Principles of Diffusion - Rickaby

Question 1

What are some possible causes of respiratory insufficiency if ventilation is adequate?

Answer 1

Lung interstitial defects, problems relating to blood (ie Anemia)

Question 2

What are the factors that contribute to diffusion which can change in disease states?

Answer 2

Pressure (increases diffusion)

Distance (decreases diffusion)

Area (increases diffusion)

Question 3

What is the typical surface area available for gas exchange in an adult human?

What is the usual path length (thickness of respiratory membrane)?

Answer 3

80 square meters

0.6 µm

Question 4

How is oxygen carried by blood? How much via each method?

Answer 4

Direct dissolution (follows Henry’s law, 0.18ml/100ml)

Bound to hemoglobin (lecture says 5ml/100ml, should be closer to 19-20ml under normal conditions)

Question 5

Compare the relative diffusion behaviors of CO₂ and O₂.

Answer 5

CO₂ diffuses more easily, as it is more soluble in water/plasma. Carbon dioxide generally does not saturate under physiological conditions.

Question 6

How can oxygen requirement be calculated?

Answer 6

Examine the extraction of oxygen (oxygen level at either end of a vascular circuit)

For total usage, look at arterial and venous oxygen, and cardiac output.

(Fick principle?)

Question 7

Why is carbon monoxide used to measure someone’s oxygen D_L?

Answer 7

Calculation of D_L is nearly impossible since P_c cannot be measured. carbon monoxide almost completely binds hemoglobin, thus, its capillary gas pressure stays close to zero.

The D_LCO can be converted back to D_LO2 afterwards.

Question 8

Distinguish between steady-state and single breath D_L measurements.

Answer 8

Steady state involves breathing a low concentration CO gas mixture and then collecting exhalate. Single breath does this over a single full VC inspiration/expiration.

Steady state is preferable for those unable to complete single breath testing, and can be used during exercise studies.

Question 9

How do D_LCO and D_LO2 correlate?

Answer 9

D_LO2 is 1.23 times D_LCO

Question 10

How can lung volumes be measured without using a spirometer?

Answer 10

A pneumotachometer can measure volume by integrating a flow rate over time.

Question 11

Why is methane a part of a diffusion test?

Answer 11

Methane is not absorbed at all, its relative dilution in the lungs is used to calculate total lung volume, which is needed for calculation of D_LCO

Question 12

What parameters can affect the measurement of D_L values?

Answer 12

Age, height, gender. Also anemia, altitude, other blood problems, etc

Question 13

How does D_L change in exercise? How?

Answer 13

D_L increases by a factor of 3, mostly by increasing the pressure gradient by increasing pulmonary perfusion (only modest changes in surface area, thickness etc)

Question 14

How would the following conditions change D_L?

Hemorrhage

Anemia

Answer 14

Hemorrhage will decrease diffusion.

Anemia will also decrease diffusion.

Recall the equation: 1/D_L = 1/D_M + 1/(Qc * θ)

Question 15

How do D_L and D_L/VA change in restrictive and obstructive lung diseases?

Answer 15

Restrictive: D_L and VA both decrease, so D_L/VA remains normal.

Obstructive: DL decreases more than VA, so D_L/VA decreases.

Question 16

How does V/Q mismatch affect diffusion capacity?

Answer 16

V/Q mismatch would reduce diffusion capacity. Recall that there will be no net diffusion in complete shunting (no ventilation) or dead space (no perfusion.

Question 17

Describe how diseases like pneumonia or emphysema might affect diffusion capacity.

Answer 17

Pneumonia: Fluid in the alveoli impair diffusion, as does wall thickening.

Emphysema: Loss of surface area, decreases diffusion capacity.