Respiratory failure

In order to diagnose this, you need to do an arterial blood gas

The bits that aren't working properly in respiratory failure have been crossed out. They're known as lungs. They be needin' a' mendin'!

Definition

P_aO₂<8kPa. There are two types which are defined based on the P_aCO₂.

Type I

P_aCO₂ is normal or low. Basically, hypoxia without hypercapnia (low oxygen, not high carbon dioxide). It doesn't matter if the CO₂ is low or normal - if it's not high and the patient is hypoxic, it's type I respiratory failure.

Type II

P_aCO₂ > 6kPa i.e. it's high. In words, it's hypoxia with hypercapnia (low oxygen with high carbon dioxide).

Epidemiology

Since both of these are caused by common disease like asthma and COPD, they're dead common. Learn about them!

Pathophysiology

If this picture doesn't help you differentiate between the two types of respiratory failure, I don't know what will.

Type I is caused by anything which results in a ventilation/perfusion (V/Q) mismatch. In normal ventilation the amount of air coming into the lungs (ventilation) is just enough to oxygenate the amount of blood coming into the lungs (perfusion). In V/Q mismatch, that ratio is out of whack causing the body to breath more quickly to make up for it. Causes include: pneumonia, pulmonary oedema, PE, asthma, emphysema, fibrosing alveolitis and ARDS

As for type II, there are a bunch of theories on how this works. The theory of hypoxic drive (where oxygen becomes the primary stimulus for breathing) has become more controversial recently. So in terms of pathophysiology, it's a bit difficult to be sure what's right or not. If nothing else, remember the "hypoxia with hypercapnia (P_aCO₂ > 6kPa)" bit and nothing else. It's basically anything with respiratory failure where they are hypoventilating (breathing too slowly). Causes include:

• Lung - asthma, COPD, pneumonia, pulmonary fibrosis, sleep apnoea
• Brain - sedative drugs, CNS tumour, trauma
• Neuro/Muscle - Cervical cord lesion, paralysed diaphragm, polio, myastehnia gravis, Guillain-Barre syndrome
• Bones - kyphoscolisosis, any deformity of the chest wall that would stop efficacious chest expansion on inspiration

Risk Factors

Having any of the diseases listed above, really.

Clinical Features

Essentially, they can be split into those caused by hypoxia and those causes by hypercapnia.
Hypoxia will present with: dyspnoea, restlessness, agitation, confusion and central cyanosis. Longstanding, it will give polycythaemia, pulmonary hypertension and cor pulmonale.

Hypercapnia will present with: headache, peripheral vasodilatation, tachycardia, bounding pulse, tremor (CO₂ retention tremor), papilloedema, confusion, drowsiness, coma

Investigations

Bloods

• FBC - WCC↑ in infection, platelets↑ in inflammation, Hb↓ in anaemia
• U+Es - provide baseline if they become very ill
• CRP - raised in inflammation
• D-dimer if PE suspected
• ABG - determines nature of respiratory failure
• Blood culture if febrile - determine infection

Imaging

• CXR - pneumonia, pulmonary oedema
• CTPA if PE suspected and D-dimer is positive

Others

• Spirometry
• Sputum culture - infection

Management

Treat underlying cause. Whether it's type I or type II, you need to figure out what's causing it and deal with it.

Type I - give oxygen (35-60%) and assisted ventilation if P_aO₂<8kPa. You are looking to get oxygen saturation up (92-96% depending on age).
Type II - controlled oxygen (24%). You also need to recheck ABG after 20 mins. If the P_aCO₂ remains the same or gets lower, increase O₂ to 28%. If the P_aCO₂ continues to fall consider assisted ventilation or a respiratory stimulant (doxapram 1.5-4mg IVI). If all this fails, intubate and ventilate.