Transcutaneous measurement of pCO ₂ (p tc CO ₂ ): Time delay from change in alveolar pCO ₂ to first response in p tc CO ₂

Introduction: The p tc CO 2 delay has been estimated to 16 sec (healty subjects) [1] and <60 sec (hypercapnic patients) [2]. We simultaneously record polysomnography (PSG) and p tc CO 2 from COPD patients and needed the response time in our setup. Material: 9 stable COPD patients (6 male) with mean FEV1 41% of pred. (SD 20). Method: Patient in supine position wearing face mask connected to a stopcock, inlet selecting either room air or a bag with 4% CO 2 in air. P tc CO 2 measured by Radiometer TOSCA 500, probe on the earlobe. Arterial samples from arterial catheter analyzed by Radiometer ABL 500. 3 test-phases, each 200 sec: 1) stable phase breathing room air, 2) increasing phase after switching to 4% CO 2 , 3) decreasing phase after switching back to room air. TOSCA p tc CO 2 was read every 5. sec for 120 sec, then every 10. sec. Arterial samples were drawn 3 times during phase 1), every 5. sec the first 30 sec of phase 2) and 3), then every 30. sec for a total of 150 sec. Results: First response time (T fr ) meaning time from change in alveolar pCO 2 to p tc CO 2 >2SD off stable phase. Mean T fr (SD) in increasing phase: 54 (5,6) sec. Mean T fr (SD) in decreasing phase: 57 (15) sec. For arterial pCO 2 : 13,3 (5,6) sec and 11,7 (2,5) sec, accordingly. Conclusion: Scoring PSG from COPD patients, the transcutane pCO 2 signal should be left-shifted 2 epochs (1 minute) as a respiratory event changing alveolar pCO 2 will show a first p tc CO 2 response after 54-57 sec. References: 1. Kesten S. et al Chest 1991;99:1211-15. 2. Janssens J.P. et al Chest 1998; 113:768–773.