Lecture 4: Gas Exchange Flashcards
Ve (minute ventilation) =
Vt (tidal volume) x RR
Dead space (definition and two components)
Volume of airways and lungs that do not participate in gas exchange, includes anatomical dead space (respiratory tract –> terminal bronchiole), alveolar dead space (ventilated alveolar that are not perfused)
Physiological dead space should be equal to what in healthy people?
Anatomical dead space
Dead space assumptions (3)
All CO2 in expired air comes from gas exchanged; no CO2 in inspired air; physiological dead space contributes no CO2
Dead space equation
Vd/Vt = (PaCO2 - PeCO2) / PaCO2
Alveolar ventilation equation
VCO2 = (VA x PACO2) / K
If we ventilate more, what happens to PaCO2?
Decreases
Alveolar gas equation
PAO2 = FIO2 (Patm - PH2O) - (PaCO2 / R)
Where in the lung is ventilation higher?
Bases: gravity causes resting volume of alveoli in bases to b smaller –> more compliant (small pressure difference = large change in volume) –> more air goes here
Where in the lung is perfusion higher?
Gravity causes perfusion to be highest in lung bases
Zone 1
PA > Pa > Pv
Zone 2
Pa > PA > Pv
Zone 3
Pa > Pv > PA
What is the normal value of V/Q (lung average)?
0.8
Alveolar hypoxia causes…
Pulmonary vasoconstriction
Regional hypoxia
Vasoconstriction leads diversion of blood away from poorly ventilated alveoli
Generalized hypoxia
When PAO2 is low (because PIO2 low or disease) you can get pulmonary hypertension
Where is V/Q highest and lowest?
Highest = apex; Lowest = base
V/Q = infinity F
Ventilation, no perfusion (dead space); PAO2 = 150; PACO2 = 0
V/Q = 0 F
Perfusion, no ventilation (airway obstruction); PaO2 = 40, PaCO2 = 46
Diffusion-limited gas exchange
Total amount of gas transported across the alveolar-capillary barrier is limited by the diffusion process (too slow, pressure gradient continues exists)
Perfusion-limited gas exchange
Total amount of gas transported across the alveolar-capillary barrier is limited by blood flow (perfusion; partial pressure gradient NOT maintained)
Oxygen and carbon dioxide are __________-limited
Perfusion
What does the O2-Hb dissociation curve show us? (4 conditions)
O2 preferentially released to tissues that are more metabolically active (decreased pH, increased CO2, increased temp, increased 2,3-DPG)
CO binds heme with _____ times more/less affinity. Besides blocking O2 from binding, what else does CO do?
250; more; prevents O2 release by left shifting curve
What is diffusing capacity? How does it work?
Quantitative measure of gas transfer in the lungs; low concentration of CO delivered to lungs; amount CO measured at exhale to determine diffusion
DLCO (diffusing capacity) equation
DLCO = VCO / PACO
FFF
(OR) = (dCO x A) / T
How much O2 is dissolved? How else is it carried in blood?
2%; bound to Hb
Arterial oxygen content (CaO2) equation
CaO2 = (1.34 x Hgb x O2 saturation) + (0.003 x PaO2)
Oxygen delivery (DO2) equation
DO2 = CaO2 x CO (cardiac output)
Hypoxia (def)
Decreased oxygen delivery (or utilization) by tissues
Hypoxemia (def)
Low PaO2 (can CAUSE hypoxia)
Causes of hypoxia (4)
- Low CO, 2. Low Hb (anemia), 3. Low O2 saturation (hypoxemia, CO poisoning), 4. Low O2 utilization (O2 delivered but NOT used = sepsis, cyanide)
Causes of hypoxemia (5)
- Low PiO2, 2. Hypoventilation, 3. V/Q mismatch, 4. Shunt, 5. Diffusion impairment
What can test for hypoxia/hypoexmia?
A-a gradient and effect of supplemental oygen
What is the A-a gradient? What is normal? What does hypoxemia with normal A-a gradient indicate? What does hypoxemia with increased A-a gradient indicate?
Difference between PAO2 and PaO2;
When does supplemental oxygen help? (4)
Increases FiO2, will increase PaO2 if: low PiO2, hypoventilation, V/Q mismatch, diffusion impairment
When will supplemental oxygen not help?
If hypoxemia is secondary to shunt
How does supplemental O2 help in w/ hypoventilation?
Breathing supplemental oxygen will require less ventilation to achieve higher PAO2
Why does O2 not help in case of a shunt?
Perfused areas of lung already have Hb saturated with O2, so they cannot “make up” for the not perfused portions with the extra O2
Why does O2 help in case of V/Q mismatch?
In creased FiO2 will help the less perfused alveoli increase to 100% saturation
V/Q mismatch only causes hypoxemia when…
V/Q units are low
Why is CO2 not elevated in shunt?
Because of relationship between PCO2 content and and increased ventilation F
Why does O2 help diffusion impairment?
If you have diffusion impairment (fibrosis) O2 becomes diffusion limited exchange, so supplemental O2 will increase PAO2 and O2 pressure gradient, which will help
When does diffusion impairment manifest?
Exercise: CO increases, speed of blood increases, less time in alveolar capillary –> not enough time for gas to diffuse
