Mechanism of pnd
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"paroxysmal nocturnal dyspnea"
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paroxysmal nocturnal dyspnea mechanism heart failure Merck Manual
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PND = paroxysmal nocturnal dyspnea: sudden severe breathlessness that wakes a person from sleep, typically 1 to 2 hours after lying down, and improves on sitting upright or standing.
Mechanism
Usually indicates left-sided heart failure:
- Lying flat increases venous return from the legs and splanchnic circulation to the chest.
- During sleep, interstitial fluid from dependent tissues is reabsorbed into the intravascular compartment.
- A failing left ventricle cannot handle the increased preload.
- Left atrial, pulmonary venous, and pulmonary capillary hydrostatic pressures rise.
- Fluid transudes into the pulmonary interstitium and alveoli, producing pulmonary congestion/edema.
- This reduces lung compliance and stimulates pulmonary receptors, causing acute dyspnea and cough.
- Sitting upright reduces venous return and redistributes blood to the lower body, so symptoms improve.
Additional contributors can include reduced nocturnal adrenergic support and diminished respiratory drive during sleep.
Contrast with orthopnea: orthopnea occurs soon after lying down; PND classically awakens the patient after a period of sleep.
Source: Goldman-Cecil Medicine, “Shortness of Breath,” lines 2317-2323.
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