Acute or chronic severe malfunction in gas exchange between the lungs and the blood causing hypoxia with or without hypercapnia. Hypoxic respiratory failure (type 1 respiratory failure) is hypoxia without hypercapnia and with an arterial partial pressure of oxygen (PaO2) of <60 mmHg (8 KPa) on room air at sea level. Hypercapnic respiratory failure (type 2 respiratory failure) is hypoxia with an arterial partial pressure of carbon dioxide (PaCO2) of >50 mmHg (6.5 KPa) on room air at sea level.
Acute respiratory failure
Acute respiratory failure develops over hours, and is characterized by an acute lack of oxygen delivery to the blood by the respiratory system or acute failure of the respiratory system to remove carbon dioxide (CO2) from the blood. 
Chronic respiratory failure
Chronic respiratory failure develops over longer periods of time and is usually defined as long-term lack of oxygen delivery to the blood by the respiratory system. It causes progressively worsening respiratory acidosis (carbonic acid accumulation resulting from inefficient expulsion of CO2 from the respiratory system) in someone with chronically decreased arterial partial pressure of oxygen (PaO2) or chronically elevated arterial partial pressure of CO2 (PaCO2) levels. The failure of the respiratory system to remove sufficient CO2 from the blood over time leads to renal compensation, with elevation of the concentration of serum bicarbonate (HCO3) to bind with hydrogen ions caused by increased systemic CO2. Baseline oxygen and carbon dioxide levels in arterial blood may be abnormal and compensated for by the renal system (increasing bicarbonate to compensate for respiratory acidosis) and blood system (increased red cell volume to compensate for decreased oxygen). 
Acute trauma, surgery, or shock respiratory failure
Acute failure to provide oxygen and carbon dioxide exchange between the lungs and the blood as a result of injury or lack of perfusion to the respiratory system. 
Hypoxic respiratory failure
Also known as type 1 respiratory failure. This occurs when the PaO2 is <60 mmHg. The PaO2 of patients with chronic lung disease can be as low as 50 mmHg normally, and in these patients respiratory failure occurs when there are 10% decreases in baseline arterial oxygen measurements. 
Hypercapnic respiratory failure
Also known as type 2 respiratory failure. This occurs when there is hypoxia associated with a PaCO2 that is >50 mmHg.