## Objectives

- 1.

Describe the significance of a normal ventilation/perfusion ratio ( <SPAN role=presentation tabIndex=0 id=MathJax-Element-5-Frame class=MathJax style="POSITION: relative" data-mathml='V˙/Q˙’>V˙/Q˙˙V/˙Q

V˙/Q˙

˙V/˙Q

V ˙ / Q ˙

).

- 2.

Characterize the <SPAN role=presentation tabIndex=0 id=MathJax-Element-6-Frame class=MathJax style="POSITION: relative" data-mathml='V˙/Q˙’>V˙/Q˙˙V/˙Q

V˙/Q˙

˙V/˙Q

V ˙ / Q ˙

at the apex and at the base of the lung in upright individuals.

- 3.

Calculate the alveolar–arterial oxygen difference (AaDo

_{2 }) and describe how to use it.

- 4.

List the four major causes of hypoxemia and describe their anatomy and physiology.

- 5.

List the two major causes of hypercarbia and describe their anatomy and physiology.

- 6.

Distinguish pathophysiologic processes associated with hypoxemia using 100% oxygen.

Although ventilation and pulmonary blood flow (perfusion) are important individual components in the primary function of the lung, the relationship between ventilation and perfusion—specifically the ratio of ventilation to perfusion, defined as <SPAN role=presentation tabIndex=0 id=MathJax-Element-7-Frame class=MathJax style="POSITION: relative" data-mathml='V˙/Q˙’>V˙/Q˙˙V/˙Q

V˙/Q˙

˙V/˙Q

V ˙ / Q ˙

, is the major determinant of normal gas exchange. Before reading about ventilation–perfusion relationships, review ventilation ( Chapter 5 ) and perfusion ( Chapter 6 ).

## Ventilation/Perfusion Ratio

Both ventilation ( <SPAN role=presentation tabIndex=0 id=MathJax-Element-8-Frame class=MathJax style="POSITION: relative" data-mathml='V˙’>V˙˙V

V˙

˙V

V ˙

) and lung perfusion ( <SPAN role=presentation tabIndex=0 id=MathJax-Element-9-Frame class=MathJax style="POSITION: relative" data-mathml='Q˙’>Q˙˙Q

Q˙

˙Q

Q ˙

) are essential elements in the normal functioning of the lung, but they are insufficient to ensure normal gas exchange. For example, consider the situation in which blood is perfusing an area of the lung that has no ventilation ( Fig. 7.1 ). Overall ventilation and overall perfusion in the lung may be normal, but in this specific area of the lung, normal gas exchange does not occur because there is no ventilation. Thus, without ventilation, the blood entering and leaving the area would be unchanged and would remain deoxygenated. Similarly, imagine an area of the lung with normal ventilation but no perfusion. Gas entering and leaving the alveoli in this area would be unchanged; that is, it would not participate in gas exchange because there is no blood flow to the area.

The ventilation/perfusion ratio ( <SPAN role=presentation tabIndex=0 id=MathJax-Element-13-Frame class=MathJax style="POSITION: relative" data-mathml='V˙/Q˙’>V˙/Q˙˙V/˙Q

V˙/Q˙

˙V/˙Q

V ˙ / Q ˙

) is the ratio of ventilation to blood flow. It can be defined for a single alveolus, for a group of alveoli, or for the entire lung. At the level of a single alveolus, it is defined as the alveolar ventilation ( <SPAN role=presentation tabIndex=0 id=MathJax-Element-14-Frame class=MathJax style="POSITION: relative" data-mathml='V˙A’>V˙A˙VA

V˙A

˙VA

V ˙ A

) divided by the capillary blood flow ( <SPAN role=presentation tabIndex=0 id=MathJax-Element-15-Frame class=MathJax style="POSITION: relative" data-mathml='Q˙C’>Q˙C˙QC

Q˙C

˙QC

Q ˙ C

). At the level of the lung, it is defined as the total alveolar ventilation divided by the cardiac output.

