Inspiratory muscle training: where are we?

Inspiratory muscle dysfunction has been shown to be related to hypercapnia and hence, to ventilatory insufficiency in patients with chronic obstructive pulmonary disease (COPD) [1]. Accordingly, treatment of inspiratory muscle dysfunction has gained interest in the last decade. Among other treatment modalities, such as pharmacotherapy and respiratory muscle rest, training of the inspiratory muscles is frequently applied in clinical practice, but its benefits largely remain a matter of debate [2]. Since LEITH and BRADLEY [3] showed increases in strength and endurance after training of inspiratory muscles in healthy subjects, there has been a substantial number of studies addressing the effects of inspiratory muscle training in various disorders, including: neuromuscular diseases, COPD, weaning failure, kyphoscoliosis and thoraco-abdominal surgery. These disorders all have weakness of inspiratory muscles in common and hence, are frequently associated with ventilatory failure at rest or during exercise. Although a large number of studies have been published, only a few of these studies have a randomized-controlled design. The results from other studies should be considered preliminary and interpreted with great caution. In general, most studies show improvements in strength or endurance of the inspiratory muscles [4–11]. The transfer to outcome variables relevant to patients, such as reduced dyspnoea, improved exercise tolerance, better performance of activities of daily living (ADL), reduced postoperative pulmonary complications, or successful weaning from mechanical ventilation, is less obvious. Two types of training, i.e. "normocapnic hyperpnoea" (NCH) and "inspiratory resistive training" (IRT) are practiced at the present time. During NCH the patient is required to achieve a supernormal target ventilation for 15–20 min, while PaCO2 is kept constant [3]. Conceptually, NCH seems an appropriate technique to train the respiratory muscles since hyperpnoea also occurs during exercise. The equipment for this type of training is complicated and not available for home treatment. Therefore, NCH is difficult to apply on a large scale and consequently it will not be further discussed in the present editorial. During IRT the patient inspires through a mouthpiece with a two-way valve and a resistance in the inspiratory line. This resistance is usually flow-dependent. Appropriate training intensity is only achieved if an adequate target pressure is obtained [7]. A flow-independent resistance was also developed, a "threshold load". In this system a valve is opened when a critical mouth pressure is reached [12]. Strictly, this type of loading requires build up of negative pressure before flow occurs, and hence, is inertive in nature. Whether resistive loading or this inertive loading produces different training effects, remains to be studied. In patients with tetraplegia, improvements in strength and endurance of inspiratory muscles were suggested in an uncontrolled study [13]. ESTRUP et al. [14] and recently WANKE et al. [11] showed improvements in strength and endurance of the inspiratory muscles in patients with progressive muscular dystrophy. Only the latter was a controlled study. Inspiratory muscle training was further studied in two cases of kyphoscoliosis and both improvements in strength and endurance were reported [15]. Inspiratory muscle training was also applied in surgical patients. GROSS [16] observed improvements in endurance time and lung function (forced vital capacity and maximum voluntary ventilation) in cardiac patients who had trained their respiratory muscles prior to surgery in comparison with a control group. In addition, the postsurgical duration of mechanical ventilation was significantly less. Training of inspiratory muscles during weaning in ventilator-dependent hypercapnic patients was studied in an uncontrolled design, showing improvements in strength and endurance of the respiratory muscles [17]. It was suggested that these improvements were related to the ability to wean more than half of the patients to spontaneous breathing or nocturnal ventilation. Most studies deal with respiratory muscle training in the rehabilitation of COPD patients. Specific effects of inspiratory muscle training such as an increase in maximum inspiratory pressure (PImax), were found in five, statistically valid studies in which training intensity was carefully controlled [4–8]. The endurance capacity of the inspiratory muscles improved in a number of studies, also suggesting a specific treatment effect [8, 9, 18]. Despite the improvements in strength of the inspiratory muscles in the above-mentioned studies, improvement in exercise capacity, a transfer effect, was only observed in two statistically valid true experiments, i.e. controlled randomized studies [4, 8]. Improvements in exercise capacity with IRT alone were generally smaller than with exercise conditioning and IRT combined [4, 5]. In this issue of the European Respiratory Journal, the study by WANKE et al. [19] supports this finding. They observed EDITORIAL