The recording of dead space will give information on how much of total ventilation that reaches both ventilated and perfused alveoli and thus allows gas exchange between alveoli and pulmonary blood. Realising that CO2 retention can be an effect not only of low total ventilation but also of increased dead space is one important information. Moreover, dead space will give insight into the matching of ventilation and perfusion. This is because dead space is affected by a number of factors: 1) tubings and valves that the subject has to rebreath through (apparatus dead space), 2/ Airways (anatomical dead space), 3/ Non-perfused but ventilated alveoli, e.g. pulmonary embolus (alveolar dead space), 4/ Excessive ventilation of alveoli in relation to their perfusion that can be seen in chronic obstructive lung disease (another form of alveolar dead space), and 5/ So called "shunt dead space" that is an erroneous description of right to left lung shunt that brings the higher CO2 concentration in venous blood to the arterial side thereby producing an arterial-to-end-tidal CO2 difference. The dead spaces 2-5 are called physiological dead space. The recording of dead spaces can be done according to the Riley three-compartment model or by analysis of the expired CO2 curve. However, both are subjected to potential errors that have to be considered to make a dead space recording meaningful. A correct measurement and calculation of the dead space will give valuable information on the ventilatory support of the critically ill patient and can also be a valuable diagnostic tool. It should therefore not be forgotten in the intensive care setting.