Apnea-Hypopnea Index indicates apnea and hypopnea events that may happen while the person is sleeping. AHI provides the severity of sleep apnea. AHI is common part of results that sleep tests such as Polysomnogram or home sleep testing.
Apnea-Hypopnea Index Calculation
The Apnea-Hypopnea index or AHI indicates how severe is the sleep apnea that patient is suffering from. Apnea or aponea is a brief suspension of breathing process in a sleeping individual. The lungs do not expand or inhale air during this period. Apnea can be mimicked by voluntarily holding breath for a brief period. In case of hypopnea, shallow respiration happens. Not to mention here, some part of air inhaled during hypopnea.
Hope you would have understood apnea vs hypopnea briefly. In both instances, oxygen saturation level in blood drops. At the moment of apnea, respiration stops while in case of hypopnea, the count of respiratory rate lowers. Many times the person suffering from such disorders remain unaware of the situation. An overnight sleep study determines the presence and severity of apnea and hypopnea. At least 5 episodes of sleep apnea required for concluding the diagnosis.
The optimal variables considered in the measurement of the apnea-hypopnea index are debatable.
The considerations for defining actual apnea or hypopnea event vary. Patient age, gender, demography etc. elements impact the threshold values. However, the apnea-hypopnea index (AHI) and respiratory disturbance index (RDI) considered as primary efficacy measures in many clinical trials undertaken to conclude sleep apnea.
Sleep Study details
The sleep studies are carried out in a lab termed as Polysomnography. The sleep physician may advice sleep study in a home also. The person remains hooked up to various devices. The sleep study records brain activities, lung movements, oxygen saturation levels in blood and limb movements during the sleep. The resulting data then quantified and analyzed by an expert sleep physician.
Below are few important sleep study measurements.
- TST: This factor indicates total sleep time or actual amount of time the person was sleeping during the study. It is based on recordings obtained from brain waves.
- Sleep onset latency: It is the actual time required for a person to sleep once the initiated. Usually, a normal person takes 15 to 30 minutes to fall asleep.
- Sleep efficiency: This is a ratio between the total sleep time and the total recording time (TRT)
- Sleep Stages: The Non-REM and REM sleep patterns recorded during the sleep study. The results include progress of sleep from stage 1 to stage 4.
- Sleep interruptions: The sleep study also records in-between awakenings and arousals. Arousal lasts for 5 to 15 seconds. Awakening is arousal lasting more than 15 seconds.
- Oxygen Saturation Level in Blood: The decrease in saturation level of oxygen in blood indicates the event of apnea or hypopnea. Normal oxygen saturation is around 95 percent. A drop in blood oxygen saturation to 86 percent is mild. A reduction in-between 80 to 85 percent is moderate, and a drop to 79 percent or less is severe.
- Heart rhythm: Monitoring of heart rhythm helps to rule out underlying cardiac pathologies.
- Leg movements: The study also observes leg movements while sleeping to rule out the possibility of another sleep disorder.
Analysing Sleep Study Results
The data generated from these electrical systems used for concluding the diagnosis. However, clinical judgment is necessary to correlate the sleep study results for arriving at the diagnosis. It is important that the person specializing in concluding sleep study results reviews the generated data.
Apnea-hypopnea index (AHI) and oxygen saturation levels in blood indicate the severity of OSA. AHA computes apnea and hypopnea events occur during the sleep study. AHA is expressed apnea and hypopnea events occurred during each hour.
Categorization of Apnea-hypopnea index (AHI) values
- Normal: AHI less than 5 per hour
- Mild Sleep Apnea: AHI ≥ 5 but < 15 per hour
- Moderate Sleep Apnea: AHI≥ 15 but < 30 per hour
- Severe Sleep Apnea: AHI≥ 30 per hour
Respiratory disturbance index
Respiratory disturbance index or RDI is another formula used in sleep studies. Unlike apnea-hypopnea index (AHA), it also includes respiratory-effort related arousals. In other words, RDI includes more subtle breathing irregularities. In the same sleep study, RDI can be higher than the apnea-hypopnea index of the same individual.
Respiratory Disturbance Index = (RERAs + Hypopneas + apneas) X 60 / TST (Total Sleep Time in minutes).
Hope you would have understood the details of Apnea-hypopnea index (AHI) in diagnosing sleep disorders. The need of conducting sleep study should be identified in the early stage of sleep disorders. Additionally, the results of sleep study should be interpreted by an expert specialist.