THE use of pulse oximetry to continuously monitor blood oxygenation (Spo2) is accepted as the standard of care during anesthesia1and in the postanesthesia care unit, but the pulse oximeter, like any other monitoring device, is not perfect. Problems fall into two basic categories: (1) Data failure or dropout, when no Spo2reading is obtainable, occurs because of either too little signal or too much noise, i.e. , low signal/noise ratio. (2) The Spo2reading displayed is spurious, i.e. , it does not accurately predict the fractional (Hbo2%) or functional (Sao2%) oxygen saturation of hemoglobin in the arterial blood. In the search for the best available signal/noise ratio, the forehead offers several potential advantages over other pulse oximetry sensor placement sites. The skin of the lower forehead just above the eyebrows may be a better location for the sensor because its blood supply is from the supraorbital artery and therefore well maintained, and the area shows less vasoconstrictor response to cold or other stimuli compared with other peripheral sites.2Unfortunately, the forehead site is also associated with spuriously low Spo2readings in some patients. In this issue of Anesthesiology, Agashe et al. 3report how use of a headband that applies up to 20 mmHg pressure on the forehead pulse oximeter sensor decreases the incidence of spuriously low Spo2readings that are likely related to venous pulsation artifact. These authors disclose that they are all full-time employees of Nellcor Puritan Bennett, Tyco Healthcare (Pleasanton, CA), the sponsor of this study. Nellcor manufactures the Max-Fast forehead reflectance sensor and headband that were used.
Agashe et al. 3studied healthy volunteers breathing room air in the supine position and two levels of Trendelenburg positions using the forehead sensor with the headband adjusted to its maximum and minimum recommended pressure limits. Spo2readings obtained from the forehead sensor with the subjects supine and the headband in place were used as a baseline to compare the effects of Trendelenburg on Spo2reading accuracy with and without use of the headband. Occurrences of spuriously low Spo2readings detected by forehead sensors were compared with those from digit sensors. Agashe et al. 3found no difference between Spo2readings obtained from the forehead sensor in the supine and Trendelenburg positions when the headband was used. When it was not used, forehead Spo2readings obtained while subjects were in the Trendelenburg positions were significantly lower than the Spo2readings when the subjects were supine.
Pulse oximetry failure occurs frequently. Reich et al. 4reviewed 9,203 electronic anesthesia records at The Mount Sinai Medical Center and found a pulse oximetry failure rate of 9.18%. Independent intraoperative predictors of failure included hypothermia and hypotension. Almost all of these patients had been monitored using sensors placed on the fingers or toes.
There are anecdotal reports of forehead pulse oximetry working when sensors at other sites have failed; indeed, Nellcor advertises the Max-Fast forehead sensor as “most likely to succeed in challenging conditions.” However, to date, there is no published clinical study that compares the failure rate of forehead pulse oximetry sensors with other peripherally (i.e. , finger, toe, earlobe) placed sensors.
To properly assess the validity of data displayed by a physiologic monitor, in this case the Spo2reading, the clinician should understand the principles underlying the technology. The traditional two-wavelength pulse oximeter is an optical plethysmograph that measures the ratio of pulse-added absorbance (AC) to fixed absorbance (DC) of radiation at wavelengths of 660 and 940 nm. The “ratio of ratios,” often termed R, where R =[(AC660/DC660)/(AC940/DC940)], is used to determine the Spo2reading via an empiric algorithm created by the pulse oximeter manufacturer. The pulse-added signal is produced by changes in volume in the vascular bed at the sensor site, due to pulsatile arterial (oxygenated) blood flow during the cardiac cycle.5If there are also pulsations in venous blood at the sensor site, a lower Spo2reading will result because the instrument is unable to distinguish arterial from venous blood pulsations. When there is a continuous column of blood between the right heart and the forehead sensor site (i.e. , jugular vein valve absent), venous pulsations can be transmitted from the chest.6
Spuriously low Spo2readings are therefore most likely to occur during positive-pressure ventilation, in the head-down (Trendelenburg) position, and when venous drainage from the neck is impeded. The Spo2underreading can be significant, depending on the venous pulse pressure and the distensibility of the venous system at the probe site. Barker7reported one case (anterior neck surgery) in which the Max-Fast read an Spo2of 60–70% for the entire case while an earlobe sensor and arterial blood gas analysis indicated saturation percents in the mid-90s.
Shelley et al. 8studied the plethysmographic waveforms from reflectance pulse oximetry sensors (Max-Fast) placed on the finger, ear, and forehead of 25 patients undergoing general anesthesia. In 20 of the 25 patients, the forehead probe generated signals that were similar to the finger and the ear. In 5 patients, a more complex signal with an intermittent venous component was recorded. This component was exacerbated when the patient was placed head-down. Application of pressure to the forehead probe eliminated the venous component, whereas relieving pressure from the ear probe clip induced the venous component. The amount of pressure applied and use of a headband were not studied.8
The results of Agashe et al. 3suggest that use of a headband that applies 10–20 mmHg pressure to the forehead probe provides a potential solution to the problem of spuriously low Spo2readings in patients in whom venous pulsations are likely to occur. Their study has several limitations. First, the subjects were awake, healthy volunteers rather than potentially very sick patients undergoing general anesthesia with positive-pressure ventilation. Second, the subjects breathed room air and had baseline Spo2values of 98%. Because the pulse oximeter is used clinically to detect hypoxemia, performance under such conditions must be evaluated. Third, it is unclear from this study whether a headband tension of 10–20 mmHg reliably prevents spurious readings in all patients. The current Nellcor sensor application guide that describes how the sensor and headband are to be applied states, “Forehead sensors are contraindicated for patients in Trendelenburg’s (head-down) position.” Fourth, headband pressure may cause injury to the tissue under the forehead sensor, particularly in patients where perfusion is suboptimal. Indeed, Shelley et al. 8noted that use of their setup in a subsequent study resulted in a burn on the forehead of one of their research subjects. This occurred with the probe secured by a Tegaderm dressing and without application of external pressure. When such a sensor is used, the skin at the site must be checked at regular intervals.
The place of forehead reflectance pulse oximetry continues to be the subject of discussion. We look forward to the results of further investigations. In the meantime, the educated user will recognize the potential advantages of forehead pulse oximetry as well as the limitations and potential hazards, and interpret the data accordingly.
Mount Sinai School of Medicine of New York University, New York, New York. james.eisenkraft@mssm.edu
