Common causes of hypoxemia Flashcards
1. Define and distinguish between hypoxia and hypoxemia 2. Be able to calculate the A-a gradient, define its normal range and describe its significance in distinguishing between the common causes of hypoxemia 3. Describe the clinically important causes of arterial hypoxemia
Hypoxia
Failure of oxygenation at the tissue and cellular level
Hypoxemia
Low partial pressure of oxygen in the blood
Measuring hypoxemia
Specifically hypoxemia is determined by measuring partial pressure of oxygen in the arterial blood (PaO2)
PaO2 of the arterial blood and alveolar gas in the perfect lung + reality
- PaO2 of arterial blood would be the same as that in alveolar gas in the perfect lung
- in reality always be small differences between PAO2 and PaO2 (approx by 10-20 mmHg)
Why small differences in PAO2 and PaO2
1) incomplete diffusion (very small)
2) ventilation and perfusion not perfectly matched (even in healthy -especially in different areas of lungs)
3) small percentage of bronchial arterial blood is collected by the pulmonary veins after it has perfused the bronchi and its O2 has been partly depleted
Where is ventilation/perfusion the best matched
At the bases of the lungs
A-a gradient
-alveolar-arterial difference in oxygen (A-a DO2)
Determining A-a DO2
1) Obtain PaO2 and PaCO2 levels by sampling arterial blood gases
2) Use PaCO2 and the alveolar air equation to calculate PAO2
3) Calculate A-a DO2 = PAO2 - PaO2
What do large values for A-a DO2 indicate
Problems with gas exchange
Estimating normal range for A-a DO2 by age
Age/4 + 4
Increased age affects A-a gradient
Causes of hypoxemia - 2 main categories of mechanisms
1) Those that increase A-a DO2
2) Those where A-a DO2 is preserved
Five causes of hypoxemia
- Low inspired oxygen
- Alveolar hypoventilation (low alveolar ventilation)
- Diffusion impairment (diffusion abnormality)
- Ventilation -perfusion inequality (ventilation - perfusion mismatch) - V/Q mismatch
How low inspirated oxygen causes hypoxemia
PiO2 = (PB-PH2O) FiO2 -where PiO2 is decreased FiO2 = Fraction of inspired oxygen PiO2 = partial pressure of inspired O2 PB = barometric pressure PH2O= partial pressure of water vapour in lungs
A-a DO2 if hypoxemia caused by low inspired oxygen
-will be normal as PAO2 and PaO2 will be decreasing at the same rate
Causes of low inspired oxygen
1) Decrease in barometric pressure (breathing at high altitude)
2) Decrease in FlO2 - accidental (anaethetist does not supply enough O2 r leak in breathing circuit)
PaCO2 in situations of low inspired oxygen + mechanism
1) Peripheral chemoreceptors sense low arterial PO2 and increase ventilation through input to respiratory centers
2) Hyperventilation
3) PaCO2 is decreased (hypocapnia)
How alveolar hypoventilation (low alveolar ventilation) causes hypoxemia
-breathing less (mechanically)-volume entering and exiting alveoli per unit time is reduced
A-a DO2 if hypoxemia is caused by alveolar hypoventilation
A-a DO2 is normal as PAO2 and PaO2 will be decreasing at the same rate
PaCO2 if hypoxemia is caused by alveolar hypoventilation
-PaCO2 is elevated (hypercapnia) - because not exhaling as much either and body’s primary means of eliminating CO2 is through the lungs
PaCO2 proportional to VCO2/ VA
(alveolar PACO2 is directly proportional to the amount of CO2 produced by metabolism and delivered to the lungs VCO2 and inversely proportional to the alveolar ventilation VA)
-reduction of alveolar ventilation = increased alveolar PCO2 (PACO2) because gas is not being exchanged at normal rate
-in turn CO2 in blood perfusing the alveoli will begin to accumulate and increase the PaCO2
Treatment of hypoventilation
Supplemental O2
-increasing fraction of inspired oxygen can alleviate the hypoxemia and the hypercapnia
Potential causes of hypoventilation (9)
- Depression of CNS by drugs
- Inflammation, trauma or hemorrhage in the brainstem
- Abnormal spinal cord pathway
- Disease of the motor neurons of the brain stem/spinal cord
- Disease of the nerve supplying the respiratory muscles
- Disease of the neuro muscular junction
- Disease of the respiratory muscles
- Abnormality of the chest wall
- Upper airway obstruction
Diffusion impairment - how it causes hypoxemia
-O2 diffusion from alveoli to arteries is reduced
A-a DO2 if hypoxemia is caused by diffusion impairment + why
- A-a DO2 is normal at rest but may be elevated during exercise
- because at rest transit time for RBC in pulmonary capillary > then time for PO2 in mixed venous blood to reach eq with alveolar gas
- during exercise is > blood flow so transit time is decreased - in the healthy individual would still be sufficient time for PO2 in mixed venous blood to reach equilibrium with alveolar gas but if have impaired diffusion time to reach eq will be further increased and cause increase PAO2 and decreased PaO2 (or in elite athelete who achieve very high CO during exercise resulting in large decrement in pulmonary transit time)
Why does impaired diffusion occur
-When there is an increase in the thickness of the physical seperation between alveolar gas and pulmonary capillary blood + shortened pulmonary transit times (i.e. performing exercise)
Treatment of diffusion impairment
- supplemental oxygen
- increasing fraction of inspired oxygen -increases the partial pressure gradient driving O2 across the altered blood air barrier
Ventilation-perfusion inequality - why causes hypoxemia
-for efficient gas exchange to occur air must reach the regions of the lung that are being appropriately perfused
A-a DO2 in hypoxemia caused by V/Q mismatch
A-a DO2 is elevated
-because alveoli not sufficiently perfused so PAO2 increases and PaO2 does not
Average alveolar ventilation
4L/min
Average CO
5L/min
Normal range V/Q ratio
0.8- 1
Why V/Q mismatch occurs even in healthy lungs
- ventilation and blood are both gravity -dependent
- i.e. both increase as move down the lungs
- blood flow shows about 5-fold difference between top and bottom while ventilation shows about a 2-fold difference
- this causes gravity-dependent regional variations in V/Q ratio - range from 0.7 at the base of the lungs to 3.0 at the apex
- this mismtch accounts for approx 2/3 of the normal A-a DO2 seen in healthy individuals
At top of the lungs what is the V/Q ratio + consequence
-at top of lungs V/Q is high and is more ventilation compared to perfusion
At base of lungs what is the V/Q ratio + consequences
- V/Q is low
- is more perfusion compared to ventilation
Treament of hypoxemia caused by V/Q mismatch
-supplemental oxygen will help correct hypoxemia because will provide additional O2 to low V/Q regions (bases)
(but will have little affect on high V/Q regions - blood in these regions will already be nearly saturated)
-but will improve both saturation and PaO2 in blood coming from regions with low V/Q ratio
How shunting causes hypoxemia
Shunt = a condition in which deoxygenated blood from the venous system is directed to the arterial system without receiving oxygen from the lungs
Shunts can occur naturally or be the result of a disease process
A-a DO2 in hypoxemia caused by shunting
-will be elevated
Supplemental oxygen efficacy for hypoxemia caused by shunting
Will not correct hypoxemia - because shunted blood will never be exposed to supplemental oxygen
Common causes of shunting
- congenital abnormalities: tetrology of fallot,
- intrapulmonary fistulas (direct communication between a branch of pulmonary artery and pulmonary vein)
