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In hypoxemic respiratory failure (type I) the arterial oxygen tension (PaO2) is less than 60 mm Hg with a normal or low arterial carbon dioxide tension (PaCO2). This common form of respiratory failure generally results from fluid filling or collapse of alveolar units. Examples of type I respiratory failures are pulmonary oedema, pneumonia, and pulmonary haemorrhage.
Hypercapnic respiratory failure (type II) is characterised by a PaCO2 higher than 50mm Hg. Hypercapnia results from either:
Failure of central drive e.g. drug overdose
Mechanical failure due to weakness such as neuromuscular disease, chest wall abnormalities
Severe airway obstruction (e.g. asthma and chronic obstructive pulmonary disease (COPD).
NB: If a patient has Hypercapnic hypoxaemia (type II failure) do not attempt a trial of breathing room air in the hope their "hypoxic drive" will reduce their PaO2. In most patients there is little evidence any such hypoxic drive exists, furthermore hypercapnia causes alveolar hypoxia.
The relationship between the arterial blood gas and inspired oxygen is described in the following equations:
*Saturated vapour pressure of water at 37°C = 47 mmHg
Consider an 80 year old man with COPD breathing room air whose PaCO2 is 72 mmHg, where:
PaO2 = 760 mmHg Aagradient = 24 mmHg (Age/4+4) RQ = 0.8
Therefore, from (1) and (2):
PaO2 = 0.21 x (760-47) = 150 mmHg
PaO2 = 150 – 72/0.8 – 24 = 44 mmHg
Therefore, the patient will be hypoxaemic
Pulmonary oedema (hydrostatic-cardiogenic, or leaky capillary – ARDS)
Distal: COAD, asthma, bronchiectasis, sputum retention
Loss of chest wall integrity: flail chest
Loss of chest wall elasticity: sever kyphosis or scoliosos
Pneumo / Haemothorax, pleural effusions
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