52. Dyspnoea

Dyspnoea is defined as the subjective feeling of difficulty to breathe. Patients usually complain about an unpleasant feeling of awareness of respiratory efforts, shortness of breath and angina-like thoracic pressure.

Dyspnoea isn’t always accompanied by respiratory failure, however they often occur together. It always involves an increased work of breathing.

It can be caused by:

• Exercise (not pathological)
• Increased respiratory drive
  • Hypoxaemia
  • Metabolic acidosis
  • Stimulation of J-receptors
    • Pulmonary infiltration by cancer
    • Pulmonary hypertension
    • Pulmonary oedema
• Impaired ventilation
  • Obstructive ventilatory disorders (extra or intrathroacic)
    • Bronchial asthma
    • Emphysema
    • Bronchitis
    • Trachea or larynx obstruction
    • endobronchial tumor
  • Restrictive ventilatory disorders
    • Pulmonary fibrosis
    • Backward left ventricular failure
    • pneumothorax
  • Decreased thoracic/abdominal compliance
    • Kyphoscoliosis
    • Obesity
    • Pregnancy
    • abdominal tumor
    • pleural callus
• Weakness of ventilatory muscles
  • Neurological disorders (Guillan – Barre syndrome)
• Enhanced dead-space ventilation (increased V/Q ratio)
  • Capillary destruction
    • Emphysema
    • Interstitial lung diseases
  • Narrowing of pulmonary vessels (embolism, pulmonary vasculitis)
• Anxiety

J-receptors (or C-fibre receptors) are receptors located in the alveolar wall. They’re activated by the factors listed above and are innervated by vagus. Stimulation of these receptors causes a reflex increase in respiratory drive (increased breathing rate). The reflex is also part of the sensation of dyspnoea.

Types

Dyspnoea of cardiac origin and dyspnoea of pulmonary origin have different symptoms, so we can look at them as two different types.

Cardiac dyspnoea occurs due to pulmonary congestion or oedema and induces alveolar hyperventilation, which causes hypocapnia and hypoxaemia. The hypocapnia can cause cerebral vasoconstriction which may lead to psychosomatic symptoms, a disorder called hyperventilation syndrome.

Pulmonary dyspnoea occurs in most lung diseases. Hypoxaemia can be caused by many things, like V/Q mismatching or diffusion abnormalities. Global respiratory failure usually develops, causing hypercapnia rather than hypocapnia.

In both types can pulmonary hypertension develop due to widespread vasoconstriction in the lungs due to hypoxia.

Pathomechanism

Dyspnoea develops when the brain understands that there is a mismatch between how ventilation should be and how the ventilation currently is. The brain compares the afferent signals from chemoreceptors, J-receptors, muscle spindles in respiratory muscles, proprioceptors and other peripheral and central receptors and the efferent signals going from the respiratory centre to the respiratory muscles. When there’s a mismatch between the respiratory drive and the actual ventilation will the sensory cortex induce the sensation that “something’s wrong with the respiration”.