Brief reportPulse Oximetry Using a Disposable Finger Sensor Placed on the Forehead in Hypoxic Patients
Introduction
Non-invasive measurement of oxygen saturation is frequently used in the Emergency Department (ED) and has been referred to as the “fifth vital sign,” joining blood pressure, pulse, respiratory rate, and temperature. Due to its ease of use, it has virtually eliminated the need for arterial blood gas analysis to evaluate patients for hypoxia.
Non-invasive oxygen saturation can be measured from any skin surface using a pulse oximeter. The finger is commonly used for adults in the ED. A device designed for one body part (such as the finger) may not be reliably used on another site (such as the forehead), although it is unknown if the change in accuracy would be clinically significant. Despite this, disposable finger probes are occasionally placed on the forehead if an adequate signal cannot be obtained from the finger. An informal survey we conducted suggested that this is not an uncommon practice at our institution as well as four surrounding hospitals. Specific forehead (reflectance) probes are marketed. However, if a single sensor type could be used on multiple body sites, then stocking would be simplified.
The purpose of this study was to determine the agreement between a disposable finger oximetry sensor placed on the finger and placed on the forehead in hypoxic ED patients. To our knowledge, there have been no previously published studies addressing this issue.
Section snippets
Methods
This was approved by the Institutional Review Board at our hospital. We utilized a prospective observational study design. The study occurred in the ED at an academic urban hospital with an annual census of over 105,000 patient visits.
A convenience sample of adult patients was approached during times when two research volunteers were present in the ED from November 2000 to January 2001. Patients met inclusion criteria if they had a room air oxygen saturation value at or below 92% measured by a
Results
Twenty-five patients were approached for inclusion. One declined to give informed consent. Four were excluded due to incomplete datasets (inadequate waveform for data collection or failure to reach a stable oximetry value). Eleven of the 20 subjects (55%, 95% confidence interval 32–77%) had an absolute difference between the finger and forehead oximetry values ≥ 5%. This was our a priori definition of clinically significant. Table 1 shows the characteristics of patients who did and did not meet
Discussion
The pulse oximeter estimates the percentage oxygen saturation of hemoglobin in arterial blood (SpO2). These devices need to isolate the light absorption by hemoglobin and oxyhemoglobin from the confounding effects of the light absorption and scattering by non-arterial blood and other tissues. Thus, different algorithms are used if the sensor transmits its light beam through tissue (such as finger), or if it reflects the light off of tissue (such as the forehead) (3). Transmittance and
Limitations
There were several limitations to our study. The research assistants were not trained, or instructed, to identify interference from abnormal venous pulsations. The study was stopped before the planned sample size was reached. The sample size limited our ability to stratify our dataset. The final ED diagnosis was not assessed for correctness.
Conclusions
Despite providing a seemingly satisfactory waveform and a stable oxygen saturation measures, the use of a disposable (transmittal) finger oxygen saturation sensor on the forehead provided inaccurate readings in 11 out of 20 hypoxic ED patients.
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