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LETTERS TO THE EDITOR

Is Continuous Intraarterial Blood Gas Monitoring Reliable During One-Lung Ventilation?

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

1. Zaugg M, Lucchinetti E, Zalunardo MP, et al. Substantial changes in arterial blood gases during thoracoscopic surgery can be missed by conventional intermittent laboratory blood gas analyses. Anesth Analg 1998;87:647–53.[Abstract/Free Full Text]
2. Ishikawa S, Makita K, Nakazawa K, Amaha K. Continuous intra-arterial blood gas monitoring during oesophagectomy. J Anaesth 1998;45:273–6.

Response

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

1. Zaugg M, Luccinetti E, Zalunardo MP, et al. Substantial changes in arterial blood gases during thoracoscopic surgery can be missed by conventional intermittent laboratory blood gas analyses. Anesth Analg 1998;87:647–53.
2. Zollinger A, Spahn DR, Singer T, et al. Accuracy and clinical performance of a continuous intra-arterial blood-gas monitoring system during thoracoscopic surgery. Br J Anaesth 1997;79:47–52.[Abstract/Free Full Text]
3. Clutton-Brock TH, Fink S, Markle D, et al. The evaluation of a new intravascular blood gas monitoring system in the pig. Monit 1994;10:387–91.
4. Venkatesh B, Clutton-Brock TH, Hendry SP. A multiparameter sensor for continuous intra-arterial blood gas monitoring : a prospective evaluation. Crit Care Med 1994;22:588–94.[Web of Science][Medline]
5. Venkatesh B, Clutton-Brock TH, Hendry SP. Evaluation of the Paratrend 7 intravascular blood gas monitor during cardiac surgery : comparison with the C4000 in-line blood gas monitor during cardiopulmonary bypass. J Cardiothorac Vasc Anesth 1995;9:412–9.[Web of Science][Medline]
6. Hatteril M, Tibby SM, Durward A, et al. Continuous intraarterial blood-gas monitoring in infants and children with cyanotic heart disease. Br J Anaesth 1997;79:665–7.[Abstract/Free Full Text]
7. Bacher A, Young Kwon J, Zornow MH. Effects of temperature on cerebral tissue oxygen tension, carbon dioxide tension, and pH during transient global ischemia in rabbits. Anesthesiology 1998;88:403–9.[Web of Science][Medline]
8. Larsen CP, Vender J, Seiver A. Multisite evaluation of a continuous intraarterial blood gas monitoring system. Anesthesiology 1994;81:543–52.[Web of Science][Medline]
9. Desiderio DP, Wong G, Shah NK, et al. A clinical evaluation of pulse oximetry during thoracic surgery. Anesth 1990;4:30–4.
10. Zollinger A, Zaugg M, Weder W, et al. Video-assisted thoracoscopic volume reduction surgery in patients with diffuse pulmonary emphysema : gas exchange and anesthesiological management. Anesth Analg 1997;84:845–51.[Abstract]
11. Ishikawa S, Makita K, Nakazawa K et al. Continuous intraarterial blood gas monitoring during oesophagectomy. Can J Anaesth 1998;45:273–6.[Web of Science][Medline]

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