Intrapulmonary Shunting and Deadspace

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Intrapulmonary Shunting and Deadspace

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

FIG. 8-1 The theoretical respiratory unit. A, Normal ventilation, normal perfusion. B, Normal ventilation, no perfusion. C, No ventilation, normal perfusion. D, No ventilation, no perfusion.

A Ideal alveolar-capillary unit: An alveolar-capillary unit in which perfusion and ventilation are normal; theoretically, a unit with a / ratio of 1.0.

B Deadspace unit: An alveolar-capillary unit in which ventilation is normal but perfusion is diminished or absent; a unit with a / ratio >1.0.

C Shunt unit: An alveolar-capillary unit in which perfusion is normal but ventilation is diminished or absent; a unit with a / ratio <1.0.

D Silent unit: An alveolar-capillary unit without perfusion or ventilation and therefore a / ratio of 0.0.

II Intrapulmonary Shunting

A 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.

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

FIG. 8-2 Concept of physiologic shunting (see text). t is cardiac output per unit time; c is the portion of the cardiac output that exchanges perfectly with alveolar air; s is the portion of the cardiac output that does not exchange with alveolar air; and PaO₂ is the alveolar oxygen tension.

FIG. 8-2 Concept of physiologic shunting (see text). t is cardiac output per unit time; c is the portion of the cardiac output that exchanges perfectly with alveolar air; s is the portion of the cardiac output that does not exchange with alveolar air; and PaO₂ is the alveolar oxygen tension.

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

a. Normally approximately 2% to 5% of the cardiac output bypasses the pulmonary capillaries because the following veins empty into the left side of the heart:

b. Increases in anatomic shunt may occur as a result of:

(1) Vascular pulmonary tumors

(2) Arterial venous anastomosis

(3) Congenital cardiac anomalies

(4) Severe liver disease

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

FIG. 8-3 Alveolar-capillary diagram of intrapulmonary (capillary) shunting showing why supplemental O₂ fails to correct hypoxemia. Only O₂ exchange is shown, and P(A-a)O₂ is assumed to be zero. A, With room air, although blood leaving the normal alveolar-capillary unit is normally saturated, blood passing the capillary on the right ‘sees’ no O₂ because its alveolus is unventilated and it leaves the unit unsaturated. When the two streams of blood mix, the resulting Pao₂ is determined by the average of their O₂ contents, not by their PO₂ values. B, Addition of 40% oxygen fails to correct the hypoxemia because O₂ content is not significantly increased in the normal unit and capillary blood in the unventilated unit still ‘See’ no O₂. Even 100% O₂ could not completely reverse the oxygenation defect in this example; very different from the effect with a low / as illustrated in Figure 8-4.

FIG. 8-3 Alveolar-capillary diagram of intrapulmonary (capillary) shunting showing why supplemental O₂ fails to correct hypoxemia. Only O₂ exchange is shown, and P(A-a)O₂ is assumed to be zero. A, With room air, although blood leaving the normal alveolar-capillary unit is normally saturated, blood passing the capillary on the right ‘sees’ no O₂ because its alveolus is unventilated and it leaves the unit unsaturated. When the two streams of blood mix, the resulting Pao₂ is determined by the average of their O₂ contents, not by their PO₂ values. B, Addition of 40% oxygen fails to correct the hypoxemia because O₂ content is not significantly increased in the normal unit and capillary blood in the unventilated unit still ‘See’ no O₂. Even 100% O₂ could not completely reverse the oxygenation defect in this example; very different from the effect with a low / as illustrated in Figure 8-4.

a. Normally capillary shunting does not exist.

b. Capillary shunting is caused by:

(1) Atelectasis

(2) Consolidating pneumonia

(3) Complete airway obstruction

(4) Pneumothorax

(5) Pulmonary edema

(6) Any pathophysiologic process that eliminates ventilation to perfused alveoli

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

FIG. 8-4 Hypoxemia caused by / mismatch showing the effect of supplemental O₂. / is normal on the left side of each idealized lung unit and low on the right. Only O₂ exchange is shown, and the P(A-a)O₂ is assumed to be zero. A, With room air, not enough O₂ reaches the poorly ventilated alveolus to fully saturate its capillary blood. B, With 40% O₂, Pao₂ in this alveolus is increased enough to make its capillary PO₂ nearly normal. Note that the Pao₂ in the mixed blood from the two capillaries is determined by the average of the O₂ contents of the two blood streams, not by their Pao₂ values.

