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1 Ventilation-perfusion Ratio

2 Ventilation/perfusion Ratio or V/QThis relationship between ventilation and PERFUSION is called the ventilation-perfusion ratio (V/Q ratio). On average,alveolar ventilation is about 4 l/min and pulmonary capillary blood flow, ( cardiac output) is about 5 l/min, making the overall ratio of ventilation to perfusion 4:5, or 0.8. The V/Q ratio varies throughout the lung.

3 How the V/Q Ratio affects Alveolar GasVentilation and Perfusion (V/Q) has a profound effect on the partial pressures of O2 and CO2 in the alveolus. Pa02 and PaCO2 levels average about 100 mmHg and 40 mmHg respectively. Only in a small portion of the lungs are those numbers true. They are actually an average

4 Ventilation -------4 l/min.Perfusion l/min. Normal V/Q = 4.5 =0.8 ( all over the lung)

5 When V (alveolar ventilation) is normal for a given alveolus and Q (blood flow) is also normal for the same alveolus, the ventilation-perfusion ratio (V/Q ) is normal.

6 When the ventilation (V) is zero, but there is still perfusion (Q ) of the alveolus, the V /Q is zero. When there is adequate ventilation (V) but zero perfusion (Q), the ratio V/Q is infinity.

7 When V/Q is equal to zero—that is, withoutany alveolar air and because the blood that perfuse the capillaries is venous blood returning to the lungs from the systemic circulation the Po2 is of 40 mm Hg and a Pco2 of 45 mm Hg in alveoli that have blood flow but no ventilation.

8 When VA/Q Equals Infinity The air that is inspired loses no oxygen to the blood and gains no carbon dioxide from the blood. And because normal inspired and humidified air has a Po2 of 149 mm Hg and a Pco2 of 0 mm Hg, these will be the partial pressures of these two gases in the alveoli.

9 At a ratio of either zero or infinity, there is no exchange of gases through the respiratory membrane of the affected alveoli.

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11 When there is both normal alveolar ventilationand normal alveolar capillary blood flow (normal alveolar perfusion), exchange of oxygen and carbon dioxide through the respiratory membrane is nearly optimal, and alveolar Po2 is normally at a level of 104 mm Hg, which lies between that of the inspired air (149 mm Hg) and that of venous blood (40 mm Hg).

12 “Physiologic Shunt” Whenever V/Q is below normal, there is inadequate ventilation to provide the oxygen needed to fully oxygenate the blood flowing through the alveolar capillaries. Therefore, a certain fraction of the venous blood passing through the pulmonary capillaries does not become oxygenated. This fraction is called shunted blood.

13 Some additional blood flows through bronchial vessels rather than through alveolar capillaries, normally about 2 % of the cardiac output; (unoxygenated, shunted blood).

14 The total quantitative amount of shunted blood per minute is called the physiologic shunt.The greater the physiologic shunt, the greater the amount of blood that fails to be oxygenated as it passes through the lungs.

15 When V /Q is greater than normalWhen ventilation of some of the alveoli is great but alveolar blood flow is low, (more oxygen in the alveoli be wasted), anatomical dead space areas of the respiratory passageways is also wasted.

16 The sum of these two types of wasted ventilation is called the physiologic dead space.This is measured in the clinical pulmonary function laboratory by making appropriate blood and expiratory gas measurements

17 Abnormal V/Q in the upper and lower normal lungIn a normal person in the upright position, both pulmonary capillary blood flow and alveolar ventilation are considerably less in the upper part of the lung than in the lower part; (blood flow is decreased more than ventilation is. At the top of the lung, V /Q is as much as 2.5 times as great as the ideal value, which causes a moderate degree of physiologic dead space in this area of the lung

18 In the bottom of the lung, there is slightly too little ventilation in relation to blood flow, with V /Q as low as 0.6 times the ideal value. In this area, a small fraction of the blood fails to become normally oxygenated, and this represents a physiologic shunt.

19 During exercise, blood flow to the upper part of the lung increases and less physiologic dead space occurs.

20 -Emphysema causes many of the alveolar walls to be destroyed.-In smokers two abnormalities occur to cause abnormal V/Q First, because many of the small bronchioles are obstructed, the alveoli beyond the obstructions are unventilated, causing a V/Q that approaches zero.

21 Second, in those areas of the lung where the alveolar walls have been mainly destroyed but there is still alveolar ventilation, most of the ventilation is wasted because of inadequate blood flow to transport the blood gases.

