alveolar gas equation/oxygen deprivation/ V/Q Flashcards
alveolar gas equation
PAO2 = PIO2 - PaCO2/R = 150mmHG - PaCO2/0.8
PA02 = alveolar PO2
PIO2 = PO2 in inspired air
PaCO2 = arterial PCO2
R=respitatoy quotient = CO2 produced/O2 CONSUMED
what is R in alveolar gas equation
R=respitatoy quotient = CO2 produced/O2 CONSUMED
normally is 0.8
PO2 in inspired air in see level (and why)
150mmHg
Ptotal (760) - correction because of trachea (47)
= 713
713 x 0.21 = 150 mmHg
A-a gradient:
equation
A-a gradient = alveolar PO2 - arterial PO2
A-a gradient - normal values
10-15 mmHg
increased A-a gradient may occur in ….
situations:
hypoxemia
causes include: shunting, V/Q mismatch, fibrosis (impairs diffusion)
oxygen deprivation in tissues - types
- hypoxemia
- hypoxia
- Ischemia
what is hypoxemia
decreased arterial PCO2
what is hypoxia
decreased O2 delivery to tissues
what is ischemia
loss of blood flow
causes of hypoxemia (divide by A-a gradient)
normal A-a gradient: a. high altitude, hypoventilation (eg opioid)
increased A-a gradient: a. V/Q mismatch b. Diffusion limitation (eg. fibrosis) c. Right-to-left shunt
causes of hypoxia
- HYPOXEMIA
- anemia
- decreased cardiac output
- CO poisoning
causes of ischemia
- impeded arterial flow
2. decreased venous drainage
V/Q means
ventilation/perfusion
V/Q ideally
ideally ventilation is matched to perfusion (V/Q = 1) for adequate gas exchange
V/Q lung zones and explanation
V/Q at apex = 3 –> wasted ventilation
V/Q at base = 0.6 –> wasted perfusion
V/Q: P artery (Pa) P venous (Pv) P alveoli (PA)
bigger, smaller in the apex of the lung
PA>Pa>PV
V/Q: P artery (Pa) P venous (Pv) P alveoli (PA)
bigger, smaller in the middle of the lung
Pa>PA>PV
V/Q: P artery (Pa) P venous (Pv) P alveoli (PA)
bigger, smaller in the base of the lung
Pa>PV>PA
V/Q=0 - explanation
airway obstruction (SHUNT) 100% O2 does not improve alveolar PO2
V/Q=0 –> if 100% O2–> alveolar PO2
does not improve
V/Q = infinity
blood flow obstruction (physiologic dead space)
assuming 100% O2?
It improves PaO2
V/Q – > response to exercise
increased cardiac output –> vasodilation of apical capillaries –> V/Q ratio approaches 1
organism that thrive in high O2- area in the lungs
and example
apex
ex. TB
V, Q, V/Q ratio distribution in apex, base and middle of the lung
V/Q as apex as higher (0.6 at base, 3 at apex)
Both V and Q are greater at the base, but the increasing of Q from apex to base is greater from the increasing of V and thats why V/Q is decreased in the base (0.6)
hypoventilation causes hypoxemia - why
hypoventilation –> increased PaCO2–>increased PACO2 –> decreased PAO2
PaCO2 - equation
PaCO2 = K x VCO2 / VA
VA: alveolar ventilation
VCO2: rate of elimination
increased VCO2 - situations
fever
exercise
overconsumption of carbohydrates
chill tetanus
