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St. Boniface General Hospital, Winnipeg, MB
Correspondence to: Meir Kryger, MD, FCCP, Director, Sleep Disorders Centre, St. Boniface General Hospital, University of Manitoba, R2034, 351 Tache Ave, Winnipeg, MB, R2H 2A6 Canada
To the Editor:
I am writing to comment on the article by Davila et al1 (November 2002), which highlighted the importance of understanding instruments, mainly oximeters in his example, when acquiring data to be used in medical diagnosis. Davila and colleagues1 showed that, depending on the acquisition parameters set on the instrument and the mechanism of storage of the data, the results of variables such as oxygen desaturation index could vary tremendously, with the result that the settings on the instrument can result in patients being misdiagnosed. There are many oximeters on the market, and there are many data acquisition systems that have oximeters built in, and many people using these systems are not aware of the problems that Davila and colleagues1 have pointed out in their article.
Although response time of the oximeter and mechanism of storage are very important, we showed that oximeter and sensor type and location may also dramatically affect the oximetry signal.2 Using the same instrument, we demonstrated that an ear sensor provided better performance than a finger sensor. In addition to trying to correlate instantaneous oximetry readings with findings on more rapidly responding channels, for example the EEG, there may be a much longer delay in the oximetry signal when a finger sensor is used than an ear sensor. We believe that an ear sensor is preferable to a finger sensor, although it is a bit more inconvenient to use; but because of the faster response, we are able to assess variables such as heart-to-ear circulation time, which is a useful measure, for example in patients with cardiac failure.3 In those few patients in whom obtaining a signal from the ear is problematic because of perfusion problems, application of a tiny amount of topical vasodilator improves the signal. We applaud Davila et al1 for reminding us all once again how important it is to understand the instruments we are using to diagnose our patients. The fine print in the instruction manual really is important.
References
Baptist Health Sleep Disorders Clinic, Little Rock, AR
Correspondence to: David G. Davila, MD, FCCP, Baptist Health Sleep Disorders Center, 9601 Interstate 630, Exit 7, Little Rock, AR 72205-7299
To the Editor:
Our study focused on downstream issues, in terms of differential averaging, storage, and display of data once it had reached the oximeter. As pointed out by Kryger, it is important to realize that upstream factors, relating to differences in the sensor types and locations, can be significant sources of variability in the data as well. We agree that it is important for the clinician to appreciate the vulnerabilities in all the links of the chain of detection, transmission, computation, storage, and display of oximetry data.
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