ch5 vent-perf-notes-

ch5 vent-perf-notes- - Ventilation-Perfusion Questions 1....

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Ventilation-Perfusion Questions 1. Why does non-uniformity of ventilation and perfusion affect overall gas exchange? 2. What is alveolar-arterial oxygen gradient? In normal lungs ventilation and perfusion are fairly uniform throughout, but there are many clinical situations where this is not the case. The pictures below compare normal and a patient with pulmonary embolism (blood clots lodging in the lungs) following venous injection of a tracer. The alveolar gas equation used in HW 1 can be derived from a mass balance equations based on the figure below. VA*PIO2-MRO2=VA*PAO2 MRCO2=VA*PACO2 The respiratory quotient R == Resulting in PACO2=R*(PIO2-PAO2) which is the simplified alveolar gas equation. It is simplified because it is based on the assumption of equal inspired and expired VA, which is not strictly true especially because MRCO2 and MRO2 are not necessarily equal.
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Similar mass balances can be written for the tissue compartment. The corresponding blood mass balance leads to: R = = CaCO2=CvCO2-R*(CaO2-CvO2) The above equation is the blood line. The main difference from the alveolar gas equation is the use of concentration rather than partial pressure. Concentration is a function of partial pressure, but especially for oxygen, it is highly non-linear. These functional relationships are called dissociation curves. Oxygen is carried primarily by hemoglobin so it is customary to plot the oxygen dissociation curve in terms of % saturation=S, concentration C=S*[Hb] where {Hb] = concentration of hemoglobin.
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The dissociation curve for carbon dioxide is approximately linear, especially over the normal physiological range (30-50 mm Hg). By combining the MRCO2 from the gas equation and MRO2 from the blood equation. Te ventilation-perfusion ratio can be calculated from R= =
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VA/Q = R*(CaO2-CvO2)/PACO2 In West, there is an additional factor multiplying R of 8.63. This factor is to take care of differences in units. For example, MRO2 and MRCO2 are measured in ml/min STPD while VA is measured in liters/min BTPS. So VA/Q can be calculated from the above somewhat
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This note was uploaded on 02/15/2012 for the course BME 403 taught by Professor Yamashiro during the Spring '07 term at USC.

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ch5 vent-perf-notes- - Ventilation-Perfusion Questions 1....

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