Lec 2-3 Gas Exchange Flashcards
What is minute ventilation?
step1
volume of gas moved through nose/mouth in one minute
Ve = Vt * RR
Vt = tidal volume = volume/breath RR = respiratory rate
What is normal tidal volume?
step1
500 ml
what is normal respiratory rate?
12-14 breaths/min
What are the 2 equations for minute ventilation?
Ve = Vt * RR = tidal vol * resp rate
Ve = VA [alveolar] + Vd [dead space]
What is the equation for alveolar ventilation?
step1
VA = (Vt - Vd)*RR
=( tidal - dead space) * resp rate
What is dead space ventilation?
the portion of minute ventilation that does not participate in gas exchange = wasted ventilation
due to anatomic +/- functional dead space
What is alveolar ventilation?
the portion of minute ventilation that does participate in gas exchagne
What is anatomic dead space?
the volume of the respiratory tract that does not participate in gas exchange = conduction zone
What is normal anatomic dead space?
step1
150 mL [1/3 of tidal volume]
What is functional dead space?
step1
the wasted ventilation that occurs when alveoli are ventilated but not perfused so cannot participate in gas exchange
due to pulm embolism or other block in blood flow to that portion of the lung
What is the physiologic dead space?
step1
physiologic dead space = anatomic dead space + functional dead space
What is normal functional dead space?
0 in normal person; higher in disease state
How can you measure dead space?
step1
measure difference between O2 in expired air compared to pure alveolar air
use arterioal PCO2 to stand in for PaCO2
What is equation for dead space?
step1
Vd = Vt * (PaCO2 - PECO2) / PaCO2
PaCO2 = arterial PCO2 PECO2 = expired PCO2
What is equation for Vd/Vt ratio?
Vd/Vt = (PaCO2 - PeCO2 ) / PaCO2
PaCO2 = arterial PCO2 PECO2 = expired PCO2
What is the alveolar ventilation equation in terms of rate of CO2 production etc?
VA = VCo2 * K / PACO2
conversely:
PACO2 = VCO2 * K / VA
K = contant 863 for BTPS
What is relationship PACO2 and VA?
PACO2 = CO2 in alveoli
is inversely proportional to
VA = ventilation to alveoli
What happens if VCO2 doubles in strenuous exercise?
only way to maintain normal PACO2 is for VA to double also
–> when VA is doubled; PACO2 is halved
What is the alveolar gas equation?
step1
PAO2 = PIo2 - PaCO2/R
PAO2 = alveolar PO2 PIO2 = PO2 in inspired air PaCO2 = arterial PCO2 R = respiratory quotient = CO2 produced/O2 consumed
What is PIO2? How do you determine it?
step1
PIO2 = FIO2 * (Pb - PH2O)
PIO2 = PO2 in inspired air FIO2 = fraction of O2 in inspired air [normal = 0.21] Pb = barometric pressure [normal = 760] PH2O = pressure of water [normal = 47]
What is normal value for PIO2?
step1
150
What is respiratory quotient? Normal value?
step1
R = CO2 production / O2 consumption
normal = 0.8
What is normal PH2O in air?
47
What happens to PACo2 and PAO2 if alveolar ventilation is halved?
- PACO2 is doubled
- PAO2 is reduced
A man has a rate of CO2 production that is 80% of rate of O2 consumption. If his arterial PCO2 = 40 mmHg and PO2 in humidified tracheal air is 150 mmHg, what is his alveolar PO2?
PAO2 = inspired - PACO2 / R
= 150 - 50 = 100 mmHg
With each inspiration, where does the air go?
pre-inspiration have 150mL old gas in dead space
when you inspire
- -> that 150 mL old gas goes to alveoli
- -> have 300 new mL fresh air that go to alveoli
- -> have 150 mL fresh air in dead space
What is normal PaCO2?
