RESPIRATORY PHYSIOLOGY
Student Questions and Comments

Question: Why are high VA/Q lung regions called "Alveolar Dead Space"?

Dr. Brown,
In your notes, you have down that the alveolar dead space refers to the areas in the lungs that are over ventilated and under perfused. Shouldn't it be those that are under ventilated and over perfused because that are the equivalent to a shunt. I thought that areas that were over ventilated and under perfused were able to contribute a normal O2 partial pressure to the systemic arterial system.

Answer:

The term "Dead Space" refers to a volume in the respiratory system that is ventilated with air but does not exchange gases with pulmonary capillary blood. Overly ventilated/under perfused regions fit the dead space definition only approximately because they do exchange some gas with the pulmonary capillaries. However, the amount exchanged is relatively small because of the low blood flow (there simply isn't much blood with which to exchange) and, because of the low flow, blood from this region makes only a minor contribution to the total amount of blood entering systemic arteries.

So, because these regions contribute little to the total gas exchange between alveoli and blood, most of the ventilation to the high ventilation/low perfusion regions is, in a sense, "wasted". Therefore their volume is classified as alveolar dead space.

The term "Shunt", or, more precisely, right-to-left shunt, refers to blood that passes from systemic veins to systemic arteries without exchanging gas with alveolar air. Under ventilated/over perfused pulmonary regions do exchange some gas but not much because their low ventilation rate provides little gas to exchange. Therefore, blood flow through these regions approximate a true shunt.