Assessment of Continuous Measurement of Transcutaneous CO2 for Evaluation of Alveolar Dead Space During Exercise

Patients in intensive care, equipped, for their routine clinical management, with an arterial catheter blood line, will have a simultaneous determination of PaCO2 and PtCO2: * 1\) at rest; 2 arterial CO2/PtCO2 determinations will be carried out, 10 minutes apart * 2\) During the muscular exercise part of their routine rehabilitation program (whatever it might be, limbs active mobilization, walking, etc...) : several arterial /PtCO2 samples will be collected during the first minutes of their muscular routine: 30 sec, 1 minute, 1 ½ minute, 2, 2 ½ , 3, 4, 5 minutes, 10 minutes (end of exercise). This will generate several couples of blood/transcutaneous CO2 determinations

Repeated pairs of PtCO2 and PaCO2 (through arterialized capillary sampling), will be measured at the following timing: rest (0), then 30 sec post beginning of hyperventilation (breathing with accelerated frequency e.g. 30/min), 1 minute, 1 ½ minute 2, 2 ½ , 3, , 4, 5 minutes; recovery of this hyperventilation will be followed, for 5 minutes, with the PtCO2, during 3 minutes

an exercise test, according to the routine protocol used in the clinic: initial phase of 3 minutes gentle pedaling (10 to 50 watts, according to subject capabilities), then stepwise increase in the workload, by increments of 20-40 watts every minute, until maximum tolerated workload (VO2max). Repeated pairs of PtCO2 and PaCO2 (through arterialized capillary sampling), will be measured at the following timing: rest -10 minutes, 0, then 30 sec post beginning of exercise, 1 minute, 1 ½ minute 2, 2 ½, 3 minutes, then 3 ½ minutes, then at mid-point of every step increase in workload, until maximum tolerated workload. Recovery will be followed with the PtCO2, for 5 minutes