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Flashcards in V/Q ratio Deck (21)
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1

Partial Pressure of:

Inspired Air

Alveolar Air

Venous Blood

Arterial Blood

  • Inspired AIr
    • O2=150
    • H20=47
  • Alveolar Air
    • PAo2=100
    • PAco2=40
  • Venous Blood:
    • PVo2=40mmHg
    • PVco2=47
  • Arterial Blood:
    • PaO2=90
    • Paco2=40

2

V/Q ratio

  • Alveolar Ventilation and Alveolar Perfusion should MATCh, BUT DONT
    • V=4.2 L/min
    • Q=6.0 L/min
  • Ideal V/Q=1
    • average=0.8
  • V/Q is greater at Apex
  • Both Ventilation and perfusion increase from Apex to Base
    • But Q increases more than V
    • @ apex: V/Q>1
    • @ base: V/Q<1
  • V/Q matching is a proportion not an absolute

3

At the Whole body level:

  • O2 Delivery (Ventilation)=CO2 production (Perfusion)
    • Total Ventilation=CO

4

V/Q>1

  • Ventilation exceeds perfusion
    • no hypoxemia 
  • Hyperventilation
    • Pco2<40mmHg
      • Increase Po2
      • Decrease Pco2

5

V/Q<1

  • When perfusion exceeds ventilation
  • Hypoventilation
    • PaCO2>40mmHg
      • Decrease Po2
      • Increase Pco2

6

Dead Space vs Shunt

  • Dead space
    • ventilation with no perfusion
    • Extreme of hyperventilation
    • Acts like inspired air
      • Po2=150mmHg
      • Pco2=0mmHg
  • Shunt
    • perfusion without ventilation
    • extreme of hypoventilation
    • Acts like Venous blood
      • PO2=40mmHg
      • PCO2=47mmHg
  • Both represent the extremes of V/Q imbalance or mismatch

7

V/Q ratio determines

  • alveolar and arterial pressure of O2 and CO2

8

Autoregulation of V/Q

  • Autoregulation of both:
    • airway resistance
    • vascular resistance 
  • Self-correct local V/Q mismatch

9

Autoregulation of Low V/Q

  • Hypoventilation-->Decreased V/Q
  • Increased Pco2
    • Bronchodilation--> Increase V
  • Decreased Po2
    • Vasoconstriction--> Decrease Q
    • Hypopoxic Pulmonary Vasoconstriction (HPV)
  • Increase V/Q

 

Global Lung Hypoxia=Cor Pulmonale

  • chronic hypoxia

10

Autoregulation of High V/Q

  • Hyperventilation-->Increase V/Q
    • self limiting
  • Decrease Pco2
    • Bronchoconstriction-->Decrease V
  • Increase Po2
    • Vasodilation-->Increase Q
  • Decrease V/Q

11

Normal Range for Pao2 and PaCo2

  • Pao2
    • 80-105mmHg
  • PaCo2
    • 35-43

12

Hypoxemia vs Hypoxia

  • Hypoxemia
    • Low PaO2 (<80mmHg)
    • arterial blood
    • most common cause of hypoxia
  • Hypoxia:
    • O2 delivery to tissues do not meet metablic demand
    • not the same as hypoxemia 

13

Hypercapnia vs Hypocapnia

  • Hypercapnia:
    • High Paco2 (>43mmHg)
  • Hypocapnia
    • Low Paco2 (<35mmHg)

14

A-a O2 gradient

  • A-a O2 gradient=PAo2-Pao2
    • normal=(age/4)+4
      • 10mmHg usually
  • Hypoxemia + Normal A-a gradient
    • low barometric pressure (altitude)
    • Hypoventilation (Increase Paco2)=Hypercapnia
  • Hypoxemia + increased A-a gradient
    • V/Q mismatch-O2 helps
    • Shunt(R to L)=No O2 helps
    • Impaired diffusion-O2 helps

15

Pulmonary Embolism

  • occlusion of pulmonary arterial vessel by a clot (thrombus) formed following venous stasis
  • EXAMPLE OF Dead space: V/Q ratio=infinite
    • ventilated but no perfusion
  • Blood goes to other areas
    • V/Q in other areas would decrease

16

O2 Transport

  • To exit Alveoli
    • O2 must dissolve into liquid on surface of airways
    • diffuse into plasma
    • diffuse into RBC
    • bind to HB

17

Diffusion formula

  • Diffusion=(A/dX)*dC
    • A=Surface area
    • dX=thickness 
    • dC=concentration difference across barrier
  • where dC=dPxS 
    • S=solubility
      • SO2=0.3
      • SCO2=0.6
        • 20x more soluble than O2

18

O2 Perfusion Limited vs O2 Diffusion limited

  • O2 perfusion limited
    • Nitrous oxide (N2O)
  • O2 Diffusion Limited:
    • Carbon Monoxide (CO)

19

Single Breath DLco Determination

  • Diffusion capacity of the lung for CO
  • Normal Range=20-30
  • Exhales to RV
  • Takes deep breathe
    • alveoli filled with 0.3% CO and 10% He
  • Holds breathe for 10 seconds
    • Fraction of CO will diffuse into blood
    • He will stay in lungs=determine RV
  • Exhale
    • 1st=air from conducting airways
    • Last=alveolar air=CO measurement

20

Lung Diffusing Capacity (DL)

  • Measure of gas diffusion in the lungs
    • CO binds to Hb keeps plasma Pco low, so only have to know concentration in alveoli
  • Determine if hypoxemia is explained by impaired diffusion
    • <20=alveolar impaired diffusion
  • Controls:
    • Surface Area
      • lung volume=corrected
    • HB level
      • -corrected for Hb

21

PO2

partial pressure of oxygen gas