have increased levels of CO2

17
Q

what conditions can cause hypoventilation?

A
  1. medulla issues like from sedative medications/overdose
  2. cervical spinal cord damage like whiplash injury
  3. motor neuron diseases like ALS or polio
  4. phrenic and intercostal efferent nerve damage like during central line insertion, open heart surgery, drugs
  5. NMJ diseases like Curare or myasthenia graves
  6. weakened diaphragm/ respiratory muscles like myopathies, muscular dystrophies
  7. lung damage like COPD or pulmonary fibrosis
  8. thorax deformities like kyphosciolosis
  9. obesity
18
Q

what are CO2 levels during hypoventilation?

A

hypercapnia = CO2 levels are over 45 mmHg

this lead to respiratory acidosis

19
Q

what are CO2 levels during hyperventilation?

A

hypocapnia = CO2 levels are under 35 mmHg

this lead to respiratory alkalosis

20
Q

what is the pathophysiology of respiratory acidosis?

A

during hypercapnia CO2 levels are over 45 mmHg which leads to respiratory acidosis

arterial pH is below 7.35

most common causes of hypoventilation are drug overdose, myasthenia graves or severe lung disease like asthma or pneumonia

hypoventilation will have a normal A-a and if it’s abnormal, suspect another secondary condition

21
Q

what is obesity hypoventilation syndrome?

A

obese patients have hypoventilation syndrome

their breathing center is not as responsive to low oxygen so they retain CO2 easily

this is exacerbated when they receive sedatives

22
Q

what is the rspitory quotient?

A

RQ = CO2 produced/oxygen uptake

depends on diet

23
Q

what is the relationship between CO2 level and pH?

A

for every +Δ10 in pCO2, pH decreases by:
0.08 (in acute resp. acidoses)

0.03 (in chronic resp. acidoses)

for every -Δ10 in pCO2, pH increases by:
0.08 (in acute resp. alkaloses)

0.03 (in chronic resp. alkaloses)

24
Q

28 year old presents to the ER with shallow and deep breathing. On physical exam the patient is somnolent . Pupils are pinpoint and track marks are present on both arms. ABG showed a pH of 7.30 and a PaCO2 of 55. to relieve the metabolic disturbance you decided to give Naloxone IV of 0.4 mg.

diagnosis?

A

overdose leading to hypoventilation and respiratory acidosis

PaCO2 is high (35-45 is normal)

25
Q

what is the approach for evaluating the cause of hypoxemia in a patient?

A
  1. check A-a gradient

adjusted for age and FiO2

  1. if the A-a is normal, it’s either hypoventilation or low atmospheric pressure

if the A-a is elevated, check to see if the O2 saturation is corrected by giving 100% oxygen

  1. if it’s not fixed, it’s being caused by a shunt

if it is fixed by giving 100% oxygen, it’s either being caused by V/Q mismatch and/or impaired diffusion

26
Q

what 3 things can caused hypoxemia with an elevated A-a gradient?

A
  1. impaired diffusion
  2. V/Q mismatch
  3. right-left shunt
27
Q

which conditions can cause impaired pulmonary diffusion?

A
  1. pulmonary fibrosis (honeycomb MRI)
  2. interstitial lung disease
  3. reduced lung volume
  4. emphysema
  5. pulmonary resection
  6. anemia

this will cause hypoxemia with an elevated A-a gradient

impaired diffusion is reserved for pathologic states characterized by increased thickness of the alveolar-capillary membrane

28
Q

which conditions can cause a V/Q mismatch?

A
  1. pulmonary edema
  2. pneumonia
  3. COPD
  4. ARDS/ALI
  5. PE
  6. pneumothorax
  7. pulmonary HTN
  8. pulmonary contusion

this will cause hypoxemia with an elevated A-a gradient

29
Q

which conditions will cause a pulmonary shunt?

A
  1. R–>L intracardiac shunt
  2. atelectasis
  3. mucus plugging
  4. pulmonary AVM
  5. diffuse alveolar hemorrhage
  6. hepatopulmonary syndrome

this will cause hypoxemia with an elevated A-a gradient

will NOT correct with 100% oxygen

30
Q

what is a V/Q mismatch?

A

V = minute ventilation = TV x respiratory rate

Q = CO; perfusion

a normal V/Q = 4/5 = .8

a good lung tries to maintain the same perfusion and ventilation rate! if someone has walking pneumonia, the ventilation will be disturbed in the sick part of the lung so the blood vessels in that area will constrict to decrease perfusion so that it matches the decreased ventilation –> the areas of the lung with better ventilation will then receive higher blood flow

with people who have COPD or are old, they can’t compensate like this and there’s a V/Q mismatch

31
Q

what are the 3 zones of perfusion in the lungs?

A

zone 1: V&raquo_space;>Q so V/Q will be really high like 3.4

zone 2: V = Q so V/Q will be average around .8

zone 3: Q&raquo_space;»V so V/Q will be low like .63

32
Q

how does pulmonary edema cause a V/Q mismatch?

A

water in the alveoli will decrease ventilation

V/Q will decrease

33
Q

how does pulmonary embolism cause a V/Q mismatch?

A

there’s a disruption of perfusion so Q will decrease

V/Q will increase

34
Q

what is a pulmonary shunt?

A

alveoli lack ventilation and the blood passes and receives no oxygen – the blood perfuses but won’t get oxygenated because of the absence of ventilation like if there’s a mucous plug

increasing the respiratory rate will not resolve the hypoxemia and neither will giving 100% oxygen

shunt does not result in increase in CO2

the treatment is to resolve the atelectasis or the Interruption of ventilation

35
Q

what is the difference between an anatomical vs. physiologic pulmonary shunt?

A

physiologic = blood travels in area of the lung which is not ventilated

anatomic shunt = RV deoxygenated blood goes to the aorta through the VSD instead of the PA – or if there’s a pulmonary arteriovenous fistulae

36
Q

A 30-year-old male is brought to the emergency department after being found unresponsive at home with a syringe in his arm. The patient is still unresponsive, blood pressure is 120/60 mmHg, and pulse is 100 bpm. His pupils are very small and unreactive. He appears cyanotic, and his respiratory rate is 8 breaths/min. Arterial blood gas on room air shows a pH 7.22 (normal 7.35–7.45), pCO2 of 72 mmHg (normal 34–45 mmHg), and a pO2 of 50 mmHg (normal 80–100 mmHg).

Which of the following is responsible for this patient’s hypoxemia?

A. Alveolar hypoventilation plus low V/Q
B. Alveolar hypoventilation alone
C. Alveolar hypoventilation plus right-to-left shunt
D. Low V/Q ratio
E. Low diffusion capacity
A

PAO2 = 150-(72/.8) = 60

A-a = 60-50 = 10 so it’s normal

so his hypoxemia is being caused by hypoventilation alone

Pulmonary Week 2 (9 decks)

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