Slide 1BME 501 Lecture Notes – Feb 5Alveolar Gas EquationAlveolar Gas EquationAlveolar Gas EquationVentilation-Perfusion RatioVentilation-Perfusion RatioVentilation-Perfusion RatioVentilation-Perfusion RatioVentilation-Perfusion RatioRegional Gas Exchange in LungRegional Gas Exchange in LungRegional Gas Exchange in LungRegional Gas Exchange in LungRegional Gas Exchange in LungWasted VentilationPA-a,O2 Difference CalculationBME 501Advanced Topics in Biomedical SystemsSpring 2014Dr. KayBME 501 Lecture Notes – Feb 5Ventilation 流流 -Perfusion 流流 Relationships•Alveolar 肺肺 Gas Equation•Ventilation-Perfusion Ratio•Regional 肺肺肺 Gas Exchange in Lung•Wasted Ventilation•PA-a,O2 Difference CalculationAlveolar Gas EquationSet-up of alveoli for gas exchangeAlveoli 流流MRCO2MRO2Alveolar Gas EquationDerivationAlveoliMRCO2MRO2Alveolar Gas EquationDerivationVentilation-Perfusion RatioR and V-P ratioAlveoliMRCO2MRO2Ventilation-Perfusion Ratio•Gas and blood equations both based on R•R = MRCO2/MRO2•Overall R is set by metabolism (0.7 to 1.0)•Individual alveolar R can range from 0 to infinity•Easier to work with ventilation-perfusion ratio•Must solve non-linear equationsGas and Blood R-curve IntersectionVentilation-Perfusion Ratio•Most common cause of hypoxemia 肺肺肺肺•If ventilation and blood flow are mismatched in lung, gas-exchange is impaired 肺肺肺•Ventilation-perfusion ratio determines efficacy肺肺 of exchangeVentilation-Perfusion InequalityVentilation-Perfusion Ratio•Partial pressure of any gas present in lungs is determined by ratio of ventilation to blood flow•Altering V-P ratio alters PO2 and PCO2 in lung unitVentilation-Perfusion RatioRegional Gas Exchange in Lung•V-P ratio varies along height of upright lung•Ventilation increases slowly from top to bottom of lung•Perfusion increases rapidly from top to bottom of lung•V-P ratio is very high at top of lung and much lower at bottom of lungRegional Gas Exchange in LungRegional Gas Exchange in Lung•V-P inequality 肺肺肺 causes less effective O2 and CO2 transfer•Produces lower Pa,O2 and higher Pa,CO2Effects of V-P Inequality on Overall Gas ExchangeRegional Gas Exchange in Lung•Normal alveolar-arterial O2 difference only ~5 mmHg due to V-P inequality•Lung disease can cause PA-a,O2 difference to be much greaterEffects of V-P Inequality on Overall Gas ExchangeRegional Gas Exchange in Lung•Normal subject has fairly well-match ventilation and perfusion•Subject with chronic bronchitis or emphysema has many regions of V-P mismatchDistribution of V-P RatiosWasted Ventilation•In the presence of V-P inequalities, PCO2 often remains at normal values•Lung units with abnormally high V-P ratios are inefficient at eliminating 肺肺 CO2 •Chemoreceptors 肺肺肺肺肺 sense rise in PCO2 and drive increased ventilation•Excess ventilation much less effective at increasing PO2PA-a,O2 Difference Calculation•Patient at sea level•Pa,O2 = 50 mmHg; Pa,CO2 = 60 mmHg•R = 0.8•What is