paroxysmal nocturnal dyspnea

Paroxysmal nocturnal dyspnea, usually shortened to PND, is a sudden attack of severe breathlessness that jolts a person awake at night, typically one to three hours after falling asleep. Unlike ordinary orthopnea, which eases the moment you sit up, PND can take many minutes of sitting on the edge of the bed — sometimes throwing open a window for air — before the suffocating feeling settles.

The cause is the same congestion that drives orthopnea, but it builds up gradually during sleep. Lying down slowly returns pooled fluid to the circulation, and during sleep the body's breathing reflexes and stress hormones are dialed down, so the failing heart falls behind without the early warning of an awake, upright person. Fluid accumulates in the lungs until the rising pressure abruptly triggers the panicked, gasping awakening that defines the syndrome.

PND is a strong pointer toward left-sided heart failure and tends to indicate more significant congestion than orthopnea alone. It should be distinguished from other nighttime breathing problems such as sleep apnea or asthma, which can also disturb sleep but follow different patterns and respond to different treatments.