Definition

Neuromuscular disorders such as amyotrophic lateral sclerosis and myopathic disorders most often result in respiratory morbidity and mortality caused by weakness of the respiratory muscles. The three respiratory muscle groups are the inspiratory muscles, the expiratory (predominantly abdominal and upper chest wall) muscles for coughing, and the bulbar-innervated muscles. Whereas the inspiratory and expiratory muscles can be completely supported by physical aids such as continuous noninvasive ventilation (NIV), as presented here, and some patients have used this for more than 50 years without resort to tracheostomy (Fig. 150.1), there are no effective noninvasive measures to assist bulbar-innervated muscle function [1,2].

Symptoms

Patients with diminished ventilatory reserve who are able to walk complain of exertional dyspnea; but wheelchair users’ symptoms may be minimal except during intercurrent respiratory infections, when they complain of anxiety, inability to fall asleep, and possibly dyspnea. Morning headaches, fatigue, sleep disturbances, and hypersomnolence result from nocturnal hypoventilation or the decreased ability to recruit accessory respiratory muscles during sleep [3].

Physical Examination

Signs of inspiratory muscle impairment and hypoventilation can include tachypnea, paradoxical breathing, hypophonia, nasal flaring, accessory respiratory muscle use, cyanosis, flushing or pallor, anxiety, and airway secretion congestion. Lethargy, obtundation, and confusion signal carbon dioxide (CO₂) narcosis. Often there are no signs at all despite symptoms of hypoventilation. The rest of the physical examination is usually typical for neuromuscular disorders (see, for example, Chapters 132 and 135).

Functional Limitations

With diminished respiratory reserve, walking becomes limited by dyspnea in going up stairs or walking long distances. Once a patient is wheelchair dependent and incapable of independently performing few activities of daily living, there are no additional functional limitations associated with chronic hypoventilation. However, symptoms of hypoventilation and CO₂ levels may increase until patients become obtunded or develop CO₂ narcosis.

Diagnostic Studies

Respiratory muscle dysfunction and hypoventilation are diagnosed by end-tidal CO₂ monitoring (capnography), oximetry, spirometry, and assessment of cough peak flows. The end-tidal CO₂ is 2 to 6 mm Hg less than arterial PCO₂. The vital capacity (VC) is measured in sitting and supine positions and should not be significantly different in either position. Because hypoventilation is worse during sleep, the supine VC is more important. Orthopnea is common when the VC is less than 25% of normal or the VC in the supine position is at least 20% less than in the sitting position. The normal difference in VC between sitting and the supine position is less than 7%. Patients wearing thoracolumbar bracing should have the VC measured both with the brace on and with it off because a brace that fits well can increase VC, whereas one that restricts respiratory muscle movement can decrease it.

Patients are taught glossopharyngeal breathing (GPB), and its progress is measured spirometrically, as is air stacking. Air stacking and GPB are used for lung volume recruitment. Patients air stack by receiving consecutively delivered volumes of air through a manual resuscitator or volume-cycled ventilator that are held by the glottis to the greatest volume possible. The maximum volume that can be held by the glottis, the maximum insufflation capacity (MIC), is determined spirometrically. Likewise, GPB can often provide volumes of air to or beyond those achieved by air stacking and is also measured spirometrically [4]. A nasal interface or oral-nasal interface can be used for air stacking when the lips are too weak for effective air stacking through the mouth (Fig. 150.2).

Cough peak flows are measured with a peak flow meter (Access Peak Flow Meter; Healthscan Products Inc, Cedar Grove, NJ). Cough peak flows of 160 L/min are minimal and often ineffective [4]. The attainment of (unassisted) cough peak flows above 160 L/min is the best indicator for successful tracheostomy tube removal irrespective of remaining pulmonary function [4]. Patients with VCs of less than 1500 mL have assisted cough peak flows measured from a maximally air stacked volume of air with an abdominal thrust delivered simultaneously with glottic opening [5]. A cough produced from a deep air stacked volume and with an abdominal thrust applied concomitantly with glottis opening is a “manually assisted cough”; its efficacy is also determined by peak flow meter.

For the stable patient without intrinsic pulmonary disease, arterial blood gas sampling is unnecessary. Besides the discomfort, 25% of patients hyperventilate as a result of anxiety or pain during the procedure [6].

For symptomatic patients with normal VC, an unclear pattern of oxyhemoglobin desaturation, and no apparent hypercapnia, polysomnography is warranted [7]. Polysomnography is unnecessary for symptomatic patients with decreased VC because it is programmed to interpret every apnea and hypopnea as resulting from central or obstructive events rather than from inspiratory muscle weakness.

Whereas all clearly symptomatic patients with diminished lung volumes require a trial of NIV to ease symptoms, if symptoms are questionable, nocturnal continuous capnography and oximetry are useful and most practically done in the home. A questionably symptomatic patient with decreased VC, multiple nocturnal oxyhemoglobin desaturations below 95%, and elevated nocturnal CO₂ should be encouraged to undergo a trial of nocturnal NIV.

The initial intervention goals are to promote normal lung and chest wall growth for children; to maintain lung and chest wall compliance, lung growth, and chest wall mobility; and to increase cough peak flows to prevent intercurrent respiratory tract infections from developing into pneumonias and respiratory failure.

Pulmonary compliance is diminished and chest wall contractures and lung restriction occur when the lungs cannot be expanded to predicted inspiratory capacity. As the VC decreases, the largest breath one can take expands only a fraction of lung volume. Like limb articulations, regular mobilization is required. This can be achieved only by providing deep insufflations, air stacking, or nocturnal NIV [8]. The primary objectives of lung expansion therapy are to increase VC and voice volume, to maximize cough peak flows (Fig. 150.3), to maintain pulmonary compliance, to diminish atelectasis, and to master NIV because anyone who can air stack through a mouthpiece can use mouthpiece NIV and therefore be successfully extubated and decannulated even if not ventilator weaned (Table 150.1).