What is the typical negative inspiratory pressure (NIP) or force in a normal healthy adult?

From the Guidelines

In a normal healthy adult, the negative inspiratory pressure (NIP) or maximal inspiratory force (MIF) typically ranges from -97 to -128 cmH2O for females and males, respectively, with average values of approximately 97 cmH2O for females and 128 cmH2O for males, as measured by maximal inspiratory pressure (MIP) 1.

Key Points

• The measurement of NIP or MIF reflects the strength of the inspiratory muscles, primarily the diaphragm and external intercostal muscles, and their ability to generate negative pressure during inhalation.
• The negative pressure is what allows air to flow into the lungs during breathing.
• Values less negative than these average values may indicate respiratory muscle weakness, which could be caused by neuromuscular disorders, fatigue, or other conditions affecting respiratory function.
• This measurement is clinically important for assessing a patient's ability to breathe independently, determining readiness for extubation in mechanically ventilated patients, and monitoring progression of diseases affecting respiratory muscle strength.

Clinical Considerations

• The most widely utilized measure of respiratory muscle performance is MIP, which is a measure of peak inspiratory muscle strength 1.
• MIP is typically measured using noninvasive handheld manometers, which possess excellent reliability and are affordable and accessible 1.
• The European Respiratory Society statement on respiratory muscle testing provides further details on MIP and other forms of respiratory muscle testing 1.

From the Research

Negative Inspiratory Pressure in Healthy Adults

• The typical negative inspiratory pressure (NIP) in a normal healthy adult is not explicitly stated in the provided studies, but we can infer some information from the available data 2, 3, 4, 5, 6.
• A study on maximal inspiratory mouth pressures (PIMAX) in healthy subjects found that the mean values of PIMAX were 9.95 kPa for men and 7.43 kPa for women 6.
• Another study measured the lowest mouth pressures developed during maximum static inspiratory efforts and found that the pressures increased (became less negative) with advancing age in healthy adults 3.
• The study also found that women generated pressures that were between 80 and 90% of the values found in men, and children generated pressures that were as low as those developed by younger adults 3.
• A study on the effects of negative airway pressure breathing on steady-state end-expiratory lung volume and breathing pattern found that the changes in the timing of respiratory events occurred at -5 cmH2O, whereas tidal volume compensation was not seen until -15 cmH2O 2.
• Another study on ventilatory and diaphragmatic EMG changes during negative-pressure ventilation in healthy subjects found that tidal volume increased in most subjects, more with high than with low pressures, whereas diaphragmatic electromyogram (DEMG) increased, decreased, or showed no change 4.

Factors Influencing Negative Inspiratory Pressure

• Age, sex, height, weight, and body mass index (BMI) were found to be significant predictors of maximal inspiratory mouth pressures (PIMAX) in healthy subjects 6.
• Smoking status and smoked pack-years were not independent predictors of inspiratory pressures 6.
• The study on the effects of negative airway pressure breathing found that the responses to negative airway pressure breathing were an increase in breathing frequency, due to a shortened expiratory time, and an increase in inspiratory activity 2.