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Canadian Journal of Anesthesia 52:379-382 (2005)
© Canadian Anesthesiologists' Society, 2005
General Anesthesia

The Stealth StationTM Image Guidance System may interfere with pulse oximetry

[Le système de guidage par imagerie Stealth StationTM peut nuire à la sphygmo-oxymétrie]

Johannes H. van Oostrom, PhD, Michael E. Mahla, MD and Dietrich Gravenstein, MD

From the Department of Anesthesiology, University of Florida College of Medicine and Departments of Biomedical Engineering, and Electrical and Computer Engineering, University of Florida College of Engineering, Gainesville, Florida, USA.

Address correspondence to: Dr. Johannes H. van Oostrom, Department of Anesthesiology, P.O. Box 100254, Gainesville, Florida 32610-0254, USA. Phone: 352-846-0914; Fax: 352-392-6407; E-mail: hans{at}anest.ufl.edu


   Abstract
TOP
 Abstract
Introduction
 Technical features
 Discussion
 Conclusion
 References
 
Purpose: Interference on pulse oximetry can come from many sources. We found an additional source of interference from the Stealth StationTM. This article gives an overview of sources of pulse oximeter interference so that clinicians can better prevent them.

Technical features: This article discusses the infrared disturbances caused by the Stealth StationTM. The Stealth StationTM is a frameless stereotactic positioning system that utilizes a three dimensional location system to measure the position of the patient and the surgical tools, and to relate those positions to previously recorded imaging. To understand the disturbance caused by the Stealth StationTM, we discuss its operation and that of pulse oximeter monitors. Pulse oximeter interference can come from volume artifacts, electrical and light noise, and can be caused by issues related to the patient. Because the passive Stealth StationTM contains a strong infrared light source, interference caused by light is a likely reason for the interference we noted. Pulse oximeters rely on the time-variant light signal modulated by arterial volume variations in the finger. Although relatively immune to static light sources, pulse oximeters are extremely sensitive to time-varying light sources. The light emitted by the passive Stealth StationTM is time-varying at 4 Hz and this is causing the pulse oximeter to provide invalid results. Shielding can generally be used to stop the light from the Stealth StationTM from being picked up by the pulse oximeter sensor.

Conclusion: Infrared light interference can be very common, but is easily dealt with if one is aware of it.


   Introduction
TOP
 Abstract
 Introduction
Technical features
 Discussion
 Conclusion
 References
 
HEMOGLOBIN oxygenation percentages as calculated by the pulse oximeter are generally trusted. However, a number of factors can cause invalid saturations to be calculated, or cause other failures of the pulse oximeter. Limitations of pulse oximeter measurements can be grouped as: volume artifacts, noise from light interference or electrical interference, and patient related interference. While the factors interfering with the saturation measurement are well understood,1 we discovered another interference that can occur when a Stealth StationTM Image Guidance System (Medtronic Sofamor Danek, Memphis, TN, USA) is in use. The Stealth StationTM interfered with the pulse oximeter causing it to give invalid readings of around 80%. We investigated the cause of this problem, and are reporting it in this article.


   Technical features
TOP
 Abstract
 Introduction
 Technical features
Discussion
 Conclusion
 References
 
The Stealth StationTM Image Guidance System is a frameless stereotactic surgical positioning system, frequently used for neurological cases.2 Magnetic resonance images (MRI) are recorded previously with fiducial markers placed on the patient’s scalp. The position of the fiducial markers can be tracked in the operating room by an infrared (IR) imaging system. This system contains an IR light source and a camera, both mounted on the ceiling or on a pole. The light source illuminates the surgical field and the IR camera records the location of the markers in three dimensional space. In addition, surgical instruments outfitted with markers are also used and tracked. Because the relationship between the fiducial markers connected to the patient and the MRI is known, and the relationship between the position of the surgical instruments and the patient is known, it is possible to relate the position of the surgical instruments with the MRI. The surgical instruments’ position and orientation are represented in the three-dimensional image of the patient’s head and allows accurate planning of the surgical approach to the tumour.

During several cases using the Stealth StationTM, oxygen saturation was monitored using a Philips CMS (Philips, Andover, MA, USA) pulse oximeter integrated monitor. The hardware revision of the pulse oximeter module was M1020A and the CMS contained software revision number 17.62. Interference on the pulse oximetry was noted (Figure 1Go). This interference appeared as an approximate 4 Hz disturbance, which caused the pulse oximeter to display saturations below 80%. We found that the interfering signal was coming from the Stealth StationTM Image Guidance System. There is no indication of a light source on the Stealth StationTM camera arm, because the system is IR light-based, which is not visible to the human eye.



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  		FIGURE 1 Pulse oximeter plethysmogram with interference (bottom) and electrocardiogram (top).

 
To eliminate the interference, we attempted to shield the pulse oximeter probe from the Stealth StationTM IR light sources. We first used a blue cloth towel wrapped around the hand (Figure 2aGo), which reduced the amplitude of the interfering signal, but did not eliminate it. We then used the cover of an alcohol pad, which is a paper cover with an aluminium lining inside. We slid this cover over the pulse oximetry probe and the interference was eliminated (Figure 2bGo).



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  		FIGURE 2a Pulse oximeter plethysmogram with interference while covered with a towel (bottom) and corresponding electrocardiogram (top).

 


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  		FIGURE 2b Pulse oximeter plethysmogram without interference after covering with a metallic sheath (bottom) and corresponding electrocardiogram (top).

