Respiratory failure is the inability of the respiratory system to facilitate adequate gas exchange and meet the metabolic needs of the body. Failure of the respiratory system can be caused by abnormalities in the respiratory, neuromuscular, or central nervous system. Respiratory distress is defined as signs of increased work of breathing, nasal flaring, use of accessory muscles, and inspiratory retractions. This does not always accompany respiratory failure, because respiratory failure can be due to lack of respiratory effort. Within the pulmonary system, there can be hypoventilation, diffusion impairment, intrapulmonary shunting or a ventilation-perfusion mismatch. Abnormal arterial pO₂ and pCO₂ is found in respiratory failure, but it is not necessary to measure and document these values prior to initiating care. Physical exam is the key to determining respiratory failure and treatment should not be delayed for blood gas analysis.