Chapter 12: Ventilation-Perfusion Relationships

Info from 'Respiratory Care Anatomy and Physiology' 5th edition

overall V_A/Q_c ratio

• important for maintaining gas exchange

• resting V_A ~4 L/min

• resting Q_c ~5 L/min

• V_A/Q_c ratio ~0.8

V/Q as determinant of P_AO₂

• ratio affects P_AO₂ and P_ACO₂

• normal P_AO₂ = 100 torr

• normal P_ACO₂ = 40 torr

conditions when V/Q is high

• ↑P_AO₂ and ↓P_ACO₂

• causes: pulmonary emboli, obstructed pulmonary artery or arterioles, extrinsic pressure on pulmonary vessels, ↓Q

conditions when V/Q is low

• ↓P_AO₂ and ↑P_ACO₂

• causes: obstructive/restrictive lung disorders, hypoventilation

alveolar O₂-CO₂ diagram

• absolute shunt (V/Q = 0)

  • perfusion without ventilation

  • relative shunt (V/Q > 0 but < 1)

• absolute dead space (V/Q = ∞)

  • ventilation without perfusion

  • relative dead space (V/Q < ∞ but > 1)

regional P_ACO₂ and P_AO₂ in lungs

• hypoventilation → ↑P_ACO₂ → ↑P_AO₂

• V/Q distribution in normal lung

  • blood flow and ventilation larger at bases, less at apices

  • base to apex: ↓blood flow > ↓ventilation

    • apex usually over-ventilated

    • apex to base: P_AO₂ ↓ by 40 mmHg and P_ACO₂ ↑ by 15 mmHg

normal gas exchange

• room air: P_A-aO₂ ~7-14 mmHg

• 100% O2: P_A-aO₂ ~50-100 mmHg

• shunting disease: ↓V/Q and ↑P_A-aO₂

why P_A-aO₂ increases when F_iO₂ increases

Hb saturated to capacity when PO₂ is 100-663 mmHg

mechanisms of hypoxemia

• hypoventilation

  • ↑PCO₂ with ↓PO₂

  • causes: muscle paralysis/weakness, drug-induced respiratory center depression

• absolute shunt

  • R-to-L shunting (venous admixture)

    • anatomical

    • intrapulmonary

    • doesn’t respond well to O₂ therapy

• V/Q mismatch (V/Q > 0, < 1 [relative shunt])

  • most common cause, responds well to O₂ therapy

variable effect of V/Q imbalances on gas exchange

• normal O₂ consumption + normal CO₂ production + hypoventilation = ↑P_aCO₂ + ↓P_aO₂

• V/Q mismatch and shunt → normal or ↓ P_aCO₂ within limits

  • medullary response to ↑P_aCO₂ → hyperventilation

• ↑V_E lowers PCO₂ without ↑PO₂

  • equilibrium curve shapes different for O₂ and CO₂

effect of increased V/Q on P_aO₂ and P_aCO₂

• hyperventilation as cause of V_D

  • ventilation increases out of proportion to blood flow

• ↓pulmonary blood flow as cause of V_D

  • shock, embolism, overdistended alveoli

  • blood flow diminished/absent in ventilated alveoli

physical compensatory responses to dead space and shunt

• shunt (↓P_aO₂)

  • local hypoxic pulmonary vasoconstriction

  • blood flow diverted to well-ventilated alveoli

  • poorly-ventilated units have less blood flow

    • less hypoxemic effect on arterial blood

• dead space (↓P_aCO₂)

  • local alveolar duct constriction

  • ↑R_aw and hypoventilation match V/Q better

indicators of shunt

• P_A-aO₂

  • most common index of O₂ transfer efficiency

  • ↑A-a gradient: ↑shunting

• P_a-AO₂

  • more stable than A-a gradient when F_iO₂ changes

  • normal ratio: 0.80-0.95

  • used to predict F_iO₂ needed to achieve desired P_aO₂

• P_aO₂/F_iO₂ (oxygenation ratio)

  • normal: 380-475 mmHg

  • poor indicator

  • affected by changes in P_aCO₂

• P_A-aO₂, P_aO₂/P_AO₂, P_aO₂/F_iO₂ are sensitive to pulmonary factors, don’t take C_vO₂ into account

shunt equations

$$\frac{Q_{S}}{Q_{T}}=\frac{C_{c}O_2-C_{a}O_2}{C_{c}O_2-C_{v}O_2}$$

$$C_{c}O_2=\left(P_{A}O_2\cdot0.003\right)+\left(Hb\cdot1.34\cdot S_{c}O_2\right)$$

(Q_s = shunted cardiac output, Q_T = total cardiac output, [C_cO₂ − C_aO₂] = O₂ lost from Q mixing, [C_cO₂ − C_vO₂] = total O₂ uptake)

shunt estimation formulas

$$\frac{\left(A-a\right)\cdot0.003}{\left\lbrack\left(A-a\right)\cdot0.003\right\rbrack+5}$$

$$\frac{C_{c}O_2-C_{a}O_2}{3.5+\left(C_{c}O_2-C_{a}O_2\right)}$$

significance of shunt

shunt fraction %	clinical significance
<10%	normal lungs
10-19%	seldom requires significant ventilatory support
20-29%	significant abnormality; requires PEEP/CPAP
≥30%	severe disease; life-threatening; mechanical ventilation with PEEP