Skin reflectance pulse oximetry: In vivo measurements from the forearm and calf

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Abstract
This study describes the results from a series of human experiments demonstrating the ability to measure arterial hemoglobin oxygen saturation (SaO2) from the forearm and calf using a reflectance pulse oximeter sensor. A special optical reflectance sensor that includes a heating element was interfaced to a temperature controller and a commercial Data-scope ACCUSAT pulse oximeter that was adapted for this study to perform as a reflectance pulse oximeter. The reflectance pulse oximeter sensor was evaluated in a group of 10 healthy adult volunteers during steady-state hypoxia. Hypoxia was induced by gradually lowering the inspired fraction of oxygen in the breathing gas mixture from 100 to 12%. Simultaneous SaO2 measurements obtained from the forearm and calf with two identical reflectance pulse oximeters were compared with SaO2 values measured by a finger sensor that was interfaced to a standard Datascope ACCUSAT transmittance pulse oximeter. The equations for the best-fitted linear regression lines between the percent reflectance, SpO2(r), and transmittance, SpO2(t), values in the range between 73 and 100% were SpO2(r)=−7.06+1.09 SpO2(t) for the forearm (n=91,r=0.95) and SpO2(r)=7.78+0.93 SpO2(t) for the calf (n=93,r=0.88). The regression analysis of the forearm data revealed a mean ± SD error of 2.47±1.66% (SaO2=90−100%), 2.35±2.45% (SaO2=80–89%), and 2.42±1.20% (SaO2=70–79%). The corresponding regression analysis of the calf data revealed a mean ± SD error of 3.36±3.06% (SaO2=90–100%), 3.45±4.12% (SaO2=80–89%), and 2.97±2.75% (SaO2=70–79%). This preliminary study demonstrated the feasibility of measuring SaO2 from the forearm and calf in healthy subjects with a heated skin reflectance sensor and a pulse oximeter.