LA

Chapter 7- Gas Diffusion

What is Diffusion

  • Net movement of molecule

- From high to low concentration

- Diffusion gradient

  • Continues until equilibrium

  • Diffusion across alveolar-capillary membrane and tissue

Disunion Gradients of Respiratory Gases

  • Inspired air contains about 21% O2 and essentially no CO2

  • PO2 in the conducting airway gas is lower than PO2 in the atmosphere

- Gas in the lung is 100% saturated with water vapor

- At body tempature, the partial pressure of water vapor in the lung (PH2O) is 47mmHg

- PO2 in conducting airways is about 150mmHg

  • PIO2= 760mmHg x 0.21= 158mmHg

  • PIO2 (humidified)= (760-47) x 0.21=149.73 149mmHg

  • Respiratory exchange ratio (R=0.8)

- R= VCO2/VO2

O2 consumption= 250mL/min

CO2 production= 200mL/min

  • Alveolar air equation

- PAO2= PIO2-PACO2[FIO2+(1-PIO2/R)]

PAO2= PIO2- (PaCO2)1.2

PAO2=PIO2-PaCO2(if FIO2>0.60)

Laws of Governing Diffusion

  • Fick’s Law

Vgas=[AxDx(P1-P2)]T

A=Surface Area

D=Diffusion coefficient (solubility)

(P1-P2)= diffusion gradient

T= membrane thickness

This means the greater the surface area, solubility coefficient and pressure gradient, the greater the diffusion rate; the greater the membrane thickness, the slower the diffusion rate

  • Physicalgas characteristics and diffusion

- Grams Law: Gas diffusion rate s inversely proportional to the square root of its gram molecular weight (or density); lighter gas=faster diffusion ratw

- Henry’s Law: Gas diffusion is directly proportional to he gas partial pressure (greater pressure, greater diffusion)

- CO2 diffuses 20 times faster than O2 across alveolar-capillary membrane because of its much greater solubility (it is actually a heavier molecule)

Limitations of Oxygen Diffusion

  • Effects of partial pressure gradient and capillary blood transit time

- Capilary transit time =0.75 seconds

- Alveolar- capillary equilibrium= within first 0.25 second

  • Perfusion and diffusion limitations to O2 transfer

- Increased blood flow

CO= diffusion limited

N2O= perfusion limited

  • Diffusion path length

- From alveolar gas to red blood cells

- Path distance <0.1 micrometers

  • Fibrotic thickening of alveolar and capillary walls

  • Interstitial edema fluid, separating alveolar can capillary membranes

  • Fluids in the alveoli

  • Interstitial fibrotic process that thicken the interstitial space

  • Dilated, engorged capillaries, which allow RBCs to flow side by side. Interstitial edema fluid

  • Diffusion surface area

- The total area of contact between ventilated alveoli and perfumed apilaries

- Decreased the diffusion surface area and the lungs diffusion

  • A decrease in the number of open, perfumed capillaries

  • The decrease in the number of open, ventilated alveoli

Measuring Difusión Capacity

  • General Principles

- Single-breath CO diffusion tests (DLCO)

-DLCO=Ml CO transferred to blood/min - mean PACO-mean PCCO

  • Normal Values

- 20 to 30mL/min/mmHg

- DLCO-DLCO x 1.23= about 32mL/min/mmHg

- Notice unit of diffusion is opposite of resistance: flow/pressure= coonductance

Factors Affecting Measured Carbon Monoxide Diffusion in the Lung

  • ody

  • Age

  • Lung Volume

  • Exercise

  • Body position

- DLCO 15% to 20% higher in supine

  • Alveolar PO2 and PCO2

  • Hemoglobin concentration

  • Pulmonary diseases

Conditions That Decrease Diffusion Capacity

  • Increased diffusion path distance

- Interstitial or alveolar edema

- Intestinal or alveolar fibrosis

- Dilated, engorged capillaries (CHF)

  • Decreased diffusion surface area

- Destrction of alveolar capillaries bed (emphysema)

- Reduced capillary blood flow

- Pulmonary embolus

- Low cardiac output (heart failure and blood loss)

- Tumors

  • Decreased uptake by red blood cells

- Anemia

- low pulmonary capillary blood volume

  • Ventiltion-perfusion mismatch

- Obstructive lung diseases (reduced regional ventilation)

- Atelectasis (small airway and alveolar collapse)

- Pneumonia (fluid-filled alveoli)

Clinical se of DLCO

  • Assessment of gas diffusion across alveolar-capillary membrane

  • More sensitive that PaO2 to detect O2 transfer problems

  • DLCO clarifies mechanisms of arterial hypoxemia

- If DLCO is normal, diffusion is not a contributing factor