The apnea–hypopnea index (AHI) is commonly used to diagnose obstructive sleep apnea (OSA), but it doesn’t fully capture physiological variations that may contribute to health risks. Respiratory event duration, which reflects arousal threshold, may be a more precise predictor of mortality.

The studies have categorized respiratory event duration into quartiles (Q1 to Q4), representing the range of durations from the longest to the shortest events. Here’s a summary of the quartiles: (https://doi.org/10.1164/rccm.201804-0758oc)

  1. Quartile 1 (Q1 – Longest Event Duration):
  • Event duration range: 24.1 to 57.6 seconds
  • This served as the reference group in statistical analyses.
  1. Quartile 2 (Q2):
  • Event duration range: 20.6 to 24.1 seconds
  1. Quartile 3 (Q3):
  • Event duration range: 17.7 to 20.6 seconds
  1. Quartile 4 (Q4 – Shortest Event Duration):
  • Event duration range: 11.2 to 17.7 seconds

  • Participants in the shortest event duration quartile (Q4) had the highest risk of all-cause mortality compared to those in the longest quartile (Q1).
  • Hazard Ratios (HRs) indicated a significant increase in mortality risk for Q4 across adjusted models.

Key Findings of Sleep Heart Health study (SHHS) (https://doi.org/10.1164/rccm.201804-0758oc)

  • Shorter respiratory event durations during sleep are associated with an increased risk of all-cause mortality, independent of apnea–hypopnea index (AHI) and other confounders.
  • This association was observed in both men and women, with the strongest risk seen in individuals with moderate sleep apnea (AHI 15–30).

2. Significance of Event Duration:

  • Event duration reflects physiological traits like arousal threshold, which is linked to sleep fragmentation and increased sympathetic activity.
  • Unlike AHI, event duration is a more sensitive measure of the physiological stress caused by sleep-disordered breathing.

3. Gender Differences:

  • Mortality risk associated with shorter event durations was significant for both men and women, while AHI predicted mortality primarily in men.
  • Women with OSA often exhibit symptoms like insomnia and sleep fragmentation, which may not be fully captured by AHI but are reflected in event duration.

4. Mechanistic Insights:

  • Shorter events are linked to greater arousability, which can lead to:
    • Sleep fragmentation.
    • Increased sympathetic tone.
    • Ventilatory instability.
  • These physiological traits may increase vulnerability to adverse cardiovascular outcomes and mortality.

5. Clinical Implications:

  • Event duration could serve as a practical and easily derived metric from polysomnography to improve risk stratification in OSA patients.
  • This metric may help identify at-risk individuals, particularly those with intermediate AHI levels, who might benefit from targeted interventions.

6. Study Strengths:

  • The large sample size, community-based cohort, and long follow-up period provided robust evidence for the findings.
  • Sensitivity analyses confirmed that the results were consistent across different event subtypes and statistical models.

7. Limitations:

  • The study relied on a single-night polysomnography, which may not fully capture habitual sleep patterns.
  • The causal mechanisms linking event duration to mortality remain unclear.

8. Future Directions:

  • Further research is needed to explore the biological pathways linking short event durations to mortality, including the role of sympathetic activation and arousal-mediated stress.
  • Studies should investigate whether treatment strategies targeting event duration or arousal threshold can reduce mortality risk in OSA patients.

The SHHS study explored the impact of different respiratory event subtypes (apneas and hypopneas) on the association between event duration and mortality. Here’s a summary of the findings: (https://doi.org/10.1164/rccm.201804-0758oc)

1. Event Subtypes:

  • Apneas: Complete cessation of airflow.
  • Hypopneas: Partial reduction in airflow, often accompanied by oxygen desaturation.
  • Events were further categorized by the degree of associated oxygen desaturation (e.g., >3% desaturation).

2. Correlation Across Subtypes:

  • Event duration was strongly correlated between different subtypes, particularly between hypopneas and combined apnea-hypopnea durations (correlation coefficient (r = 0.74 – 0.98)).
  • Apnea durations had a weaker correlation with combined durations ((r = 0.51 – 0.72)).

3. Impact on Mortality by Event Subtype:

  • The association between shorter event duration and increased mortality risk was consistent across event subtypes.
    • The risk associated with hypopneas was slightly stronger than for apneas, reflecting the predominance of hypopneas in the sample

4. Non-REM vs. REM Events:

  • Event duration was analyzed separately for non-REM (NREM) and REM sleep: The mortality association was significant in both sleep states, with minimal differences in effect size.

5. Subtype-Specific Findings:

  • Hypopneas, particularly those associated with oxygen desaturation, were more strongly associated with increased mortality risk compared to apneas.
  • This underscores the importance of hypopnea characteristics, such as event duration and desaturation severity, in evaluating OSA-related mortality risk.

Conclusion of SHHS: (https://doi.org/10.1164/rccm.201804-0758oc)

Shorter respiratory event durations, whether in apneas or hypopneas, predict higher mortality. Hypopneas with oxygen desaturation were particularly impactful, suggesting that these events may play a critical role in adverse outcomes associated with obstructive sleep apnea.

Other studies have shown that Obstructive apneas led to more severe SpO2 desaturation compared to hypopneas. Increased event duration lead to increase in the severity of the related SpO2 desaturation. In addition to considering event duration, obstructive apneas should have more weight than hypopneas when estimating severity of OSA and associated long-term cardiovascular risk. (https://doi.org/10.1007/s11325-017-1513-6) This study shows that apneic events associated with desaturation leads to more severe desaturation as compared to hypopneas but they have not studied any correlation between longer events leading to more desaturation depth and their association with increased CVD morbidity and mortality. This study also does not clarify regarding apneas without desaturation and their significance in relation to hypopneas in increasing the risk of CVD.