Dead space and shunt, ventilation perfusion

[wuzzdaddy]

---

Members don't see this ad.

I understand that in the hypothetical case where the perfusion to an entire lung is impaired, e.g. pulmonary embolism, there is an increase in dead space because while there is adequate ventilation in the embolized lung, there is no perfusion. I understand that in this case the dead space is doubled so our minute ventilation goes from lets say 5 l/m to 2.5 l/m. Now, in the hypothetical case where an entire lung is filled with fluid, e.g. Pneumonia there is impaired ventilation but adequate perfusion and my physiology text book says that there is no change in minute ventilation. I have a hard time understanding why there is no change in minute ventilation because in this case, all the alveola in the affected lung are full of fluid; therefore, these alveola won't be ventilated and it seems that minute ventilation would also go from 5l/m to 2.5 l/m. But my physiology book disagrees. I appreciate if someone could clarify this for me.

 

[jdh71]

I understand that in the hypothetical case where the perfusion to an entire lung is impaired, e.g. pulmonary embolism, there is an increase in dead space because while there is adequate ventilation in the embolized lung, there is no perfusion. I understand that in this case the dead space is doubled so our minute ventilation goes from lets say 5 l/m to 2.5 l/m. Now, in the hypothetical case where an entire lung is filled with fluid, e.g. Pneumonia there is impaired ventilation but adequate perfusion and my physiology text book says that there is no change in minute ventilation. I have a hard time understanding why there is no change in minute ventilation because in this case, all the alveola in the affected lung are full of fluid; therefore, these alveola won't be ventilated and it seems that minute ventilation would also go from 5l/m to 2.5 l/m. But my physiology book disagrees. I appreciate if someone could clarify this for me.

Your physiology textbook is FoS in the real world. There will be an increase in VE if you fill up an entire lung because the body will compensate for the reasons you have already surmised.

Though you should probably think of the problem more in the abstract, not the practical. Think of it a snap shot at the very immediate moment of filling an entire lung. Right then. Before compensation.