Shunts And Venous Admixture

Matching of the lung's airflow and blood flow is not perfect. On one side of the alveolar-capillary membrane there is "wasted air" (i.e., physiological dead space), and on the other side there is "wasted blood" (Fig. 20.14). Wasted blood refers to any frac-

Ventilation Blood Flow

Profiles for alveolar ventilation and blood 'flow in healthy adults. The y-axis represents flow (either blood flow or airflow) in L/min. The ventilation-perfusion ratio is shown on the x-axis, plotted on a logarithmic scale. The optimal Va/Q ratio is 0.8 in healthy lungs. (Adapted from Lumb AB. Nunn's Applied Respiratory Physiology. 5th Ed. Oxford: Butterworth-Heinemann, 2000.)

Perfusion The Lungs

Effect of regional differences of ventilationperfusion ratios on blood gases in the apex and base of the lungs.

"Wasted air"

Alveolar-capillary membrane

"Wasted air"

Alveolar-capillary membrane

Mixed Venous Blood Gases

Expired gas

Mixed venous / / blood / I PO2 = 40 mm Hg Pco2 = 46 mm Hg

End-pulmonary capillary blood Po2 = 102 mm Hg PCO2 = 40 mm Hg

Systemic arterial blood

"Wasted air" and "wasted

"blood." The plumbing on both sides of the alveolar-capillary membrane is imperfect. On one side there is "wasted air" and on the other side there is "wasted blood." The total amount of wasted air constitutes physiological dead space and the total amount of wasted blood (venous admixture) constitutes physiological shunt.

tion of the venous blood that does not get fully oxygenated. The mixing of unoxygenated blood with oxygenated blood is known as venous admixture. There are two causes for venous admixture: a shunt, and a low Va/Q ratio.

An anatomic shunt has a structural basis and occurs when blood bypasses alveoli through a channel, such as from the right to left heart through an atrial or ventricular septal defect or from a branch of the pulmonary artery connecting directly to the pulmonary vein. An anatomic shunt is often called a right-to-left shunt. The bronchial circulation also constitutes shunted blood because bronchial venous blood (deoxygenated blood) drains directly into the pulmonary veins that are carrying oxygenated blood.

The second cause for venous admixture is a low regional Va/Q ratio. This occurs when a portion of the cardiac output goes through the regular pulmonary capillaries but there is insufficient alveolar ventilation to fully oxygenate all of the blood. With a low regional Va/Q ratio, there is no abnormal anatomic connection and the blood does not bypass the alveoli. Rather, blood that passes through the alveolar capillaries is not completely oxygenated. In a healthy individual, a low Va/Q ratio occurs at the base of the lung (i.e., gravity dependent). A low regional a/ ratio can also occur with a partially obstructed airway (Fig. 20.15), in which underventilation with respect to blood flow results in regional hypoventilation. A fraction of the blood passing through a hypoventilated region is not fully oxygenated, resulting in an increase in venous admixture.

The total amount of venous admixture as a result of anatomic shunt and a low Va/Q ratio equals physiological shunt and represents the total amount of wasted blood that does not get fully oxygenated. Physiological shunt is analogous to physiological dead space; the two are compared in Table 20.1, in which one represents wasted blood flow and the other represents wasted air. It is important to remember that, in healthy individuals, there is some de-

Normal Pao2 = 102 mm Hg Paco2 = 40 mm Hg

Normal Pao2 = 102 mm Hg Paco2 = 40 mm Hg

Normal Po2
PO2 = 40 mm Hg PCO2 = 46 mm Hg

Local low Va/Q Pao2 < Normal Paco2 > Normal

Low Venous Po2
PO2 = 40 mm Hg PCO2 = 46 mm Hg

Local high Va/Q Pao2 > Normal Paco2 < Normal

Local high Va/Q Pao2 > Normal Paco2 < Normal

Low Venous Po2
PO2 = 40 mm Hg PCO2 = 46 mm Hg

Abnormal ventilation-perfu-"sion ratios. Airway obstruction (middle panel) causes a low regional ventilation-perfusion (Va/Q) ratio. A partially blocked airway causes this region to be underventilated relative to blood flow. Note the alveolar gas composition. A low regional Va/Q ratio causes venous admixture and will increase the physiological shunt. A partially obstructed pulmonary arteriole (right panel) will cause an abnormally high Va/Q ratio in a lung region. Restricted blood flow causes this region to be overventi-lated relative to blood flow, which leads to an increase in physiological dead space.

^ABLE 20.1^ Shunts and Dead Spaces Compared

Shunt

Dead Space

Anatomic

Anatomic

Low Va/Q ratio

Alveolar

Physiological shunt (calculated

Physiological dead space (calculated

total "wasted blood")

total "wasted air")

gree of physiological dead space as well as physiological shunt in the lungs.

In summary, venous admixture results from anatomic shunt and a low regional Va/Q ratio. In healthy individuals, approximately 50% of the venous admixture comes from an anatomic shunt (e.g., bronchial circulation) and 50% from a low Va/Q ratio at the base of the lungs as a result of gravity. Physiological shunt (i.e., total venous admixture) represents about 1 to 2% of cardiac output in healthy people. This amount can increase up to 15% of cardiac output with some bronchial diseases, and in certain congenital disorders, a right-to-left anatomic shunt can account for up to 50% of cardiac output. It is important to remember that any deviation of Va/Q ratio from the ideal condition (0.8) impairs gas exchange and lowers oxygen tension in the arterial blood. A good way to remember the importance of a shunt is that it always leads to venous admixture and reduces the amount of oxygen carried in the systemic blood.

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Responses

  • doreen
    What is venous admixture?
    1 year ago
  • kevin
    How to tell a venous admixture on a blood gas?
    7 months ago
  • MAXIMILIAN JAGER
    Where do we have venous ad mixture in pulmonary circulation?
    7 months ago
  • amy-leigh
    How does venous admixture cause a normal shunt?
    2 months ago

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