 

   Discussion
TOP
 Abstract
 Introduction
 Technical features
 Discussion
Conclusion
 References
 
Interference on pulse oximeter signals can originate from a number of sources (TableGo). Volume artifacts are present when there are volume changes at the measurement site not caused by the arterial pulse (movement artifact for example). Noise artifacts can be light interference (like in our case) or electrical interference and will typically show up in the plethysmogram as additional waveform fluctuations. Sometimes these fluctuations cause an increased calculated heart rate or change saturation calculation. Patient related interferences, like the presence of carboxyhemoglobin, can also cause incorrectly calculated saturations.


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  		TABLE
 
To examine the reasons for our interference, we need to investigate the operation of the Stealth StationTM and pulse oximeters. The Stealth StationTM registers the instruments through their IR light signature. This signature is produced either actively, that is from IR light-emitting diodes (LEDs) fixed on the instruments that emit light, or passively (like in our case) from the reflection of IR light generated near the detection cameras back from circular reflectors mounted on the instruments. The detection cameras are mounted on the ceiling or on a movable pole. Because the detector can be up to ten feet away, the power of the IR system is quite strong. The IR signals are frequency modulated to facilitate pick up at the sensor. It is this modulated signal that interferes with the pulse oximeter probe. Smith et al. provide more details on the Stealth SystemTM.2

Pulse oximeter probes contain red and IR LED on one side of the probe and a photodetector on the other side. The photodetector is used to pick up the light emitted from the LEDs that has been modulated by the pulsating volume changes in the finger. This photodetector is typically sensitive to a wide wavelength spectrum. The two plethysmograph signals (one for red, one for IR) detected by the photodetector are processed to calculate the blood oxygen saturation.

Pulse oximetry relies on the Beer Lambert law to calculate absorbency A = –ln (I/I0) where I is the detected light intensity by the detector and I0 is the intensity of light emitted by the photo diode. A (at a given wavelength) consists of absorbency due to oxygenated hemoglobin (Ao) and reduced hemoglobin (Ar), as well as a time-invariant absorbency due to other tissues such as bone, venous blood, etc. (Ax).10


To eliminate the last term a time derivative of A is taken (dA/dt), which leaves only the terms with time varying components (arterial blood). It is for this reason that pulse oximeter probes are relatively immune to fixed light signals (as long as they do not overpower the detector), but they are extremely sensitive to varying signals to facilitate detection of small vascular bed volume changes. It should also be noted that IR light sources are readily reflected by hard surfaces like walls, floors, and equipment in the operating room. As a result, the IR light sources on the Stealth StationTM do not need to be directly aimed at the pulse oximeter probe to cause significant interference.


   Conclusion
TOP
 Abstract
 Introduction
 Technical features
 Discussion
 Conclusion
References
 
Infrared interference on the pulse oximetry signal could come from many sources. Infrared communication has many applications: remote controls, synchronization (as on PalmTM devices), wireless communication between laptops and printers, etc. All of those IR light sources are modulated at some frequency, which means that they have the potential to interfere with pulse oximetry. The degree of interference will depend on the level of signal filtering present in pulse oximeters. With Stealth StationTM technology already having a rapidly growing role in orthopedic, otolaryngologic and neurosurgical procedures, this interference should be expected. Simple shielding measures will resolve this interference.


   Footnotes
 
Accepted for publication June 15, 2004. Revision accepted January 19, 2005.


   References
TOP
 Abstract
 Introduction
 Technical features
 Discussion
 Conclusion
 References
 
1 Jubran A. Pulse oximetry. Crit Care 1999; 3: R11–7.[Medline]

2 Smith KR, Frank KJ, Bucholz RD. The NeuroStationTM–a highly accurate, minimally invasive solution to frameless stereotactic neurosurgery. Comput Med Imaging Graph 1994; 18: 247–56.[Medline]

3 Jopling MW, Mannheimer PD, Bebout DE. Issues in the laboratory evaluation of pulse oximeter performance. Anesth Analg 2002; 94(1 Suppl): S62–8.

4 Goldman JM, Petterson MT, Kopotic RJ, Barker SJ. Masimo signal extraction pulse oximetry. J Clin Monit 2000; 16: 475–83.

5 Trivedi NS, Ghouri AF, Shah NK, Lai E, Barker SJ. Effects of motion, ambient light, and hypoperfusion on pulse oximeter function. J Clin Anesth 1997; 9: 179–83.[Medline]

6 Block FE Jr, Detko GJ Jr. Minimizing interference and false alarms from electrocautery in the Nellcor N-100 pulse oximeter. J Clin Monit 1986; 2: 203–5.[Medline]

7 Barker SJ, Tremper KK, Hyatt J. Effects of methemoglobinemia on pulse oximetry and mixed venous oximetry. Anesthesiology 1989; 70: 112–7.[Medline]

8 Vokach-Brodsky L, Jeffrey SS, Lemmens HJ, Brock-Utne JG. Isosulfan blue affects pulse oximetry. Anesthesiology 2000; 93: 1002–3.[Medline]

9 Ralston AC, Webb RK, Runciman WB. Potential errors in pulse oximetry. III: effects of interferences, dyes, dyshaemoglobins and other pigments. Anaesthesia 1991; 46: 291–5.[Medline]

10 Flewelling R. Noninvasive optical monitoring. In: Bronzino JD (Ed.). IEEE Biomedical Engineering Handbook, volume I, 2nd ed. Boca Raton, Florida: CRC Press; 1999: 86-4–5.




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