Resp14 - Hypoxaemia & Respiratory Failure

Question 1

Difference between hypoxaemia and hypoxia

Normal oxygen ranges

Answer 1

1.) Hypoxaemia - low pO2 in the blood

2. ) Hypoxia - O2 deficiency in the tissues
   - tissues can be hypoxic without hypoxaemia
3. ) Normal Oxygen Ranges
   - O2 SATS: 94-98%, hypoxia when < 90%
   - paO2: 9.3 - 13.3 kPa, hypoxia when < 8kPa

Question 2

4 effects of hypoxaemia

CNS Function
Cardiac
Hypoxic Vasoconstriction
Cyanosis

Answer 2

1. ) Impaired CNS Function - leads to confusion, irritability, agitation
2. ) Cardiac Problems - cardiac ischaemia –> arrhythmia
3. ) Hypoxic Vasoconstriction - of pulmonary vessel
   - occurs in chronic hypoxaemia
   - leads to pulmonary hypertension –> cor pulmonale
4. ) Cyanosis - due to presence of 4-6 g/dl of deoxyHb
   - can be central or peripheral

Question 3

6 causes of hypoxaemia and effect of inhaled oxygen

Low Inspired O2 x2
Hypoventilation
V:Q Mismatch x6
Diffusion Defects x2
Intra-Lung Shunt
R to L Shunt

Answer 3

1. ) Low Inspired O2 - e.g. high altitude, drowning
   - fully corrected with inhaled oxygen
2. ) Hypoventilation - causes type 2 respiratory failure
   - fully corrected w/ inhaled O2 but leads to hypercapnia
3. ) V:Q Mismatch - (6 causes of V:Q mismatch)
   - partially corrected with inhaled oxygen
4. ) Diffusion (alveolar capillary wall) Defects
   - fibrotic lung disease (thickened alveolar wall)
   - pulmonary oedema (fluid in interstitial space)
   - partially correcred with inhaled oxygen
5. ) Intra-Lung Shunt - extreme V:Q mismatch where there is absolutely no ventilation
   - occurs in ARDS where you get collapsed alveoli
   - not corrected by inhaled oxygen (oxygen doesn’t get to the alveoli at all)
6. ) Right to Left Shunt - in the heart
   - causes cyanotic heart disease
   - not corrected by inhaled oxygen

Question 4

4 features of type 1 respiratory failure

Definition
Aetiology
Causes x2
Treatment

Answer 4

1. ) Definition - impairment in gas exchange causing hypoxaemia WITHOUT hypercapnia
2. ) Aetiology - imparied gas exchange at the alveolar capillary memebrane
3. ) Causes - V:Q mismatch or diffusion defect
   - V:Q mismatch: mixed blood has low pO2 w/ normal pCO2
   - diffusion defect: CO2 more soluble so O2 diffusion is more affected –> low pO2, normal pCO2
4. ) Treatment - oxygen therapy
   - only partially corrects hypoxaemia until underlying pathology is corrected

Question 5

3 features of type 2 respiratory failure

Definition
Aetiology
Treatment

Answer 5

1.) Definition - impairment in gas exchange causing hypoxaemia WITH hypercapnia

2. ) Aetiology - respiratory pump failure (hypoventilation)
   - alveolar/arterial pO2 falls –> hypoxaemia
   - alveolar/arterial pCO2 rises –> hypercapnia
3. ) Treatment - oxygen therapy
   - must be controlled in patients w/ chronic hypercapnia

Question 6

6 causes of acute hypoventilation (type 2 respiratory failure)

Brainstem x2
Spinal Cord
Neuromuscular Junction
Pleural Cavity x2 
Lungs
Airways x3

Answer 6

1. ) Brainstem - opiate overdose, head injury
2. ) Spinal Cord - trauma to cervical spinal cord
3. ) Neuromuscular Junction - myasthenia gravis
4. ) Pleural Cavity - pneumothorax, large pleural effusions
5. ) Lungs - ARDS in the new born (stiff lungs)
6. ) Airways - life-threatening asthma, laryngeal oedema, foreign body in upper airway

Question 7

4 causes of chronic hypoventilation

Respiratory Muscles x2
Chest Wall x3
Lungs
Airways

Answer 7

1.) Respiratory Muscles - myopathy, muscular dystrophy

2. ) Chest Wall - severe obesity, kyphoscoliosis
   - scoliosis: sideways curvature of the spine
   - kyphosis: excessive outwards curvature of the spine
3. ) Lungs - late stage lung fibrosis (stiff lungs)
4. ) Airways - late stage COPD

Question 8

4 effects of hypercapnia

CNS Function
Vasodilation
Acidity
Central Chemoreceptors

Answer 8

1. ) Impaired CNS Function - leads to drowsiness, confusion, flapping tremors, coma
2. ) Peripheral and Cerebral Vasodilation - headaches, warm hands, bounding pulse
3. ) Respiratory Acidosis
4. ) Reset Central Chemoreceptors - chronic hypercapnia causes them reset to higher CO2 levels
   - respiratory drive now driven by hypoxia (via peripheral chemoreceptors)