physiological dead space

If anatomical dead space is the wasted air sitting in the corridors, physiological dead space is the full accounting of all wasted breathing — corridors plus any rooms that are aired but empty of guests. It is the total volume of each breath that fails to exchange gas, whether because it never reached an air sac or because the air sac it reached had no blood flowing past it.

Formally, physiological dead space equals anatomical dead space plus alveolar dead space, the latter being air sacs that are ventilated but underperfused or unperfused. In a healthy person the two are nearly equal, because almost all ventilated air sacs are also well perfused, so alveolar dead space is tiny. Disease is what pries them apart: anything that reduces blood flow to ventilated lung adds alveolar dead space and so enlarges the physiological total.

Because it captures wasted ventilation from any cause, physiological dead space is the clinically meaningful quantity. A rising dead-space fraction — the share of each breath that is wasted — signals worsening gas-exchange efficiency and, in critically ill patients, predicts a harder fight to clear carbon dioxide. It can be estimated at the bedside by comparing the carbon dioxide in exhaled gas with that in arterial blood.