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TECHNOLOGY, COMPUTING, AND SIMULATION

The Effect of Venous Pulsation on the Forehead Pulse Oximeter Wave Form as a Possible Source of Error in Spo₂ Calculation

Department of Anesthesiology, Yale University, New Haven, Connecticut

Address correspondence and reprint requests to Kirk Shelley MD, PhD, Department of Anesthesiology, 333 Cedar Street, PO Box 208051, New Haven, CT 06520–8051. Address e-mail to kirk.shelley{at}yale.edu.

	Abstract

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Reflective forehead pulse oximeter sensors have recently been introduced into clinical practice. They reportedly have the advantage of faster response times and immunity to the effects of vasoconstriction. Of concern are reports of signal instability and erroneously low Spo₂ values with some of these new sensors. During a study of the plethysmographic wave forms from various sites (finger, ear, and forehead) it was noted that in some cases the forehead wave form became unexpectedly complex in configuration. The plethysmographic signals from 25 general anesthetic cases were obtained, which revealed the complex forehead wave form during 5 cases. We hypothesized that the complex wave form was attributable to an underlying venous signal. It was determined that the use of a pressure dressing over the sensor resulted in a return of a normal plethysmographic wave form. Further examination of the complex forehead wave form reveal a morphology consistent with a central venous trace with atrial, cuspidal, and venous waves. It is speculated that the presence of the venous signal is the source of the problems reported with the forehead sensors. It is believed that the venous wave form is a result of the method of attachment rather than the use of reflective plethysmographic sensors.

	Introduction

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The widespread and vital role of pulse oximetry in clinical care has prompted manufacturers to attempt to improve the reliability of the signal in the context of potentially complicating factors. A pulse oximeter manufacturer has recently introduced a reflective forehead probe (Max-Fast; Nellcor, Pleasanton, CA). These probes not only appear to respond faster than finger probes to changes in saturation (1) but also, and more importantly, appear to be less likely to be compromised by vasoconstriction (2). The sensor, which is placed above the eyebrow in the region of the supra-orbital artery, is attached in a manner similar to that of an adhesive bandage. Unfortunately, there have been reports of intermittent low saturation readings with the Nellcor Max-Fast system (3–5). The cause of these spuriously low readings is yet to be determined, which prompted us to compare the finger, forehead, and ear plethysmographic tracings obtained in 25 patients undergoing surgery at our institution. This identified a uniquely complex forehead tracing in five of the patients, prompting a test to determine if this difference was attributable to the observation that finger and ear plethysmographs are applied with a clip whereas the forehead probe is applied with a low-pressure dressing. Consideration was also given to the possibility that the complex wave form might be attributable specifically to the site of measurement (i.e., the complex wave form would only be detectable at the forehead site). It was hypothesized that the method of attachment might allow for a low-pressure component to appear in the plethysmographic wave form in either the forehead or the ear site.

	Methods

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With approval from the Human Investigations Committee, 25 ASA physical status I–II patients (20 females and 5 males) scheduled for elective surgery (gynecological and urological procedures) at Yale-New Haven Hospital were studied. None of the patients studied had a history of congestive heart failure or valvular heart disease. Verbal consent was obtained for the placement of the additional monitors. Written consent was obtained to review any relevant medical records. All patients received midazolam, 1–3 mg IV, for premedication. On arrival in the operating room, routine clinical monitoring devices were placed. In addition, each patient had a reflective infrared plethysmographic sensor placed on his or her finger, ear, and forehead as described in detail below. Anesthesia consisted of an IV induction (propofol 2.0 – 3.0 mg/kg IV), and tracheal intubation was facilitated with vecuronium 0.1 mg/kg IV. Sevoflurane 1%–3% in combination with nitrous oxide 60% in oxygen was administered for maintenance of anesthesia. Additional medications (fentanyl, morphine and ondansetron) were given at the discretion of the anesthesia care team. An upper body Bair Hugger warming unit (Model 505; Augustine Medical, Inc., Eden Prairie, MN) was applied at the start of the procedure and used throughout the case. At the end of surgery, inhaled anesthetics were discontinued and residual neuromuscular blockade was reversed with neostigmine 0.05 mg/kg IV and glycopyrrolate 0.01 mg/kg IV.

