What are normal and abnormal Apnea-Hypopnea Index (AHI) scores?

Normal and Abnormal AHI Scores

An Apnea-Hypopnea Index (AHI) score below 5 events per hour is considered normal, while scores of 5 or above indicate obstructive sleep apnea (OSA) with increasing severity levels: mild (5-14), moderate (15-30), and severe (>30 events per hour). 1, 2

Understanding AHI Measurement

• AHI represents the average number of apneas (complete breathing cessations) and hypopneas (partial breathing reductions) per hour of sleep measured during polysomnography 1
• Different scoring criteria for hypopneas can significantly affect AHI values and diagnosis thresholds 1
• The American Academy of Sleep Medicine (AASM) has established standard definitions for scoring respiratory events during sleep 1

AHI Severity Classification

• Normal: AHI < 5 events per hour 1
• Mild OSA: AHI 5-14 events per hour with no or mild symptoms 2
• Moderate OSA: AHI 15-30 events per hour with occasional daytime sleepiness 2
• Severe OSA: AHI > 30 events per hour with frequent daytime sleepiness that interferes with normal activities 2
• Extreme OSA: Some research recognizes AHI > 100 events per hour as an extreme category associated with increased comorbidities 3

Important Considerations in AHI Interpretation

Scoring Method Variations

• Different hypopnea definitions can result in substantially different AHI values for the same patient 1
• Using the AASM-recommended 30% flow reduction with ≥4% oxygen desaturation criteria results in AHI values approximately 70% lower than when using Chicago criteria (50% flow reduction with ≥3% desaturation or arousal) 1
• An AHI of 5 using the AASM-recommended criteria would correspond to approximately 10 using the alternative criteria and 15 using Chicago criteria 1

Clinical Context

• AHI should be interpreted alongside clinical symptoms and comorbidities 2
• Even with the same AHI, symptom severity may vary between patients 2
• Factors associated with higher AHI include hypertension, increased neck circumference, and oxygen desaturation 3
• Time spent with oxygen saturation below 90% (T90>10%) is strongly associated with extreme OSA (OR=19.68) 3

Population Differences

• AHI interpretation may need adjustment for specific populations:
  • Lean patients may have clinically significant hypopneas with minimal desaturation 1
  • Adolescents scored using adult criteria may have significantly lower AHI values than when scored using pediatric criteria 1
  • Different scoring methods can result in significantly different diagnosis rates, especially in borderline cases 1

Clinical Implications of AHI Scores

• Untreated moderate to severe OSA (AHI ≥15) significantly increases risk of cardiovascular disease, stroke, and cardiac arrhythmias 2
• Treatment effectiveness is often measured by reduction in AHI 4
• The "Effective AHI" concept accounts for sleep-disordered breathing during both PAP-on and PAP-off periods to better assess treatment effectiveness 5
• Successful treatment response is typically defined as AHI <20 events per hour and >50% reduction from baseline 4

Common Pitfalls in AHI Interpretation

• Relying solely on AHI without considering symptoms may lead to undertreatment or overtreatment 2
• Not accounting for scoring method differences when comparing studies or patient results 1
• Failing to consider position-dependent variations in AHI 5
• Not accounting for time spent with PAP therapy when evaluating treatment effectiveness 5

Understanding these nuances in AHI interpretation is crucial for accurate diagnosis and appropriate treatment planning for patients with suspected sleep-disordered breathing.