A reply

Abstract

We are grateful to Drs McCormack and Kelly for their interest in our publication on the accuracy of non-invasive carbon dioxide monitoring. They correctly point out that transcutaneous Pco2 (Ptcco2) measurement may be influenced by low cardiac output, and that we did not obtain cardiac index values at the time of the measurements. Because most of our study patients had normal cardiac function, insertion of a pulmonary arterial catheter was rarely indicated or justified. Therefore, cardiac index values were available only in a small minority of our patients. However, pre-existing normal cardiac function and haemodynamic stability make low cardiac output a very unlikely confounder of our study. Drs McCormack and Kelly have also missed data on the effects of vasopressor therapy on Ptcco2 values in our study. We have not reported these data in detail but stated in the Results section that vasopressors did not influence the accuracy of Ptcco2 monitoring in our study patients. As mentioned in the discussion, these findings are in agreement with those of two recent studies [1, 2] that also failed to find an influence of administered of vasopressors on Ptcco2 measurements. Drs McCormack and Kelly postulate that interpretation of data from patients with pulmonary pathologies may be difficult because of the disturbed pulmonary carbon dioxide transfer. We agree that pulmonary pathologies may influence the arterial Pco2 level. However, why should transcutaneous carbon dioxide transfer be influenced by pulmonary disease, unless the latter is combined with major haemodynamic consequences? The same holds true for low PEEP (2–8 mmHg), as used in our patients. PEEP has been found to influence Ptcco2 measurements only at higher PEEP levels [3]. Drs McCormack and Kelly correctly state that findings of industry-sponsored trials may not necessarily be invalid. However, conflicts of interest may arise at various levels when industry representatives perform or co-author studies involving their own products, thus questioning the reliability of their study results [4]. We are grateful to the manufacturers for providing the monitors and all disposables based on a contract that confirmed our unrestricted right to interpret and publish the data. We fully support Drs McCormack and Kelly's call for further scientific evaluation of transcutaneous Pco2 monitoring. We only tested its usefulness in the specific setting of patients during and shortly after major surgery. Our conclusion may not apply to patients in different clinical settings.