40 = realtively constant
What does hypoventialtion do to arterial PaCO2?
increases it
What does hyperventilation do to arterial PaCO2?
decreases it
What will pulmonary embolism do to PaCO2?
in most people remains normal even though we would have expected an increased due to wasted ventilation
b/c most individuals with PE will increase total minute ventilation to adjust for the increased dead space –> can maintain normal alveolar ventilation but appear tachypneic
How is ventilation distributed in the lungs?
more ventilation to alveoli at bottom [when standing/sitting]
- due to difference in intrapleural pressure at bottom of long
- these differences are due to gravity
Where does functional residual capacity of lung mostly reside?
in apex of lung
What are the 3 zones of perfusion? their order of hydrostatic pressures Pa [arterial] vs PA [alveolar] vs Pv [venous]?
zone 1 = apex
PA > Pa > Pv
zone 2 = middle
Pa > PA > Pv
zone 3 = base
Pa > Pv > PA
What is intrapleural pressure at top vs bottom of lungs?
top = -10 cm H20 bottom = -2cm H2O
avleoli at apex = more distended than at base
What is positive pressure in lungs? negative?
positive = outwardly directed distending pressure
negative = inwardly directed collapsing pressure
Are alveoli in lung apex or base bigger?
bigger in apex due to difference in pleural pressure + b/c base = compressed by weight of lung above = slinky model
What is PO2 and PCo2 of blood as it enters the pulmonary arteries?
this is mixed venous blood
PO2 = 40
PCO2 = 46
oxyhemoglobin sat = 75%
What are unique aspects of pulm vasculature that allow it receive more blood flow in exercise without increasing resistance?
- distensibility = more distensible than ystemic, less smooth muscle
- recruitability = in normal resting state lots of pulm vascular bed not being used so can recruit when needed
- capacity for vasodilation
Does pulm arterial pressure rise after a pneumonectomy?
nope! this is a sign of the extreme capability for recruiting new vessels
What is V/Q at apex of lung?
step1
3 = wasted ventilation
What is V/Q at base of lung?
Step1
0.6 = wasted perfusion
Where is greatest ventilation in lung? what about perfusion?
step1
both ventilation and perfusion are greater at the base of the lung than the apex
What is V/Q in airway obstruction?
step1
approaches 0 = shunt
What is V/Q in blood flow obstruction?
step1
approaches infinity = physiologic dead space
What is relationship Pa and Pv in healthy lung?
Pa always > > Pv
pulm arterial hydrostatic P is always greater than pulm venous P in healthy lung
Why is alveolar pressure higher than pulm artery pressure in zone 1 of lung?
the pulm artery pressure is insufficient to reach/perfuse the top part of the lung
little blood flow
What determines blood flow in zone 2?
perfusion pressure = Parterry - Palveoi
What part of lung has highest V/Q? Why?
highest V/Q in zone 1
because regional variations in ventilation arent as great as regional variations in perfusion
What part of lung has highest PaO2?
zone 1
What part of lung has highest PaCO2?
zone 3
What is PCO2 and PO2 in pulmonary veins?
PaCO2 = 40 mmHg PaO2 = 100 mmHg
What is normal alveolar ventilation?
4 LPM
What is normal lung perfusion?
5 LPM
What is normal avg V/Q ratio?
0.8
What does high V/Q mean?
high ventilation relative to perfusion
blood flow decreased; pulm capillary blood from this region has high O2 and low CO2
alveolar gas looks like inspired air [PAO2 = 150; PACO2 = 0]
== dead space
What does low V/Q mean?
low ventilation relative to perfusion
ventilation decreased
pulmonary capillary blood has low PO2 and high PCO2
=== shunt
What is PAO2/PACO2/PaO2/PaCO2 in high V/Q?
PAO2 = 150; PACO2 = 0 reflect outside air = no gas exchange b/c not enough perfusion
PaO2/PaCO2 = not applicable b/c no blood flow
== dead space
What is PAO2/PACO2/PaO2/PaCO2 in low V/Q?
PAO2/PCO2 = not applicable b/c no ventilation
PaO2 = 40; PaCO2 = 46; reflect same as mixed venous blood b/c no gas exchange
== shunt
What is the A-a gradient? normal value
PAO2 - Pao2 = alveolar - arterial
normally PaO2 is slightly lower than we would calculate if we did alveolar gas equation
normally = 10-15 mmHg
When do you get high A-a gradient?
hypoxemia due to shunting, V/Q mismatch, fibrosis [impaired diffusion], etc
What is PO2 in each of the following places:
- dry inspired air
- humidified tracheal air
- alveolar air
- mixed venous blood in pulm artery
- systemic arterial blood in pulm vein
- dry inspired air = 160
- humidified tracheal air = 250
- alveolar air = 100
- mixed venous blood in pulm artery = 40
- systemic arterial blood in pulm vein = 100
What is PAO2/PACO2 in alveoli?
