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V/Q ratio

Physiology and Pharmacology

QuestionAnswer
Pulmonary circulation Low pressure Arterial pressure 25/15 mmHg Circulation has to take entire cardiac output 5 l/min at rest up to 20 l/min during exercise The pressure only increases slightly as there is a fall in vascular pulmonary resistance
Regional blood flow Inject Xe133 into peripheral vein First pass through lung it leaves the blood and enters alveoli proportional to blood flow These counts give regional blood flow Normalised to volume by rebreathing to equilibrium and taking more counts
Theoretical zones of the lung Zone 1 - PA>Pa>Pv - capillary squashed so no perfusion Zone 2-Pa>PA>Pv - dynamic changes in flow as capillary opens and closes under pressure Zone 3 -Pa>Pv>PA - normal scenario where capillary is open Position of zones varies in cardiac/breathing cycle
Hypoxic pulmonary vasoconstriction Systemic - hypoxia causes vasodilation to match flow to metabolism Pulmonary - hypoxia causes vasoconstriction to match perfusion to ventilation Global hypoxic vasoconstriction increases pulmonary pressure and causes right sided heart failure
Effect of VQ mismatch In extreme situations - normal blood flow and normal ventilation but no gas exchange since blood never comes near fresh gas
VQ ratios Low VQ - increased Pco2 and decreased Po2 Hight VQ - increased Po2 and decreased Pco2 Ideal point - correct exchange as alveoli and arterial Po2 and Pco2 equilibrate
VQ in a perfect lung Partial pressure of CO2 in alveolar gas leaving the lung and in the blood leaving the lung is the same
VQ in inhomogeneous lungs Partial pressures of CO2 in alveolar gas and arterial blood are not equal Increase arterial Pco2 - flow to the lungs will be altered to meet demands Loss of efficient CO2 production
O2 and VQ mismatch The dissociation curve for O2 is not linear in physiological range The content of O2 is lowered more by a given Pao2 fall for low VQ units than it is elevated by a rise in Pao2 by high VQ units Effects of mismatch is worse for O2 than other gases
Three compartment model of the lungs The effects of any VQ distribution in the lung on alveolar and arterial Pco2 and Po2 can be mimicked exactly by a lung with three components An area of dead space - VQ = infinity An area of shunt - VQ = 0 Normal area - VQ = 0.8
Causes of VQ mismatch Pneumothorax Emphysema Adult and neonatal respiratory distress syndrome Asthma Pneumonia Pulmonary embolism
Pure shunt An extreme case of VQ mismatch where VQ = 0 Shunt fraction is typically 0.05 but is increased in disease Low VQ due to shunts is resistant to O2 therapy Can normally increase effectiveness of V by increasing O2 - no effect in VQ = 0
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