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Department of Anesthesiology and Critical Care Medicine, Tokyo Medical and Dental University School of Medicine Tokyo, Japan 113-8519
I read the report by Zaugg et al. (1) with interest, but I question whether the PaO2 data displayed on the Paratrend 7 (PT7) were reliable during one-lung ventilation (OLV) for several reasons.
First, the bias for the sequential changes of PaO2 between two consecutive time points (
PaO2) differed significantly from the hypothesized ideal value of 0, although the authors failed to indicate this. Second, the exclusion criteria for the PaO2 data were not clear. Some PaO2 (blood gas analysis) values may have been much higher (e.g., by 
40% of the PaO2 on the PT7) than the PaO2 on the PT7 data that were recorded simultaneously (underestimation of the true PaO2 by PT7). If such PaO2 data were not excluded from the study, the authors must have overestimated the underestimation of the PaO2, which the authors maintained, were detected by PT7. Third, the insignificant correlation between SpO2 and the PaO2 value on the PT7 suggests that the PaO2 data were not reliable. The authors should have measured the PaO2(BGA) to confirm that pulse oximetry did not discriminate changes in PaO2. Finally, we found that PaO2 data as measured on the PT7 are not always reliable during OLV because of rapid changes in PaO2 and slow response time of the sensor (2).
PT7 is a useful monitor for showing trends even during OLV (2). However, I believe that PaO2 data as measured by the PT7 should be interpreted more carefully during OLV, and the authors should have explained the reliability of that PaO2 value in more detail.
References
Institute of Anesthesiology University Hospital Zurich Zurich, Switzerland
We thank Dr. Ishikawa for his interest in our study (1). We would like to reply to his comments as follows.
The Paratrend 7 monitoring system (PT7), which was used in our study, is a widely validated and accepted method of continuous intraarterial blood gas measurement with good accuracy and performance. Apart from our own results in patients undergoing thoracoscopic interventions with one-lung ventilation (2), this device has been validated in an experimental study (3). In the intensive care unit (4), and during cardiac surgery (5). Furthermore, this device was used by two other groups, and their results have also been published (6,7). Nevertheless, in our study, we provided ample data on the good agreement of PT7 data with laboratory blood gas analyses. In fact, whenever a laboratory blood gas analysis was performed, PT7 values were recorded simultaneously and used for bias/precision analysis. We found an overall limit of agreement for bias/precision of -3.4/15.9 mm Hg in the clinically most important range of PaO2 values <100 mm Hg. Therefore, a PaO2 value of 65 mm Hg obtained by PT7 could be as low as 45.7 mm Hg or as high as 77.5 mm Hg. However, both values clearly indicate hypoxemia under an inspired oxygen fraction of 1.0 and, thus, represent a critical medical condition.
More importantly, the above-calculated range is made by assuming that differences between the PT7 continuous blood gas monitoring system and the laboratory blood gas analysis are a priori due to errors in PT7 measurements. We (and others) do not believe that this assumption is correct. Neither method—PT7 or laboratory blood gas analysis–provides unequivocally correct measurements, and bias and precision only assess the degree of agreement. Therefore, laboratory blood gas analysis should not be considered the "gold standard" anymore (8).
Furthermore, the assumption that the poor correlation between pulse oximetry and the PaO2 values obtained by PT7 would be due to inaccurate PT7 PaO2 values is not correct. Inaccuracy of pulse oximetry, particularly during thoracic surgery, is well known. Reference 5 in our article (9) represents a carefully conducted clinical evaluation of pulse oximetry during thoracic surgery. The authors were motivated by their experience of a poor correlation between SpO2 and arterial oxygen saturation (notably measured by a laboratory blood gas analyzer).
Because the maximal and minimal values obtained by PT7 between two consecutive laboratory blood gas analyses can be determined only retrospectively by analyzing the recorded PT7 data, the proposed simultaneous laboratory blood gas analyses are not feasible with the given study design. Based on our experience, the accuracy of the PT7 multisensor system in measuring arterial blood gases is not significantly affected by the response time even during the rapid and extreme blood gas changes associated with thoracoscopic surgery (2,10).
Finally, we were surprised about the high incidence of clot formations in Dr. Ishikawa's report on 12 patients undergoing esophagectomy (3 of 12, 25%) (11). It seems likely that these clots significantly affected the accuracy of the sensor. Prevention of clot formation is crucial to obtain accurate measurements and largely depends on the correct handling and positioning of the PT7 sensor. We did not observe any clot formation in this study (1) or in any of our previous studies (2,10).
References
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