Journal of clinical monitoring and computing
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J Clin Monit Comput · Jun 2004
The influence of larger subcutaneous blood vessels on pulse oximetry.
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. ⋯ 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.
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J Clin Monit Comput · Jun 2004
Increased tidal volume variability in children is a better marker of opioid-induced respiratory depression than decreased respiratory rate.
During opioid administration, decreasing respiratory rate is typically used as a predictor of respiratory depression. Prior to opioid-induced apnea, progressively irregular breathing patterns have been noticed. We hypothesize that opioid administration to children will increase tidal volume variability (TV(var)) and that this will be a better predictor of respiratory depression than a decrease in respiratory rate. ⋯ TV(var) increases as children approach opioid-induced respiratory depression. This is a more useful predictor of respiratory depression than a fall in respiratory rate because the TV(var) increase is 10 times the drop in respiratory rate. A TV(var) increase also correctly predicts respiratory depression twice as often as decreased respiratory rate and is independent of age-related alterations in physiologic respiratory rates.