Respiratory Muscle Strength Assessment

Normal Values and Measurement Techniques

The American Thoracic Society recommends widely accepted lower limits of normal for Maximum Inspiratory Pressure (MIP) as 75 cm H2O for men and 50 cm H2O for women, and for Maximum Expiratory Pressure (MEP) as 100 cm H2O for men and 80 cm H2O for women 1
Maximum static transdiaphragmatic pressure (PI,di,max) has a mean of 108 cm H2O for men and 65 cm H2O for women, and transdiaphragmatic pressure during sniff (Pdi,sn) has a mean of 148 cm H2O for men and 121 cm H2O for women 2
For proper testing protocol, a patient should be seated with a cylindrical mouthpiece, incorporating a small leak to prevent glottic closure, and MIP should be measured at or near residual volume (RV) while MEP should be measured at or near total lung capacity (TLC) 3, 4, 5
Each effort should be maintained for at least 1-1.5 seconds, and the maximum value of three maneuvers that vary by less than 20% should be recorded 4, 5

MIP values numerically less than 70 cm H2O in males or 60 cm H2O in females suggest significant inspiratory muscle weakness, and Pdi,sn values less than 100 cm H2O in males and 80 cm H2O in females suggest clinically significant diaphragm weakness 2
Respiratory muscle testing should be considered for patients at increased risk for severe COVID-19 outcomes and those with persistent dyspnea following acute COVID infection 6
COVID-19 can affect respiratory muscle performance through various mechanisms, potentially explaining persistent dyspnea in some patients despite undamaged lung parenchyma 6

MIP and MEP measurements are highly dependent on patient effort, have a significant learning curve, and are static measurements that don't account for velocity of muscle shortening 1
Values are critically dependent on lung volume and muscle length, and in children, there's significant variability and a learning effect; multiple measurements (at least 5) are recommended 1, 3
Wide normal ranges reflect natural variation in muscle strength between individuals, and for comprehensive assessment, consider additional tests like sniff nasal inspiratory pressure or transdiaphragmatic pressure measurements in specific clinical scenarios 2, 7