School of Medicine

Ventilation-Perfusion Relationships

I. Ventilation-perfusion relationships ( )

A. Concept: for optimal gas transfer to occur in the lung, ventilation and perfusion must be matched.

1. For the whole lung, alveolar ventilation and pulmonary blood flow are both about 5 L/min.

2. Therefore, for the whole lung  is about 0.8 - 1.2.

3. However, ventilation and perfusion must be matched on the alveolar-capillary level.

4. Local airway responses and hypoxic pulmonary vasoconstriction help match ventilation and perfusion.

B. Consequences of high and low (Levitzky Fig. 5-1):

1. Normal

a. Inspired air entering alveoli:

PO₂ = 150 mmHg; PCO₂ = 0 mmHg

b. Mixed venous blood entering pulmonary capillaries:

PO₂ = 40 mmHg; PCO₂ = 45 mmHg

c. Result:

Alveolar PO₂ = 100 mmHg;

Alveolar PCO₂ = 40 mmHg

2. Low 

a. Extreme case - perfusion, but no ventilation ( = 0): Complete airway obstruction or atelectasis leading to intrapulmonary shunt

b. Result: PAO₂ = 40 mmHg; PACO₂ = 45 mmHg; no gas exchange

3. High 

a. Extreme case - ventilation, but no perfusion (= infinity): no perfusion, leading to alveolar dead space

b. Result: PAO₂ = 150 mmHg; PACO₂ = 0 mmHg; no gas exchange

4. There is a continuum of 's ranging from zero to infinity, resulting in a range of PO₂'s and PCO₂'s, as shown on the O₂ - CO_​2 diagram (Levitzky Fig. 5-2). Low  lower PO₂'s, higher PCO₂'s; high  's higher PO₂'s, lower PCO₂'s.

C. Tests of non-uniform gas distribution:

1. Single breath of O₂ test. Monitor expired N₂ concentration in "alveolar plateau" phase (Levitzky Fig.3-14). Steep plateau = non-uniform gas distribution.

2. N2 "washout" with O₂. In a normal person alveolar N₂ concentration is less than 2.5% after 7 minutes (Levitzky Figs. 5-3, 5-4).

3. Trapped Gas. Difference between FRC as determined by helium dilution and body plethysmograph techniques.

4. Single breath of ¹³³Xe. Location of poorly ventilated areas by lung scan.

5. Technetium labelled aerosolized DTPA.

D. Tests for non-uniform pulmonary blood flow:

1. Pulmonary angiogram

2. Lung scan after injection of ¹³¹I or technetium labelled macroaggregates of albumin

3. Lung scan after intravenous ¹³³Xe E. Tests of mismatched ventilation and perfusion:

1. Physiologic shunts:

a. Anatomic shunts - normally <2% of the cardiac output; bronchial veins, thebesian veins, and pleural veins.

b. Absolute intrapulmonary shunts

c. Shunt-like states - low 

d. The shunt equation (Pulmonary venous admixture):

2. Physiologic Dead Space:

a. Anatomic Dead Space

b. Alveolar Dead Space

c. The Bohr Equation:

d. There is normally no arterial-alveolar CO₂ difference. Arterial PCO₂'s greater than alveolar PCO₂'s (= end-tidal PCO₂'s, see Levitzky Fig. 3-9) indicate alveolar dead space (or possible diffusion block).

3. Alveolar-arterial O₂ difference (Levitzky Table 5-2). Normally 5-20 mm Hg or age divided by 4 plus 4 mm Hg. Increased  by mismatch, shunts, and diffusion block. Also increased by high FIO₂'s, low PO₂'s and shifts in oxyhemoglobin dissociation curve.

4. Alveolar plateau of the expired CO₂ trace (Levitzky Fig. 3-9).

5. Lung scans after inspired and injected radioactive xenon or technetium.

6. Multiple Inert Gas Elimination Technique (Levitzky Fig. 5-5). 7.

II.  Regional    differences in the normal upright lung and their consequences (Levitzky Figs. 5-6, 5-7, 5-8).

 

        

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Copyright 2000 M. G. LEVITZKY

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Last updated Friday, July 19, 2013 4:10 PM

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