As expected, the loss of inspiratory muscle function is related to the level of injury as illustrated in Figure 1. Dyspnea, defined as a subjective report of breathlessness or shortness of breath, is common in people with SCI and is greatest in people with tetraplegia (Ayas et al. 1999).  Approximately two-thirds of the prevalence of dyspnea in this group is attributed to the inspiratory muscle loss (Spungen et al. 1997).  Improved inspiratory muscle strength and endurance could potentially improve cough and maximal exercise ventilation in addition to decreasing dyspnea.  The inspiratory muscles can be trained similar to the limb muscles with inexpensive devices that increase the resistive or threshold inspiratory load on the inspiratory muscles (Reid et al. 2004).  Table 2 outlines common measures that are indicative of respiratory muscle strength and endurance.  In neuromuscular disorders like SCI, maximal lung volumes that measure inspiratory capacity also can reflect increased inspiratory muscle strength.

Table 2 Measures of respiratory muscle strength and endurance

Commercially available hand-held devices can be used for inspiratory muscle training.  The two main types of devices are the resistive and threshold trainers (Figure 3) (see Reid et al. 2004 for details of these training techniques).  Both of these devices have a one-way valve that closes during inspiration so that the subject must breathe through a small diameter hole for the resistive trainer or against a spring loaded valve for the threshold trainer. The one-way valve opens during expiration such that no load is imposed during the expiratory phase of respiration.  Evidence showing decreased dyspnea and improved strength and endurance after IMT is well documented in people with other health conditions like chronic obstructive pulmonary disease (COPD) (Reid et al. 2004; Geddes et al. 2005). 

Two main types of commercially available trainers are used to improve the strength and endurance of the inspiratory muscles.  Both trainers have a one way valve that opens during expiration such that no load is imposed during the expiratory phase of respiration.

Figure 3: Inspiratory muscle trainers

Table 3: Inspiratory Muscle Training

Discussion

A recent RCT provides level 1 evidence (Van Houtte et al. 2008) that respiratory muscle training improves respiratory muscle function and decreases respiratory function.  Previous to this study, most reports were not comparable and could not be combined in a meta-analysis (Brooks et al. 2005) because of research design, heterogeneity of subject characteristics or differences in training techniques.  None of these previous studies that used an RCT design incorporated an optimal IMT protocol.  In particular, several used an inspiratory resistive device with no target to control for decreasing resistance with slower flows so the training methods may have induced an alteration in breathing pattern towards slower inspiratory flows rather than a training response against higher inspiratory pressures.  The few studies that utilized a RCT design also showed improvement in both control (or sham) and training groups.  Comparable improvement in measures of inspiratory muscle and lung function in the control and IMT groups may reflect learning of testing maneuvers, benefit from other rehabilitation or lifestyle activities, and/or natural progression of improvement after SCI. 

The single subject report by Ehrlich et al. (1999), utilized the threshold trainer, which imposes a constant inspiratory load regardless of breathing pattern.   Given that threshold IMT has consistently shown improvements in inspiratory muscle strength and endurance in people with chronic obstructive pulmonary disease, this technique warrants a larger RCT to determine its benefit for people after SCI.

The recent RCT by Van Houtte et al. (2008) clearly provides level 1 evidence that respiratory muscle training has the potential to not only improve respiratory muscle performance but also to dramatically reduce respiratory infections.

Future research to determine a potential treatment effect of IMT after SCI, should utilize:  1) larger samples; 2) a research design that controls for the influence of learning or recovery from SCI on IMT outcome measures of inspiratory muscle strength and endurance, and dyspnea; 3) optimal training techniques of threshold loading, targeted resistive devices, or normocapnic hyperpnea, a form of endurance training of the respiratory muscles; 4) outcomes of inspiratory muscle strength and endurance; dyspnea; quality of life; daily function; 5) a comparison of the effectiveness of IMT relative to or as an adjunct to other rehabilitation interventions. Of equal importance, overly aggressive prescription of IMT has the potential to fatigue and injure the inspiratory muscles, which can increase the person’s predisposition to respiratory compromise.  The article by Reid et al. (2004) provides a table that outlines parameters to monitor during IMT in order to avoid untoward responses such as muscle fatigue and hypercapnia. Parameters include: intensity of load, mode of load, duration, frequency and length of training to ensure adequate training protocol; blood pressure, heart rate, respiratory rate, other signs and symptoms of respiratory distress or inability to tolerate exercise load as signs of exercise intolerance; discoordinate chest wall movement, excessive dyspnea during training, long lasting complaints of fatigue after training sessions to avoid inspiratory muscle fatigue; signs of delayed-onset muscle soreness, reduced strength and endurance to avoid muscle injury; and end-tidal CO2, SpO2 and signs of headache, confusion to avoid hypercapnea (Reid et al. 2004). Van Houtte et al. (2008) provided 48 hours rest after their participants were unable to tolerate an overly intense workload.

For inspiratory muscle training to improve ventilation, decrease dyspnea, and to improve daily function after SCI, parameters to optimize IMT are only available for people with other respiratory conditions.  For people with chronic obstructive pulmonary disease, the optimal IMT protocol should utilize threshold or targeted resistive trainers, at an intensity of 30-70% of MIP, for a duration up to 30 minutes per session, performed continuously or in intervals, 4-6 days/week and be continued indefinitely (Geddes et al. 2006).  Progression of intensity (MIP) should not exceed 5% per week.

Conclusion

• There is level 1 evidence based on an RCT, and  level 4 evidence from several studies to support IMT as an intervention that might decrease dyspnea and improve inspiratory muscle function in some people with SCI

• Respiratory muscle training improves respiratory muscle strength and endurance in people with SCI.