Introduction

Cough is an important host-defense mechanism, because it clears foreign material and secretions from the airway and prevents aspiration of food and fluid [1–3]. A voluntary cough consists of three phases, inspiratory, compressive, and expiratory [2]. A cough cannot be effective if even one of these phases is defective [4].

The cough peak flow (CPF) is an index of voluntary cough strength [5]. CPF values are affected by several factors, including lung volume and respiratory muscle strength, [4, 6–12] thorax expansion which is correlated with lung volume, [13, 14] and posture [15, 16]. In addition, a study of the effect of different body positions on CPF found that values were significantly higher for the standing position than for the supine position. Therefore, the standing position is encouraged for patients needing to cough to clear respiratory secretions [15]. However, elderly individuals and seriously ill patients who lie in the supine position may not be able to change their body position and produce an effective cough. Furthermore, pressure relieving mattresses have been recommended for preventing the development of pressure ulcers; [17, 18] and the interface pressure of the mattress has been usually set on low for elderly individuals or patients whose overall status has deteriorated [17, 19]. A pressure relieving air-mattress that is set on low leads to increased surface area of the body in contact with the mattress and decreased pressure on the skin in contact with the mattress. Therefore, clarifying whether or not pressure relieving air-mattress firmness levels affect the CPF of individuals in the supine position is important. To our knowledge, there have not been any published reports on the relationship between pressure relieving air-mattress firmness levels and the CPF.

In this study, we clarified whether or not the CPF of healthy volunteers in the supine position was affected by different firmness levels of a pressure relieving air-mattress.

Materials and Methods

Measurement of body sinking distance

To quantify the amount the body sank into the pressure relieving air-mattress, the sinking distance was measured without any activity at three points on the body: the left lesser tubercle of the humerus, left anterior superior iliac spine (ASIS), and left patella. A marker was placed at each bony protuberance, and the distance from the edge of the bed to the marker was measured with the pressure relieving air-mattress in the “soft” and “hard” mode, by a three-dimensional motion analysis system (ICpro-2DdAF for Windows, Hu-Tech, Tokyo, Japan). The difference in the sinking distances of the “soft” and “hard” modes was calculated using the formula that follows.

Results

The characteristics and spirometric parameters of the participants were as follows: age 23.6 ± 5.3 years, height 171.5 ± 6.2 cm, weight 64.1 ± 9.7 kg, body mass index 21.8 ± 3.0 kg/m², percent body fat 18.6 ± 5.5%, percent of predicted forced vital capacity 96.2 ± 15.9%, percent of predicted forced expiratory volume in 1 s 92.7 ± 16.2%.

Levels of the inner pressure of the pressure relieving air-mattress in the “soft” mode were significantly lower than pressure levels in the “hard” mode (soft 25.0 ± 1.9 hPa vs. hard 35.0 ± 1.9 hPa, P < 0.001). Table 1 shows the parameters related to CPF values at the 2 different pressure relieving air-mattress firmness levels. The values of all parameters for the “soft” mode were significantly lower than those for the “hard” mode.

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Table 1. Cough parameters measured at two different pressure relieving air-mattress firmness levels.

https://doi.org/10.1371/journal.pone.0146714.t001

Fig 1 shows the differences in the sinking distances of the participants’ body between the pressure relieving air-mattress in “soft” and “hard” mode at three points on the body. The differences in the sinking distance at the left lesser tubercle of the humerus, left ASIS, and left patella were -2.7 ± 1.0 cm, -4.2 ± 1.4 cm, and -2.1 ± 0.8 cm, respectively. The differences in the sinking distances between the three points were statistically significant.

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Fig 1. Differences in the sinking distances of three points between “soft” and “hard” mattress modes.

ASIS, anterior superior iliac spine. *: P < 0.05; †: P < 0.001.

https://doi.org/10.1371/journal.pone.0146714.g001

Discussion

In this study, we studied whether pressure relieving air-mattress firmness levels affected the CPF of young healthy volunteers. The CPF, FVC, PEmax and PImax values of the participants with the pressure relieving air-mattress in the “soft” mode were significantly lower than the values with the pressure relieving air-mattress in the “hard” mode. These findings indicate that a harder mattress might be associated with a higher CPF, which is important for host defense.

To the best of our knowledge, this is the first study that demonstrated that cough strength was affected by different pressure relieving air-mattress firmness levels. To clarify the mechanisms involved in our CPF findings, we assessed several parameters associated with the production of cough with relation to the pressure relieving air-mattress firmness levels. The inspiratory phase of cough is affected by an increase in lung volume, which is produced by thoracic expansion and related to the strength of the inspiratory muscles [4, 14]. The compressive and expiratory phases of cough are affected by an increase in the intrathoracic pressure, as well as the strength of the expiratory muscles [2]. In this study, we measured the FVC, PImax, and PEmax, which are indices of lung volume, inspiratory muscle strength, and expiratory muscle strength, respectively.

The pressure relieving air-mattress firmness levels might change the normal curvature of the spine, which could affect the expansion of the thorax, lung volume, and degree of force exerted by the inspiratory and expiratory muscles. Yamamoto et al [21] reported that lower pressure levels of a pressure relieving air-mattress led to deeper immersion of the lumbar spine and buttocks with spinal curvature in healthy adults. Culham et al [22] reported that women with accentuated kyphosis of the spine demonstrated significantly lower vital capacity, lower total lung capacity, and decreased lateral expansion and vertical excursion of the ribs, compared to healthy women of the same age. Furthermore, two studies demonstrated that lung volume is correlated with both PImax and PEmax levels [23, 24]. In our study, the difference in the sinking distances of the ASIS measured at the “soft” and “hard” modes were larger than the differences for the lesser tubercle of the fumerus and patella. The FVC, PImax and PEmax values of the participants with the pressure relieving air-mattress in the “soft” mode were significantly lower compared to the values with the pressure relieving air-mattress in the “hard” mode. These results suggest that the deeper immersion of the lumbar spine and buttocks with accentuated spinal curvature of participants with the pressure relieving air-mattress in the “soft” mode might limit the expansion of the thorax, as reflected by the decreased FVC and PImax levels, and which resulted in lower PEmax.

The results of this study suggest that pressure relieving air-mattress firmness levels may affect the parameters associated with production of cough, including lung volume, inspiratory and expiratory muscle strength, and ultimately, the CPF.

Study Limitations

The limitations of this study were as follows: first, our study participants were healthy young men; and second, we did not assess the details of the participants’ posture, such as spinal curvature. Studies that better delineate and assess such parameters as posture are needed for further evaluation of the effects of different pressure relieving air-mattress firmness levels on the cough strength of elderly individuals and patients. Such studies could provide information that aids in the selection of pressure relieving air-mattress firmness levels suitable for the elderly or severely ill patients, and suitable for pulmonary rehabilitation.

Conclusions

This study of healthy young men in the supine position, who produced a voluntary cough, showed that the CPF, which is an index of cough strength, may be affected by different pressure relieving air-mattress firmness levels. The results suggest that a harder mattress may result in increased CPF values.

Acknowledgments

We thank all the participants who volunteered to take part in this study.