Type I respiratory failure is a failure of oxygenation. Patients experience hypoxaemia (PaO2 <7.3kPa, or 8kPa in a patient with other confounding factors) with normal or abnormally low carbon dioxide. Type I respiratory failure is generally caused by lung disease, for example pneumonia, interstitial lung disease and exacerbation of asthma.
Type I respiratory failure is generally an abnormality of the PaO2 only. For this reason, the rest of the blood gas result looks fairly normal.
Diagram: Type I Respiratory Failure
Type I respiratory failure is often accompanied by hypocapnia (low PaCO2). This phenomenon occurs because the body is trying to compensate for hypoxaemia by increasing ventilation. Where the oxygen levels in the blood are reduced by a gas exchange abnormality, the carbon dioxide moves much more freely from the blood into the alveoli. The compensatory increase in ventilation has a small effect on increasing the oxygen levels in the blood, but a bigger effect on reducing the carbon dioxide content of the blood. This compensatory hypocapnia causes an elevation of the blood pH; this is type I respiratory failure with respiratory alkalosis.
Type I Respiratory Failure with Respiratory Alkalosis
After a prolonged period of respiratory alkalosis, the metabolic system will begin compensating for this acid-base imbalance by removing bicarbonate (HCO3–). This pattern of metabolic compensation can be seen below.
Type I Respiratory Failure with Respiratory Alkalosis and Metabolic Compensation
Low blood oxygen levels are dangerous because it results in low tissue oxygen levels. Certain tissues, such as the myocardium and central nervous system are particularly vulnerable to hypoxia, and irreversible damage can result within a very short time period of hypoxaemia.