The two major functions of respiration are to add oxygen to and remove carbon dioxide from the blood.
Acute respiratory failure is the relatively sudden decline in either or both of these functions. If an acute insult is of sufficient magnitude, acute respiratory failure may occur even though lung function before the insult was normal.
If chronic compensated respiratory failure is present because of pre-existing lung disease, even a minimal insult may lead to acute or chronic respiratory failure. All causes of respiratory failure result in hypoxemia. The causes of mia can be divided into four classes. ventilation perfusion mismatch, (2) alveolar tilation, (3) right-to-left shunt, and(4) decreased inspired oxygen tension(e.g., at high altitude or in fires where oxygen is rapidly consume tension is, by definition due to alveolar hypoventilation. Hypercapnia is commonly present in patients with severe obstructive lung disease, neuromuscular weakness, a depressed central nervous system. In patients with normal lungs hypoventilation can cause hypoxemia owing to lowering of alveolar oxygen tension(PAO2) by displace oxygen by the increased carbon dioxide.
Hypoxemia in this situation is easily reversed by increasing the concen- tration of oygen in the air being breathed in all but the most extreme cases of hypoventilation. In this situation when the patient is receiving supplemental oygen, direct measurement of arterial oxygen tension(Paoa), arterial carbon dioxide tension(PCO2), and pH is the only way to determine the degree of respiratory failure.
Most patients with ventilatory failure also have significant unerlying lung disease, creating hypoxic as well as hypercapnic respiratory failure.