A-a gradient

Question 1

Explain the reasons for the normal alevolar-arterial oxygen difference?

Answer 1

Normal A-a gradient is 5-10mmHg. This is because

• There is a lower V/Q at the base of the lungs compared to the apex, meaning that blood is perfusing alveoli with low PAO2.
• Shunt: bronchial artery blood (perfusing lung tissue, delivering oxygen to it, and being collected by pulmonary veins, depleted of oxygen), and coronary blood (coronary veins drain blood from myocardium and some is directly conveyed to the LV via the thebesian veins, depleted of oxygen).

Question 2

Give a definition for shunt.

Answer 2

Shunt refers to blood that enters the systemic arterial system without going through ventilated areas of the lung.

Question 3

Give some examples of shunt.

Answer 3

Normal:
- Bronchial blood
- Coronary blood (thebesian veins)
Abnormal:
- Pulmonary arteriovenous fistula
- VSD
- ASD
- PDA