The term respiratory failure can also be used more generically to describe the failure of external or internal respiration
For example, if the cardiovascular system is unable to circulate blood at a sufficient rate to meet the body’s metabolic demands, oxygen transport will be inadequate and tissues may suffer from hypoxia.
Although this example more accurately describes a condition of circulatory insufficiency, it also represents a dysfunction of the breathing apparatus.
Oxygenation insufficiency refers more specifically to conditions characterised by the presence of an arterial blood oxygen partial pressure (PaO2) of less than 60 mmHg (hypoxemia), despite an increase in the concentration or fraction of oxygen in the inspired air (FiO2) of 0.50 or more.
Oxygenation insufficiency is generally considered
- mild with PaO2 between 60 and 79 mmHg;
- moderate with PaO2 between 40 and 59 mmHg;
- severe with PaO2 below 40 mmHg.
Ventilatory insufficiency refers more specifically to inadequate ventilation between the lungs and the atmosphere, which ultimately results in an inappropriate elevation of the partial pressure of carbon dioxide in arterial blood (PaCO2) to values above 45 mmHg (hypercapnia).
Ventilatory failure is generally considered to be
- mild: PCO2 between 45 and 60 mmHg;
- moderate: PCO2 between 60 and 90 mmHg;
- severe: PCO2 over 90 mmHg.
When PCO2 exceeds 100 mmHg, coma may occur and, above 120 mmHg, death.
PCO2 is measured by haemogasanalysis.