Type 1 and 2 resp failure, V/Q mismatch, ABGs, Obstructive sleep apnoea

Question 1

How is breathing controlled?

Answer 1

Neural controls:

• Cerebral cortex: conscious control of breathing
• Medulla oblongata: breathing centre

Chemical Control

• Chemoreceptors detect rising CO2 (inc acidity, H+) and low O2
• CO2 + H20 <=> HCO3 + H+
• These are found in the medulla, carotid bodies, aortic bodies

Question 2

What is V/Q ratio?

Answer 2

V/Q ratio

• V = Alveolar ventilation i.e. amount of air that reaches the alveoli
• Q = Perfusion. Pulmonary blood flow that reaches the alveoli
• V/Q influences gas exchange where CO2 leaves and O2 enters the blood

Question 3

What is V/Q mismatch?

Answer 3

(1. ) Either ventilation or perfusion limits the availability of O2 and CO2 exchange. Examples of V/Q mismatches:
(2. ) Reduced perfusion + normal ventilation e.g. PE
(3. ) Reduced ventilation + normal perfusion e.g. pulmonary oedema, bronchoconstriction
(4. ) Alveolar-artery gradient can be used to assess the present of inefficient gas exchange
(5. ) V/Q mismatch usually causes type 1 resp failure (low O2)

Question 4

What would you expect to see in an ABG for metabolic acidosis and alkalosis

Answer 4

(1. ) metabolic acidosis
- low pH
- normal PaCO2
- low HCO3

(2. ) metabolic alkalosis
- high pH
- normal PaCO2
- high HCO3

Question 5

What would you expect to see in an ABG for respiratory acidosis and alkalosis

Answer 5

(1. ) resp acidosis
- low pH
- high PaCO2
- normal HCO3

(2. ) resp alkalosis
- high pH
- low PaCO2
- normal HCO3

Question 6

Causes of acidaemia

Answer 6

• less CO2 leaving blood (e.g. hypoventilation)
• loss of HCO3 (salicylate poisoning)
• increase in H+ production (ketoacidosis - kussmaul breathing)

Question 7

What would you expect see in CO2 and O2 levels in type 1 and 2 resp failure

Answer 7

(1. ) Type 1 resp failure = Low pO2 + normal/low pCO2

(2. ) Type 2 resp failure = Low pO2 with high pCO2

Question 8

What causes type 1 resp failure? (4)

Answer 8

(1. ) V/Q mismatch e.g. pneumonia, pulmonary oedema, PE, asthma, emphysema, pulmonary fibrosis, ARDS. Air can enter but alveoli is full of blood/pub/water which prevents oxygen crossing into the circulation.
(2. ) Hypoventilation
(3. ) Abnormal diffusion
(4. ) Right to left cardiac shunt

Question 9

What causes type 2 resp failure? (4)

Answer 9

(1. ) Airway obstruction or to alveolar hypoventilation (so reduced oxygenation and reduced CO2 elimination from blood). Causes:
(2. ) Airway obstruction e.g. asthma, COPD, pneumonia, end-stage pulmonary fibrosis, obstructive sleep apnoea
(3. ) Reduced compliance e.g. pneumonia, rib fracture
(4. ) Reduced strength of resp muscles e.g. myasthenia gravis, GBS, MND
(5. ) Drugs acting on resp centres (opiates)

Question 10

Clinical features of resp failure

Answer 10

Depends if type 1 or type 2

(1. ) Hypoxia Sx
- dyspnoea, restlessness, agitation, confusion

(2. ) Hypercapnia Sx
- headache, peripheral vasodilation, tachycardia, tremor/flap,

(3. ) Airway obstruction = wheeze, pursed lips, intercostal indrawing
(4. ) Cor pulmonale = raised JVP, peripheral oedema, hepatomegaly, ascities

Question 11

Ix for resp failure

Answer 11

Aimed at determining the underlying causes

(1. ) Blood tests: FBC, U&E, CRP
(2. ) ABG
(3. ) CXR
(4. ) Sputum and blood culture if febrile
(5. ) Spirometry if COPD, neuromuscular disease, GBS

Question 12

Mx for resp failure

Answer 12

(1.) Treat underlying causes

(2. ) Type 1:
- Give 40-60% oxygen by facemask

(3. ) Type 2:
- Give controlled oxygen therapy at 24% (uncontrolled can worsen condition and lead to cardiac arrest)
- Recheck ABG after 20mins: if dec in pCO2 = inc oxygen therapy to 28%, if inc in pCO2 = assisted ventilation

Question 13

What is Obstructive Sleep Apnoea? RF?

Answer 13

(1. ) Disorder characterised by intermittent closure/collapse of the pharyngeal airways causing apnoeic episodes during sleep.
(2. ) RF = large adenoids and obesity

Question 14

Clinical features of Obstructive Sleep Apnoea?

Answer 14

Typical pt is obese, middle ages man who presents because of snoring or daytime somnolence (drowsiness).

(1. ) Loud snoring
(2. ) Daytime somnolence
(3. ) Poor sleep quality
(4. ) Morning headache
(5. ) Decreased libido
(6. ) Nocturia
(7. ) Dec Cognitive performance

Question 15

Tx of Obstructive Sleep Apnoea?

Answer 15

(1. ) Weight mx
(2. ) Avoidance of alcohol and smoking
(3. ) CPAP via a nasal mask during sleep is effective and recommended for those with moderate to severe disease