Steady-state response characteristics of a pulse oximeter on equine intestine

The steady-state response characteristics of a pulse oximeter were evaluated on intestinal segments of seven clinically normal halothane-anesthetized horses. Arterial oxygen tension > 200 mm of Hg, end tidal carbon dioxide from 30 to 35 mm of Hg, and systemic mean arterial pressure > 70 mm of Hg were maintained throughout the recording periods. Values for percentage of pulse oximeter oxygen saturation, pulsatile blood flow, and percentage of signal strength were recorded from jejunum, ileum, cecum, left ventral colon, left dorsal colon, and descending colon. Probe placement on intestinal segments was recorded as over or not over visible subserosal or transmural vessels. There was no significant difference between median values on the basis of vessel codes for pulse oximeter oxygen saturations, pulsatile flow, and signal strength. Median values recorded for pulse oximeter oxygen saturation were 93% from jejunum and ileum and 95% from cecum, left ventral colon, left dorsal colon, and descending colon; median values for pulsatile flow were 576 from jejunum, 560 from ileum, 560 from cecum, 574 from left ventral colon, 578 from left dorsal colon, and 560 from descending colon; median values for signal strength were 50% from jejunum, 67.5% from ileum, 60% from cecum, 75% from left ventral colon, 50% from left dorsal colon, and 52.5% from descending colon. Median values obtained from each anatomic location were not significantly different for pulsatile flow or signal strength. Median pulse oximetry oxygen values recorded from jejunum and ileum were significantly lower than values obtained from other intestinal segments. When calculated arterial oxygen saturation was compared with oxygen saturation determined by the pulse oximeter, pulse oximeter oxygen saturation was consistently lower by 6.7% (jejunum and ileum) and 4.7% (cecum, left ventral colon, left dorsal colon, and descending colon). Equine and human absorption spectra were generated and compared for reduced hemoglobin and oxyhemoglobin at wavelengths of 600 nm (red) to 950 nm (infrared). Extinction coefficients calculated at wavelengths used by the pulse oximeter (660 nm and 940 nm) were nearly identical. The pulse oximeter is a self-calibrating instrument that displays oxygen saturation, heart rate, plethysmographic waveform, and signal strength indicator. Probe application was rapid and easy. Response time for the appearance of a plethysmographic waveform ranged from 5 to 25 seconds.