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The partial pressure of oxygen in the tracheal air of a resting, young, healthy patient is known to be 150 mmHg; however, when the alveolar oxygen partial pressure is tested, the pressure reads 145 mmHg. This finding most likely indicates which of the following about this patient?
A. Normal alveolar ventilation and perfusion
B. Poor alveolar ventilation
C. Poor alveolar perfusion
D. Diffusion-limited gas exchange
F. Arteriovenous blood shunting
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C is correct
Under normal conditions, the pO2 of inspired air is approximately 160 mmHg. This value decreases to approximately 150 mmHg in the trachea due to the partial pressure of watery vapor at that location. After the inspired air enters the alveoli the pO2 rapidly drops to 104 mmHg and the pCO2 increases from a value near zero to approximately 40 mmHg. At this point the alveolar air has reached equilibrium with the pO2and pCO2values in the blood. Under normal resting conditions, the diffusion of both O2 and CO2 across the alveolar membrane is perfusion limited. This means that the rapid it with which the alveolar air equilibrates with the partial pressure of oxygen and carbon dioxide in the blood depends on how quickly a given volume of blood can course through the alveolar capillaries. In cases where perfusion is poor, equilibrium will occur slowly or may not occur at all. In the ultimate example of a ventilation/perfusion mismatch, a pulmonary embolus can block all blood flow to a given segment of lung that aerates normally. The result is complete failure of gas diffusion. In this situation the tracheal air and the alveolar air would have approximately the same composition. The patient described in the question stem has a tracheal pO2 of 150 mmHg and an alveolar pO2 of 145 mmHg. Thus, this patient is suffering from very poor alveolar perfusion evidenced by the failure of the alveolar gas to reach equilibrium with the blood gas.
(Choice A) In normal alveolar ventilation and perfusion the po2 in the alveoli rapidly reach equilibrium with the pO2 of the blood at a value of approximately 100 mmHg.
(Choice B) An alveolar gas sample in cases of poor alveolar ventilation would show a gas composition identical to that of the blood due to the extended amount of time that the volume of air in the poorly ventilated alveoli would have to reach equilibrium with the blood.
(Choice D) In diffusion-limited gas exchange the alveolar gas does note equilibrate with the blood gas by the time that a given volume of blood reaches the end of the alveolar capillary. The diffusion of O2 is normally perfusion-limited not diffusion-limited. A situation where O2 becomes diffusion limited is in disease states such as emphysema and pulmonary fibrosis, and occurs physiologically in states of very high pulmonary blood flow such as during exercise. The question stem specifically states that this patient is young and at rest, so these situations are unlikely.
(Choice E) Arteriovenous blood shunting would cause changes in the pO2 of the venous blood but would not affect the equilibration of alveolar gas with blood gas. If the venous pO2 were increased by an A/V shunt then more blood would need to flow through the pulmonary capillaries before equilibrium could be reached.
Educational Objective:
The diffusion of O2 in a normal individual at rest is perfusion limited and the equilibrium value of O2 in the alveoli is approximately 100 mmHg. Situations where the exchange of O2 is diffusion limited are during exercise in normal individuals (high pulmonary blood flow) and also present in certain diseases such as emphysema and pulmonary fibrosis.
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