In addition to the plethysmographic wave forms, CO₂ wave forms, airway pressure wave forms, patient position (supine, Trendelenburg, or lithotomy), and estimated blood loss were recorded. All tracings were recorded with a data acquisition system (Powerlab/16SP; AD Instruments, Colorado Springs, CO) and a quad bridge amp (ML112) that can receive simultaneous input from up to four infrared plethysmographic transducers (MLT1020).¹ The plethysmographic transducers use reflective technology with the infrared (940–960 nm) LED next to the photoelectric diode. All the plethysmographic transducers are identical except for the method by which they attach to the body. The finger transducer attaches via a finger clip, the ear transducer via an ear clip, and the forehead transducer was secured with a Tegaderm (3M Health Care Ltd., St. Paul, MN) adhesive on the forehead just above the eyebrow (in the region of the supra-orbital artery). All plethysmographic sensors were light shielded. All wave forms were sampled at 100 Hz.

Data analysis was performed with Igor Pro (Wavemetics, Lake Oswego, OR), a data analysis and graphics package. The tracings from the different plethysmographs were assessed by an investigator in real time who inspected the wave forms as displayed by the data acquisition system and hence was blinded as to the site of placement. When a difference between the wave forms from the forehead and ear sensors was identified, we attempted to eliminate the difference between the forehead and ear signals by applying external pressure to the forehead probe and/or by relieving pressure from the ear clip. The former was achieved by application of a pressure dressing with a force that was less than the force that compromised the arterial amplitude. The latter was achieved by attaching the ear probe with an adhesive dressing, Tegaderm, instead of the ear clip. As above, the blinded investigator compared the tracings.

	Results

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As expected, the finger and ear clips generated typical "pulse oximetry" signals throughout the monitoring period in all patients. The forehead probe generated a signal similar to that from the finger and ear probes in 20 of the 25 patients, but it generated a more complex signal with a prominent venous component intermittently in 5 patients (Figs. 1 and 2). The aberrant forehead wave forms were characterized by configuration highly suggestive of a central venous wave form (Fig. 3). It was noted that the complex forehead wave form occurred exclusively in supine patients and was exacerbated when one of the five patients was placed in the Trendelenburg position at the request of the surgeon.

View larger version (30K): [in this window] [in a new window]   Figure 1. Simultaneous plethysmographic tracings from three sites (finger, ear, and forehead) using identical sensors. Data from one of the five study patients who demonstrated a pronounced venous component in the forehead tracing. X-axis depicts time (s); y-axis depicts output (volts). The finger and ear sensors are attached via a clip whereas the forehead is attached with a Tegaderm adhesive.

View larger version (28K): [in this window] [in a new window]   Figure 3. The effect of a pressure dressing on the forehead wave form.

View larger version (45K): [in this window] [in a new window]   Figure 2. The forehead plethysmographic tracing from five different patients all demonstrating venous characteristics.

Our hypothesis that the venous component of the forehead signal was attributable to less occlusive pressure at that site was confirmed by the elimination of the venous component by the application of external pressure to the forehead probe (Fig. 4) and by the appearance of the venous component when pressure was relieved at the ear probe (Fig. 5). The conversion of the complex venous wave form to an arterial wave form, with pressure, occurred with all five patients in whom the complex wave form was observed. The amount of pressure required to eliminate the venous pulse was not measured but appeared to be of a minimal amount. It did not cause blanching of the surrounding tissue.

View larger version (25K): [in this window] [in a new window]   Figure 4. The effect of removing the clip from the ear sensor and using a Tegaderm adhesive for attachment.

View larger version (18K): [in this window] [in a new window]   Figure 5. Annotated tracing of a patient’s simultaneous ear and forehead tracing demonstrating central venous morphology. The traditional method of labeling the central venous wave form is used with highlighting the three peaks (a - atrial, c - cuspidal, v - venous) and two descents (x, y).

The conversion of the complex wave form to the expected arterial wave form with the application of pressure (via pressure dressing or ear clip) is taken as evidence of the detection of an underlying venous wave form. In addition, the unmodified forehead wave form demonstrates features normally associated with the central venous wave form (Fig. 5).

	Discussion

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All pulse oximeters are fundamentally photoelectric plethysmographs that reflect changes in the blood volume of a peripheral vascular bed during the cardiac cycle. This technology has been shown to be quite sensitive to small amounts of pulsatile blood flow (6), and the determination of oxygen saturation is based on this feature. At first, it might seem surprising that the weaker central venous signal could have a pronounced effect on the plethysmographic wave form in the presence of the high pressure arterial wave form. However, the amplitude of the plethysmographic wave form is directly proportional to the vascular distensibility (7), which is significantly greater (up to 10-fold) in the arterial system. If it reaches the threshold for a pulsation, the venous signal can have significant impact on the calculation of the Spo₂ (8). This should be considered in the design of pulse oximetry probes and the interpretation of pulse oximetry signals.

