respiratory effort as well, such as opioids, sedative-hypnotics, and alcohol. These lower respiratory drive by blunting the response of the respiratory center of the brain to hypercapnia. Metabolic disorders could also decrease respiratory effort. Hypoglycemia and hypotension depress the central nervous system and compromise the respiratory system. Respiratory muscle weakness: Neuromuscular disorders may lead to respiratory muscle weakness, such as spinal cord injury, neuromuscular diseases, and neuromuscular blocking drugs. Respiratory muscle fatigue can also lead to respiratory muscle weakness if patients breathe over 70% of their maximum voluntary ventilation. Breathing over an extended period of time near maximum capacity can cause metabolic acidosis or hypoxemia, ultimately leading to respiratory muscle weakness. Diagnosis Diagnosis requires clinical evaluation, as detailed below. Initial assessment After determining the scene is safe, approach the patient and attempt to converse with him or her. If the patient responds verbally, you have established that there is at least a partially patent airway and that the patient is breathing (therefore not currently in respiratory arrest). If the patient is unresponsive, look for chest rise, which is an indicator of active breathing. A sternal rub is sometimes used to further assess for responsiveness. Initial assessment also involves checking for a pulse, by placing two fingers against the carotid artery, radial artery, or femoral artery to ensure this is purely respiratory arrest and not cardiopulmonary arrest. Checking a pulse after encountering an unresponsive patient is no longer recommended for non-medically trained personnel. Once one has determined that the patient is in respiratory arrest, the steps below can help to further identify the cause of the arrest. Clearing and opening the upper airway The first step to determining the cause of arrest is to clear and open the upper airway with correct head and neck positioning. The practitioner must lengthen and elevate the patient's neck until the external auditory meatus is in the same plane as the sternum. The face should be facing the ceiling. The mandible should be positioned upwards by lifting the lower jaw and pushing the mandible upward. These steps are known as head tilt, chin lift, and jaw thrust, respectively. If a neck or spinal injury is suspected, the provider should avoid performing this maneuver as further nervous system damage may occur. The cervical spine should be stabilized, if possible, by using either manual stabilization of the head and neck by a provider or applying a C-collar.