L08 Alveolar Gas Equation Flashcards Preview

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Flashcards in L08 Alveolar Gas Equation Deck (55):

What is Pulmonary Ventilation V̇ ?

= amount of air moved into / out of lungs per minute (minute volume)


V̇ is calculated by?

V̇ = f x TV

f = frequency of breathing (breaths/min)
TV = tidal volume (L)


what is normal value for V̇?

~ 6 L/min at rest


Define Alveolar Ventilation V̇A.

= amount of air reaching functioning alveoli (exchange surface) per minute = minute volume for gas exchange


How much of Pulmonary Ventilation is Alveolar Ventilation?

so Alveolar Ventilation Normal value ~ 4.2 L/min at rest


What is the other third of pulmonary ventilation?

Dead Space Vent.


Equation for dead space ventilation?

TV = VA + VD


Equation for Alveolar Ventilation?

V̇A = f x VA = f x (TV - VD)

VA = volume of air reaching the functioning alveoli (L)
VD = volume of dead space (L)


Define dead space volume.

= space in lungs where gas exchange cannot take place


2 types of dead space? Explain each

1. Anatomical dead space (born with it)
= parts of respiratory tract acting as passageways (nose, pharynx, trachea, bronchi, bronchioles) = 150 mL

2. Physiological dead space (physiological) = anatomical dead space PLUS:
a) Space that does not receive blood supply e.g. diseased lung and/or
b) Space in which ventilation is in excess of need to arterialize the blood

Normal total dead space volume = 150 mL


is alveolar dead space important?

No, normally negligible


Equation linking PACO2 and V̇A

V̇CO2 = amount of CO2 Exhaled per min.
V̇A = alveolar ventilation
FACO2= fractional conc. of CO2 (partial pressure of CO2)

V̇CO2= V̇A x FACO2


FACO2 depends on what?

The amount of CO2 into capillary and CO2 out of airwayw


How does PACO2 relate to FACO2?

FACO2 = k1 x PACO2

k1 is a constant

therefore PACO2 = partial pressure of CO2 in alveolar air


V̇CO2= V̇A x FACO2
V̇CO2= V̇A x k1 x PACO2
PACO2 = 1/k1 x V̇CO2 / V̇A
what does this mean?

At a constant level of CO2 production / metabolic activity (i.e. also constant): increase level of alveolar ventilation V̇A means PACO2 falls

PACO2 varies inversely with V̇A


For body pH to be kept at normal physiological value ~ 7.4, what is the PACO2?

Isocapnic line: PACO2 need to be kept at 40 mmHg for pH = 7.4


What conpensatory measures are taken to counter rise in PACO2? (normal at rest)

ventilation in excess to metabolic demand > ↓ PCO2 in alveolar and systemic blood

Same amount produced in body but less CO2 remains


What measures to counter drop in PACO2? (normal at rest)

inadequate ventilation to meet metabolic demand >
↑ PCO2

> CO2 builds up in arterial systemic blood


Exercise changes metabolic status. What changes to meet metabolic demand?

increased ventilation to meet metabolic demand (e.g. moderate exercise) > normal PCO2 > normal pH

(curve on PACO2/V̇A graph shifted to right. Higher V̇A needed to maintain same PACO2 as at rest)


PAO2- V̇A relationship.

What is the equation for V̇O2?

V̇O2 = k1 x V̇A (PIO2 - PAO2)

PIO2 = inspired PO2
PAO2 = alveolar PO2


Rearranging V̇O2 equation gives?

What does that mean?

How does it compare to PACO2?

V̇O2 = k1 x V̇A (PIO2 - PAO2)

PAO2 = PIO2 - (1/k1) V̇O2/V̇A

This means PAO2 varies directly with V̇A, whilst PACO2 varies inversely with V̇A.


Explain V̇A and PACO2 and PAO2 relationship.

Increase V̇A causes decrease in PACO2, increase in PAO2


At a constant level of O2 consumption / metabolic activity and a fixed inspired O2 concentration: increase level of alveolar ventilation casues PAO2 increases. What is the normal PAO2?

100mmHg at iso-oxic line


Hyperventilation and hypoventilation causes what to PO2?

Hyper = increase PO2
Hypo = decrease PO2

During exercise, metabolic demand for O2 increases. So increased V̇A is needed for the same PAO2


At steady state, amount of CO2 exhaled per min =?
Amount of O2 uptake per min = ?

 Amount of CO2 exhaled per minute = amount of CO2 produced per minute

 Amount of O2 uptake per minute = amount of O2 consumed per minute


Divide V̇CO2 equation by V̇O2 equation.

V̇CO2 = k1 x V̇A x PACO2
V̇O2 = k1 x V̇A (PIO2- PAO2)


R is respiratory quotient


What is respiratory quotient?

R = respiratory quotient or respiratory exchange ratio = CO2 production / O2 consumption


What is R determined by? give RQ of fat, carb and protein

determined by metabolism of tissues, e.g. food taken in:
Carb= 1.0
Fat = 0.7
Protein = 0.6


What is RQ of mixed diet?
(What is V̇CO2/ V̇O2?)

what is normal PACO2?

RQ mixed = 0.8
from 200/250

normal PACO2 = 40mmHg
PIO2 = 150 mmHg
PAO2 = 100 mmHg


How is PIO2 calculated?


