Ventilation, perfusion and V/Q mismatch

Normal ventilations and perfusion:

Dyspnea, hypoxemia, hypercapnia, acidosis… lungs are failing… but why and how? In order to make a better diagnosis and properly approach to the cause of the problem, 2 main issues can be identified:

Ventilation and perfusion: Main interaction between the alveolar sac and the pulmonary capillary bed. Gas exchange occurs here. O2 goes into the blood stream as CO2 is delivered back to the alveoli so it can be exhaled.

1. Air is not reaching the alveoli. Either not enough air is being inhaled (obstruction due to bronchospasm) or alveolar sacs are full of fluids (pneumonia, hemorraghe). This means VENTILATION is failing. Blood in the capillary bed is not receiving any oxygen and co2 is not being delivered into the alveoli. This co2 rich blood later joins the rest of the circulation where ventilation has been good and therefore oxygenation too. This means there is a mix of o2 rich and co2 rich blood with a hypoxemic net effect. This mix is also called “a shunt”.

Ventilation failure: Either air can’t properly reach the alveoli or the alveoli are occupied by something like fluids or blood. Gas exchange can’t occur so blood in the capillary stays CO2 rich. As blood stream circulates it reaches other areas with O2 rich blood from other regions of the lung. This mixing is also called “shunt”.

2. A second possibility is that air IS reaching the sacs, but there’s not enough blood in the capillaries to exchange gases with. Circulation and perfusion failure happens for different reasons: It’s typical to have reduced blood flow due to intense vasoconstriction, which happens as a physiologic response to hypoxemia. In other words, a bad case of sleep apnea could cause such constriction, perfusion can’t be good. This is also true for ARDS’ hypoxemia. Another classic cause is PE, as the emboli interrupts normal capillary circulation. Finally we can’t forget the destroyed and fibrotic alveoli in enphysema from bad and chronic COPD and pulmonary fibrosis. Scarry alveoli have no vessels. The sacs have no function and no gas exchange occurs. Basically it’s a useless ventilation. We call this “dead space”.

If capillary circulation fails, there also won’t be gas exchange. The alveolar sac is useless as air just moves with no respiratory function. This is also called “dead space”.

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-The Plague Doctor


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