Ventilation Perfusion Defects

- is the ratio of alveolar ventilation (V) to pulmonary blood flow (Q).

Matching ventilation and perfusion is important to achieve the ideal exchange of 02 and C02.

- If the frequency, tidal volume, and cardiac output are normal, the V/Q ratio is approximately 0.8. This situation results in an arterial Po2 of 100 mm Hg and an arterial Pco2 of 40 mm Hg.

B. Changes in V/Q ratio

1. V/Q ratio in airway obstruction

- If the airways are completely blocked (e.g., by a piece of steak caught in the trachea), then ventilation is zero. If blood flow is normal, then V/Q is zero.

- There is no gas exchange in a lung that is perfused but not ventilated. The Po2 and Pco2 of pulmonary capillary blood (and, therefore, of systemic arterial blood) will approach their values in mixed venous blood.

2. V/Q ratio in blood flow obstruction

- If blood flow to a lung is completely blocked (e.g., by an embolism occluding a pulmonary artery), then blood flow to that lung is zero. If ventilation is normal, then V/Q is infinite.

- There is no gas exchange in a lung that is ventilated but not perfused. The Po2 and Pco2 of alveolar gas will approach their values in inspired air.

C. V/Q ratios in different parts of the lung (Table 4-5; see Figure 4-10)

-Both ventilation and blood flow are nonuniformly distributed in the normal lung.

1. Blood flow is lowest at the apex and highest at the base because of gravitational effects.

2. Ventilation is also lowest at the apex and highest at the base, but the regional differences for ventilation are not as great as for perfusion.

3. Therefore, the V/Q ratio is highest (> 1.0) at the apex of the lung and lowest (< 0.8) at the base of the lung.

4. As a result of the regional differences in V/Q ratio, there are corresponding differences in the efficiency of gas exchange and in pulmonary capillary

Table 4-5. V/Q Characteristics of Different Areas of the Lung

Regional

Regional

Area of

Blood

Arterial

Arterial

Lung

Flow

Ventilation

V/Q

Po2

Pco2

Apex

Lowest

Lower

Highest

Î

Base

Highest

Higher

Lowest

li

T

V/Q = ventilation/perfusion ratio.

V/Q = ventilation/perfusion ratio.

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