essay 19 - Mechanisms Responsible for Pulmonary Gas Exchange Disorders. Changes in V/Q ratio. gas exchange abnormalities. alveolar hyperventilation syndrome. Breathing control disorders

mechanism responsible for pulmonary gas exchange disorders

• impaired ventilation;perfusion ratio

• impaired diffusion capacity

• alveolar hypoventilation

what is normal level of V/Q

(ventilation ~4L/min; perfusion ~5L/min). this balance ensures efficient gas exchange

changes in ventilation-perfusion ratio

1. decreased V/Q (<0.8, shunt/like states)

• perfusion is preserved but ventilation is reduced

• seen in = chronic bronchitis, asthma, pneumonia

• consequences = alveoli are perfused but poorly ventilated → hypoxemia and hypercapnia

2. increased V/Q (>0.8, dead space states)

• ventilation is adequate but perfusion is reduced

• seen in = pulmonary embolism, emphysema

• consequences = wasted ventilation, poor oxygen uptake, hypoxemia

pulmonary gas exchange abnormalities (impaired diffusion capacity)

• reduced transfer of gasses (especially O2) across the alveolo-capillary membrane, even with normal ventilation and perfusion

• determinants of diffusion = surface area, membrane thickness, partial pressure gradient, capillary transit time and gas solubility

• causes:
  - pulmonary fibrosis = thickened interstitium → decreased gas transfer
  - emphysema = destruction of alveolar walls → decrease surface area
  - pulmonary edema = fluid in alveoli/interstitium → thickened barrier

• consequences = hypoxemia (more prominent than hypercapnia), exercise intolerance, dyspnea

describe alveolar hypoventilation syndrome

• inadequate alveolar ventilation → hypercapnia and hypoxemia. occurs even if lung tissue is structurally normal

• Types:

1. homogenous hypoventilation:
   - primary = depressed central drive or neuromuscular weakness (e.g CNS trauma, drug overdose, myasthenia gravis)
   - secondary = mechanical limitation of breathing (e.g obesity hypoventilation syndrome, chest wall deformities)

2. hetergenous hypoventilation=
   - ventilation unevenly distributed across lung regions
   - common in advanced COPD with emphysema → areas of obstruction, air trapping and V/Q mismatch

• consequences = chronic hypercapnia, respiratory acidosis, secondary hypoxemia

breathing control disorders

• impaired regulation of respiration by the central nervous system or faulty transmission to respiratory muscles. dysfunction of the respiratory centres in the medulla/pons or impaired chemoreceptor signalling

• examples:
  - central hypoventilation = absent or reduced automatic breathing
  - congenital (“Ondine’s curse”), acquired (brainstem stroke, trauma,
  tumor), or drug-induced (opioids, sedatives)

• central sleep apnea = intermittent failure of CNS to initiate breaths → periods of apnea during sleep. seen in heart failure, opioid use, stroke

• neurodegenerative diseases = parkinson’s, ALS → impaired motor neuron function, chest wall rigidity, autonomic dysfunction → hypoventilation

• Cheyn-stokes respiration = abnormal pattern of periodic breathing characterised by a cyclical waxing and waning of tidal volume, followed by episodes of apnea (no breathing).

• consequences = hypoxemia, hypercapnia, secondary systemic hypoxia, risk of respiratory failure