Respiratory failure

Question 1

Ventilation, perfusion(Q), pressure and V/Q ratios vary throughout the lung. Describe how?

Answer 1

Pressure least at apex of lung

Tip of lung experiences absent perfusion so PA>Pa>Pv. VQ>1

Middle of lung experiences cyclic perfusion so Pa>PA>Pv.
VQ=1

Bottom of lung experiences constant perfusion Pa>Pv>PA
VQ<1

Question 2

What is hypoxia pulmonary vasoconstriction?

Answer 2

It is a mechanism that helps minimise V/Q mismatch

Incoming blood detects low pO2 in poorly ventilated areas of lung so divert to areas with higher pO2

Question 3

Describe 5 mechanisms of hypoxaemia

Answer 3

1. Low V/Q ratio(<0): can be overcome by increasing pO2
2. Shunt: anatomical (approx. 5%), intracardiac shunt (congenital, acquired disease), intrapulmonary shunt
3. Hypoventilation due to CNS disease, drug overdose, weakness
4. Reduced PIO2: at low pO2
5. Diffusion limitation: interstitial oedema, inflammation, scarring

Question 4

What is venous admixture?

Answer 4

The amount of poorly oxygenated venous blood needed to cause a drop in alveolar-artery pressure

Question 5

State three causes of intrapulmonary shunting

Answer 5

1. Arteriovenous malformations
2. Lung consolidation
3. Pneumonia

Question 6

Define minute ventilation, alveolar ventilation and dead space

Answer 6

Minute ventilation- amount of air/minute breathed into the lung

Alveolar ventilation- amount of that air that reaches the alveoli and participates in gas exchange

Dead space- amount of that air that reaches the alveoli and doesn’t participate in gas exchange

Question 7

State the causes of pulmonary hypoxia in terms of a) reduced O2 delivery and b) reduces ability to use O2

Answer 7

a) anaemia, circulatory

b) cytotoxicity as in sepsis