Alveolar Gas Exchange II Flashcards
“Normal” ventilation/perfusion inequality: Ventilation/perfusion ratio is less at the lung base, so describe the oxygenation of blood flowing through the basal region of the lung?
it is less oxygenated
What is the normal PO2 gradient in arterial blood?
15 mmHg
Why is there a problem with low DO2 in exercise, where it becomes more severe?
Because the blood has less time in pulmonary capillaries to equilibrate with alveolar gas
What type of shunts produce hypoxia? Which ones reduce systemic perfusion and increase cardiac work?
- right-to-left cause hypoxia
- Left-to-right reduce perfusion and increase cardiac work
What are the different levels that cause shunting or “venous admixture” of blood?
- Levels of the shunt
- Lungs (i.e., atelectasis with maintained circulation)
- Heart (i.e., intracardiac septal defect)
Peripheral circulation (i.e., congenital anomaly of the circulation such as patent ductus arteriosis
What are some causes of ventilation/perfusion mismatch?
- uneven lung tissue compliance
- pulmonary embolism
- unilateral pulmonary artery stenosis
Describe V/Q ratio levels in apex of lung and base of lung?
- High V/Q ratio at apex
- Low V/Q ratio at the base
What is the basal value of V/Q ratio in the lungs?
0.8
What cause may lead to a gross scatter of V/Q ratios commonly resulting in hypoxemia with CO2 usually being compensated?
diffuse pulmonary disease
Are V/Q ratios in this scenario greater than or less than 0.8?
- contributes to alveolar dead space
- alveolar gas tensions approach those of inspired air,
- alveolar O2 increases and CO2 decreases,
- as do the end pulmonary capillary pressures
R > 0.8
Are V/Q ratios in this scenario greater than or less than 0.8?
- ventilation below perfusion requirements, contributes to physiological shunt
- alveolar gas tensions approach those of mixed venous blood,
- alveolar O2 falls and CO2 increases,
- as do the end pulmonary capillary pressures
R < 0.8
If overall alveolar ventilation is normal, an elevation of PaCO2 will return levels to normal how?
elevation of PaCO2 leads to an increase in alveolar ventilation and a return to normal
What is the alveolar gas equation?
Relationship between PA O2 and PACO2 depends on the respiratory quotient R= 0.8
A decrease in alveolar ventilation reduces PAO2 slightly higher than increases PCO2
Suppose a patient is given a mixture of 50% Nitrous Oxide (a sedative gas) and 50% atmospheric air to breath. Will he become hypoxic? (Assume alveolar CO2 remains at 40 mmHg)
•Answer: He will become very hypoxic and might not survive.
Alternative: Give a mixture of 50% Nitrous Oxide and 50% Oxygen. (Calculate PAo2
A patient with normal lungs overdoses on a secobarbital, causing hypoventilation. The patient arrives at a hospital where the barometric pressure = 500 mmHg. Alveolar PCO2 rises to 80 mmHg and the respiratory exchange ratio is 1.0. Assuming that the patient’s condition remains unchanged, what percentage of inspired O2 will return the patient’s alveolar PO2 to normal (100 mmHg)?
A. 55
B. 40
C. 36
D. 28
E. 24
B. 40
A patient has a PIO2 of 147 mmHg, an R of 0.85, and a PaCO2 of 42 mmHg. What is the PAO2?
98 mmHg
Calculate the PaO2 for the following: Atmospheric pressure = 547 mmHg, FIO2 = 0.3 (30%), PaCO2 = 40 mmHg, A-a gradient (A-a ΔO2) = 50 mmHg; R=0.8.
f
