Viviane Soares
Amazonas Street, No. 294, Downtown
Anápolis, Goiás. Zip Code: 75024-080
ftviviane@gmail.com
OBJECTIVE: To evaluate respiratory function and verify whether body mass index (BMI) and physical activity level (PAL) influence muscle strength and lung function in adolescents.
METHODS: Cross-sectional study with 123 adolescents aged 13-18 years. The adolescents were classified into two groups according to BMI. BMI was calculated and the variables measured were: waist circumference, bioimpedance parameters, fat-free mass and body fat. PAL was estimated by a specific instrument, respiratory muscle strength (maximum inspiratory pressure-MIP and maximum expiratory pressure-MEP) by manovacuometry and lung function by spirometry (forced vital capacity-FVC, forced expiratory volume in the first second-FEV 1 and FEV 1 /FVC).
RESULTS: The prevalence of overweight was 26.8%. PAL did not show a significant difference between the groups (p=0.69). MIP (p=0.007) and FVC (p=0.005) were higher in overweight adolescents and the FEV 1 /FVC ratio was lower (p=0.001). Multiple regression showed that BMI and PAL adjusted for sex, age and sexual maturity were predictors of MIP, explaining the relationship by 30% and of FEV 1 /FVC by 9%. BMI was a predictor of FEV 1 (adjusted R 2 = 48%, p=0.001) and FVC (adjusted R 2 = 51%).
CONCLUSION: Overweight adolescents had higher MIP and FVC and the FEV 1 /FVC ratio was lower. It is believed that PAL and BMI directly influence respiratory muscle strength and lung function in adolescents.
▪ FFM = 1.31 + (0.61 x height (cm) 2 / resistance) + 0.25 x body mass (kg) ▪ GC = body mass – FFMRespiratory muscle strength was assessed using a digital vacuum manometer (MVD-300, Globalmed, Porto Alegre, Brazil). Maximum inspiratory pressure (Pi max ) and maximum expiratory pressure (Pe max ) were obtained from residual volume (RV) and total lung capacity (TLC), respectively 14 . The examination was performed with the adolescent in a sitting position, using a nose clip. Inspiratory and expiratory efforts were sustained for 1 second (s). Predicted values were calculated according to Domènech-clar et al. 15 . Spirometry was performed using a portable device (Micro Quark, Cosmed) and disposable mouthpieces specific for adolescents aged 12–18 years. The acceptability and reproducibility criteria were those recommended by the American Thoracic Society/European Respiratory Society 16 . The measurements collected were forced expiratory volume in one second (FEV 1 ), forced vital capacity (FVC) and the FEV 1 /FVC ratio was calculated. The classification of disorders was performed using the lower limit of 70% of the predicted value. Data were expressed as mean, standard deviation, frequency and percentages. The Student’s t-test or Mann-Whitney test were used to compare groups according to data normality. Multiple regression analysis, stepwise method , was used to evaluate PAL and BMI as predictors of respiratory muscle strength and lung function. Models were adjusted for sex, age and sexual maturity. The value considered for p was <0.05. Data were analyzed using the Statistical Package for Social Sciences (SPSS). RESULTS The study had a total sample of 123 adolescents, 55.3% of whom were male and aged between 14 and 18 years. The prevalence of overweight/obesity was found in 26.8% of adolescents. There was no significant difference in PAL between the groups (p=0.69) and, of the eutrophic and overweight adolescents, 19 (57.6%) and 59 (65.6%) were active, respectively. Pi max (p=0.007) and FVC (p=0.005) were higher in overweight adolescents, while the FEV 1 /FVC ratio was lower (p=0.001) (Table 1). The phase angle was the only parameter that did not present a significant difference (p=0.26). Figure 1 shows a comparison of maximum respiratory pressures between overweight and normal-weight adolescents. Pimax , which represents inspiratory muscle strength, was 16.3% higher in overweight adolescents. Only four adolescents in the overweight group had Pimax within the expected range. None of the adolescents evaluated had Pemax within the expected range for their age. Figure 2 shows the spirometry parameters. When comparing the groups, lung function showed FVC 11.6% higher in overweight adolescents, while the ratio was 5.5% lower. Two eutrophic adolescents and one overweight adolescent presented mixed ventilatory disorder. The PAL and BMI were tested in multiple regression as predictors of respiratory muscle strength and lung function (Table 2). The two parameters together, adjusted for sex, age and sexual maturity, were predictors of Pimax , explaining the relationship by 30% and the FEV1/FVC ratio was explained by 9%. BMI was a predictor of FEV1 ( adjusted R2 = 48 %, p=0.001) and FVC ( adjusted R2 =51%, p<0.001), while PAL was a predictor of Pemax ( R2 = 41%, p=0.02) and %FEV1 ( R2 = 10%, p=0.008). DISCUSSION This study showed that overweight adolescents had higher Pimax and FVC when compared to eutrophic adolescents, while the FEV 1 /FVC ratio was lower. Most adolescents had respiratory muscle strength below the predicted level. Regarding lung function, only two eutrophic adolescents and one overweight adolescent had mixed respiratory disorder. PAL and BMI were shown to be direct predictors of respiratory muscle strength and lung function, and BMI was an inverse predictor of the FEV 1 /FVC ratio. Pimax was higher in overweight adolescents. This result was also shown in a study that compared eutrophic and overweight individuals 6 . However, there is evidence that Pimax is similar in both groups 8,17 . The higher Pimax in overweight adolescents can be explained by biomechanical and physiological factors in which the distribution of body fat in the abdominal region would be the main mechanical factor 9 . However, in the present study, even with obese individuals presenting higher WC, the majority presented measurements within the predicted range for age and sex. It is worth noting that in this age group, the reduction in compliance and consolidation of the ribs do not yet have an influence on respiratory mechanics 9 . Regarding the assessment of lung function, this is essential to detect changes in airway conduction and the restrictive barrier imposed by obesity. FVC was higher, while the FEV 1 /FVC ratio was lower in obese adolescents. There are similar results in the literature, in which overweight Thai children and adolescents tend to have a lower FEV 1 /FVC ratio, which is consistent with airway obstruction 17 . In Canadian adolescents with and without excess weight, no differences in lung function were found 7 . And even though the obese adolescents in this study had higher FVC and lower FEV 1 /FVC ratio, only three adolescents were diagnosed with mixed ventilatory disorder (two eutrophic and one overweight), showing that changes in lung volumes only occur in people with extreme BMI (>45 kg/m 2 ), but who remain with normal partial oxygen pressure 18 . The level of physical activity assessed in min/week showed no significant difference among the adolescents, a fact also observed in another study carried out with children/adolescents 8. However, when the level of physical activity was used as a predictor of respiratory function, together with BMI, it influenced respiratory muscle strength, FEV 1 and FEV 1 /FVC ratio. BMI is the most widely used parameter to assess excess weight and is considered a general marker of health, showing a positive relationship with Pi max , FVC and FEV 1 and a negative relationship with the FEV 1 /FVC ratio. These findings corroborate studies found in the literature 7,8,19 that emphasize that at this stage of life the body is able to adapt to the condition of excess weight. However, with advancing age it is known that this relationship (excess weight and respiratory function) becomes inversely proportional and special attention is needed to food consumption and physical activity as strategies to prevent future problems. The strengths of this study are related to the study of respiratory function in adolescents, since the number of studies involving this population and the condition of excess weight is limited in the literature. Adolescents diagnosed with asthma were excluded, which made it possible to avoid selection bias due to the fact that this clinical condition already presents, to a greater or lesser degree, airway obstruction. Another point to be highlighted was the possibility of verifying the influence of BMI and PAL on respiratory muscle strength and lung function. It is important to emphasize that BMI is a general marker of obesity and it is possible that the distribution of body fat can influence respiratory function. Thus, other markers such as body fat percentage and blood levels of cholesterol and its fractions can be used to fill this gap. CONCLUSION Based on the study carried out, it is concluded that overweight adolescents had greater inspiratory force and FVC when compared to eutrophic ones, when the FEV 1 /FVC ratio is lower. Most adolescents had respiratory muscle strength below that predicted for age, body mass and height. Regarding lung function, only two adolescents had mixed ventilatory disorder. It is believed that PAL and BMI are directly related to respiratory muscle strength and lung function in adolescents and that BMI was a negative predictor of the FEV 1 /FVC ratio.
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