Unfortunately, the proprietary nature of medical devices makes it impossible to determine if this is indeed the source of the intermittent error present in the Nellcor Max-Fast system. If the presence of a venous signal is indeed the source of the error, then use of a headband or pressure dressing should eliminate the problem. Unfortunately, this may make the sensor less comfortable to wear and could eliminate the arterial pulsation, especially in hypotensive patients, and slightly increase the risk of a pressure-induced injury to the site.

It is intriguing that under certain circumstances, the morphology of the central venous pressure wave form might be available to the clinician noninvasively. In considering the utility of this finding, it is important to keep certain limitations in mind. Despite having the superficial appearance of a pressure tracing, the plethysmograph is a measure of the relative change in blood volume in a small region of tissue over time. Therefore, the amplitude of the signal is very sensitive to changes in local vascular compliance and blood flow. In addition, unlike pressure measurements, the plethysmographic signal has no standardized calibration procedure, thereby compromising interpatient comparisons. On the other hand, the timing and morphology of the central venous wave form appears to be preserved. It is interesting to speculate as to potential clinical uses of this wave form when faced with a patient with congestive heart failure, right ventricular ischemia, cardiac tamponade, or valvular heart disease (9)

Although most commercially available forehead probes use reflectance plethysmography whereas ear and finger clips use transmittance plethysmography, this difference does not appear to be responsible for the selective appearance of the venous signal in the forehead probe. In the present study, all probes used reflectance plethysmography. In addition, applying pressure to the forehead probe eliminated the venous component (Fig. 4) and relieving pressure from the ear probe generated the venous component (Fig. 5). This finding is consistent with the finding that reflective sensors attached via clips, which apply pressure to the tissue, never demonstrated the venous wave form during this study. The exact amount of pressure required to eliminate the venous pulse was not measured. Presumably the amount of pressure would be close to the central venous pressure. In further support of our conclusion, the detection of venous pulsation in the pulse oximeter wave form has been previously described (10). In that case, transmission plethysmography was being used with the sensor located on the finger with a loose fitting, disposable, Band-Aid like attachment. In the case of the forehead sensor, the use of a standard sports headband or simple pressure dressing should be sufficient to suppress the impact of venous pulsation; this was not specifically tested in this study.

In conclusion, the method of attachment of a pulse oximeter sensor may have an impact on the device’s ability to accurately determine arterial oxygen saturation. The use of a clip, headband, or pressure dressing would appear to prevent the low pressure venous wave form interfering with the oximeter’s function. This difficulty may be counteracted through the use of digital signal processing techniques; this has not yet been determined. At this point, the mere presence of a venous wave form in the oximeter signal should not be over interpreted. It is likely the result of a complex interaction between the patient’s fluid status, competence of heart and venous valves, and the position of the patient.

	Footnotes

 
¹ It should be noted that that the use of this set-up in a subsequent study resulted in a burn on the forehead of a research subject. At this time the use of this infrared plethysmographic transducer can not be recommended. This occurred with the probe attached with a Tegaderm without external pressure applied. The mechanism of injury appears to be the result of a low voltage current leak from the probe as described in Leeming MN, Jacobs RG, Howland WS. Low voltage, direct current plethysmograph burns. Med Res Eng 1971;10:19–21.

	References

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1. Cooke J, Scharf J. Improving pulse oximeter performance (abstract). Anesthesiology 2002;96:A593.
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4. Redford DT, Barker SJ, Lichtenthal PR. Evaluation of 2 forehead reflectance oximeters in intraoperative surgical patients (abstract). Anesth Analg 2004;98(S10):A26.
5. Mahajan A, Lee E, Callom-Moldovan A. Intraoperative use of forehead reflectance oximetry in pediatric patients (abstract). Anesth Analg 2004;98(S7):A20.
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7. Dorlas JC, Nijboer JA. Photo-electric plethysmography as a monitoring device in anaesthesia: application and interpretation. Br J Anaesth 1985;57:524–30.[Abstract/Free Full Text]
8. Sami HM, Kleinman BS, Lonchyna VA. Central venous pulsations associated with a falsely low oxygen saturation measured by pulse oximetry. J Clin Monit 1991;7:309–12.[Web of Science][Medline]
9. Mark JB. Central venous pressure monitoring: clinical insights beyond the numbers. J Cardiothorac Vasc Anesth 1991;5:163–73.[Medline]
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