What is a bad Negative Inspiratory Force (NIF) score?

Understanding Negative Inspiratory Force (NIF) Scores

A bad Negative Inspiratory Force (NIF) score is generally considered to be less negative than -30 cm H2O, which indicates significant inspiratory muscle weakness and potential need for mechanical ventilation. 1

Normal vs. Abnormal NIF Values

• NIF (also called Maximum Inspiratory Pressure or MIP) measures inspiratory muscle strength, with more negative values indicating stronger respiratory muscles 1
• Normal NIF values are more negative than -70 cm H2O for males and -60 cm H2O for females 1
• Values less negative than -30 cm H2O are considered severely abnormal and associated with potential respiratory failure 1
• Values between -30 and -60 cm H2O represent moderate weakness that may require monitoring 1

Clinical Significance of Abnormal NIF

• A NIF less negative than -30 cm H2O is associated with increased risk of requiring mechanical ventilation in patients with neuromuscular disorders 1
• Early studies showed that patients with NIF less negative than -20 cm H2O consistently failed weaning trials from mechanical ventilation 1
• NIF values between -20 and -30 cm H2O represent a critical range where clinical decision-making becomes more complex 1
• Poor NIF scores correlate with increased morbidity and mortality due to respiratory failure 2

Factors Affecting NIF Measurement

• Patient effort and cooperation significantly impact NIF values, potentially leading to falsely abnormal results 1
• Lung volume affects NIF measurements - values are typically most negative when measured at residual volume 1
• In patients with COPD or hyperinflation, NIF may appear abnormally low due to shortened inspiratory muscle fiber length rather than true weakness 1
• Intrinsic positive end-expiratory pressure (PEEPi) can cause underestimation of true inspiratory muscle strength 1

Clinical Applications

• In neuromuscular disorders, NIF less negative than -30 cm H2O indicates high risk for respiratory failure 1
• For ventilated patients, NIF is used to assess readiness for extubation, with values more negative than -30 cm H2O suggesting potential success 1, 3
• In myasthenic crisis, NIF values correlate with extubation outcomes - values less negative than -40 cm H2O predict extubation failure 3
• For patients with amyotrophic lateral sclerosis (ALS), declining NIF values (becoming less negative by approximately +2.9 cm H2O/month) indicate disease progression 2

Monitoring Recommendations

• Serial NIF measurements are more valuable than isolated readings for tracking disease progression 4, 2
• Home monitoring of NIF is feasible for patients with neuromuscular conditions like myasthenia gravis 4
• When NIF values are concerning (less negative than -30 cm H2O), additional respiratory parameters should be assessed, including:
  • Forced vital capacity (FVC) - values <20 mL/kg suggest need for ventilatory support 1
  • Maximum expiratory pressure (MEP) - values <40 cm H2O indicate poor cough effectiveness 1, 5
  • Peak cough flow (PCF) - values <270 L/min suggest ineffective airway clearance 1

Common Pitfalls and Caveats

• NIF measurements have high sensitivity (80%) but poor specificity (25%) for predicting weaning success 1
• A single NIF measurement may not reliably predict respiratory failure - trends are more informative 2
• Pulse oximetry and arterial blood gases may remain normal until late stages of respiratory failure, making them less reliable early indicators than NIF 1
• Standardized measurement techniques are essential - values obtained through different methods may not be comparable 1
• In patients with bulbar dysfunction, NIF may underestimate true respiratory muscle strength 5