pulmonary blood flow and VQ mismatch

pulmonary circulation

low pressure → low vascular resistance → thin blood vessel walls → less smooth muscles in blood vessel walls

Factors that affect pulmonary vascular resistance: passive regulation

cardiac output, lung volume, hydrostatic pressure of blood vessels

Cardiac output

increase cardiac output → increase blood flow → recruitment (opening of closed capillaries) and/or distention (increase in diameter of open capillaries) → decrease pulmonary vascular resistance

Lung volume

low: compressed lung tissue → decrease blood vessel diameter → increase pulmonary vascular resistance

high: inflated alveoli → decrease blood vessel diameter → increase pulmonary vascular resistance

hydrostatic pressure of blood vessels

distribution of blood flow in the lung is uneven

blood flow decreases as it travels toward apex because of gravity → less hydrostatic pressure

Zone 1: little blood flow, P Alveoli > P artery > P venous, V/Q =3.3

Zone 2: driving pressure= P artery - P alveoli, P artery > P Alveoli > P venous, V/Q=1

Zone 3: driving pressure = P artery - P venous, P artery > P venous > P Alveoli, V/Q= 0.6

As a whole unit, V/Q=1

Factor that affect pulmonary vascular resistance: active regulation

reduction in alveolar P O2 → hypoxic pulmonary vasoconstriction (HPV)

HPV: small pulmonary arteries constrict, increasing PVR, and decreasing blood flow

Blood flow is directed away from poorly ventilated areas of lung → pulmonary edema

Extensive pulmonary artery constriction → pulmonary hypertension

V/Q mismatch

High V/Q: oxygen but less/no blood flow

Alveolar dead space: ventilation of lung units without perfusion (V/Q= infinity) caused by cardiovascular shock, emphysema, pulmonary edema

Low V/Q: blood flow but less/no oxygen→ shunt

Shunt: abnormal connection between pulmonary and systemic circulations

-Left to right: “back-lead” of blood from systemic to pulmonary circulation → increase pulmonary flow → development of pulmonary hypertension

-Right to left: deoxygenated blood flows directly from pulmonary to systemic circulation to cause hypoxemia (V/Q =0), patients do not respond to 100% oxygen treatments because of perfusion limitation

Cause of hypoxemia

V/Q mismatch (most common) caused by COPD, asthma, chronic bronchitis, pulmonary edema, pneumonia

hypoventilation (increase P a CO2)

decrease PiO2 (extreme high altitude)

shunt

diffusion limitation (decrease DL CO)

All causes of hypoxemia can be corrected by giving 100% O2 except shunt

A-a gradient normal

Cause of hypoxemia: hypoventilation → increasing PaCO2, low PiO2 → extremely high altitude

A-a gradient increase

Cause of hypoxemia: V/Q mismatch, shunt, impaired diffusion → decrease DLCO → can be corrected via 100% O2