In Brazil there are few published studies on the reference values of maximal respiratory pressures. The first publication was written by Camelo Jr et al.,1 who, in 1985, was a pioneer in the field by suggesting maximal inspiratory (MIP) and expiratory (MEP) pressure values for the Brazilian adult population. A few years later, Neder et al.2 proposed predicted values of MIP and MEP, which were later questioned by Parreira et al.3 These authors reported different results in subsequent studies, suggesting that the use of the predictive values published by Neder et al.2 were not applicable to the target population. More recently, Costa et al.4 recommended new equations for the determination of MIP and MEP, and proposed that their findings could contribute to the study by Neder et al.2 by predicting, in particular, inspiratory muscle strength.

The previous studies1–4 that formulated the predicted values for the Brazilian population used an aneroid manometer to measure respiratory muscle strength. Only one single Brazilian study5 recently reported the reference values using a digital manometer. It is worth mentioning that currently the digital manometer is widely used in the clinical practice of respiratory therapy, and most importantly, the predicted values formulated with the equations developed with the aneroid manometer do not always match the results obtained with a digital manometer. In addition, the interpretation of the pulmonary function of a given individual is based on data comparisons obtained with healthy subjects, and do not take into account the effects of specific disfunctions the patients may have. As there are no specific equations for people with certain diseases, the results may not always correspond to those observed in clinical practice.

Given the lack of reference equations for patients with type 2 diabetes mellitus (DM2), in addition to the fact that there has only been one single publication conducted with a digital manometer, the objective of this study was to compare, in a population of patients with DM2, the MIP and MEP values obtained with a digital manometer against the values predicted by the equations proposed for the healthy Brazilian population, which were based on evaluations conducted with an aneroid manometer. Additionally, this study proposes new equations to determine the predicted values of MIP and MEP for patients with DM2 based on evaluations of the respiratory muscle strength conducted with a digital manometer. The hypothesis of this study was that the values obtained in patients with DM2 with a digital manometer differ from the values provided for the healthy population which were based on evaluations conduced with an aneroid manometer, and that new equations should therefore be formulated.

The values of MIP and MEP were measured in 219 patients with DM2, aged under 75, living in the urban area of Ijuí/Brazil, with calibrated digital manometer MVD-300 (Microhard System, Globalmed, Porto Alegre, Brazil). All participants underwent at least six, technically acceptable, efforts of maximum inspiration and expiration. For data analysis, the highest value was recorded, as long as there is not more than 10 percent of difference compared to the second highest value.

Among the patients studied, 64.8 percent were female, average age of 61.4±9.5 years old and body mass index of 30.9±5.9kg/m2. The average time of DM2 diagnosis was 7.4±6.7 years, ranging from 0.5 to 40 years and the average fasting blood glucose was 126.4±53.1mg/dL, with minimum value of 55.0mg/dL and maximum of 219.0mg/dL. The average of maximal respiratory pressures achieved and predicted in male and female are recorded in Table 1.

When comparing the difference between the predicted and achieved values of MIP and MEP, there was a statistically significant difference between them, in both sexes. In post hoc Bonferroni analysis, in men, the value predicted by Costa et al.4 is higher than PImax achieved by patients (p<0.0001) and it is higher than equation values predicted by Neder et al.2 (p<0.0001). The Neder et al.2 values were similar to those achieved by the patients (p>0.05). In women it was found that the value predicted by Costa et al.4 equation was statistically lower than that achieved by patients (p<0.0001). The values predicted by Neder et al.2 equation are closer to the value found in the test, but they are significantly different when compared to the scores obtained by the patients (p=0.021) (Table 1).

Regarding the MEP, the values proposed by Costa et al.4 equation showed equivalent numbers to those achieved by patients, with no significant difference between the sexes (p>0.05). On the other hand, the Neder et al.2 equation shows a considerably higher predicted value than the value achieved by patients, for both sexes (p<0.0001). There was a significant difference between the values predicted by both Costa et al.4 and Neder et al.2 equations (p<0.001) (Table 1).

The Pearson correlation coefficient was used to determine which variables best explained the maximal respiratory pressures values. For men, negative correlations were observed between MIP and age (r=−0.374; p=0.001) and between MEP and age (r=−0.379; p=0.001) and positive correlations between MEP and body mass (r=0.340; p=0.003). For women, there was a negative correlation between MIP and age (r=−0.216; p=0.021) and MEP and age (r=−0.192; p=0.027).

Based on the linear regression model, gender, age and body mass were considered predictor variables to determine the MEP for men. Table 2 describes the equations for MIP and MEP which have been proposed for patients with DM2 from the digital manometer. The equations obtained the following coefficients of determination (R2) and standard error of the estimate (SEE) for men: MIP (R2=14.0, SEE=39.1); MEP (R2=21.9, SEE=27.7) and for women: MIP (R2=4.7; EPE=27.1) MEP (R2=3.7; EPE=27.5). There is no collinearity between predictor variables MEP in men (tolerance:0.966; inflation factor of variance:1.035).

To the best of our knowledge, it is the first study to propose equations to obtain reference values for patients with DM2 of the population, and the second one to establish reference values for the maximal respiratory pressures from digital manometer measuring. Thus, the results of this study may contribute to the prediction of maximal respiratory pressures in patients with DM2, in particular, measurements by digital manometer. It is important to point out that further studies with patients from different regions of Brazil and with different characteristics are still needed, so that they can formulate new tables or equations for determining the maximal respiratory pressures in patients with DM2.