Correlation Between TcPco2 and PaC02 at Steady State
Twenty-six pairs of measurements were recorded from 26 different subjects. Because PaC02 can vary substantially even in patients in apparently stable condition, we ensured that arterial samples were taken after stabilization of TcPco2 values for at least 3 min. We also checked for the stability of these values afterwards: 3 min after blood sampling, changes in TcPco2 were minimal and ranged from 0 to 3 mm Hg (mean±SD: 1±1 mm Hg). For correlation studies, PaC02 values were compared with values of TcPco2 at time of sampling.
Possible Drift of TcPco2 During Continuous Recording
To detect any systematic drift of TcPco2 values as compared with PaC02, we continuously recorded TcPco2 in five patients for 4 h. We did not exceed 4 h to avoid any risk of skin burning as a consequence of the temperature of the sensor. Arterial samples were taken at the beginning of the recordings, then every hour for 4 h. antibiotics online
Response of Transcutaneous Measurements to Induced Changes in PaCOz
In six hypercapnic patients, we examined the response of TcPco2 to a ventilatory event that was expected to induce a change in PaC02. The event consisted in initiating NPPV in five patients and interrupting NPPV in one. During the procedure, TcPco2 was recorded continuously. Arterial blood samples were drawn 3 and I min before the event, then 1, 3, 5, 7, 9, and 20 min afterwards. Values of PaCO£ were paired with the corresponding values of TcPco2 at the time of arterial blood sampling. Values obtained 1 min before the event were considered as baseline. An exponential fit curve was computed for PaC02 and TcPco2 curves of each patient. The “90% response time” (t90%) was then calculated for each curve (ie, the time at which PaC02 or TcPco2 had reached 90% of the final equilibrated value). The mean difference between t90% for TcPco2 and tgo% for PaC02 was reported as an index of the TcPco2 response time itself. Results are reported as mean value (range).
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