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Fernandez and colleagues¹ compare forehead reflectance pulse oximetry and finger transmittance pulse oximetry in a group of critically ill patients with low cardiac index. The data appear to show that oxygen saturations from forehead oximetry were statistically closer to blood gas analysis than were data derived from finger oximetry; however, the differences are clinically irrelevant. Based on these data, the authors conclude that forehead oximetry is "better" than finger oximetry for this group of patients.

However, a methodological flaw may have affected the study’s results. Patients who required Trendelenberg positioning for shock were excluded a priori from entering the study. Unfortunately, the reader cannot know how many patients were screened and excluded for this reason, because these data are not provided. Having the patient’s head lowered for any reason (eg, postural drainage, central line insertion) can affect the accuracy of forehead reflectance oximetry due to venous congestion.^2,3

This inaccuracy would affect the statistical comparison of oxygen saturation derived from the different methods. Spuriously low values may prompt unnecessary and potentially harmful interventions. Although use of a headband to apply pressure to the sensor site has been shown to reduce the effect of slightly negative incline positioning (–15°) among normal volunteers,³ to my knowledge this has not been studied in patients with shock. Why exclude these patients?

This limitation of forehead reflectance oximetry is an obstacle that must be overcome before the technology can be used with confidence in cases in which finger oximetry now is regularly applied. By excluding patients who required negative incline positioning, Fernandez et al did not convincingly demonstrate that forehead pulse oximetry is "better" than or an acceptable replacement for finger pulse oximetry for such patients. I do agree, however, that further study is needed before we can safely make such assumptions.

Jeffrey M. Haynes, RRT, RPFT
St Joseph Hospital, Nashua, NH

REFERENCES

1. Fernandez M, Burns K, Calhoun B, Saramma G, Martin B, Weaver C. Evaluation of a new pulse oximeter sensor. Am J Crit Care. 2007;16:146–152.[Abstract/Free Full Text]
2. Jorgensen JS, Schmid ER, Konig V, Faisst K, Huch A, Huch R. Limitations of forehead pulse oximetry. J Clin Monit. 1995;11:253–256.[Medline]
3. Agashe GS, Coakley J, Mannheimer PD. Forehead pulse oximetry: headband use helps alleviate false low readings likely related to venous pulsation artifact. Anesthesiology. 2006;105(6):1111–1116.[Medline]

We thank Mr Haynes for his remarks. Although we used Trendelenberg positioning as an exclusion criterion in our study, rarely are patients in our units positioned in such a way. This may have beenevidenced by the fact that no patient was excluded from our study specifically for that reason.

Our rationale for excluding patients in the Trendelenberg position was that this study was conducted "on label," which restricts the use of the device to conditions identified by the manufacturer in the product directions for use.¹ In this case, directions included only non-Trendelenberg positions. Such exclusion is not a methodologic flaw, however, because previous reports have raised concerns about venous pooling of blood during Trendelenberg positioning, a condition that may lead to spurious underestimation of oxygen saturation due to venous blood in the local tissues.^2–6

We concur with Mr Haynes that repeating the study by Agashe et al⁶ could help to determine whether their finding of no difference in forehead sensor performance during Trendelenberg positioning in healthy volunteers would be the same for critically ill patients with low cardiac index. Because no published data exist on the performance of finger pulse oximetry during negative incline positioning, it would be ideal to examine peripheral sensor performance. Perfusion to the extremities is likely decreased in Trendelenberg positioning, potentially leading to signal dropout problems.

Marco Fernandez, RN, MSN, Kathy Burns, RN, BSN, MNED, CCRN, Beverly Calhoun, RN, BSN, Saramma George, RN, BA, Beverly Martin, RN, BSN and Chris Weaver, RN, BSN
REFERENCES

1. Directions for Use. OxiMax® MaxFast® adhesive forehead reflectance oximetry sensor. Nellcor Puritan Bennett Division, Tyco Healthcare Group LP, Pleasanton, Calif, #100001370A-1204:2–6.
2. Cooke J, Scharf J. Forehead pulse oximetry in the Trendelenberg position [abstract]. Anesthesiology. 2004;101:A583.
3. Cheng EY, Hopwood MB, Kay J. Forehead pulse oximetry compared with finger pulse oximetry and arterial blood gas measurement. J Clin Monit. 1988:4(3):223–226.[Medline]
4. Jorgensen JS, Schmid ER, Konig V, Faisst K, Huch A, Huch R. Limitations of forehead pulse oximetry. J Clin Monit. 1995;11(4):253–256.[Medline]
5. Shelley KH, Tamai D, Jablonka D, Gesquiere M, Stout RG, Silverman DG. The effect of venous pulsation on the forehead pulse oximeter wave form as a possible source of error in SpO₂ calculation. Anesth Analg. 2005;100:743–747.[Abstract/Free Full Text]
6. Agashe GS, Coakley J, Mannheimer PD. Forehead pulse oximetry: headband use helps alleviate false low readings likely related to venous pulsation artifact. Anesthesiology. 2006;105(6):1111–1116.[Medline]

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