PO2 = 100 PCO2 = 40
What is PvO2/PvCO2 in mixed venous blood?
PO2 = 40 PCO2 = 46
What is PaO2/PaCO2 in systemic arterial bloodi?
PO2 = 100 PCO2 = 40
What is PO2/PCO2 in peripheral tissue?
PO2 = 40 PCO2 = 46
What is fick’s law of diffusion?
rate of transfer of gas by diffusion directly proportional to:
- driving force [partial P dif]
- diffusion coefficient
- surface area available
inversely to:
- thickness of membrane barrier
What is measured by pulse ox?
SAO2% = oxyhemoglobin saturation percent
measures the O2 loading onto hemoglobin in arterial blood
usually correlates with the O2 content of blood
BUT: assumes normal amount of normally functioning hemoglobin
What is the relationship of PO2 to SaO2?
described by the hemoglobin saturation curve
What factors shift hemoglobin curve to the right?
acidemia
2, 3 DPG
hyperthermia
What are the rules for using PO2 and SAO2 in clinic?
- changes in PO2 above 60 mmHg usually not of therapeutic significance EXCEPT changes in PO2 may reflect significant alteration in lung function and may help diagnose
- changes in SaO2 from high 90s to low 90s are of diagnostic signifiance; may reflect significantly increased A-a even though may not have much therapeutic significance
What does it mean that we have “perfusion limited gas exchange”
total amount of gas transported across alveoli/capillary barrier is limited by blood flow [perfusion]
only way to increase amount of gas transported is to increase blood flow
Where in capillary do O2 partial pressures equilibrate with those in alveoli?
within first 1/3 of capillary
What is diffusion limited gas exchange?
total amount of gas transported across barrier is limited by diffusion
as long as partial pressure gradient is maintained, diffusion wlll continue along length of cpaillary
What types of gas are perfusion limited?
O2 under normal conditions
CO2
What types of gas are diffusion limited?
CO [binds to Hb]
transport of O2 during exercise [have increased blood flow]
What happens to oxygen diffusion in emphysema?
becomes diffusion-limited
lack of adequate surface area for normal diffusion
What happens to oxygen diffusion in pulmonary fibrosis?
thickening of alveolar capillary barrier –> increased distance for diffusion –> slow rate of diffusion –> prevents equilibration
How much of O2 in blood is free in solution/dissolved?
2% of total O2 content in blood
What is henry’s law ?
Conc = Pressure * solubility
What is solubility of O2?
0.003 mL O2/100 mL blood / mmHg
What is normal conc of dissolved O2?
0.3 mL
How much of O2 in blod in bound to HbA?
98% of O2 content of blood
each Hb bind 4 O2 molec
How much O2 can 1 gm of HbA bind when 100% saturated?
step1
1.34 mL 02
What is the O2 binding capacity of blood? equation? normal?
step1
binding capacity = Hb conc [gm/dL] * 1.34 mL O2/gm
normal = 20.1 mL O2/dL
What is equation for O2 content of blood?
step1
O2 content = O2 bound to HbA + dissolved O2
= Hb conc * 1.34 * % saturation + dissolved O2
How much Hb normally in blood?
step1
15 g/dL
At what level of deoxygenated Hb do you get cyanosis?
step1
when > 5g/dL Hb deoxygenated
If amount of Hb decreases how are the following affected
- O2 content of arterial blood
- O2 sat
- arterial PO2
step1
O2 content of arterial blood decreases
O2 sat and arterial PO2 do not
What is O2 content of the blood of a patient with anemia (Hb 10 gm/dL)?