PH20 = saturated water vapor pressure in alveoli


How is PAO2 calculated?

PAO2 = PIO2 - (1/k1) V̇O2/V̇A

150 – 40 / 0.8 = 100 mm Hg


How is PAO2 = PIO2 - PACO2/R normally used?

used to calculate “ideal” alveolar PAO2 of subject (PAO2= PO2 of lungs if there is PERFECT gas exchange)


The reality of gas exchange in lungs is not perfect. What gradient is generated?

Ideal PAO2 – actual arterial PO2 = alveolar-arterial O2 gradient


What is the alveolar- arterial O2 gradient useful for?

index for gas exchange function

- Lungs with perfect gas exchange: 0 mm Hg (no difference)
- Normal lungs: <10 mm Hg (tolerable)
- Lungs with severely impaired gas exchange: greatly increased (lower actual arterial PO2)


TV = VD + VA
What are the normal figures?

TV typically ~ 450mL
1/3 is dead space
2/3 is alveolar


What is the expected dead space volume in diseased patients?

Increased due to physiological d.s. increasing


Bohr's equation for physiological dead space.

Derive from TV = VD + VA

TV = VA + VD

FEg (fractional conc. of gas in expired gas)
FAd (Fractional con. of gas in alveolar)
FDg (Fractional con. of gas in dead space)
FIg (Fractional con. of inspired gas)

FEg x TV = VA x FAg + VD x FDg

FEg x TV = (TV - VD) FAg + VD x FIg

VD= TV (FAg - FEg) / (FAg - FIg)


Apply Bohr's equation for physiological d.s. with CO2

FICO2 is negligible = 0
(very little inspired CO2)
VD = TV (FAg - FEg) / FAg


PaCO2 is arteriole pCO2
PACO2 is Alveolar pCO2


V̇C is what?

maximal volume of air that can be taken into the lungs per minute
(maximal minute volume)


What is normal V̇C value?



How and Why is V̇C tested?

Forced expiration test to assess mechanical property of lung
E.g. how much ventilation we can increase in exercise


Name and explian 4 factors that affect V̇C.

1. Size of lungs: vital capacity depends on maximal stroke
2. Force available: respiratory muscle strength
3. Airway resistance: affects airflow: increased Raw means lower V̇C
4. Respiratory frequency: optimal frequency = 80-90 breaths/min (normal: 12-15 breaths/min)


How does exceeding respiratory frequency impact V̇C?

exceed optimum frequency, time for lung filling decrease, decrease V̇C


How do obstructive and restrictive lung diseases affect V̇C?

Obstructive = increase Raw = Decrease V̇C

Restrictive = increase elasticity = harder to distend = decrease V̇C


How is ventilation distributed regionally in the lungs? What is regional distribution mainly determined by?

Uneven- less ventilation to apex, more to base
Posture and gravity


Explain uneven regional distribution of ventilation due to posture.

Gravity > upright lung means weight of air adds pressure to bottom of lungs > uneven pleural pressure distribution> size of alveoli at EEP is different


Explain gravity effect on apex of upright lung. (think pleural pressure and gravity and PV graph)

- initial volume of apex is larger in apex than in basal

- gravity pulls on apical part of lungs more than basal part

-pleural pressure on top is more negative (-10cmH20) than base (-2.5cmH2O)

-At more negative pleural pressures, the slope of PV curve is flatter. Apex is already more filled and has more negative pleural pressure

-so volume change at apex is small, thus less ventilation to apical region


Explain gravity effect on ventilation to basal region of lungs.

Base of lung has small initial volume and less negative pleural pressure compared to apex

PV curve is very steep at less negative pleural pressures

so Volume change at base is larger than in apex

More Ventilation reaches basal region


Time constant of filling time is calculated by what?

Time constant = CL x Raw

Compliance = CL

Normal = 5s/ cycle


How does edema affect the time constant for filling? (Regional disturbance in expansion)

edema > higher tissue resistance between adjacent fibers > decrease compliance/ higher elasticity/ harder to distend > Less change in volume > less filling time needed


Fast and slow alveolus refers to what?

Fast = lower time constant due to lower compliance and /or low Raw

Slow= more time constant due to higher compliance and /or high Raw


Regional changes in elasticity can impact time constant how?

Decrease in compliance/ Increase elasticity e.g. pulmonary fibrosis = lower change in volume = decrease filling time = lower time constant (fast alveolar)

Increase in compliance/ decrease in elasticity e.g. emphysema = more change in volume = more time to fill = higher time constant (slow alveolar)


Regional obstruction e.g. bronchoconstriction can affect time constant how?

Impede airflow = increased Raw = increase filling time = slow alveolar = increase time constant


Regional Check Valve impact time constant. How?

similar to bronchoconstriction but more severe

Air does not reach alveolar due to huge increase in Raw = huge decrease in radial traction due to huge decrease in lung elasticity/ huge increase in compliance = huge increase in filling time


Summarize the factors that affect regional distribution

Summarize factors that affect filling time (3)

regional = volume change related to pleural pressure and gravity

Time constant=
regional changes in elasticity
(e.g. emphysema, fibrosis)
regional obstruction
(e.g. bronchoconstriction)
regional check valve

regional disturbances in expansion
(e.g. edema)

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