Airway, breathing, and circulation, therefore work in a cascade; if the patient's airway is blocked, breathing will not be possible, and oxygen cannot reach the lungs and be transported around the body in the blood, which will result in hypoxia and cardiac arrest. Ensuring a clear airway is therefore the first step in treating any patient; once it is established that a patient's airway is clear, rescuers must evaluate a patient's breathing, as many other things besides a blockage of the airway could lead to an absence of breathing.