L35: Respiratory Failure

Question 1

Respiratory failure and types

Answer 1

Lungs unable to maintain arterial blood gases at normal levels (PO2: 80, PCO2: 40)

• PO2 < 60mmHg
• PCO2 > 50mmHg

Type 2 (Ventilation failure / unable to ventilation compensation): ↑PCO2 and ↓PO2

• Alveolar hypoventilation (inability of ventilatory compensation —> severe hypoxaemia and severe hypercapnia)
• COPD (VQ mismatch: have ventilatory compensation but limited by high Raw —> hypoxaemia more severe)

Type 1 (Gas exchange failure): ↓/no change PCO2 and ↓PO2

• Severe interstitial disease (VQ mismatch + diffusion impairment —> severe hypoxaemia with no hypercapnia; ventilatory compensation effective: no change PCO2)
• Adult respiratory distress syndrome (VQ mismatch + diffusion impairment + shunt —> severe hypoxaemia with hypocapnia; very strong ventilatory compensation: ↓PCO2 due to very low PO2 strong hypoxic drive, J receptor in interstitium activated)

Question 2

***Causes of respiratory failure

Answer 2

1. Ventilatory failure (Type 2)
   - Defect in respiratory control pathway from medulla to respiratory muscle
   —> depressed central respiratory drive (brain tumour/narcotic drugs), defects in nerves (poliomyelitis), neuromuscular disorder (Myasthenia gravis)
   - Defect in ventilatory apparatus
   —> thoracic cage deformities (kyphoscoliosis), muscle weakness (trauma), airway obstruction (COPD), limitation to lung expansion (interstitial fibrosis)
2. Gas exchange failure (Type 1)
   - Defective alveolocapillary membrane (↑thickness, ↓SA)
   —> acute respiratory distress syndrome
   - VQ mismatch
   —> vascular, interstitial lung disease, COPD

Remember for gas exchange to occur properly:

1. Alveolar ventilation
2. Perfusion
3. Gas diffusion across alveolocapillary membrane
4. VQ matching

Question 3

Causes of hypoxaemia

Answer 3

1. Hypoventilation
2. Diffusion impairment
3. Shunt
4. VQ mismatch

Effects:

• cyanosis, tachycardia
• anaerobic glycolysis —> metabolic acidosis (lactate)
• CNS and myocardium hypoxia

Mild

• CNS: Impairment of mental performance, visual acuity, clouding of consciousness
• CVS: tachycardia, hypertension
• mild hyperventilation (hypoxic drive)

Profound

• CNS: Convulsion, retinal haemorrhage, permanent brain damage
• CVS: Bradycardia + hypotension (direct depressant on heart)
• Pulmonary hypertension (hypoxic vasoconstriction)
• Renal: proteinuria

Question 4

Causes of hypercapnia

Answer 4

1. Hypoventilation
2. VQ mismatch
3. Injudicious use O2 therapy
   - removal of hypoxic drive —> ↓V —> further hypercapnia
   - removal of hypoxic vasoconstriction but ventilation not improved due to removal of hypoxic drive
   - overall: worsening of VQ mismatch

Effects:

• Mild: Cerebral vasodilation —> increased cerebral blood flow, headache
• Severe: Narcotic, clouding of consciousness (direct depressant)
• Respiratory acidosis (after injudicious use O2 therapy)
• Metabolic acidosis (release of lactate from hypoxic tissue)

Question 5

Types of hypoxia

Answer 5

1. Hypoxemic hypoxia (↓PaO2: insufficient O2 reaching blood)
2. Stagnant hypoxia (normal PaO2: failure to transport O2 because of impaired blood flow e.g. heart failure, circulatory shock, local disruption of blood flow)
3. Anaemic hypoxia (normal PaO2: reduction of O2 carrying capacity in the blood)
4. Histotoxic hypoxia (normal PaO2: impaired utilisation of O2 by the cell e.g. cyanide poisoning, tissue oedema)

Question 6

Management of respiratory failure

Answer 6

1. Treatment of underlying cause
   - antibiotics
2. Treatment of airway obstruction
   - bronchodilator
   - removal of secretion
3. Treatment of hypoxaemia
   - oxygen therapy (25-30% for COPD, 60% for lung shock)
   - mechanical ventilation with Positive end-expiratory pressure (PEEP)
4. Treatment of hypercapnia
   - mechanical ventilation
5. Treatment of cardiac failure (causes: cor pulmonale, polycethemia, water retention)
   - diuretics
   - digitalis

Hazards of treatment:

• O2 therapy: removal of hypoxic drive, O2 toxicity (oxidative damage), absorption atelectasis (due to large pressure gradient —> extremely fast gas absorption), retrolental fibroplasia
• mechanical ventilation: pneumothorax, decrease venous return