Alveolar Ventilation Flashcards
ventilation
the movement of fresh gas into the lungs for gas exchange, and movement of that gas back out of the lungs
what is inspired air and what happens to it as it gets breathed in
environmental air; gets heated to body temperature and fully humidified when inhaled
what is FIO2
fraction of O2 in inspired gas
at sea level FIO2 = 21%
what is the partial pressure of water vapor
50 mmHg
causes a drop in PO2 in the air as it travels into alveoli
equation for inspired air PO2 (PIO2)
PIO2 = FIO2 (PB - PH2O)
PIO2 = 149 mmHg at sea level
what is barometric pressure
760 mmHg
tidal volume
the volume of air moved with each breath
measured on expiration since volume in does not equal volume out
tidal volume equation
VT = VD + VA
VD = vol of air in dead space
VA = vol of air in alveoli
normal tidal volume
10-20 mL/kg
minute ventilation
volume of air exhaled in one minute
minute ventilation equation
minute ventilation = VT x RR
anatomic dead space
amount of tidal volume that doesn’t reach alveoli - remains in conducting airways and does NOT participate in gas exchange
NORMAL - can not change
alveolar dead space
amount of tidal volume that reaches alveoli but does not participate in gas exchange due to lack of perfusion to the alveolus
ABNORMAL - all should be perfused in health
physiologic dead space
the amount of minute ventilation that goes to dead space (anatomic or alveolar)
considered wasted ventilation
is VA equal to the total volume of gas in the alveoli
NO - it is the volume that reaches the alveoli per breath
alveolar ventilation
the part of minute ventilation coming from the gas exchange regions
alveolar ventilation = VA x RR
how does CO2 diffuse
along partial pressure gradient
tissues –> blood –> lungs –> pulmonary capillaries –> alveolar gas
alveolar CO2 (PACO2)
CO2 added to alveolar gas by pulmonary arterial blood and removed by alveolar ventilation
how is PACO2 related to alveolar ventilation
INVERSE
high ventilation = low PACO2 because more gas exchange is occurring
where does all CO2 come from
tissues (do NOT inspire CO2)
arterial PCO2 (PaCO2)
CO2 added to arterial blood from production in tissues
how are arterial and alveolar PCO2 related
nearly equal because CO2 is highly diffusible
how is PaCO2 related to alveolar ventilation
INVERSE
high ventilation = low PaCO2 because more gas exchange is occurring
what is the ONLY way to measure ventilation
PCO2 in arterial or venous blood (ideally arterial)
PaCO2 should be 35-45 mmHg
hypoventilation
low alveolar ventilation leading to low CO2 output –> high CO2 remaining in body
causes hypercapnia (high PA and PaCO2)
how can hypoventilation cause hypoxemia
high PaCO2 leaves less room available for O2, causing PaO2 to decrease
is hypoventilation caused by respiratory rate or tidal volume
NO - can have any RR and still be hypoventilating
hyperventilation
high alveolar ventilation resulting in high CO2 output –> low CO2 remaining in the body
causes hypocapnia (low PA and PaCO2)
alveolar gas equation
PAO2 = inspired PO2 - (PACO2 / R)
inspired PO2 = 149 mmHg
R = 0.8 (respiratory exchange ratio)
how can you use alveolar PO2 to evaluate arterial PO2
PAO2 is the maximum value that PaO2 can be
PaO2 is usually slightly lower than PAO2 (5-15 mmHg)
what does it mean if PAO2 is lower than PaO2
low inspired O2 (altitude)
or
high arterial O2 (hyperventilation)
what does it mean if PaO2 is significantly lower than PAO2
- decreased O2 delivery
- increased O2 consumption
- Hbg deficiency/abnormality