In normal individuals, alveolar ventilation and blood flow are uniformly distributed to the gas-exchanging units with slightly less alveolar ventilation relative to pulmonary blood flow. At rest, in normal individuals, alveolar ventilation is ∼4.0 L/min and pulmonary blood flow is ∼5.0 L/min. Thus, in the normal lung, the overall ventilation/perfusion ratio is ∼0.8; however, there is a wide range of ventilation/perfusion ratios in different lung units ( Fig. 7.2 ). If ventilation and blood flow are mismatched, impairment of both O _{2 }and CO _{2 }transfer occurs. When ventilation exceeds perfusion, then <SPAN role=presentation tabIndex=0 id=MathJax-Element-16-Frame class=MathJax style="POSITION: relative" data-mathml='V˙/Q˙’>V˙/Q˙˙V/˙Q

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is greater than 1; when perfusion exceeds ventilation, <SPAN role=presentation tabIndex=0 id=MathJax-Element-17-Frame class=MathJax style="POSITION: relative" data-mathml='V˙/Q˙’>V˙/Q˙˙V/˙Q

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˙V/˙Q

V ˙ / Q ˙

is less than 1.

Ventilation–perfusion mismatching occurs with increasing age (see Fig. 7.2 ) and with various lung diseases. In individuals with cardiopulmonary disease, mismatching of pulmonary blood flow and alveolar ventilation is the most frequent cause of systemic arterial hypoxemia.

A normal ventilation/perfusion ratio does not mean that ventilation and perfusion to that lung unit are normal; it simply means that the relationship between ventilation and perfusion is normal. For example, in the presence of a lobar pneumonia, ventilation to the affected lobe is decreased. If perfusion to this area remained unchanged, perfusion would be in excess of ventilation—an example of an abnormal ventilation/perfusion ratio ( <SPAN role=presentation tabIndex=0 id=MathJax-Element-23-Frame class=MathJax style="POSITION: relative" data-mathml='V˙/Q˙<1′>V˙/Q˙<1˙V/˙Q<1

V˙/Q˙<1

˙V/˙Q<1

V ˙ / Q ˙ < 1

). However, as a result of decreased ventilation to this area, hypoxic vasoconstriction occurs in the pulmonary arterioles supplying this lobe. The result is a decrease in perfusion to the affected area and a more “normal” ventilation/perfusion ratio. However, neither the ventilation nor the perfusion to this area is normal (both are decreased); but the relationship between the two is (approaches) “normal.”

## Regional Differences in Ventilation and Perfusion

Because of regional differences in ventilation and perfusion, due largely to gravity and structural effects, even in the normal lung, <SPAN role=presentation tabIndex=0 id=MathJax-Element-24-Frame class=MathJax style="POSITION: relative" data-mathml='V˙/Q˙’>V˙/Q˙˙V/˙Q

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in different areas of the lung is greater than or less than the “ideal” normal value of 0.8. In the upright position, going from the top to the bottom of the lung, ventilation increases more slowly than blood flow increases. As a consequence, the <SPAN role=presentation tabIndex=0 id=MathJax-Element-25-Frame class=MathJax style="POSITION: relative" data-mathml='V˙/Q˙’>V˙/Q˙˙V/˙Q

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at the top of the lung is high (increased ventilation relative to blood flow in the pulmonary circulation), whereas the <SPAN role=presentation tabIndex=0 id=MathJax-Element-26-Frame class=MathJax style="POSITION: relative" data-mathml='V˙/Q˙’>V˙/Q˙˙V/˙Q

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at the bottom of the lung is “abnormally” low. This relationship between ventilation and perfusion is shown in Fig. 7.3 . The important point here is that although the overall <SPAN role=presentation tabIndex=0 id=MathJax-Element-27-Frame class=MathJax style="POSITION: relative" data-mathml='V˙/Q˙’>V˙/Q˙˙V/˙Q

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in the normal lung is 0.8, it is composed of a wide range of localized ventilation/perfusion ratios ( Fig. 7.4 ).