Neuromuscular disease (NMD) is characterized by progressive weakening of skeletal, respiratory, and or bulbar-innervated muscles.1 NMD patients have decrements in vital capacity (VC), lung and chest wall compliance, and coughing capacity. These patients cannot fully expand their chests, which leads to stiffening of the joints and tissues of the rib cage.1,2 This in turn results in a reduction in chest wall compliance.2 Reduction in lung and chest wall compliance may also be related to alterations in the elastic properties of lung tissues by chronically limited range of activity.3 Normal breathing consists of varying tidal volumes with intermittent deep breaths or sighs.1,4 Even in people with normal lungs, periodic hyperinflation is required to prevent closure of lung units.1 For NMD patients, who cannot expand the lungs fully due to respiratory muscle weakness, periodic deep insufflation by air-stacking is essential.1

Air stacking involves the use of a manual resuscitator or a volume-cycled ventilator to deliver volumes of air that are consecutively held by glottic closure until no more air can be retained.5 The maximum lung volume that can be held by air stacking is the maximum insufflation capacity (MIC).6 In 2000, Kang and Bach1 demonstrated that the MIC could significantly exceed VC for patients with NMD. However, patients presenting weak or no glottic closure due to bulbar muscle dysfunction or indwelling tracheostomy cannot air stack. Lung insufflation can only be given by bypassing glottis dysfunction in these patients, and the maximum passive lung insufflation volume achieved in this manner is defined as the lung insufflation capacity (LIC).7

In this study, we designed a device that provides external control of an artificial glottic opening and closure for patients with bulbar innervated muscle weakness or who have indwelling tracheostomy tubes. We attempted to investigate the effectiveness of air stacking by replacing dysfunctional glottises with the newly designed device.

Thirty-seven patients (24 males, 13 females; mean age, 48; range, 20-76 years) met the criteria, and were included in this study. Mean±standard deviation values of all the patients totaled were 794±391 mL for VC, 1,084±260 mL for MIC, and 1,668±493 mL for LICA measurements.

When candidates were divided by diagnosis (Table 1), only 6 from the ALS group and 1 from the cervical spinal cord injury group could somewhat air stack. The remaining 30 patients failed to air stack due to glottic dysfunction, thus MICs were checked as zero. AEGD either maximized the ability to air stack or enabled passive lung insufflation even for those who initially failed. Consequently, LICA significantly exceeded the MIC for all of the candidates (p<0.05).

In accordance with underlying pulmonary function, 37 patients were subdivided into groups 1, 2 or 3 (Table 2). Group 1 included three patients who presented severe inspiratory and expiratory muscle weakness with tracheostomies. Since their respiratory muscles were too weak to even take a deep breath in and out, VCs and MICs were essentially 0 mL. Group 2 included 27 patients whose MICs were initially 0 mL. After applying the AEGD, LICAs were successfully measured in both groups. The other 7 patients who had the minimal residual glottic function to hold and stack a little volume of air were named as Group 3. Although MIC-VC difference was greater than 0 mL, when using AEGD, the LICA was greater than MIC or VC in this group (p<0.05).

Just as limb range of motion helps avert limb contractures, assisted lung insufflations help to maintain compliance for patients with diminished VCs.1 However, NMDs with glottic dysfunction and/or tracheostomized patients can not benefit from air stacking, because they cannot prevent leakage of inhaled air from the lungs.8,9 Besides the inability to maintain pulmonary compliance, these patients also present difficulty with coughing due to respiratory muscle weakness and/or glottic dysfunction. Thus effective cough mechanisms can be hindered, which in turn results in the failure of eliminating airway secretions.10,11

Bach had already defined a simple technique to provide deep lung insufflations, and called the deep passive lung insufflations attained this way as LIC.7 He used a manual resuscitator connected to a spirometer, and simply occluded the expiratory port of the spirometer with fingers while providing an additional volume of air with the manual resuscitator bag. Through this method, he was not only able to give deep lung insufflations, but also quantified the volumes.

In this study, on the other hand, we developed a clean, mechanical device to substitute for dysfunctional glottises. Despite the methodological difference, our handheld device is conceptually identical to Bach's simple method in providing deep lung insufflations and quantifying them. Although Bach's method is convenient in that does not require any device, AEGD is a clean technique that might provide more precise control and measurement of the glottic function by preventing possible air leaks. In addition, quantification of coughing ability was also possible with AEGD by changing the measurement apparatus to a peak flow meter. Recruited candidates are another distinguishing factor. Although the previous study7 presented a larger number of patients, the majority of the patients had relatively intact bulbar muscle function whose ability to air stack was spared. However, in this study, air stacking was essentially impossible in more than two thirds of the patients. About 70% of the candidates had tracheostomy tubes, and the remaining patients were bulbar-dominant ALS who showed severe glottic dysfunction.

Limitations of this study include the fact that in the process of connecting the AEGD with measuring instruments, the value could be underestimated, because the exhaled air passes the connection system at a right-angled. Consequently, the fully assembled apparatus for measurement had a relatively long pathway for expiratory flow. Speed of the flow can also be a problem. Thus, measured MICA values may actually be greater than those reported in this study.

In conclusion, NMD patients need to practice regular deep lung insufflations. The artificial external glottic device enables passive lung insufflations even for patients with severe bulbar-innervated muscle impairment or tracheostomy tubes, whose ability to air stack is poor.