2017 Hawaii Flashcards
What is the alveolar air equation
PAO2 = [FIO2 x (Barometric Pressure - Water vapor pressure)] - PaCO2/RQ
FIO2
Fraction of inspired oxygen
Can be measured as percentage or decimal
room air is considered 21% or 0.21
PAO2
partial pressure of oxygen in the alveolus
This is the quantity of oxygen in the alveolus
Barometric Pressure
760 mmHg at sea level
with increasing altitude the pressure falls and those at 600 mmHg barometric pressure will have a lower PAO2 than those at seal level. So normal blood oxygen level will change with altitude.
These people end up making more red blood cells.
water vapor pressure
50 mmHg
RQ
respiratory quotient = CO2Production/O2 Consumption
RQ ~ 0.9 (0.7/1.0)
A normal animal using an RQ of 0.9 at sea level will have a PAO2 of 105 mmHg
PaO2
amount of oxygen in arterial blood
<80 @ sea level is considered hypoxemia
A-a gradient
PAO2-PaO2
With normal lung function the partial pressure of oxygen in arterial blood (PaO2) will be slightly lower than PAO2
An A-a gradient of room air blood gases of < 15 mmHg is considered normal
The normal is due to small quantity of deoxygenated venous blood from the bronchial and coronary circulation draining into the left side of the heart.
Define Hypoxemia & give 2 main mechanisms
PaO2 < 80 mmHg
- Low Alveolar O2
- Abnormal transfer of O2 for alveoli to the arterial blood (increased A-a gradient)
Name the clinical causes of hypoxemia
Low PAO2 (normal A-a gradient)
- low inspired oxygen (could be due to low barometric pressure at high altitude or a breathing circuit without an adequate oxygen supply
- Hypoventilation (if breathing room air)
Increased A-a Gradient
- Venous admixture
- Ventilation perfusion mismatch due to
a. low venitlation/perfusion alveoli
b. no ventilation/perfusion alveoli (intrapulmonary shunt)
- Anatomical vascular right to left shunts
- Diffusion defects
N2 = 565 mmHg
H20 = 50 mmHg
CO2 = 40 mmHg
O2 - 105 mmHg
Normal patient breathing room air
N2 = 565 H20 = 50 CO2 = 80 O2 = 65
hypoventilation breathing room air: patient is hypoxemic
N2 = 345 H20 = 50 CO2 = 80 O2 = 285
Hypoventilation Breathing 50% O2
Patinet would not be hypoxemic if normal lungs.
What is the difference between hypoxia and hypoxemia?
Hypoxemia: abnormally low arterial oxygen tension in the blood
Hypoxia: underoxygenation which is inadequate level of tissue oxygenation for cellular metabolism
What are the 4 types of hypoxia
- hypoxic hypoxia: inadequate oxygen at the tissue cells caused by low arterial oxygen tension (PaO2)
- hypoventilation - increased CO2 in alveolus displaces oxygen
- high altitude
- diffusion defects
- VQ mismatch
- R to L shunt
2. Hypoxemic Hypoxia: decreased O2 intent (CaO2) anemic hypoxia PaO2 is normal but the oxygen carrying capacity of the Hb is inadequate -decreased hemoglobin -anemia -hemorrhage -abnormal hemoglobin -carboyxhyemoglobinemia -Methemoglobinemia
- Circulatory hypoxia
stagnant hypoxia or hypo perfusion where blood flow to the tissue cells is inadequate. Oxygen delivery is not adequate to meet tissue needs
systemic= shock
ischemia=local lack of perfusion
-slow or stagnant (pooling) of peripheral blood flow
-arterial-venous shunts
-decreased cardiac output
Histotoxic hypoxia
impaired ability of the tissue cells to metabolize oxygen
- cyanide poisioning
- dysoxia: sepsis alters tissue ability to utilize oxygen
Causes of V/Q mismatch?
pulmonary disease
- oedema
- pneumonia
- pulmonary hemorrhage
optimal V/Q matching would be an equal degree of ventilation as there is perfusion and = 1
What is the rule of 120?
Analyze: room air arterial blood gases at sea level and a quick easy A-a gradient is
IF PaO2 + PaCO2 > 120 mmHg it means normal A-a gradient
IF the value is lower then there is an increased or abnormal gradient
What is the 5x rule?
PaO2 you would expect with normal lungs at sea level on a given FIO2
does not take into account big changes in PCO2
so if FIO2 is 50% then PaO2 is 250 mmHg
PaO2/FIO2 ratio?
can compare serial blood gases on differing FIO2
expressed as decimal in this ratio
FIO2 105 mmHg and room air 21%
so 105/0.21=500
~500 is consistent with normal function
200-300 is mild lung dysfn
100-200 is moderate a
<100 is severe
What is ventilation?
the tidal movement of air in and out of lungs
quantified as minute ventilation
What is minute ventilation
total amount of gas inhaled in one minute and equals RR x TV of each breath
How is PCO2 determined primarily?
arterial CO2 levels are directly proportional to the rate of CO2 production by the tissues and inversely proportional to effective alveolar minute ventilation. PCO2 doesn’t really change substantially in clinical setting so it is primarily determined by alveolar minute ventilation.
What is alveolar minute ventilation?
the portion of TV that ventilates functional alveoli.
