**3. Evaluating a patient**

On initial presentation, ambulatory patients with hypercapnic ventilatory insufficiency often initially report exertional dyspnea and later morning headaches, fatigue, sleep disturbances and hypersomnolence (Bach & Alba, 1990). On the other hand wheelchair users may report wheelchair users may report very few symptoms, e.g., anxiety, dyspnea, and difficulty with sleep, except during a respiratory infection. Physical signs of tachypnea, paradoxical breathing, hypophonia, nasal flaring, accessory respiratory muscle use, cyanosis, flushing, pallor and airway secretion and congestion are signs of increasing carbon dioxide levels. Lethargy and confusion are indicative of carbon dioxide narcosis, which can be reversed with proper management through NIV.

Evaluation can include obtaining a vital capacity (VC), cough peak flows (CPF), capnography/oximetry and a polysomnogram. VC is measured in the supine and sitting positions. The VC supine is a superior indicator of ventilatory dysfunction as hypoventilation begins and is worse during sleep. The difference between the two should be less than 7 percent, while a value greater than 20 percent indicates need for nocturnal NIV. If the patient wears a thoracolumbar brace, a VC should be recorded with and without the brace as the fit can improve or worsen the VC.

Inspiratory and expiratory muscle aids, including devices and manual assisted techniques, result in intermittent pressure changes to assist inspiratory and expiratory muscles in their natural function. Noninvasive inspiratory and expiratory muscle aids are used to maintain lung and chest wall compliance, maintain normal alveolar ventilation, and to maximize cough peak flows (CPF) thus preventing episodes of acute respiratory failure, especially during intercurrent chest infections. This allows for decreased hospitalizations and prolongs survival without tracheostomy for patients with Duchenne muscular dystrophy (DMD), Spinal Muscular Atrophy including type 1 (SMA), amyotrophic lateral sclerosis (ALS), and others.

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

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

On initial presentation, ambulatory patients with hypercapnic ventilatory insufficiency often initially report exertional dyspnea and later morning headaches, fatigue, sleep disturbances and hypersomnolence (Bach & Alba, 1990). On the other hand wheelchair users may report wheelchair users may report very few symptoms, e.g., anxiety, dyspnea, and difficulty with sleep, except during a respiratory infection. Physical signs of tachypnea, paradoxical breathing, hypophonia, nasal flaring, accessory respiratory muscle use, cyanosis, flushing, pallor and airway secretion and congestion are signs of increasing carbon dioxide levels. Lethargy and confusion are indicative of carbon dioxide narcosis, which can be reversed

Evaluation can include obtaining a vital capacity (VC), cough peak flows (CPF), capnography/oximetry and a polysomnogram. VC is measured in the supine and sitting positions. The VC supine is a superior indicator of ventilatory dysfunction as hypoventilation begins and is worse during sleep. The difference between the two should be less than 7 percent, while a value greater than 20 percent indicates need for nocturnal NIV. If the patient wears a thoracolumbar brace, a VC should be recorded with and without the

the coma of carbon dioxide narcosis and ventilatory arrest.

**2. Pathophysiology** 

the day (Bach, 2004).

**3. Evaluating a patient** 

with proper management through NIV.

brace as the fit can improve or worsen the VC.

Spirometry is also used for monitoring the maximum insufflation capacity (MIC), that is, the ability to "air stack". The ability to air stack is holding with the glottis a maximal volume by holding consecutively delivered volumes of air delivered from a manual resuscitator or volume cycling ventilator. Interfaces that can be used for air stacking include a simple mouthpiece or when the lips are too weak for this, a nasal interface or lipseal. In patients who have learned glossopharyngeal breathing (GPB) techniques, this enables them to approach or attain the MIC independently.

Cough Peak Flow (CPF), measured with a peak flow meter, is an indicator of the patient's cough effectiveness. A CPF of 160 L/m is the minimum needed for sufficiently effective coughing and airway clearance to reliably permit safe extubation (Bach & Saporito, 1996). This is also the best indicator for tracheostomy removal, regardless of pulmonary function, that is, the status of the inspiratory and expiratory muscles. Patients with a VC of less than 1,500 mL should have an assisted CPF measured using a maximal lung volume by air stacking and an abdominal thrust for a manually assisted cough. The abdominal thrust should be delivered simultaneously to the opening of the glottis.

In patients without significant intrinsic lung disease, arterial blood gas sampling is unnecessary. Many patients (25%) tend to hyperventilate due to discomfort or anxiety from the procedure (Currie et al., 1986), so accurate results can be difficult to obtain. Oximetry monitoring and capnography, which is the measurement of end tidal pCO2, provide more useful information. Most conveniently they can be performed in the home.

Patients with questionable symptoms, multiple hourly nocturnal oxyhemoglobin desaturations to below 95%, and elevated nocturnal PaCO2 should undergo a trial of nocturnal NIV. If the questionably symptomatic patient finds nocturnal NIV to be more burdensome than the symptoms of ventilatory insufficiency, the patient can discontinue NIV and should be reevaluated in 3 to 6 months.

In symptomatic patients with a normal VC, no carbon dioxide retention and no clear pattern of oxyhemoglobin desaturation, a polysomnogram is warranted to evaluate for sleep disordered breathing (Williams et al., 1991). Patients with obesity-hypoventilation also should be treated with NIV or pressure or volume control ventilation and not CPAP. Neuromuscular disease NMD patients with decreased VC have no indication for undergoing polysomnography as the device is programmed to interpret each apnea and hypopnea as having a central nervous system or obstructive etiology rather than being due to inspiratory muscle weakness. Treatment of asymptomatic NMD patients based solely on polysomnographic abnormalities with continuous positive airway pressure (CPAP) or low spans of bi-level PAP is ineffective or at the least, suboptimal.
