Fig. 2. Sequence of measurements performed at hypocapnia, normocapnia, and hypercapnia. During a phase of 15–20-min steady-state conditions, arterial carbon dioxide tension was adjusted to 30, 40, and 50 mmHg and verified by blood gas analysis. Over a period of 4 min, the first transcerebral double indicator dilution measurement was performed, followed by argon application via  the lungs. Blood samples were obtained over the next 10 min to estimate the argon content and the cerebrovenous argon content difference for cerebral blood flow calculation. At the end of the argon measurement, a second blood gas analysis as well as a second TCID measurement were performed.