Alveolar ventilation

From Iusmphysiology

• started here on 02/14/11 at 11AM.

Contents 1 Alveolar ventilation 1.1 Dalton's law of partial pressure 1.2 Partial pressure of gases 1.3 Partial pressureo of respiratory gases 1.4 Definitionns 1.5 Alveolar ventilation 1.6 Alveolar ventilation equation 1.7 Distribution of ventilation 1.8 Alveolar gas exchange 1.9 Partial pressures of alveolar gas 1.10 Alveolar gas equation 1.11 Diffusion of gas 1.12 Diffusion of oxygen 1.13 Diffusion of CO2 1.14 Diffustion limitations 1.15 Measurement of diffusion capacity 1.16 P(A-a)O2 Gradient 1.17 Question 1.18 Question 1.19 Test question from last year

Alveolar ventilation

Dalton's law of partial pressure

• Partial pressure is pressure exerted by an individual gas in a gaseous mixture
• Dalton's law states total pressure in a gas mixture is equal tothe sum of the partial pressures of all the gases in the mixture.

Partial pressure of gases

• We need oxygen for life.
• Gases in the air are nitrogen and oxygen.
  • Negligible CO2 and argon
• Barometric pressure = air pressure; at sea level = 760 mmHg
• Air: 78% N, 21% O, 0.03 CO2, 0.93% argon
  • At Mt. Everest, no change in percentage but decreased pressure.
• Finding partial pressure:
  • Take the barometric prssure and multiply by the percentage of the gas.
  • At sea: 760 * 0.2 = 160 mmHg = PO2

Partial pressureo of respiratory gases

• Barometric PN = 600 mmHg, PO2 = 160 mmHg
• Brought into lungs
• Saturated with lungs 47 mmHg.
  • So we subtract 47 from 760.
  • So 760-47 * 0.79 = 563 = PN
• Alveolar air
  • PO2 = 760-47 * 0.21

Definitionns

• Total (minute) ventilation:
  • tidal volume * respiratory rate
  • similar to stroke volume (how often moving blood * how much blood moved)
  • Usually about 7.5 liters / minute
• Alveolar ventilation:
  • (tidal volume - anatomic dead space) * respiratory rate
  • Takes into acount the dead space volume
  • 5.25 l / minute
• High minute ventilation

Alveolar ventilation

• PA = partial pressure alveolar
• Pa = partial pressure arterial
• We assume these are equal unless some imparement to diffusion
• If you breath more (alveiolar vent high) you blow off more CO2.
  • The higher your ventilation the more CO2 goes out and the lower your PCO2.
  • As alveolar vent goes up, more O gomes in so PO2 goes up.
• PaCO2 = VCO2 / Va * K
  • Va = alveolar ventilation; volume / minute
  • VCO2 = amount of CO2 made by body; volume / minute
  • K takes care of the units
• So when hypoventilating

Alveolar ventilation equation

• Clinically hypoventilation is a measure of PCO2

Distribution of ventilation

• Ventilation of the lung differs from base to apex.
• Sensors can measure ventilation at apex, middle and base.
• Ventilation is highest at the base and lowest at the apex.
• Gravity pulls lungs down such that complance is lower at apex than base so more air goes tot he base than to the apex.

Alveolar gas exchange

• Normally, the PaCO2 = PACO2 (unless told otherwise on an exam question).
• Note the values of partial pressures at teach location

Work through this slide

Partial pressures of alveolar gas

• Partial pressures int he alveoli stay relatively constant.
• During normal tidal volume exchange, about 500 ml are brought in and mixed with 2.5 L of not fresh air.
• So dilution levels off huge fluctuations of partial pressures in the alveoli.
  • That would be a "bad day".
• Decrease in PO2 in the alveoli relative to atmosphere is due to three things:
  • saturation of water removes some pressure
  • inspired air is mixed with FRC
    • Recall that the end of the expiration is the same as alveolar partial pressure.

