Intrapulmonary Shunting and Deadspace

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Intrapulmonary Shunting and Deadspace

Spectrum of Ventilation/Perfusion (image/image) Abnormalities (Figure 8-1)

II Intrapulmonary Shunting

A pathophysiologic process in which blood enters the left side of the heart without having been oxygenated by the lungs. The mixing of venous blood with oxygenated blood from the pulmonary capillaries to form arterial blood.

The total quantity of shunted blood is the physiologic shunt, which is composed of three subdivisions (Figure 8-2).

1. Anatomic shunt: The portion of the total cardiac output that bypasses the pulmonary capillary bed.

2. Capillary shunt: The portion of the total cardiac output that perfuses nonventilated alveoli (Figure 8-3).

3. Shunt effect (ventilation/perfusion inequality, low image/image, venous admixture): Any pathophysiologic process in which perfusion is in excess of ventilation; however, some ventilation is still present (Figure 8-4).

III Derivation of Classic Shunt Equation

Definition of abbreviations

1. imageo2: Volume of oxygen consumed per minute

2. images: Shunted cardiac output

3. imagec: Capillary cardiac output

4. imaget: Total cardiac output

The shunt equation is based on the Fick equation, which normally is used to calculate oxygen consumption or cardiac output:

< ?xml:namespace prefix = "mml" />V˙o2=Q˙t(Cao2Cvo2) (1)

image (1)

Because actual capillary blood flow (imagec) represents the portion of the cardiac output that actually perfuses ventilated alveoli and Cco2 is the oxygen content of blood leaving those perfused and ventilated alveoli, this equation may be rewritten as:

V˙o2=Q˙c(Cco2Cvo2) (2)

image (2)

Thus total cardiac output is equal to shunted cardiac output plus capillary cardiac output:

Q˙t=Q˙s+Q˙c (3)

image (3)

Solving equation 3 for imagec:

Q˙c=Q˙t+Q˙s (4)

image (4)

Substituting into equation 2 the equivalent of imageo2 from equation 1:

Q˙t(Cao2Cv¯o2)=Q˙c(Cco2Cv¯o2) (5)

image (5)

Substituting into equation 5 the equivalent of imagec from equation 4:

Q˙t(Cao2Cvo2)=(Q˙tQ˙s)(Cco2Cvo2) (6)

image (6)

Rearranging equation 6:

Q˙t(Cao2Cvo2)=(Q˙tQ˙s)(Cco2Cvo2) (7)

image (7)

Eliminating −imagetCimageo2 from both sides of equation 7:

Q˙t(Cao2Cvo2)=(Q˙tQ˙s)(Cco2Cvo2) (8)

imagetCao2 = imagetCco2image Cco2 + image sCimageo2 (8)

Rearranging equation 8:

Q˙t(Cao2Cvo2)=(Q˙tQ˙s)(Cco2Cvo2) (9)

imagesCco2imagesCimageo2 = imagetCco2 + imagetCao2 (9)

Simplifying equation 9:

Q˙t(Cao2Cvo2)=(Q˙tQ˙s)(Cco2Cvo2) (10)

images(Cco2 − Cimageo2) = imaget(Cco2 − Cao2) (10)

Rearranging equation 10:

Q˙s/Q˙t=Cco2Cao2Cco2Cvo2 (11)

image (11)

Equation 11 is the classic shunt equation, which states that the difference between the capillary oxygen content and arterial oxygen content divided by the difference between the capillary oxygen content and the mixed venous oxygen content equals the intrapulmonary shunt fraction.

This equation is used to calculate the total physiologic shunt.

IV Calculation of the Total Physiologic (or Intrapulmonary) Shunt

The intrapulmonary shunt is determined by calculating the capillary oxygen content, arterial oxygen content, and mixed venous oxygen content.

All oxygen content determinations are based on the following equation:

Q˙s/Q˙t=Cco2Cao2Cco2Cvo2 (12)

o2 content (vol%) = (Hb cont)(Hbo2% sat)(1.34) + (0.003)(Po2) (12)

Calculation of the arterial oxygen content requires data from an arterial blood gas.

Calculation of the mixed venous oxygen content requires data from a pulmonary artery blood gas.

Capillary oxygen content