Respiratory failure is inadequate pulmonary gas exchange causing either a failure to oxygenate, to eliminate carbon dioxide (ventilation) or a combination of both. See Breathing Anatomy and Physiology page for the differences between oxygenation and ventilation. It can be due to either acute or chronic breathing inadequacy.
Respiratory failure can be defined as:
In adults the PaO2 and PaCO2 levels are assessed using blood gas analysis. However, in children, arterial blood gases are rarely taken. Therefore pulse oximetry is often used to determine respiratory failure. Venous and capillary samples are often used to quantify PaCO2 and assess for ventilatory inadequacy. There are two types of respiratory failure depending on the PaCO2.
PaO2 < 8 kPa but normal or reduced PaCO2, (PaCO2 reference range: 4.7 - 6.0 kPa).
Hypoxic respiratory failure is very common in acutely ill children.
PaO2 < 8 kPa with raised PaCO2 (usually > 6.7 kPa).
Hypoxia is accompanied by hypercapnoea (raised CO2). Hypercapnoea occurs due to inadequate ventilation, specifically inadequate minute ventilation (MV) which is the amount of air inspired/expired in one minute. As discussed above MV = RR x TV.
Importantly, children with respiratory failure from any cause who become fatigued or exhausted will start to breathe less efficiently and retain CO2. This is a sign that ventilation may need to be assisted (e.g. non-invasive or invasive ventilation). Giving more oxygen will not treat retained CO2.