Hypoxic pulmonary vasoconstriction

V/Q Mismatch - Lungs Out of Sync

• V/Q Ratio ($V_A/Q_c$): The ratio of alveolar ventilation ($V_A$) to pulmonary blood flow ($Q_c$), which measures gas exchange efficiency. An ideal match is a V/Q ratio of ~0.8.

• Mismatch Spectrum:

  • Dead Space (V/Q → ∞): Ventilation without perfusion. Occurs with obstruction of blood flow, like in a pulmonary embolism.
  • Shunt (V/Q → 0): Perfusion without ventilation. Seen in airway obstruction or fluid-filled alveoli (e.g., pneumonia, ARDS).

• Physiological Gradient (Upright Lung):

  • Apex: ↑ V/Q. Less perfusion relative to ventilation.
  • Base: ↓ V/Q. More perfusion relative to ventilation.

⭐ Both ventilation and perfusion are greatest at the lung base. However, blood flow increases more dramatically from apex to base than ventilation does, causing the V/Q ratio to be lower at the base.

Hypoxic Pulmonary Vasoconstriction - The Lung's Smart Squeeze

A unique, protective mechanism where pulmonary arteries constrict in response to low alveolar oxygen levels ($P_{A}O_2$). This shunts blood from poorly ventilated lung regions to areas with better ventilation, optimizing V/Q matching and systemic oxygenation.

• Trigger: Low alveolar pO₂ ($P_{A}O_2$).
• Action: Smooth muscle constriction in pulmonary arterioles.
• Result: ↑ Pulmonary vascular resistance in hypoxic areas, diverting blood flow to normoxic regions.

⭐ Systemic Contrast: This is the opposite of systemic circulation, where hypoxia triggers vasodilation to increase blood flow and oxygen delivery to tissues.

Pathophysiology - When The Squeeze Goes Global

When hypoxia is widespread (e.g., high altitude, COPD), HPV occurs globally, causing diffuse vasoconstriction instead of shunting blood to better-ventilated areas.

• This leads to a massive ↑ in Pulmonary Vascular Resistance (PVR).

• The result is Pulmonary Hypertension.

• Chronic pressure overload forces the right ventricle to work harder, leading to Right Ventricular Hypertrophy and eventually Cor Pulmonale (right-sided heart failure).

• Factors that inhibit HPV:

  • Certain anesthetics (e.g., halothane)
  • Sepsis
  • Calcium channel blockers
  • Metabolic acidosis or alkalosis

⭐ At high altitude, the A-a gradient is normal. Both alveolar ($P_AO_2$) and arterial ($P_aO_2$) oxygen levels are low due to decreased inspired oxygen, but the gradient itself does not increase.

Clinical Correlations - V/Q Villains

• V/Q Mismatch: Most common cause of hypoxemia. Responds to 100% O₂ because oxygen can still reach underventilated alveoli.
• True Shunt: Blood bypasses ventilation; hypoxemia is refractory to 100% O₂.

High‑Yield Points - ⚡ Biggest Takeaways

• Hypoxic Pulmonary Vasoconstriction (HPV) is a response to alveolar hypoxia (↓ PAO₂), not systemic hypoxemia.
• It diverts blood from poorly ventilated areas to well-ventilated areas, thus optimizing V/Q matching.
• This is the opposite of systemic circulation, where hypoxia causes vasodilation.
• The mechanism involves inhibition of voltage-gated K+ channels in pulmonary artery smooth muscle.
• Chronic, diffuse hypoxia (e.g., COPD, high altitude) can lead to pulmonary hypertension.