ICL 1.5: Arterial Blood Gases Flashcards
what is hypoxia?
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
what is hypoxemia?
a reduction in the concentration of oxygen in arterial blood
how do we know the partial pressure of O2, CO2, etc. in the blood?
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
what are the normal ranges for pH, CO2, pO2, HCO3 and O2 saturation in the arterial blood?
pH = 7.35-7.45
CO2 = 35-45 mmHg
pO2 = 80-100 mmHg
HCO3 = 22-26
O2 saturation = 95-100%
what is the primary function of the lung?
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
what is the alveolar-arterial gradient?
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)
what is the alveolar gas equation?
PAO2 = FiO2(Patm-PH2O) - PaCO2/RQ
FiO2 = 0.21 Patm = 760 PH2O = 47 RQ = 0.8 normally
what happens to FiO2 if a patient is on a ventilator or on oxygen?
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
what 5 things can cause hypoxemia?
hypoxemia is a reduction in the concentration of oxygen in arterial blood
- decreased inspired oxygen tension (high altitude)
- hypoventilation (CO2 > 40-50 means PAO2 will decrease)
- ventilation/perfusion mismatch
- right to left shunt
- impaired diffusion
what does the alveolar-arterial gradient tell us about the hypoxemia patient?
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
- shunt
- impaired diffusion
what is a normal alveolar-arterial gradient?
5-15 mmHg
what are the PaO2, PaCO2, A-a and PaO2 with 100% O2 in the case of hypoventilation?
PaO2 = low
PaCO2 = high
A-a = normal
PaO2 with 100% O2 = >550
what are the PaO2, PaCO2, A-a and PaO2 with 100% O2 in the case of poor diffusion?
PaO2 = low
PaCO2 = normal-low
A-a = high
PaO2 with 100% O2 = >550
what are the PaO2, PaCO2, A-a and PaO2 with 100% O2 in the case of a right-left shunt?
PaO2 = low
PaCO2 = normal-low
A-a = high
PaO2 with 100% O2 = <550
what are the PaO2, PaCO2, A-a and PaO2 with 100% O2 in the case of V/A imbalance?
PaO2 = low
PaCO2 = normal-low
A-a = high
PaO2 with 100% O2 = >550
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?
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
what conditions can cause hypoventilation?
- medulla issues like from sedative medications/overdose
- cervical spinal cord damage like whiplash injury
- motor neuron diseases like ALS or polio
- phrenic and intercostal efferent nerve damage like during central line insertion, open heart surgery, drugs
- NMJ diseases like Curare or myasthenia graves
- weakened diaphragm/ respiratory muscles like myopathies, muscular dystrophies
- lung damage like COPD or pulmonary fibrosis
- thorax deformities like kyphosciolosis
- obesity
what are CO2 levels during hypoventilation?
hypercapnia = CO2 levels are over 45 mmHg
this lead to respiratory acidosis
what are CO2 levels during hyperventilation?
hypocapnia = CO2 levels are under 35 mmHg
this lead to respiratory alkalosis
what is the pathophysiology of respiratory acidosis?
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
what is obesity hypoventilation syndrome?
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
what is the rspitory quotient?
RQ = CO2 produced/oxygen uptake
depends on diet
what is the relationship between CO2 level and pH?
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)
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?
overdose leading to hypoventilation and respiratory acidosis
PaCO2 is high (35-45 is normal)
what is the approach for evaluating the cause of hypoxemia in a patient?
- check A-a gradient
adjusted for age and FiO2
- 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
- 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
what 3 things can caused hypoxemia with an elevated A-a gradient?
- impaired diffusion
- V/Q mismatch
- right-left shunt
which conditions can cause impaired pulmonary diffusion?
- pulmonary fibrosis (honeycomb MRI)
- interstitial lung disease
- reduced lung volume
- emphysema
- pulmonary resection
- 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
which conditions can cause a V/Q mismatch?
- pulmonary edema
- pneumonia
- COPD
- ARDS/ALI
- PE
- pneumothorax
- pulmonary HTN
- pulmonary contusion
this will cause hypoxemia with an elevated A-a gradient
which conditions will cause a pulmonary shunt?
- R–>L intracardiac shunt
- atelectasis
- mucus plugging
- pulmonary AVM
- diffuse alveolar hemorrhage
- hepatopulmonary syndrome
this will cause hypoxemia with an elevated A-a gradient
will NOT correct with 100% oxygen
what is a V/Q mismatch?
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
what are the 3 zones of perfusion in the lungs?
zone 1: V»_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»_space;»V so V/Q will be low like .63
how does pulmonary edema cause a V/Q mismatch?
water in the alveoli will decrease ventilation
V/Q will decrease
how does pulmonary embolism cause a V/Q mismatch?
there’s a disruption of perfusion so Q will decrease
V/Q will increase
what is a pulmonary shunt?
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
what is the difference between an anatomical vs. physiologic pulmonary shunt?
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
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
PAO2 = 150-(72/.8) = 60
A-a = 60-50 = 10 so it’s normal
so his hypoxemia is being caused by hypoventilation alone
