DISCUSSION
In this study, the success rate observed in children and adolescents evaluated was 92.3%. This shows that the respiratory muscle strength test is a simple resource, easy to apply and highly feasible in this age group. Further, it is a non-invasive method that supplies important information about the function of the respiratory muscles implicated in the breathing process, that are often involved in and/or at a mechanical disadvantage under several conditions or clinical situations, as in lung and neuromuscular diseases.4,5
To the best of our knowledge, this is the first study aiming to determine the success rate of the respiratory muscle strength test in different age groups of the pre-school and school population, which makes it difficult to compare the values found in those of other studies. In any case, considering other lung function tests, we believe that the success rate found in this study is high. Possible contributing factors include a higher level of explanation to the subjects evaluated, a longer measurement time and patience of the evaluator responsible for performing the tests.10Differently from spirometry, in which specific standards and guidelines were developed to allow its use in younger age groups,17 to evaluate the respiratory muscle strength in this study it was not necessary to use any methodology different from that published by the guideline,4,13 suggesting that it is not difficult to perform the test in these age groups in the clinical practice. When we compared our results to the success rates of the spirometry test, the respiratory muscle strength test is also easy and simple, since the spirometry success rates are close to the results shown here, around 85%.18-21Additionally, a review of recent literature has shown that although the preschool age group is characterized as an extremely young sample, with its difficulties and motor limitations, the success rate of the spirometry (regardless of previous experience) ranged between 71% and 92%, demonstrating the feasibility of pulmonary evaluation in this age group.22 Although both tests have different methodologies, objectives and functionality, the comparison with a well-known and extensively studied lung function test may indicate a parameter until further success rates are generated for the evaluation of pediatric respiratory muscle strength.
As expected, the success rates of the respiratory muscle strength test appear to increase with the age of the children evaluated. In addition, when compared to the variable sex, no significant difference was found. These findings are similar to those found in various studies that evaluated the lung function success rates in the pediatric population, showing that as the children grow older, the success rate also rises.21,23,24 Previous studies that aimed at generating normality values for the respiratory muscle strength test ascribe the failure of the technique to the difficulty in understanding, low reproducibility and lack of cooperation by the children in this age group.13,14 However, these studies have not demonstrated success rates separately by age groups, hindering a better knowledge and understanding of the age as an influencing factor on this outcome. In this sense, several studies that evaluated the frequency of success of the spirometry test have shown that, in preschoolers, the difficulty of obtaining an adequate test may be explained as being due to reduced attention, easy distraction, difficulty in understanding, motor coordination, and low tolerance for frustration during maneuvers. Besides, the child's emotional stage and stage of development are important factors in determining the success of the test.18,25
Although the present study does not show information about the previous respiratory history of the subjects included, it is believed that those factors would not influence the ability of an individual to perform the test, which was the main outcome of the study. Also, it is unlikely that severe respiratory conditions that could possibly alter the performance of the test would be present in such a young, healthy sample, recruited at a school environment, and with normal nutritional status. On the other hand, even if that is possible, previous studies26,27 have shown that mild to moderate asthmatic subjects do not present respiratory muscle strength reduction, which is usually associated with more severe lung disease, including the presence of hyperinflation and poor nutritional status.28,29
One of the limitations of this study is that the test is performed by an experienced evaluator, who is interested in achieving as much success as possible in the tests, which may have contributed to the high success rate found in this sample. However, these findings only strengthen the idea that it is possible to perform this test in the pediatric age range, and that at least in part, achieving success depends not only on the children and/or adolescents evaluated, but also on the effort and stimulus supplied by the evaluator. In this sense, a previous study30 recently demonstrated that the quality of spirometry tests increased from 57% to 83% after the inclusion of a training program conducted by telemedicine in 15 participating centers. New studies concerning the frequency of success in the pre-school age group should be encouraged to obtain new information about the behavior of success rates in this age group, enabling comparisons among studies and helping to achieve greater use of this resource among the child population.
In brief, the respiratory strength test presented a high rate of success among the pre-school and school population evaluated. Besides, the success rate appears to increase with age. The findings of this study show that the respiratory muscle strength test is a simple and easy to apply resource. Thus, obtaining an adequate test may ensure the quality of the measurements and consequently, success in the control and follow-up of the outcome of different respiratory involvements.