FIG. 8-4 Hypoxemia caused by / mismatch showing the effect of supplemental O₂. / is normal on the left side of each idealized lung unit and low on the right. Only O₂ exchange is shown, and the P(A-a)O₂ is assumed to be zero. A, With room air, not enough O₂ reaches the poorly ventilated alveolus to fully saturate its capillary blood. B, With 40% O₂, Pao₂ in this alveolus is increased enough to make its capillary PO₂ nearly normal. Note that the Pao₂ in the mixed blood from the two capillaries is determined by the average of the O₂ contents of the two blood streams, not by their Pao₂ values.

a. Under normal conditions, shunt effect occurs in the bases of the lung, where / ratios are <1.0.

b. Venous admixture may be increased by:

(1) Retained secretions

(2) Bronchospasm

(3) Partial airway obstruction

(4) Regional increases in fibrotic tissue

(5) Decreased tidal volumes

(6) Mucosal edema at the bronchiolar level

(7) Interstitial edema

III Derivation of Classic Shunt Equation

A Definition of abbreviations

1. o₂: Volume of oxygen consumed per minute

2. s: Shunted cardiac output

3. c: Capillary cardiac output

4. t: Total cardiac output

5. Cco₂: Capillary oxygen content

6. Cao₂: Arterial oxygen content

7. Co₂: Mixed venous oxygen content

8. Pao₂: Alveolar oxygen partial pressure

9. Pao₂: Arterial oxygen partial pressure

B 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(Cao2–Cv–o2) (1)

(1)

C Because actual capillary blood flow (c) 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(Cco2–Cv–o2) (2)

(2)

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

Q˙t=Q˙s+Q˙c (3)

(3)

E Solving equation 3 for c:

Q˙c=Q˙t+Q˙s (4)

(4)

F Substituting into equation 2 the equivalent of o₂ from equation 1:

Q˙t(Cao2–Cv¯o2)=Q˙c(Cco2–Cv¯o2) (5)

(5)

G Substituting into equation 5 the equivalent of c from equation 4:

Q˙t(Cao2–Cv–o2)=(Q˙t–Q˙s)(Cco2–Cv–o2) (6)

(6)

H Rearranging equation 6:

Q˙t(Cao2–Cv–o2)=(Q˙t–Q˙s)(Cco2–Cv–o2) (7)

(7)

I Eliminating −tCo₂ from both sides of equation 7:

Q˙t(Cao2–Cv–o2)=(Q˙t–Q˙s)(Cco2–Cv–o2) (8)

tCao₂ =

tCco₂ −

Cco₂ +

sC

o₂ (8)

J Rearranging equation 8:

Q˙t(Cao2–Cv–o2)=(Q˙t–Q˙s)(Cco2–Cv–o2) (9)

sCco₂ −

sC

o₂ =

tCco₂ +

tCao₂ (9)

K Simplifying equation 9:

Q˙t(Cao2–Cv–o2)=(Q˙t–Q˙s)(Cco2–Cv–o2) (10)

s(Cco₂ − C

o₂) =

t(Cco₂ − Cao₂) (10)

L Rearranging equation 10:

Q˙s/Q˙t=Cco2–Cao2Cco2–Cv–o2 (11)

(11)

M 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.

N This equation is used to calculate the total physiologic shunt.

IV Calculation of the Total Physiologic (or Intrapulmonary) Shunt

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

B All oxygen content determinations are based on the following equation:

Q˙s/Q˙t=Cco2–Cao2Cco2–Cv–o2 (12)

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

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

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

E Capillary oxygen content

1. Because a blood sample from an ideally functioning alveolar-capillary unit is impossible to obtain, this calculation is based on the assumption that the end pulmonary capillary oxygen tension (Pco₂) is equal to the alveolar oxygen tension (Pao₂) in an ideally ventilated and perfused alveolar-capillary unit.

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