The influence of larger subcutaneous blood vessels on pulse oximetry.

OBJECTIVE: Recent studies have renewed interest in reflectance pulse oximetry, specifically for monitoring the patient's forehead. Blood circulation on the forehead immediately above the eyebrow is fed by arteries that branch from the internal carotid artery and lack the vasoconstrictor response present in more peripheral regions. Some investigators question, however, the reliability of monitoring SpO2 on the forehead due to prior reported inaccurate readings with reflectance sensors. The present study evaluates pulse oximetry accuracy when reflectance sensors are placed over potentially pulsing or moving larger arterial vessels, or over more homogeneous microvasculature devoid of larger subcutaneous vessels. METHODS. Ten healthy adult volunteers were fitted with reflectance pulse oximetry sensors and exposed to a controlled desaturation to 70%. Sensors were placed immediately above the left and right eyebrows as well as over the temple. Additionally, numerical modeling was used to simulate light signals and photon migration through a homogeneous tissue bed with an added static or dynamic artery. RESULTS: Sensors placed above the eyebrows tracked one another with significantly better accuracy than when comparing temple with the brow placement (RMS of the Differences = 1.12% vs. 4.24%, respectively). Photon migration simulations indicate that the detected light bypasses the interior of larger vessels, while vessel presence affects the red and IR light pulse amplitudes independent of SaO2. CONCLUSIONS: Placement of reflectance pulse oximetry sensors directly over larger cardio-synchronously pulsing or moving vasculature can significantly degrade SpO2 reading accuracy. Reflectance sensors placed low on the forehead directly over the eyebrow and slightly lateral to the iris appear to avoid such vasculature and provide consistent and accurate estimates of SaO2.

The effects of motion artifact and low perfusion on the performance of a new generation of pulse oximeters in volunteers undergoing hypoxemia.

INTRODUCTION: Motion artifact and low perfusion often lead to faulty or absent pulse oximetry readings in clinical practice. OBJECTIVE: Determine the impact of motion artifact and low perfusion on newly introduced pulse oximetry technologies during hypoxemic episodes in healthy volunteers. METHODS: Five different pulse oximeters from 4 manufacturers (the Datex Ohmeda 3900P; the Agilent; the Nellcor N-3000; the Nellcor N-395; and the Schiller OX-1, which is the European version of the Ivy SatGuard 2000 with Masimo SET) were compared with respect to their ability (separated or in combination) to provide accurate readings in the presence of motion artifact and low perfusion. Four of these oximeters represent the latest available oximetry technology, and one (the N-3000) represents a previous generation of oximeters. Oxygen saturation values (S(pO(2))) and pulse rate from the oximeters were recorded during episodes of induced hypoxemia in 10 healthy volunteers. Standardized and repeatable motion artifacts were generated by a motion machine and by having the test subject perform tapping and scratching motions. Perfusion to the finger was reduced by an inflatable balloon impinging on the brachial artery. The pulse oximetry readings from the test oximeters were compared to readings from control pulse oximeters on the unperturbed reference hand. The pulse rates from the test oximeters were compared to the electrocardiographically-measured heart rate. RESULTS: The frequency of faulty readings was increased by increasing motion interference and decreasing perfusion. The S(pO(2)) deviation was within +/- 3% of the reference reading > 95% of the time for all instruments during the control desaturation period in the absence of motion and with normal perfusion. With the combination of motion and low perfusion, the S(pO(2)) error was within +/- 3% less than 62% of the time for all oximeters tested. A significant difference in the frequency of large S(pO(2)) errors was observed only in the direct comparison of the N-395 and N-3000. The N-395 exhibited less frequent S(pO(2)) error exceeding 6% of S(pO(2)) in the combination of the most challenging situations (motion and motion with reduced perfusion). In the same situation the Datex-Ohmeda 3900P and Nellcor N-3000 showed significantly higher pulse rate errors than the other devices (Datex-Ohmeda 3900P 53% of the time and N-3000 37% of the time). CONCLUSIONS: The established model of creating motion artifact and low perfusion is capable of simulating a hierarchy of severe clinical situations. With solely motion or solely reduced perfusion the percentage of errors exceeding +/- 3% of S(pO(2)) increased by 20% and 10%, respectively, compared to the control period. Simultaneous presence of motion and reduced perfusion leads to a relative incidence of > 35% of errors > 3% of S(pO(2)) for the various oximeters. In this situation the N-3000 and the Datex-Ohmeda 3900P exhibited differences between estimated pulse rate and electrocardiographically-measured heart rate > 25 beats/min > 37% of the time.