Hypopneas are a specific form of disordered breathing that occurs during sleep, representing a partial collapse of the upper airway. These events are central to the diagnosis of conditions grouped under the term Sleep Disordered Breathing (SDB), most notably Obstructive Sleep Apnea (OSA). Sleep medicine specialists quantify hypopneas and complete breathing cessations to assess the quality of nocturnal respiration. This quantification helps determine the severity of a sleep disorder and guides therapeutic interventions.
Defining Hypopneas
A hypopnea is technically defined in the context of a sleep study by precise criteria that involve a reduction in airflow. To be officially counted, a respiratory event must involve a decrease in airflow of at least 30% from the baseline breathing rate. This shallow breathing event must persist for a minimum duration of 10 seconds.
The event is only scored as a hypopnea if the reduction in airflow is accompanied by a measurable physiological consequence. This consequence is typically either a drop in blood oxygen saturation (3% or more) or a brief shift in brain wave activity known as an arousal. This dual requirement ensures that only clinically meaningful partial airway obstructions are scored.
Hypopneas Versus Apneas
The primary distinction between a hypopnea and an apnea lies in the degree of airflow reduction. A hypopnea is characterized by a partial obstruction, where the air inhaled is significantly reduced but not completely stopped, generally quantified as a 30% or greater decrease. The respiratory effort continues, but it is insufficient to maintain normal breathing patterns.
An apnea, by contrast, involves a nearly complete or total cessation of airflow, typically defined as a reduction of 90% or more from the baseline. Both events must last for at least 10 seconds to be counted as a discrete episode in adults.
Measuring Severity
The frequency of hypopneas and apneas is quantified using the Apnea-Hypopnea Index (AHI), which is the standard metric for diagnosing and classifying the severity of Obstructive Sleep Apnea. The AHI is calculated by summing the total number of apneas and hypopneas recorded during a sleep study and dividing that number by the total hours of sleep. This calculation yields an average number of respiratory events per hour.
The AHI score is used to categorize the severity of the sleep disorder using established clinical thresholds:
- An AHI between 5 and 15 events per hour is classified as mild sleep apnea.
- A score ranging from 15 to 30 events per hour indicates moderate sleep apnea.
- Scores of 30 or more events per hour are categorized as severe sleep apnea.
Some sleep studies also report the Respiratory Disturbance Index (RDI), which is a broader measure than the AHI. The RDI includes apneas and hypopneas, but also incorporates Respiratory Effort-Related Arousals (RERAs). RERAs are periods of increasing respiratory effort that do not meet the criteria for an apnea or hypopnea but still result in a sleep arousal.
Common Causes and Consequences
The most common cause of hypopneas is a mechanical obstruction of the upper airway that occurs when the muscles relax during sleep. This relaxation causes the soft tissues in the throat, such as the tongue and the soft palate, to collapse inward. Anatomical features, including an increased neck circumference, enlarged tonsils, or specific jaw shapes, can predispose an individual to this partial blockage.
The immediate physiological consequences of a hypopnea are twofold: oxygen desaturation and sleep fragmentation. The partial airway collapse prevents sufficient oxygen from reaching the lungs, leading to a temporary drop in blood oxygen saturation. Simultaneously, the body initiates a subtle arousal, a brief shift toward wakefulness, to increase muscle tone and restore normal breathing. This cycle fragments the sleep architecture, preventing the deep, restorative stages of sleep.