5. Diffusion capacity, blood gas evaluation (normal values, deviations). Evaluation of the pulmonary circulation

Question 1

How do we interpret DLco findings in lung disease patients?

Diffusion Limitation of CO

Answer 1

DLco = Surface Area/Alveolar wall thickness
Thus anything decreasing the surface area or increasing the alveolar wall thickness will decrease the DLco

COPD from Emphysema:
- Decreases DLco due to decreased surface area for gas exchange
- Surface area is decreased because of destruction of the acinus [panacinar or centrilobular]

Restrictive disease caused by ILD:
- Decreases DLco due to increased alveolar wall thickness
- Wall thickness increases due to fibrosis of the wall, making it difficult for CO to pass into pulmonary circulation

We determine patient has COPD based on Plethysmography, Forced Spirometry, Pharmacodynamic tests:
- If DLco decreased = Emphysema
- If DLco unchanged = Chronic Bronchitis/Asthma [in most textbook cases]

We determine patient has Restrictive lung disease based on Plethysmography, Forced Spirometry:
- If DLco decreased = Interstitial Lung Disease [Intrinsic cause]
- If DLco unchanged = Extrinsic cause

Question 2

What deviations can we observe in ABG results?

Answer 2

• Decreased PaO2
• Increased PaCO2
• Decreased PCO2
• Carboxyhemoglobinemia
• Methemoglobinemia
• Respiratory acidosis
• Resporatory alkalosis
• Metabolic acidosis
• Metabolic alkalosis

Question 3

Causes of decreased PaO2

Answer 3

Hypoxemia caused by
- Low V/Q ratio [COPD/Asthma]
- Right to Left shunting [AV malformations, PFO]
- Impaired Diffusion Capacity [ILD, Exercise, High altitudes]
- Hypoventilation [neuromuscular disorders, severe obesity, drug OD, met alkalosis compensation, severe COPD]

Question 4

Causes of increased PaCO2

Answer 4

Hypercapnia caused by
- Hypoventilation [disorders that decrease rr and breathing depth, or as compensation to met alkalosis]

Question 5

Causes of decreased PCO2

Answer 5

Hypocapnia caused by
- Hyperventilation [Pulmonary edema, PE; HF; Met acidosis; Aspirin, Progesterone; CNS infections and tumours, bleeding, Increased ICP; Pain, pregnancy]

Question 6

What does Increased PaCO2 with Decreased pH mean

Answer 6

Respiratory Acidosis

Question 7

What does Decreased PaCO2 with Increased pH mean

Answer 7

Respiratory Alkalosis

Question 8

What does Decreased HCO3- with Decreased pH mean

Answer 8

Metabolic Acidosis

Question 9

What does Increased HCO3- with Increased pH mean

Answer 9

Metabolic Alkalosis