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Dr. Bach is Professor of Physical Medicine and Rehabilitation, Department of Physical Medicine and Rehabilitation, and Professor of Neurosciences, Department of Neurosciences, UMDNJ-The New Jersey Medical School. Dr. Kang is from the Department of Physical Medicine and Rehabilitation, UMDNJ-The New Jersey Medical School and Yonsei University College of Medicine, Seoul, South Korea.
Correspondence to: John R. Bach, MD, FCCP, Professor and Vice Chairman, Department of Physical Medicine and Rehabilitation, University Hospital B-403, 150 Bergen St, Newark, NJ 07103; e-mail: bachjr{at}umdnj.edu
The article by Misuri et al, "Mechanism of CO2 Retention in Patients With Neuromuscular Disease," in this issue of CHEST (see page 447) points out something concerning the lungs of patients with neuromuscular disorders that has long been recognized concerning their limbs: the loss of function results from a combination of weakness and increases in soft tissue elastance. The latter is caused by the failure to fully mobilize the soft tissues and joints because of muscle weakness.1 Muscle strength also diminishes as soft tissues adaptively shorten over time. As muscle loses its normal flexibility and weaker muscles are stretched by their stronger antagonists, changes in their length-tension relationships result in decreased peak tensions, strength, and endurance.2 3 When a foreshortened position of a muscle is maintained for > 5 to 7 days, the loose connective tissue in the muscle belly shortens and then gradually changes into dense connective tissue.1 These tissues lose their normal elasticity and plasticity, resulting in the loss of range of motion (ROM) and joint contractures.2 A joint is contracted when it lacks full passive ROM. The muscle and other soft tissue limitations that result in joint contractures can also cause bony deformities, particularly in young, growing patients.1
Upper limb contractures cause discomfort and diminish the ability to perform activities of daily living.4 Lower limb contractures cause the premature loss of the ability to walk.5 For example, contractures of hip flexors reduce hip extension, thereby shortening stride and requiring the patient to walk on the balls of the feet with increased lumbar lordosis and a consequent increase in energy consumption.1 Knee flexion contractures of 30° increase the work of the calf muscles and the knee extensors by 50%. It has been demonstrated that while the combination of moderate leg weakness and contractures can result in frequent falls and wheelchair dependence, when leg contractures are prevented by ROM mobilization or surgery, the same patients can walk longer without assistance and without falls despite worsening weakness.5
Flexibility exercises performed three times a week for 10 to 15 min in healthy but inactive subjects are sufficient to maintain the optimal resting lengths of the long muscles that would otherwise not be put through full ROM during normal daily activities. However, the independent performance of flexibility exercises requires that the subjects have normal strength. When strength is diminished, passive ROM with a sustained terminal stretch can be effective in preventing contractures if applied for 20 to 30 min bid.1
Normally, people take deep breaths or sigh regularly. These actions stretch the respiratory structures. Patients with chronic respiratory muscle weakness have reductions in lung volumes and vital capacity (VC) and can develop hypercapnia that greatly exceeds what might be anticipated from the loss of muscle force; and they may have decreases in lung distensibility that contribute to the disproportionate hypercapnia from lung volume restriction.6 7 As shown by Mizuri et al, failure to fully expand the lungs causes increases in lung tissue and chest wall elastance and decreases in compliance.
The total mechanical work of breathing (WOB) is the sum of the work of overcoming both the elastic and frictional forces opposing inflation. In healthy adults, about two thirds of the WOB can be attributed to elastic forces opposing ventilation. The remaining third is due to frictional resistance to gas and tissue movement. In diseased states, the WOB can dramatically increase. In patients with restrictive lung disease, work is the integration of the volume-pressure breathing curve. The increase in the WOB is a function of tissue elastance and an inverse function of pulmonary compliance.8 9
Failure to take periodic deep breaths can change alveolar surface forces and increase the tendency for alveolar collapse.10 Gross muscle weakness alters the passive recoil of the thoracic cage, modifying the neutral position at which lung and cage recoil pressures are balanced.11 This results in altered inspiratory muscle length-tension relationships, just as the stretching of weak lower limb muscles causes a decrease in their peak tensions by altering their optimal length-tension relationships. The lungs and chest walls are also, therefore, susceptible to the effects of incomplete regular mobilization. The tendons and ligaments of the rib cage and the costovertebral and costosternal articulations stiffen, and the latter ankylose,11 as the intercostal and other respiratory muscles become fibrotic and contracted.11 12
While it is common for patients with neuromuscular disease to undergo an intensive home program of limb joint mobilization, lung and chest wall mobilization is invariably ignored. Yet, without deep insufflations these patients first develop microatelectasis. Microatelectasis can develop in 1 h when normal tidal volumes cannot be increased.13 The long-term inability to take deep breaths, or chronic hypoinflation, results in permanent pulmonary restriction. For children, this causes the underdevelopment of lung tissues and decreased chest wall elasticity14 15 and static pulmonary compliance.6 16 17 Thus, decreased pulmonary compliance results initially from microatelectasis and ultimately from increased stiffness of the chest wall and lung tissues themselves.15 The presence of scoliosis can exacerbate the loss of compliance which, in turn, further increases the WOB.
