As a consequence of SCI, especially injury to the cervical and upper thoracic parts of the spinal cord, functioning of the respiratory muscles is disrupted, and leads to lowered lung volume parameters (Linn et al. 2000), in addition to other respiratory complications, such as decreases in compliance of the chest wall, changes in breathing patterns, sleep-disordered breathing (SDB), and ventilator dependency.
For individuals with SCI who have impaired autonomic function and impaired inspiratory muscle weakness, SDB may occur (Bonekat et al. 1990). In general, the incidence of SDB, characterized by sleep apnea, is estimated to be at least twice that reported in the general population (Schilero et al. 2009). Respiratory complications lead to significant morbidity and mortality in people with SCI (DeVivo et al. 1993; Cotton et al. 2005).
Among the general population, age associated changes in the respiratory system involve loss of elastic recoil of the lung. Similarly, among individuals with SCI, decreases in the compliance of the chest wall, and strength of the respiratory muscles are observed (Janssens et al. 1999; Janssens 2005). Complications resulting from SCI may therefore hold important respiratory implications as people age.
In this section, 5 longitudinal studies and 4 cross-sectional studies (see Table 6) on the respiratory system after SCI are reviewed.
Several of the identified studies highlight that SDB and other respiratory complications are higher in persons with SCI than in the general population and appear to increase with age (see Table 6). In a five-year longitudinal study to assess changes in SDB, Bach and Wang (1994) measured oxygen desaturation, which is characteristic of sleep apnea, in 10 individuals with tetraplegia; six individuals had oxygen desaturation below 90%. At the five-year follow-up, 5 of the 10 individuals had increased patterns of oxygen desaturation, leading to the conclusion that oxygen desaturation is common among people with tetraplegia and increases with age. In another longitudinal study (Berlowitz et al. 2005) sleep apnea, defined as an apnea-hypopnea index (AHI) of >10 events per hour, was found in 62% of the sample in the first month, peaking at 83% at 13 weeks, and falling to 68% and 62% at weeks 26 and 52 respectively.
Snoring is another important indicator of sleep apnea and appears to be age related. In a large case-control study, 29% of men (N = 331) and 21% of women with SCI (N = 77) snored daily or almost daily compared to 18.2% of the control group (N = 339) representing the normal population of Denmark (Biering-Sorensen & Biering-Sorensen 2001). In addition, those who snored daily or almost daily in the SCI group were significantly older than those with SCI who snored less frequently.
After SCI, there are temporal changes in pulmonary functioning. Forced vital capacity (FVC), inspiratory capacity (IC), and maximum inspiratory mouth pressure (Pimax) are lowered in the acute stage of SCI, and then gradually improve over time. In a five-year longitudinal study, Postma et al. (2013) showed improvements to FVC. Loveridge and colleagues (1992) showed that seated positioning imposes greater stress on the respiratory system in the acute stages of SCI than the supine position. While breathing patterns in the supine position at all measured time points one-YPI were comparable to the controls (N= 18), breathing patterns in the seated position had to be adjusted in order to maintain minute ventilation. Over time, however, improved breathing patterns were observed in the seated position, so much so that differences initially observed between the seated and supine positions became insignificant. Such improved breathing pattern is speculated to be due to increased accessory muscle function, improved chest wall stability, thoraco-abdominal coupling, or a combination of these factors over time.
An increasingly shallow breathing pattern resulting from a lack of deep breaths, and other factors associated with SCI such as obesity and decreased chest wall compliance, may lead to hypercapnia, or excessive amounts of carbon dioxide in the blood, and possibly ventilatory failure (Bach & Wang 1994). Despite these results from this prospective longitudinal study, it is unclear if breathing patterns change as a result of the injury or due to aging with SCI. The former may be the case as the study followed adjustments only during the first YPI.
Sustaining a SCI often leads to an initial respiratory insufficiency and necessitates a need for long-term mechanical ventilation. In some instances, individuals may be weaned from the ventilator. Wicks and Menter (1986) conducted a 10-year retrospective study of ventilator-dependent patients with tetraplegia (N = 134) to determine factors associated with weaning and long-term survival rate. Despite similar levels of injury, patients over 50 years of age had a 20% mortality rate compared to 6% for those younger than 50, and that ventilator weaning is less successful for those over the age of 50. This suggests that ventilator-dependency among SCI individuals who are older than 50 possess a much greater risk of negative health outcomes (Wicks & Menter 1986).
Although there are additional factors that can affect respiratory health long-term for the individual with SCI (i.e. level and completeness), there are several preventative activities that can be done to minimize the aging of the respiratory system, such as not smoking, minimizing exposure to polluted air, and controlling body weight (Wilmot & Hall 1993). Further research is needed to better understand SDB in persons aging, especially in terms of their implications for for cardiovascular health.
There is Level 2 evidence from a longitudinal study with AB controls (Loveridge et al. 1992) that seated breathing patterns are compromised immediately post injury but recover over time. As well, persons with tetraplegia do not take deep breaths as often as AB individuals.
There is Level 4 evidence from a longitudinal study that adults over the age of 50 who are aging with ventilator dependency are at greater risk of death and are less likely to be weaned from their ventilators than younger adults aging with a ventilator (Wicks & Menter 1986).
There is Level 4 evidence from one longitudinal study (Postma et al. 2013) that forced vital capacity improves 5 years after inpatient rehabilitation.
Sleep disordered breathing may increase or persist with the aging process in persons with SCI.
Seated breathing patterns after tetraplegia appear compromised early post-injury but may recover over time.
Adults who are older (50 years +) and ventilator dependent have a higher mortality rate and lower weaning rate than adults who are younger and who are ventilator dependent.