**expire...?

Alveolar gas equation

• This tells us the determinants of alveolar PO2:
  • Atmospheric PO2 (PIO2 = Partial pressure of oxygen in inspired air)
  • Level of ventilation (controlled by PaCO2)
  • R = respiratory quotient; oxygen consumed and CO2 produced
  • Second term gives estimate of overall ventilation
• PIO2 knwon by barometric pressure - the water vapor pressure tiems the fractionn of O2
• R = VCO2 / VO2
  • Usually about 0.8 (80%)
  • Diet can change this quotient
  • Assume 0.8 unless told otherwise

Will definitely have to calculate this on a test

• Normal PACO2 = 40
• Emphysema:
  • PACO2 = 64 mmHg
• COPD:

Figure these out.

Diffusion of gas

• Factors affecting diffusion across membrane: ATDP
• Surface area:
  • Usually as much as a tennis cort
• Thickness of membrane:
  • Thicker = longer time of difusion
• Diffusion constant:
• Pressure gradient
  • Affects spead of diffusion
• Emphysema
  • decreases SA
  • thicker membrane decreases diffusion

Diffusion of oxygen

• In mixed venous blood, PO2 = 40
  • This is as it enters the alveolar blood.
• PO2 in alveolar is 100.
• So oxygen moves from alveolus to the RBC.
  • Within 1/4 of one second, the blood will have equilibriated with the alveolar pressure.
  • RBC is usually in the cap bed for 3/4 of a second.
  • Lag time is good because during fibrosis, O moves from alveolar air to RBC more slowly.
  • During exercise, CO goes up and time in the cap bed goes down to like 1/4 second so that'd convenient.
• Now if alveolar PO2 were 50:
  • Equilibriation takes a longer time: 1/2 second.
  • This is because the pressure gradient is lower.
  • So if something equilibriates quickly, it is called "perfusion limited" or "the amoutn of oxygen delivered to tissue is determined primarily by the overall level of perfusion".
  • So there isn't usually a diffusion problem when you're not getting enough O (because the pressure gradient is usually high) but an issue of how much blood perfusionn is occuring.

Diffusion of CO2

• CO2 is going out, remember.
• PCO2 in mixed venous blood is 46.
• PCO2 in alveolar gas is 40.
• Equilibriates pretty close to O, even though the pressure gradient is much smaller.
• This is becasue CO2 diffuses much faster because it is much more soluble.
• Also, CO2 has to be made from HCO3-.
• CO2 is also considered perfusion limited.
• Note that both CO2 and O2 are ventilation limited, too, but that should go without saying.

Diffustion limitations

• N2O = nitrous oxide
• Also perfusion limited
• Equilibriates very quickly.
• So amount delivered to tissue is determined by perfusion (which is affected by CO).
• Diffusion limited:
  • CO is diffusion limited.
  • CO rapidly, quickly, tightly binds to Hb.
  • So PCO in plamsa stays low; never equilibriates with PCO in the air.
  • So when pressure changes never really changes the only other factor affecting how much is taken in is the diffusion gradient.

Measurement of diffusion capacity

• CO is unique, so it can be used to measure some diffusion capacity.
• So if you give a small, known amount of CO to the pt, and measure how much comes out, you can calcualte the DLCO = diffusion limitation of CO.
• This will tell you how well CO diffuses.
• This can tell you if the diffusion is the problem.
• Normally about 25 ml / min / mmHg.
  • In emphysema goes down because of less surface area.

P(A-a)O2 Gradient

Will definitely being on the test!

• Clinical measurement of diffusion.
• PaCO2 and PACO2 should be equal if everything is ok.
• So the gradient, when not 13(ish), identifies disease.
• Measure PaO2 and PaCO2 and calcualte the PAO2.
• Normal gradient is no higher than 12-15.
• The gradient can be increased by diffustion impairment, anatomical shunt (venous blood is shunted into blood that does have oxygen), imbalance between ventilation and perfusion.

Question

• Everything is good

Question

Know the A-a gradient.

Test question from last year

• stopped here on 02/14/11 at 12PM.