# Maximal dynamic inspiratory pressure: S-Index prediction values and diagnosis accuracy

## TL;DR

This study establishes reference values for the S-Index, a measure of inspiratory muscle strength, and explores its diagnostic potential in identifying muscle weakness.

## Contribution

The first reference equations for the S-Index in a Brazilian healthy adult population and its novel application in diagnosing inspiratory muscle weakness.

## Key findings

- The S-Index strongly correlates with FEV1 and FVC, with age, sex, weight, and height explaining 62% of its variation.
- The S-Index shows potential for ruling out inspiratory muscle weakness in both men and women, particularly when below the lower limit of normal.
- Prediction equations for the S-Index performed well in distinguishing between COVID-19 survivors with different MIP levels.

## Abstract

To establish reference values and prediction equations for the strength index (S-Index), in order to meet the growing demand for clinical application and diagnostic understanding of maximal dynamic inspiratory pressure.

This was a prospective study of 120 healthy subjects between 18 and 80 years of age. The S-Index, measured from RV to TLC after at least eight reproducible maximal maneuvers with a < 10% difference, was obtained. The MIP was also measured, and differences between S-Index and MIP values were analyzed. A multiple linear regression model estimating the S-Index value was based on clinically significant independent variables. For model cross-validation and diagnostic accuracy, we used a separate sample of COVID-19 survivors to compare the observed and predicted S-Index values.

The S-Index strongly correlated with the FEV1 and FVC. However, sex, age, weight, and height retained their significance in all final models, collectively explaining 62% of the variation in the observed values. The performance of the prediction equation was satisfactory in suggesting differences between COVID-19 survivors with an MIP < 80 cmH2O and those with an MIP ≥ 80 cmH2O. For both sexes, the S-Index exhibited the potential for ruling out, rather than confirming, inspiratory muscle weakness. If below the lower limit of normal, further evaluation is important, especially in men.

To our knowledge, this is the first set of reference equations for the S-Index based on a healthy adult population across various age groups in Brazil. Its potential as an adjunct index in evaluating inspiratory muscle strength was also explored for the first time.