Paroxysmal Nocturnal Dyspnea (PND)

Definition

PND refers to episodes of sudden, severe shortness of breath that awaken a patient from sleep, associated with feelings of anxiety and suffocation, requiring the patient to sit upright or stand for relief. - Harrison's Principles of Internal Medicine 22E

PND vs. Orthopnea - Key Differences

Feature	Orthopnea	PND
Onset after lying down	Within 1-2 minutes	After prolonged recumbency (typically ~1 hour or more)
Predictability	Fairly predictable	Less predictable
Relief	Raising head/pillows	Sitting upright; may take 30 min or longer
Severity	Gradual	Sudden, paroxysmal
Associated features	Cough	Cough + wheezing (cardiac asthma)

Pathophysiology

When the patient lies down for a prolonged period:

Increased venous return - fluid redistributes from the splanchnic circulation and lower extremities to the central circulation
Mobilization of interstitial fluid - edema fluid accumulated in dependent tissues during the day is reabsorbed into the circulation at night
This raises pulmonary capillary wedge pressure and causes alveolar edema
Reduced sensitivity of the respiratory center during sleep allows congestion to worsen before the patient is awakened - hence the delayed onset compared to orthopnea

Goldman-Cecil Medicine; Harrison's Principles of Internal Medicine 22E

Cardiac Asthma

PND episodes are frequently accompanied by coughing and wheezing, a phenomenon called "cardiac asthma."

Mechanism: increased bronchial arterial pressure causes airway compression + interstitial pulmonary edema increases airway resistance
Clinically important to distinguish from true bronchial asthma:
- Cardiac asthma patients are more likely to have diaphoresis and cyanosis
- In cardiac disease the patient must sit upright for relief; in bronchial asthma, secretions can be relieved by coughing/expectoration
- Associated signs of heart failure (JVD, S3, bilateral crackles) point to cardiac origin
Harrison's Principles of Internal Medicine 22E

Associated Features / Complications

Cheyne-Stokes respiration and central sleep apnea can precipitate PND in heart failure - related to increased sensitivity of the respiratory center to arterial PCO₂ and prolonged circulatory time
Acute pulmonary edema - the extreme end of the spectrum; marked elevation of pulmonary capillary wedge pressure manifests as severe dyspnea + pink, frothy sputum

Clinical Significance

PND is relatively uncommon compared to exertional dyspnea and orthopnea
When present, it almost always represents severe heart failure
It is associated with increased mortality
It is a cardinal symptom of left-sided heart failure (pulmonary venous congestion)
Goldman-Cecil Medicine

Causes

PND is primarily a marker of left ventricular failure from any cause:

Ischemic cardiomyopathy (post-MI)
Dilated cardiomyopathy
Hypertensive heart disease
Valvular heart disease (especially mitral stenosis/regurgitation, aortic stenosis)
Heart failure with preserved ejection fraction (HFpEF)

Quick Memory Aid

PND = "Nocturnal Pulmonary Flood" - fluid that pooled in the legs during the day rushes back to the lungs after hours of lying flat, floods the alveoli, and the patient wakes up gasping.

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