Respiratory Failure

When the process of gas exchange fails to maintain normal levels of oxygen and carbon dioxide, it is termed as respiratory failure.

Classification

The simplest way of thinking of respiratory failure is to consider the purpose of the respiratory system.

1. There is the process of ventilation which is achieved by the thorax and various muscles
2. There is the process of gas-exchange which is achieved by the lungs.
3. The aim of these processes is to maintain high levels of oxygen, and low levels of carbon dioxide.

Type I Respiratory Failure

• Hypoxia: Arterial oxygen low (PaO2 < 8kPa)
• Normo/hypocapnia: Arterial carbon dioxide is normal or low (PaCO2 <=6kPa)

Type I respiratory failure occurs due to a failure in gas-exchange. 

• A region of damaged/non-functioning lung is perfused as normal, but gas-exchange by the alveoli in that area is inadequate.
• Thus, the blood remains low in oxygen and high in carbon dioxide.
• Since the ventilatory mechanism is intact, the body can excrete excess carbon dioxide by increasing ventilation. 
• However, oxygen uptake cannot be increased since whatever haemoglobin is passing through the healthy parts of the lungs is already saturated.
• The combination of these factors leads to a hypoxia with either normal or low levels of carbon dioxide.

Causes of Type I Respiratory Failure

As described above, ventilation/perfusion mismatches lead to type I respiratory failures (inadequate oxygenation but sufficient ventilatory action to expel excess carbon dioxide). Examples of conditions which can cause this include:

• Pneumonia (fluid in alveoli interrupts gas exchange)
• Asthma
• COPD
• Pulmonary Embolism (blood flow does not reach certain parts of the lung – V/Q mismatch)
• Pulmonary Oedema (fluid in alveoli interrupts gas exchange)
• Acute Respiratory Distress Syndrome
• Pneumothorax
• Right to left cardiac shunt
• Pulmonary Fibrosis

Type II Respiratory Failure

• Hypoxia: Arterial oxygen low (PaO2 < 8kPa)
• Hypercapnia: Arterial carbon dioxide high (PaCO2 > 6kPa)

If there is widespread ventilation-perfusion mismatch to the point there is inadequate amounts of normal lung tissue to excrete excess carbon dioxide, and/or there is disease affecting ventilation, a type II respiratory failure occurs.

Causes of Type II Respiratory Failure

• COPD
• Neuromuscular diseases e.g. Guillain-Barré syndrome, motor neuron disease, myasthenia gravis
• Chest wall deformity e.g. kyphosis
• Drug-induced respiratory depression e.g. opioids, benzodiazepines
• Asthma
• Pneumonia
• Sleep apnoea

Clinical Features

The correct identification of signs is crucial for a patient suffering from respiratory failure.

• Use of accessory muscles of respiration
• Tachycardia
• Tachypnoea: Do not underestimate the importance of respiratory rate – it is an incredibly sensitive indicator of respiratory difficulty.
• Dyspnoea
• Confusion
• Cyanosis
• Features of Hypercapnia
  • Headache
  • Bounding pulse
  • Asterixis
  • Papilloedema
  • Warm extremities (due to peripheral vasodilation).

Investigations

An arterial blood gas (ABG) is the mainstay of diagnosis as it will confirm the presence of hypoxia and/or hypercapnia. In addition to this, the underlying cause of the respiratory failure needs to be diagnosed for which the following investigations may be useful:

Bedside

• Sputum/blood culture: If infection is the underlying cause
• Peak flow: If suspecting asthma to be the cause
• ECG: Signs of a pulmonary emoblism/pulmonary oedema secondary to heart failure

Bloods

• ABG: Diagnostic
• FBC: Looking for infection

Imaging

• Chest x-ray: To look for lung pathology
• CT Pulmonary Angiogram: If suspecting a pulmonary embolism

Special Tests

• Spirometry
• Bronchoscopy

Management of Type I Respiratory Failure

In an acute presentation, it is crucial to remember the A-E approach (Airway, breathing, circulatory, disability and exposure) prior to focusing on the presentation.

Supplementary oxygen is given to manage type I respiratory failure. However, it is extremely important to identify and control the underlying cause of the respiratory failure e.g. if an acute asthma attack is causing the respiratory failure, salbutamol nebulisers, steroids etc should be administered. Invasive ventilation may be needed if PaO2 fails to improve despite oxygen.

Management of Type II Respiratory Failure

When managing type II respiratory failure, differentiating the underlying cause is of extreme importance as chronic carbon dioxide retainers e.g. patients with chronic COPD may have a tolerance for hypercapnia and thus rely on their hypoxic drive to breathe – by providing high concentrations of oxygen, you may inadvertently cause respiratory depression!

Supplementary oxygen should be given with caution, typically starting with a 24-28% Venturi mask and regularly monitoring the ABG e.g. every 20 minutes to check for a response. If there is a further rise in carbon dioxide following oxygen administration, non-invasive positive pressure ventilation or intubation may be necessary. 

References

https://www.ncbi.nlm.nih.gov/books/NBK526127/ 

Kumar and Clark's Clinical Medicine, 10th Edition.