Gas_Exchange_10 - GasExchange 1 n n...

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Gas Exchange 1
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 Measures of Ventilation n Effectiveness of    PACO2 n Minute ventilation n Total amount of air enters or leaves lung/ min n TV  x Respiratory rate n Tidal Volume = Volume in dead space +  Volume in alveolus.  n Dead space volume (no gas exchange) n Anatomic dead space n % of TV in conducting airways n 1 ml/lb body wt. n Alveolar dead space n % of TV in underperfused alveoli n Normal: 20-50 ml at apex of lung 2
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Physiologic (functional) dead  space n Volume of lung that does not participate in gas  exchange (does not eliminate CO2) n Anatomic dead space + Alveolar dead space n Healthy:  physiologic = anatomic n Increased dead space n Mechanical ventilator n Ventilation/perfusion mismatch n Insufficient perfusion n Right to Left shunt n Pulmonary embolism n Congenital cyanotic cardiac disorders n Tetralogy of Fallot     (Pulmonary stenosis, VSD, Aorta over-ride,       Right Ventricular Hypertrophy) 4
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Assess dead space n  VD = VT x PaCO2 – PeCO2                  PaCO2 n Fraction = dilution of alveolar PCO2 by dead space  air n  VD/VT n Normal ratio:  0.25-0.35 n Respiratory failure n Respiratory system can’t sustain metabolic  needs of the body 6
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Alveolar Ventilation n Air available for gas exchange w/ blood n Volume of fresh air reaching alveoli / min n VA = (VT – VD) X breathing rate n Minute ventilation corrected for physiologic dead  space n Rate and Depth of Breathing n Depth is more effective at increasing alveolar  ventilation n Shallow breath fills dead space first n Increasing rate increases dead space volume 7
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Alveolar Ventilation Equation n VA = VCO2 x K/ P ACO2 n VCO2: rate of CO2 production n P ACO2 =  VCO2 x K/ VA n Describes inverse linear relationship  between alveolar ventilation and the  alveolar partial pressure of carbon  dioxide n P ACO2 determined by alveolar ventilation n When CO2 production is constant 9
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Alveolar ventilation: directly proportional to volume of CO2 produced to maintain a normal PaCO2. Elimination CO2 > Production Hypocapnia Elimination CO2 < Production Hypercapnia 10
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Alveolar Capillary Gas  Exchange n Gas Exchange n Diffusion properties of alveolar-capillary  membrane n Partial pressure gradient n Gas flows to point of equilibration n Pulmonary capillary blood flow 12
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Fick’s law n VX = DA∆P             ∆x n Vx: volume of gas transferred per time n ∆P:  partial pressure difference n A:  membrane surface area n D:  diffusion coefficient (solubility + MW) n ∆x:  membrane thickness 13
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This note was uploaded on 02/18/2012 for the course MPAS PA 602 taught by Professor Dr.laird during the Fall '10 term at Chatham University.

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Gas_Exchange_10 - GasExchange 1 n n...

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