22 In chronic obstructive lung disease, some areas of the lung exhibit serious physiologic shunt, and other areas exhibit serious physiologic dead space. Both these conditions decrease the effectiveness of the lungs as gas exchange organs, &this is the most prevalent cause of pulmonary disability .

23 Effect of the ventilation-perfusion ratio on alveolar gas concentrationTwo factors determine the Po2 and the Pco2 in the alveoli: (1) the rate of alveolar ventilation and (2) the rate of transfer of oxygen and carbon dioxide through the respiratory membrane.

24 Schematic of a single lung unit for the introduction of concepts of ventilation and perfusion relationships. V̄, mean ventilation; PV̄, partial pressure in venous blood; CV̄, content in venous blood; V̇a, alveolar ventilation; Pa, alveolar pressure; Q̇, O2 leaving the alveolus by way of blood flow; V̇o2, O2 consumption. Schematic of a single lung unit for the introduction of concepts of ventilation and perfusion relationships.

25 relationships between V̇a, V̇o2, and the local Po2 in the alveolusBathtub analogy for demonstrating the relationships between V̇a, V̇o2, and the local Po2 in the alveolus (\batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(P\mbox{\textsc{a}}_{O_{2}}\) \end{document}). Note that the spigot represents ventilation for O2. \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(P\mbox{\textsc{i}}_{O_{2}}\) \end{document}, inspired Po2. relationships between V̇a, V̇o2, and the local Po2 in the alveolus ©2008 by American Physiological Society

26 Three different lung regions with ventilation-to-perfusion ratios (V/Q ratios) of 0 (left), 1 (middle), and ∞ (right). The expected Po2s are shown for each region. \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(Pv_{O_{2}}\) \end{document}, venous Po2. Three different lung regions with ventilation-to-perfusion ratios (V/Q ratios) of 0 (left), 1 (middle), and ∞ (right).

27 Distribution of lung regions with different V/Q ratios throughout the normal lung.Distribution of lung regions with different V/Q ratios throughout the normal lung. Most of the lung has regions with V/Q ratios near 1.0. Shunt (V/Q = 0) and dead space (V/Q = ∞) represent the ends of the distribution. \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(Cc_{O_{2}}\) \end{document}, capillary O2 content; \batchmode \documentclass[fleqn,10pt,legalpaper]{article} \usepackage{amssymb} \usepackage{amsfonts} \usepackage{amsmath} \pagestyle{empty} \begin{document} \(Ca_{O_{2}}\) \end{document}, arterial O2 content.

28 How the V/Q Ratio affects Alveolar GasPa02 is determined by the amount of 02 entering the alveoli and its removal by capillary blood flow. If capillary blood flow is low Pa02 will remain high. This occurs in the apices; Ventilation with very little perfusion results in a high Pa02

29 How the V/Q Ratio affects Alveolar GasPaC02 is determined by: the amount of capillary blood perfusing an alveolus, allowing the CO2 to diffuse out of the capillary bed and into the alveolus and the amount of ventilation that alveolus receives. Areas of high perfusion and low ventilation have higher PaC02’s. ie: The bases.

30 Affects of Increased V/QWhen the V/Q ratio increases, the PaO2 rises and the PaCO2 falls. The PaCO2 decreases because it washes out of the alveoli faster than it is replaced by venous blood. The PaO2 increases because it does not diffuse into the blood as fast as it enters the alveolus. This V/Q relationship is present in the upper segments of the upright lung. (zone I)

31 Affects of Decreased V/QWhen the V/Q ratio decreases, the PaO2 falls and the PaCO2 rises. The PaO2 decreases because oxygen moves out of the alveolus and into pulmonary blood faster than it is replenished by ventilation. The PaCO2 increases because it moves out of the blood and into the alveolus faster than it is washed out. This is seen in the lower lung segments.(zone III)

32 Respiratory Quotient Gas exchange between the systemic capillaries and the cells is called internal respiration. About 250 ml of O2 are consumed by the tissues during 1 minute. The cells produce about 200 ml of CO2. The ratio between the volume of O2 consumed and the volume of CO2 produced is called the respiratory quotient.

33 Respiratory Exchange RatioGas exchange between the pulmonary capillaries and the alveoli is called external respiration. The quantity of O2 and CO2 exchanged during a period of 1 minute is called the respiratory exchange ration (RR). Under normal condition,s the RR equals the RQ.