w6- mechanics of ventillation

Question 1

how do you calculate dead space

Answer 1

VD= Vt x ((PaCO2 – PECO2)/ PaCo2)

Question 2

what is FRC

Answer 2

The volume of gas in the lungs at the end of a normal tidal expiration

Question 3

what drives alveoli to expand passively

Answer 3

increased transpulmonary pressure

Question 4

name the determinants of diffusion

Answer 4

1.Partial pressure difference

•e.g. High altitude

2. Membrane surface area

•e.g. Lung removal, emphysema

3. Membrane thickness

•e.g. Edema, fibrosis

4. Diffusion coefficient

Question 5

what is the equation for whole body oxygen content caO2

what does it depend on

Answer 5

•CaO2= Hb (gm/dl) x 1.34 ml O2/gm Hb x SaO2+ PaO2x (.003 ml O2/mm Hg/dl)

amount of Hb

the saturation of that Hb

the amount of dissolved oxygen in blood.

Question 6

what does oxygen saturation SaO2 depend on

Answer 6

indicates the percentage of hemoglobin binding sites occupied

•dependent on dissolved O2in arterial blood (i.e. PaO2)and the binding affinity of the Hbpresent, but is not dependent on the concentration of Hbin the blood.

Question 7

what does the Partial pressure of oxygen in arterial blood (PaO2) depend on

Answer 7

• PaO2is dependent on the molecules of free oxygen present, which in turn is driven by the alveolar partial pressure and the determinants of diffusion.
• The oxygen bound to Hb does not contribute to PaO2, nor does the amount of Hb(normally). This explains why patients with severe anemia or carbon monoxide poisoning may have a normal PaO2.

Question 8

If Hbis reduced 50%,

How will SaO2, PaO2, and CaO2 be affected?

Answer 8

PaO2unchanged, SaO2unchanged, CaO2reduced

Question 9

what is the alveolar gas equation and the A-a gradient

Answer 9

Question 10

what conditions would have reduced PaO2

which of these could be treated by supplying supplemental oxygen

Which of these would have a large A-a gradient of more than 20 indicating a gas transfer defect

Answer 10

large AA gradient hypoxemia:

• ventilation-perfusion (V-Q) mismatch
• reduced alveolar ventilation
• right-to-left shunt (could be tx with o2)

Regular A-a gradient hypoxemia:

• AMS (could be tx with o2)
• hypoventillation

Question 11

what can a reduced Dlco indicate

Answer 11

1. Decreased volume of pulmonary capillary blood or hemoglobin volume
2. Decreased surface area integrated between capillaries and alveoli
3. Ventilation/perfusion mismatching or intrapulmonary shunting from atelectasis

Question 12

how can you measure O2

Answer 12

•CO diffusing capacity x 1.23 = O2diffusing capacity

Because Carbon monoxide (CO) forms so quickly with hemoglobin in blood, if a small amount of CO is breathed in, its pressure gradient is essentially its partial pressure. By measuring CO concentration in blood over time and dividing by the partial pressure, one can determine the diffusing capacity for CO. This can be used to determine the diffusing capacity of O2by multiplying the CO value by 1.23