Respiratory failure

Definition:

Respiratory failure occurs when pulmonary gas exchange is sufficiently impaired to cause hypoxaemia, with or without hypercapnia. It can be divided into 2 types:

• Type I respiratory failure (‘acute hypoxaemic failure’):
• P_a0₂ <8KPa
• P_aCO₂ is normal or low
• Type II respiratory failure (‘ventilatory failure’):
• P_aO₂ <8kPa
• P_aCO₂ >7kPa

Common causes of type I failure:

• Pulmonary oedema
• Pneumonia
• ARDS
• Pulmonary fibrosing alveolitis
• Right-to-left shunts
• V/Q mismatch

Common causes of type II failure:

• CNS (e.g. drugs, tumour, trauma, infection)
• PNS (e.g. phrenic nerve damage, myasthenia gravis, Guillain-Barre syndrome)
• Muscular (e.g. dystrophies, hypokalaemia, hypophosphataemia)
• Chest wall (e.g. fractured rib, thoracic surgery, obesity)
• Airway (e.g. obstruction, bronchospasm)

Disturbances in acid-base balance:

Remember:

H⁺ + HCO₃^- CO₂ + H₂O

There are 4 situations in acid-base disturbance:

• Respiratory acidosis (e.g. respiratory depression)
• Respiratory alkalosis (e.g. anxiety attacks)
• Metabolic acidosis (e.g. DKA)
• Metabolic alkalosis (e.g. vomiting)

Normal ranges for blood gases:

• pH 7.35-7.45
• PO₂ 10-13.3kPa
• PCO₂ 4.7-6.4kPa
• HCO₃^- 20-25mmol/L
• S_aO₂ 95-100%

Management of respiratory failure:

Always seek to treat the underlying cause. There are 5 aims in management of respiratory failure:

1. Supplemental O₂
2. Control of secretions
3. Treatment of pulmonary infection
4. Control of airways obstruction
5. Measures to limit pulmonary oedema
6. Correction of electrolytes/malnutrition

Oxygen therapy:

Oxygen is, initially delivered via a face mask or nasal specs. If this is proving inadequate, there are several other techniques for respiratory support available:

Non-invasive positive pressure ventilation:

• Bi-level positive airway pressure (BPAP) (insp and exp pressures can be set separately)
• Continuous positive airway pressure (CPAP) via a face mask (constant pressure but can vary volumes)

Invasive positive pressure ventilation:

• Intermittent positive pressure ventilation (IPPV) (may be given with positive end expiratory pressure – PEEP)
• CPAP via an endotracheal tube
• High frequency jet ventilation (HFJV) (useful for patients with a lung leak)

IPPV:

This is achieved by intermittently inflating the lungs with a positive pressure delivered by a ventilator and applied via an endotracheal tube or a tracheostomy.

Beneficial effects:

• Improved CO₂ elimination
• Relief from exhaustion
• Reduction in total body 0₂ demand

Dangers of IPPV:

• Airway complications
• Mechanical faults with ventilator
• CVS complications (the intermittent application of positive pressure impedes venous return and distends alveoli, thereby stretching the pulmonary capillaries and causing an increase in pulmonary vascular resistance
• Pulmonary infection
• Barotrauma
• Abdominal distension associated with an ileus (unknown cause)
• Increased ADH secretion, therefore causing salt and water retention

Complications of endotracheal intubation:

Immediate:

• Tube in one or other (usually the right) bronchus
• Tube in oesophagus

Early:

• Migration of tube out of trachea
• Leaks around the tube
• Obstruction of the tube due to kinks or secretions

Late:

• Mucosal oedema and ulceration
• Tracheal narrowing and fibrosis

Complications of tracheostomy:

As for endotracheal tubes, plus the following:

Early:

• Pneumothorax
• Haemothorax
• Subcutaneous emphysema

Intermediate:

• Erosion of tracheal cartilages
• Erosion of innominate artery (may lead to fatal haemorrhage)
• Infection

Late:

• Tracheal stenosis at level of stoma
• Collapse of tracheal rings at level of stoma