33. Mechanisms responsible for disturbed gas exchange. Changes in the V/Q ratio, disturbed diffusion of gases through the alveolo-capillary barrier, alveolar hypoventilation syndrome, disturbances in respiratory control.

Mechanisms responsible for disturbed gas exchange. Changes in the V/Q ratio, disturbed diffusion of gases through the alveolar-capillary barrier, alveolar hypoventilation syndrome, disturbances in respiratory control.

Changes in the V/Q ratio

• ⭐ V/Q ratio - Defined as the ratio of the amount of air reaching the alveoli to the amount of blood reaching the alveoli
• ❏ Normal value = 0.8 → apex of lung > 0.8; base of lung <0.8
• ❏ V/Q can be lower due to decreased ventilation or increased perfusion
• This can occur in:
• ★ Low arterial pO2
• ★ Excretion of CO2 is impaired
• ★ Asthma, COPD, acute pulmonary edema
• ★ Anything that causes suppression of breathing - drugs, alveolar hypoventilation
• ❏ A high v/q ratio is due to increased ventilation or decreased perfusion
• This can happen in:
• ★ Loss of alveolar surface area → more ventilation available per perfusion area → decreased arterial pO2
• ★ Blood is not oxygenated enough due to impaired circulation e.g. embolism
• ★ Increased anatomical dead space

V/Q mismatch

• ❏ Occurs when areas of the lung are ventilated but not perfused or when areas are perfused but not ventilated
• ❏ Conditions = pulmonary embolism - perfusion or ventilation

Disturbed diffusion through the alveolar-capillary barrier

• ❏ Diffusion impairment describes a condition in which gas exchange between the alveoli and the red blood cells is impeded because of:
• An increase in the distance for diffusion
• Decrease in the permeability of the alveolar capillary membrane to the movement of gases
• Disturbed concentration gradient
• ❏ The diffusion capacity of CO2 is higher than O2 → can travel better through alveoli
• ❏ Therefore a high partial pressure of O2 is needed for its diffusion
• ❏ Diffusion capacity reference ranges:
• 15-20 ml/min/mmHg
• 200 ml/min/mmHg
• ❏ Disturbances:
• Thickening of alveolar-capillary membrane - fibrosis
• Widening of the alveolar wall - pulmonary edema → cardiac asthma
• Change in O2/Hb binding curve
• Reduced surface area of alveoli - emphysema → destruction of alveoli
• Disturbed concentration gradient - High altitudes → low O2 conc in alveoli/ anemia → low RBCs → low O2 concentration in blood vessels
• ❏ Hypoxemia resulting from impaired diffusion can be partially or completely corrected by the administration of 100% oxygen
• ❏ This occurs because the increase in alveolar oxygen establishes a large alveolar-capillary diffusion gradient that overcomes the resistance to diffusion

Alveolar hypoventilation syndrome

• ❏ Alveolar hypoventilation is caused by several disorders that are collectively referred to as hypoventilation syndromes
• ❏ Insufficient ventilation leading to hypercapnia, which is an increase in the partial pressure of carbon dioxide