Definition of Apnea in Obstructive Sleep Apnea
An apnea is defined as a complete cessation of airflow lasting at least 10 seconds. 1
Technical Definition
Apnea represents the complete absence of airflow (as opposed to hypopnea, which is a partial reduction) during sleep that must persist for a minimum duration of 10 seconds to be scored as a respiratory event 1, 2
In obstructive sleep apnea specifically, the apnea occurs despite ongoing respiratory effort—meaning the chest and abdomen continue attempting to breathe, but the upper airway is completely blocked 1, 2
Physiological Consequences
Each apneic event results in hypoxemia (drop in blood oxygen levels) and triggers changes in autonomic nervous system activity, causing increases in both systemic and pulmonary arterial pressure 1
Apneas are typically terminated by an arousal (brief awakening from sleep), which fragments sleep architecture and contributes to excessive daytime sleepiness and neurocognitive impairment 1
Clinical Context in OSA with Obesity Hypoventilation Syndrome
Approximately 90% of patients with obesity hypoventilation syndrome (OHS) have coexistent obstructive sleep apnea, with nearly 70% having severe OSA defined as an apnea-hypopnea index (AHI) >30 events per hour 3, 4
The presence of apneas in OHS patients carries worse prognostic implications than in eucapnic obese patients with OSA alone, including higher mortality rates, increased risk of pulmonary hypertension (30-88% prevalence), and greater likelihood of chronic heart failure and cor pulmonale 3, 4
Distinction from Hypopnea
While apnea requires complete cessation of airflow, hypopnea is defined as a ≥50% reduction in airflow (or a discernible drop) lasting ≥10 seconds, associated with either ≥3% oxygen desaturation or an arousal 1
Both apneas and hypopneas are counted together to calculate the apnea-hypopnea index (AHI), which determines OSA severity: mild (5-15 events/hour), moderate (15-30 events/hour), or severe (>30 events/hour) 1, 3