**2. Pathophysiology**

Respiratory muscle groups include inspiratory muscles (predominately the diaphragm), expiratory muscles (predominately abdominal and chest wall muscles used for coughing) and bulbar-innervated muscles (used to protect the airway). Many patients with ventilatory insufficiency manage for years without ventilator use but at the cost of orthopnea and hypercapnia which can result in compensatory metabolic alkalosis which depresses central ventilatory drive. As a result the brain becomes accustomed to the hypercapnia without obvious symptoms of ventilatory failure. Patients not introduced to NIV are oftentimes prescribed supplemental oxygen which exacerbates hypercapnia and eventually results in the coma of carbon dioxide narcosis and ventilatory arrest.

Patients with inspiratory and expiratory muscle weakness can be sustained using NIV. Ventilatory insufficiency/failure spans the spectrum from those with only diaphragm dysfunction (resulting in nocturnal ventilatory insufficiency/failure when in bed) to complete inspiratory muscle failure. Patients with complete inspiratory and expiratory muscle failure (with as little as 0 mL of vital capacity) can be completely supported using NIV for over 50 years without tracheostomy (Bach, 2004). Some of them use only nocturnal ventilatory aids and use glossopharyngeal breathing (GPB) to maintain ventilation during the day (Bach, 2004).
