“…Various types have been proposed, depending on the mode of recirculation of air in the catheter between measurement intervals, and one is commercially available (Tonocap; Datex, Helsinki, Finland) [1,5,6]. This automated semicontinuous technique (involving an ex vivo infrared sensor) may both over- and underestimate saline PCO 2 tonometry, depending, among other reasons, on correction factors for incomplete equilibration times and the bias of the blood gas machine used for the latter [1,3,4,6,7]. …”