Influence of water immersion on the airway impedance measured by forced oscillation technique
Introduction
Water immersion causes acute changes in the pulmonary physiological systems. Previous studies have reported that water immersion cause decreased respiratory function such as forced vital capacity (FVC), forced expiratory volume in one second (FEV1), and maximal respiratory muscle pressure (Buono, 1983; de Andrade et al., 2014). Therefore, these findings suggest that compared with the dryland condition, water immersion places an extra load on respiratory system and may be used as a therapeutic method for improving the respiratory function.
Previous research has reported the following two mechanisms that water immersion can directly and indirectly influence the respiratory system (e.g., airways, alveolars, and respiratory muscles). First, the increase of hydrostatic pressure on the abdominal and chest cavities induces vertical diaphragm displacement and restricts the thorax (e.g., alveolar) expansion. As a result, respiratory muscles contract to overcome the force of water pressure (Agostoni et al., 1966). Second, hydrostatic pressure during water immersion shifts blood from the lower extremities to the chest cavity and increases venous return and central blood volume (Carter et al., 2014), which stiffens pulmonary tissue and decreases lung compliance (Pendergast and Lundgren, 2009). While there have been many reports of the effects of water immersion on respiratory muscles and lung compliance, the effects of water immersion on the airway are not well understood due to methodological limitations. For example, FEV1, a clinical index of airway obstruction measured by spirometry, decreases with clavicle water immersion. Water immersion at the neck level has also been shown to increase closing volume when compared with dryland (Bondi et al., 1976). However, spirometry data are difficult to evaluate airways independently because it provides a comprehensive index of the respiratory system including respiratory muscle strength and lung compliance. In addition, closing volume may reflect decreased alveolar contractility rather than an airway obstruction (Kitaoka and Kawase, 2007). Moreover, these studies have not examined the effect of shallower immersion levels. The water depth of public swimming pools is set at 1.1–1.5 m and aquatic therapy and exercise are performed at such shallower water level. Therefore, further study is necessary to determine the effect of shallower water immersion on respiratory system.
Forced oscillation technique (FOT) is an airway evaluation method that can solve the limitations of conventional spirometry measurements. FOT measurement does not require the breathing effort used in spirometry tests and can exclude the effects of respiratory muscle and lung compliance. FOT measures the impedance of the respiratory system from the spectral relationship between the resultant pressure and airflow. The impedance is composed of respiratory system resistance (Rrs) and reactance (Xrs). Rrs reflects the resistance influenced by airway caliber and Xrs shows elastic and inertial properties of the lung, which reflects an abnormality in lung parenchyma or airways (Shirai and Kurosawa, 2016). In the bronchodilator reversibility test, the Rrs reflects airway reversibility more sensitively than FEV1 when using a spirometer (Niimi et al., 2003). The frequency dependency of Rrs is presumed to reflect ventilation inhomogeneity with peripheral airway lesions (Grimby et al., 1968). Therefore, the FOT is useful evaluation method to reflect the airway resistance during immersion.
The aim of this study was to evaluate the effects of water immersion with different water depths on airway impedance. We hypothesized that the deeper water immersion may present higher airway impedance than shallower water immersion.
Section snippets
Participants
Eleven healthy young men were recruited (23.2 ± 1.5 years old) in this study. The participant’s average heights and weights were 171.7 ± 3.6 cm and 65.5 ± 4.7 kg, respectively. The average body mass index (BMI) was 22.3 ± 1.6 kg/m2. All participants had no presence or history of pulmonary, cardiovascular, or neuromuscular disease, medication use, current or a history of smoking. This study was carried out in accordance with the Declaration of Helsinki and was approved by the Research Ethics
Results
Due to technical problems erroneous data, such as Fres = 0, were shown in two subjects. Therefore, nine subjects were analyzed for Fres, X5 and ALX.
Fig. 1 shows the response of respiratory impedance to water immersion. R5 exhibited significant condition differences (F = 9.882, P = 0.001), and was significantly higher in the water immersion conditions (CL and XA) than DL condition (P = 0.005 and P = 0.024, respectively). R20 exhibited significant condition differences (F = 5.400, P = 0.013).
Discussion
The main findings from this study are as follows. Respiratory impedance (R5, X5 and Fres) was significantly higher during water immersion above the xiphoid appendix than in the dryland condition. These findings suggested that even a water immersion at xipohid level used in public swimming pool may place a load on respiratory system. Below we discuss the potential mechanisms and clinical implications of our findings.
Funding
This work was supported by the Japan Society for the Promotion of Science [grant numbers 20K11303, KW]. The data in this study are presented clearly, honestly, and without fabrication, falsification, or inappropriate data manipulation.
Declaration of Competing Interest
The authors declare that they have no potential conflict of interest.
Acknowledgements
The authors would like to thank all participants who took in this study.
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