Assuming normal lungs hence normal PAO2 of 100 mmHg and normal PaO2 of 100 mmHg
Hb is 98% saturated at PaO2 of 100 mmHg
O2 bound to Hb = 10 gm/dL x 1.34 mL O2 / gm Hb X 98% (saturation) = 13.1 mL O2/100 mL blood
Total O2 content = above value + dissolved O2
Dissolved O2 = PaO2 X solu = 100 mmHg X 0.003 mL O2/100 mL/mmHg = 0.3 mL O2/100 mL blood
Total O2 content = sum of above two = 13.1 + 0.3 = 13.4 mL O2/100 mL blood
What is equation for O2 delivery to tissues?
O2 deliver = blood flow * O2 content of blood
== CO * (O2 bound Hb + dissolved O2)
What happens to each of the following in CO poisoning
- Hb level
- %O2 sat
- dissolved O2 [PaO2]
- total O2 content
- Hb level: same
- %O2 sat: decrease [CO competes wtih O2]
- dissolved O2 [PaO2]: same
- total O2 content: decrease
What happens to each of the following in polycythemia
- Hb level
- %O2 sat
- dissolved O2 [PaO2]
- total O2 content
- Hb level: increase
- %O2 sat: normal
- dissolved O2 [PaO2]: normal
- total O2 content: increase
What happens to each of the following in anemia
- Hb level
- %O2 sat
- dissolved O2 [PaO2]
- total O2 content
- Hb level: decrease
- %O2 sat: same
- dissolved O2 [PaO2]: same
- total O2 content: decrease
What is P50?
PO2 at which Hb is 50% saturated
usually 25 mmHg
What things shift O2-Hb dissociation curve to the right? hint. first aid mnemonic
step1
BAT ACE
- BPG
- altitude
- temperature [increase]
- acid [H+]
- CO2
- exercise
What does a shift to the right in Hb curve mean?
step1
- decreased affinity of Hb for O2
- increase in P50
- O2 unloading is facilitated
What is 2,3 BPG?
step1
byproduct of glycolysis in RBC
under hypoxic conditions, binds Hb and reduces affinity for O2
What does a shift to the left in Hb curve mean?
step1
- increased affinity of Hb for O2
- decrease in P50
- decreased unloading of O2 to tissues
How does fetal Hb differ from O2?
step1
- higher affinity for O2 than adult Hb
- dissociation curve is shifted left
How does CO affect Hb O2 dissociation curve?
step1
CO has much higher affinity for Hb than O2
presence of CO decreases available units to bind with O2
shift left in binding curve
What is hypoxemia?
step1
decrease in arterial PaO2?
What is hypoxia?
step1
decrease O2 delivery to tissue
What is ischemia?
step1
loss of blood flow
How do each of the following change in high altitude:
- paO2
- A-a gradient
- effect of supplemental O2
- paO2: decrease = hypoxemia
- A-a gradient: normal
- effect of supplemental O2: improves
How do each of the following change in hypoventilation:
- paO2
- A-a gradient
- effect of supplemental O2
- paO2: decrease = hypoxemia
- A-a gradient: normal
- effect of supplemental O2: improves
How do each of the following change in diffusion defect:
- paO2
- A-a gradient
- effect of supplemental O2
- paO2: decrease = hypoxemia
- A-a gradient: increase
- effect of supplemental O2: improves
How do each of the following change in V/Q defect:
- paO2
- A-a gradient
- effect of supplemental O2
- paO2: decrease = hypoxemia
- A-a gradient: increase
- effect of supplemental O2: improves
How do each of the following change in R–> L shunt:
- paO2
- A-a gradient
- effect of supplemental O2
- paO2: decrease = hypoxemia
- A-a gradient: increase
- effect of supplemental O2: does not improve
How does high altitude affect O2?
low barometric pressure –> decreased PIO2 –> decrease PAO2
normal diffusion –> normal A-a gradient
giving extra O2 increases PIO2 and improves
How does hypoventilation affect O2?
decreases alveolar PAO2
supplemental O2 will improve
What is effect of giving extra O2 on pt with R to L shunt?
- deoxygenated blood mixed with oxygenated “non-shunted” blood and dilutes it
giving supplemental O2 doesnt really help b/c the shunted blood still keeps diluting the normal oxygenated blood
How does cyanide poisoning cause hypoxia?
decrease O2 utilization by tissues
What are some examples of things that cause hypoxia but not hypoxemia?
- decreased cardiac output
- anemia
- CO poisoning