The airways normally divide 27 times by 8 years of age. The alveoli and respiratory exchange membrane continue to grow, reaching 80 m2,18 the equivalent surface area of the football fields, by late adolescence.9 Lung growth peaks with the plateauing of the VC at 19 years of age. With severe or advanced neuromuscular disease, upper thoracic volumes can decrease during inspiration.19 For infants with spinal muscular atrophy, for example, this can result in the underdevelopment of the lungs and chest wall and in the boney deformity, pectus excavatum.
It is ironic that although few patients die from stiff ankles and wrists, the inability to attain a volume of air > 1,500 mL invariably results in diminished spontaneous and assisted cough flows,20 and this greatly increases the risk of pulmonary morbidity and mortality.21 Patients with VCs of 300 mL who have predicted VCs of perhaps 3,000 mL can expand < 10% of their lungs on their own. Incentive spirometry is, therefore, useless for them. However, these patients can receive insufflations delivered to them via oral, nasal, or oronasal interfaces from a manual resuscitator, volume-cycled ventilator, or cough machine (In-exsufflator; J. H. Emerson Co; Cambridge, MA).20 Lung hyperinflation has been shown to reverse acute lung compliance reductions in dogs as well as in humans.22 23 However, the only studies of the effects of long-term regimens of deep insufflations on static pulmonary compliance in humans were instituted only after patients already had severe pulmonary restriction (VCs < 50% of predicted normal), involved the use of insufflation pressures < 30 cm H2O (grossly inadequate to fully expand the lungs), and called for the use of insufflations for only minutes each day.24 Although short periods of mechanical hyperinflation20 25 can briefly increase the dynamic pulmonary compliance associated with airway changes and reverse acute atelectasis,23 26 27 multiple daily periods of mechanical hyperinflations of < 30 cm H2O pressure do not improve static compliance in adults.24 Studies on paralyzed animals with lungs comparable to those of human babies suggest that insufflation pressures of 40 cm H2O may be needed to reverse atelectasis.28
The maximum insufflation capacity (MIC) is a measure of the maximum volume of air that can be held with a closed glottis and then expelled.29 The MIC is obtained by some combination of the airstacking of mechanically delivered insufflations or glossopharyngeal breathing. The MIC is a function of pulmonary compliance and strength of oropharyngeal and laryngeal muscles. A MIC of at least 500 to 1,000 mL is necessary to achieve adequate manually assisted peak cough flows to prevent airway mucus accumulation, atelectasis, and pneumonia during chest infections.29 While the MIC can be limited to the VC or little more than the VC in patients with severely dysfunctional bulbar musculature or those not regularly receiving deep insufflation therapy, it can become much greater than the VC and even improve with practice as VCs decrease with progressive disease (S. W. Kang and J. R. Bach; unpublished data; March 1999). This can help maintain patients free of respiratory complications and tracheostomy despite having little or no remaining VC and requiring continuous noninvasive ventilation.
In summary, the lungs and chest walls of patients with muscle weakness require regular ROM mobilization, as do weak limbs. This can by provided by the delivery of maximal insufflations or stacking air volumes to the MIC. If patients have sufficient bulbar muscle control to stack volumes of air so that their MICs exceed their VCs, they can use noninvasive ventilation long term as an alternative to tracheostomy, even in the absence of measurable VC (S. W. Kang and J. R. Bach; unpublished data; March 1999). For children, positive pressure insufflations that are provided as necessary can reverse pectus excavatum and, by preventing this boney deformity, promote more normal lung growth (V. Niranjan and J. R. Bach; unpublished data; July 1999).
References
This article has been cited by other articles:
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  N. Laws and A. Hoey Progression of kyphosis in mdx mice J Appl Physiol, November 1, 2004; 97(5): 1970 - 1977. [Abstract] [Full Text] [PDF]
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